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Our new to us Leon 1.4tsi is missing it's undertray.
I am noticing that in the cold weather the engine cools down very quickly if not working hard - it has readout for coolant temp which normally sits at 90*c, but queuing or even just a longer downhill it can drop 10*c in a minute, and the yesterday in -8 air it was struggling to sit at 80*c. The heater is noticeably cooler when it does this and I am finding I have cold feet.
Cold weather also knocks 30% off the usually excellent fuel economy.
So:
- I am expecting an undertray will help with it staying warm? (as well as keeping crap out, better aero etc)
- Is this engine really twitchy for outside temperatures? Maybe it is so small and efficient?
- Does the Leon have a crap heater? I am comparing to our older Ibiza and the V70 which was nuclear strength.
Thermostat stuck open?
Thermostat stuck open?
Or it could be the thermostat opening and introducing cooler water registering on the gauge - this would only happen once though.
I doubt the undertray has any effect tbh (heat rises and you've got a big hole at the front of the engine bay) - unless you're at the traffic lights a very very long time I doubt a biggish bit of aluminium/iron is going to cool down THAT quickly, but who knows... not me! Maybe a small engine will always cool down quicker?
More efficient engines waste less energy so therefore produce less heat.
It might just be the cold weather.
Thermostat stuck open?
It seems just fine in warm or average weather - coolant temp never moves.
It might just be the cold weather.
Indeed, -5 and more is an issue!
Our V70 has always had its undertray missing since we've owned it - has never effected it's heater/engine temperature and it does make access and cleaning easier.
The undertray does affect airflow yes. But not all cars have them - my Prius never did. I had an old petrol Passat (no, not that one) that was missing the undertray it should have had, and in wet weather it would barely run (and sometimes didn't) because of the amount of water being sprayed into the engine.
Re the coolant - remember that the heat in the coolant is waste heat, so a more efficient engine puts less heat into it. That said, your engine still should have lots of waste heat, just less than a normal petrol engine. It does sound like there's an issue.
Yes it will affect it.
Yes there's a big hole in the front to let air through the radiator - but the tray (along with the bonnet and wings) restrict the flow through of air - meaning its warmer in the engine bay. Remove the tray and there's a whole lot more Ir flow and hence cooling of both the water (in the radiator) and the engine block generally.
The undertray does affect airflow yes.
Remove the tray and there’s a whole lot more Ir flow
Although not while sat in a queue!
(And that's when the temp is dropping).
Start/stop would affect it and some engines will drop to idle revs when coasting downhill.
Both could affect it.
Although not while sat in a queue!
Probably. The undertray probably reflects a lot of heat back into the engine bay even when stationary. I would not expect that to have the significant effect you describe, but it's plausible. I'd say definitely head onto eBay and pick one up.
The heater is noticeably cooler when it does this and I am finding I have cold feet.
That might be a blockage in the coolant circuit, because I would expect that 80C coolant should still be able to heat air up to 22C.
My old Golf is missing its undertray. Whenever I've mentioned getting one to mechanics/MOT guys, the response has always been "wouldn't bother mate". I suppose it can help to stop excessive water ingress and stones flying up and damaging engine components and sensors. Likewise I was trying to think of the point of all these plastic engine covers. They're such an annoyance having to remove every time you need to do a job. The only thing I could think of is that they protect electrical/ignition components from rain when the bonnet is open... anyone?
Thats not the under tray causing the drop in temp. -8 is not cold for any car, its something not right.
Dont buy an under tray until you have spent money getting the real problem solved.
I suppose it can help to stop excessive water ingress and stones flying up and damaging engine components and sensors
Honestly, on my old car every time I went through a big puddle it would basically stall and I'd have to rely on momentum to keep me going until it dried out enough to restart. In a wet autumn on country roads this was every few miles. For the money, definitely get a new one, it's a no brainer.
I'd replace it, but I'd also want to understand why it's missing; that may provide insight around the heating/cooling issue.
That's a good point. Things like this can sometimes show how much the previous owners cared about the car.
Thermostat stuck open?
This would be the first thing I'd look at. The thermostat should keep the engine temperature above a lower limit no matter how much airflow is going through the radiator. As soon as the engine coolant temperature drops, the thermostat closes and shuts off coolant flow.
It's possible that the previous owner removed the thermostat because the radiator is partially blocked so overcooling the engine prevented it from overheating. I would have a flow test done on the radiator and have the thermostat checked as the first step.
Honestly, on my old car every time I went through a big puddle it would basically stall and I’d have to rely on momentum to keep me going until it dried out enough to restart. In a wet autumn on country roads this was every few miles. For the money, definitely get a new one, it’s a no brainer.
