Strange Heating Problem

[guy.joseph]

Hi
I've got a strange heating problem that I'm currently trying to get to the bottom of on my 2.0 E85.
I replaced the water pump a few months ago, with a coolant flush and change as part of this process. I didn't change the thermostat which I now potentially regret!
The temperature gauge on the car has always behaved perfectly, warming up then sitting bang in the middle.
As the winter season came in, I started to notice that the cabin heating was not blowing out a lot of heat. A bit of research and thinking an airlock was the issue, I ran through a bleeding process of the system which didn't make a massive difference.
I then connected to INPA and found an error for the Aux. water pump shorting to ground. A (used, but believed to be working) replacement pump fitted and the error has disappeared from INPA. However, the heater seems to output very little heat still.
INPA also showed the "Coolant Output Temperature" at 4 degrees.... pretty much ambient temperature. I find this strange, potentially a dodgy sensor, but it gave me another thought...
I took the car on my usual commute today, which is 25 miles on the motorway at 70mph. Temperature gauge bang in the middle. The heating was still barely warm though. When I stopped, the hoses seemed firm, as if the system was properly pressurised, but the bottom radiator hose was COLD - like not even lukewarm. The ambient temperature today was like 0 degrees, and the top of the radiator is hot, gradually getting cooler from the top corner to the bottom. Is this normal or not? I also cannot ever remember hearing the fan come on, even after a long drive through town.

My question is what is wrong now - and currently wondering about any of:
1) There is still an airlock in the system. I am going to try the bleeding technique with a big bottle above the filler cap that I've seen on here.
2) The temp sensor - I have ordered a new one as it's a cheap one to try and replace, but if the lower radiator is cold maybe it's accurate
3) The coolant filler cap? Could be a pressurisation issue?
4) The thermostat is stuck closed so there is no flow through the radiator? Although in which case, why hasn't it overheated?
5) Something else?