I've had a number of VAG group cars with missing undertrays and none have behaved like that.
When the car is absolutely stone cold, open the coolant tank and look to see if there is a bag full of silica inside (or it might say 'mit silikat' which also confirms it). If that bag is split/empty etc, then that's your issue. It is unfortunately a very common fault. The silica beads leak out of the bag and block the heater matrix (and cause other problems in the cooling system).
I've binned the undertrays off my cars before as they're usually mounted with small plastic fitting that break, which mean you end up drilling holes and bodging with zipties and it just makes servicing a PITA. I would have thought that any negative effects were offset by letting some relatively cool air get up into the engine bay and cool the exhaust header/turbo.
When the car is absolutely stone cold, open the coolant tank and look to see if there is a bag full of silica inside (or it might say ‘mit silikat’ which also confirms it). If that bag is split/empty etc, then that’s your issue. It is unfortunately a very common fault. The silica beads leak out of the bag and block the heater matrix (and cause other problems in the cooling system).
First things first, the cooling system actually takes very little of the waste heat out of the engine, almost all of it is lost via the exhaust. So engine "efficiency" doesn't really impact on how quickly it gets upto temperature.
Secondly, the cabin heater is more than capable of removing more heat than the cooling system absorbs from the engine. And it's usually bypassing the thermostat. So when cold the engine is pumping through the engine then through the heater matrix even when the thermostat is stopping it going through the main radiator. My old C-Max had such a powerful heater that it would drag the coolant temperature down when set to max heat, max fan and intake rather than circulation at anything less than motorway speeds. Cold weather just makes that worse as it's got an even greater differential temp between the incoming air and the coolant.
Turn the heater down, wear hat/gloves/coat, then the engine can run at the temperature it's supposed to and you'll get better mpg while producing less pollution.
For what it'll cost you I would put an undertray on. But it sounds like something is wrong with your engine too.
So engine “efficiency” doesn’t really impact on how quickly it gets upto temperature.
A whole lot of work goes into making the engine heat up as fast as possible. Like some cars having vents that close on the grill. This is to reduce cold start emissions.
Likewise I was trying to think of the point of all these plastic engine covers.
They're mostly for engine noise reduction (NVH).
Rapid cooling of the engine in cold weather may be down to missing or malfunctioning air inlet devices. Many cars have thermostatically controlled vanes behind the grill that reduce airflow in cold temperatures.
Sounds like we need to re-invent radiator blinds
Only a few years ago, it wasn't uncommon to see people driving around in winter with a piece of card blocking off half the radiator.
Edit: Or perhaps they're still around, as per @greybeard 's post^
A whole lot of work goes into making the engine heat up as fast as possible. Like some cars having vents that close on the grill. This is to reduce cold start emissions.
But running the cabin heater on a cold day whilst sat stationary is more than capable of undoing that as it's taking heat directly from the engine even before any passive cooling takes place.
Sounds like we need to re-invent radiator blinds
You needed warmth in the engine bay back in the 80s with carburetion as the jets would freeze.
The only place I can see it benefiting a modern car is if the oil cooler isn't thermostatically controlled or doesn't use an oil/water cooler.
Cheers all - I will go look for all sorts including [s]cardboard[/s] active vanes near radiator...
I do feel it may be an efficiency issue coupled with missing undertray.
I have had a couple of cars with dicky thermostats before, one refused to warm up in the first place and one just kept overheating.
Worth checking all sorts in case.
Undertray will also make engine a bit quieter too.
Engine top cover = NVH, looks pretty, some thermal insulation, some (not many) have an impact on pedestrian safety.
Undertray = NVH (radiated noise off the road surface), aerodynamics (a properly designed full length tray has a measurable impact), impact/abrasion damage from road junk (vertical surfaces near the front/bottom of the engine take a lot of abrasive damage, even from things like dry salt and dust, some versions of the engine may have auxiliary coolers mounted low down (unlikely on a 1.4tsi though)), some thermal (stops cold air coming in at the bottom).
I tore one of a Mazda 3 on a snowdrift once. I didn't really think about it till then engine bay starting filling with snow and accreted ice, and it failed to restart till this had been melted off and out by sitting in a garage
thisisnotaspoon
Full MemberI’ve binned the undertrays off my cars before as they’re usually mounted with small plastic fitting that break, which mean you end up drilling holes and bodging with zipties and it just makes servicing a PITA. I would have thought that any negative effects were offset by letting some relatively cool air get up into the engine bay and cool the exhaust header/turbo.
When the car is absolutely stone cold, open the coolant tank and look to see if there is a bag full of silica inside (or it might say ‘mit silikat’ which also confirms it). If that bag is split/empty etc, then that’s your issue. It is unfortunately a very common fault. The silica beads leak out of the bag and block the heater matrix (and cause other problems in the cooling system).
First things first, the cooling system actually takes very little of the waste heat out of the engine, almost all of it is lost via the exhaust. So engine “efficiency” doesn’t really impact on how quickly it gets upto temperature.
Secondly, the cabin heater is more than capable of removing more heat than the cooling system absorbs from the engine. And it’s usually bypassing the thermostat. So when cold the engine is pumping through the engine then through the heater matrix even when the thermostat is stopping it going through the main radiator. My old C-Max had such a powerful heater that it would drag the coolant temperature down when set to max heat, max fan and intake rather than circulation at anything less than motorway speeds. Cold weather just makes that worse as it’s got an even greater differential temp between the incoming air and the coolant.
Turn the heater down, wear hat/gloves/coat, then the engine can run at the temperature it’s supposed to and you’ll get better mpg while producing less pollution.
Are you quoting me here by mistake? Or are you saying I'm wrong? If so go and google what I've written and you will see it is a very common problem in VAG cars and the symptoms are cool air from the heating vents and problems with the thermostat sticking etc.
example threads:
https://www.golfmk7.com/forums/index.php?threads/burst-silica-bag-cant-get-it-solved.369711/
https://www.briskoda.net/forums/topic/479871-header-tank-silica-bag-fault/
https://www.audi-sport.net/xf/threads/silica-bag-burst.415086/
possibly thermostat
possibly waterpump
possibly gunked up cabin heater
possibly gunked up radiator
possibly combination of the above
mert seems to know what he is talking about w.r.t. the undertray.
I’ve binned the undertrays off my cars before as they’re usually mounted with small plastic fitting that break, which mean you end up drilling holes and bodging with zipties and it just makes servicing a PITA.
or you could, you know, get some more of the small plastic fittings...?
Your thermostat is stuck open.
Yeah, eBay is the place to go for small plastic fittings. You can usually find the exact ones for your car as the manufacturers re-use them for decades.
Turn the heater down, wear hat/gloves/coat, then the engine can run at the temperature it’s supposed to and you’ll get better mpg while producing less pollution.
A colder engine won't produce less pollution, I don't think. It should be more than capable of heating the cabin, so you've got an issue. Pretty straightforward DIY job to replace a thermostat (in fact I think it was the first job I ever did on a car); first thing you need to do is jack it up and remove the under..oh...
It should be more than capable of heating the cabin, so you’ve got an issue.
Naa, it IME fairly standard in most cars I've driven that the heater will more than overpower the engine when idling.
That and I drive like a nun.
Naa, it IME fairly standard in most cars I’ve driven that the heater will more than overpower the engine when idling.
I've never known it, but this:
it can drop 10*c in a minute
Seems dramatic. However it is possible that it's just cold weather. Has the OP tried googling it?
Loits alreadycoverd here, but obviously two issues, first being you need to get your thermostat checked/replaced.
However - to add a ittle on undertrays. Where they have been specificed/designed in, it is for a very very good reason and they can have a very important affect on the cooling capacity of your cooling pack.
Flow through radiators is based on a pressure differential - the pressure in front must be higher than the pressure behind for air to flow though. Undertrays and ducting are critical to maintaining the low pressure on the backside of the radiator, and thus promoting flow through the rad.
Removing undertrays to 'assist' with cooling of a car thats running hot generally causes more air to be forced into the engine bay, thus reducing the pressure differential, and reducing the cooling capacity of the radiator.
If your car was designed with an undertray - use it.
If you have a car thats overheating, removing undertrays to get more air into the engine bay will only cause it to run hotter.
Context - experience of automotive cooling pack design and calcs on Mercedes Benz and Cummins engines.
Modern engines don't retain heat like old ones, so coolant temperatures fluctuate quite a bit.
However as has been mentioned, I'd be checking for coolant flow through the heater matrix, as silica sachets splitting on VAG cars is a well known issue. Also the additional electric coolant pumps are a common failure.
With the car warmed up, and the heater at max temp, check the temperature of the hoses going to/from the matrix. If it's not blocked, both hoses should be a similar temp. If one is noticeable colder, then the matrix is blocked.
A colder engine won’t produce less pollution, I don’t think. It should be more than capable of heating the cabin, so you’ve got an issue.
Engines run more efficiently at higher temperatures, plus, catalytic converters need to be hot to work. Combustion is poor when the engine is cold so the fuel-air ratio is increased to compensate. This is why fast warm-up is a big part of modern emissions regulations. If the thermostat is stuck open, the engine will overcool and the engine management system will run a sub-optimal mixture because it will be trying to warm the engine up.
the heater will more than overpower the engine when idling
A heater core is tiny compared to a radiator. If you have the heater set to recirculate air, the engine will produce much more heat then the heater can handle if you are driving, once the car interior is warmed up. If you are driving in stop-start traffic, the heater should crank out plenty of hot air once the engine is fully warmed up.
A misbehaving thermostat/blocked heater core would be the really obvious things to check.
Your thermostat is stuck open.
That usually pings a coolant light on the dash as there is a sensor monitoring coolant temperature in VAG engines (close to the thermostat block and about £30 to replace or was). Stuck open the temperature won't get to past the lower limit line let alone 80C.
Modern engines don’t retain heat like old ones, so coolant temperatures fluctuate quite a bit.
You wouldn't know that from the temp gauge in a Ford, though. Apparently, to avoid their customers worrying about the temperature rising and falling within the normal range, the software is programmed to set the gauge in the centre, provided the temperature is somewhere within the permitted range. So when it's about to overheat, you get no warning until it does. If it's an Ecoboost, it's probably too late by then.
A heater core is tiny compared to a radiator.
Isn't really of any consequence. The radiator is sized to keep the engine cool when it's being driven as hard as you like in 40+ heat.
The heater matrix isn't sized relative to the engine. And the heat transferred to the engine at idle is going to be a tiny fraction of what it is when the engine is doing any work.
You wouldn’t know that from the temp gauge in a Ford, though. Apparently, to avoid their customers worrying about the temperature rising and falling within the normal range, the software is programmed to set the gauge in the centre, provided the temperature is somewhere within the permitted range. So when it’s about to overheat, you get no warning until it does. If it’s an Ecoboost, it’s probably too late by then.
Ford temp gauges are generally 7 position, but we're not discussing a Ford here.
However lots of manufacturer's no longer display an accurate temperature by default, as almost nobody actually regularly monitors a gauge to prevent any kind of damage. It's far more effective to bring on a warning lamp and cut power (although not every brand cuts power during overheating), than it is to think anybody would notice a gauge in the red.
However lots of manufacturer’s no longer display an accurate temperature by default, as almost nobody actually regularly monitors a gauge to prevent any kind of damage. It’s far more effective to bring on a warning lamp and cut power (although not every brand cuts power during overheating), than it is to think anybody would notice a gauge in the red.
aygo/c1/108 dont even have a temp gauge - i have an OBD unit that ive plugged in to keep an eye on it for obvious reasons. more useful than a TPMS tbh
But then like the time we had a golf and the person driving it who shall remain nameless phoned me saying that the temperature gauge shot up(boiled the coolant) went back to normal (coolant evacuated) then went back to the top before the engine locked up.......
people dont how understand what they are looking at.
To be clear: I get lots of warm air in the cabin. Plenty.
I just get cold feet. Which may just be a sh*t heater routing/pipe/design, but also my feet are where the (blast) of cold air hits without an undertray. It can say 22*c on the digital doofa, and yet I still can get cold feet. Never an issue in another car.
The engine also heats up, really well. Coolant temp comes up quickly. It has been stable across 40*c in Spain, 4 up with 4 bikes on the back. It is fine down to below freezing.
I have noticed though that in (very) sub zero weather, in a gentle downhill or sitting in slow traffic etc can knock 5-10*c off the coolant temperature. As soon as we move again, or pull on the flat, all is good within a few seconds of minute. This is also coupled with poor economy in cold weather - which can be fuel, wet roads etc as well as a cold air blast.
I am splitting hairs here - not noticing a problem running, more the case that I wonder if an undertray will generally keep my feet warmer and help the engine when not working hard to stay slightly warmer - and of course keep the winter crap out of my engine bay.
However lots of manufacturer’s no longer display an accurate temperature by default, as almost nobody actually regularly monitors a gauge to prevent any kind of damage.
They haven't for years. The middle 10 degrees of the gauge would cover everything from 70 to 110, the 40 degrees either side would cover 40-70 and 110-140 (with most cars blowing off steam through the cap at about 130). That was the standard Ford gauge from the late 80's, very similar for Rover, PSA and so on.
Stopped people complaining when their accurate gauge would creep up when the engine was working harder.
That was the standard Ford gauge from the late 80’s, very similar for Rover, PSA and so on
It was the mid 90s before Ford started moving to digital dashes. Prior to that it was still old fashioned analogue gauges direct to sender, although Ford did use friction gauges to dampen movement.
Only gauge manipulation done prior to the digital dashes was anti-slosh modules to stop fuel gauges bouncing around as much.
Does warm air come out of the footwell air vent?
I have noticed though that in (very) sub zero weather, in a gentle downhill or sitting in slow traffic etc can knock 5-10*c off the coolant temperature. As soon as we move again, or pull on the flat, all is good within a few seconds of minute.
That's possible, I suppose. A TSI engine is direct injection petrol, which means it's only putting in a tiny bit of fuel around the spark plug under light load so I can imagine that when idling it would produce so little heat that the coolant might cool down. A bit of googling sees this thread repeating all over the internet. People in cold climates reporting the same thing; some people saying it's because of the cold and other saying the thermostat is broken.
Does warm air come out of the footwell air vent?
Yes, and can be very hot.
However, not a lot comes out when on 'auto' - only when you switch to 'feet' (or 'feet' and another). It is like the 'Auto' doesn't kick enough heat out to feet - or/and the lack of an undertray is robbing heat. It is only a couple of mm of steel and a carpet...
Then your answer is clear, at least for step 1. £25 on eBay.
It was the mid 90s before Ford started moving to digital dashes. Prior to that it was still old fashioned analogue gauges direct to sender, although Ford did use friction gauges to dampen movement.
This was pre digital, as far as i know the "adjustment" was done electrically (no, i don't know how). TBH, it might not have been a ford gauge, but the rest of the vehicle was a ford parts bin special! Lights, handles, switchgear, indicator stalks, seat frames, seatbelts and so on...
However, not a lot comes out when on ‘auto’ – only when you switch to ‘feet’ (or ‘feet’ and another).
Every car I've used with an auto setting does that.
They seem to think everyone really wants heat coming out the dash vents as priority. I either have it coming out the windscreen blower or the feet. Auto sucks. Wouldn't read too much into that.
Absolutely what Snotrag says. My car runs hotter without the undertray (which accidentally got binned grr).
You can see it, as it has a ye-olde temperature gauge which hasn't been processed.
However – to add a ittle on undertrays. Where they have been specificed/designed in, it is for a very very good reason and they can have a very important affect on the cooling capacity of your cooling pack.
Flow through radiators is based on a pressure differential – the pressure in front must be higher than the pressure behind for air to flow though. Undertrays and ducting are critical to maintaining the low pressure on the backside of the radiator, and thus promoting flow through the rad.
Removing undertrays to ‘assist’ with cooling of a car thats running hot generally causes more air to be forced into the engine bay, thus reducing the pressure differential, and reducing the cooling capacity of the radiator.
my feet are where the (blast) of cold air hits without an undertray
Matt out earlier.
<img src="https://images.thestar.com/M5e9HWczWsygMkh9SSDQ7WuB85k=/1086x613/smart/filters:cb(2700061000):format(webp)/
" alt="" />
Maybe you've got poor circulation 😬
Completely different motor so maybe irrelevant but my T4 van was behaving oddly temeperature wise and the heater wasn't getting warm enough. Turned out to be a crack in the thermostat.
The TFSI's thermostats are common problems, was on my golf too. Not sure if the TSI's are better? You should be able to get a 2nd undertray for cheap I'd have thought but I'd budget to get thermostat replaced.
It seems just fine in warm or average weather – coolant temp never moves.
And other comments about the temp gauge. Correct that they are non-linear in what they display. The VW clusters I worked on (mid 2000s) had a dead band from about 80 to 110 degs, and this is common approach across all manufacturers. From a drivers POV you really only need to know three conditions: Warming up, normal running, getting too hot. My Ka didn’t have a temp gauge, just a warning lamp for if it got too hot (which I never saw).
If you have the correct coolant mix, the boiling point is way above 110degs. When working on cooling systems at Ford in the early ‘90s, correct coolant mix and pressure cap fitment would boil at 131deg at sea level, and 127deg at altitude (think top of Grossglockner pass).
I had a bad thermostat in the past. It did take longer to warm up, but otherwise the symptoms were identical. Unnoticeable during the summer months, it would sit at the correct temp all day long. In winter it would drop when descending, then sit at the correct temp when revs picked up again.
Removing undertrays to ‘assist’ with cooling of a car thats running hot generally causes more air to be forced into the engine bay, thus reducing the pressure differential, and reducing the cooling capacity of the radiator.
Just a thought but this would probably make the aircon less efficient as well in the summer, as less air would be going through the condenser.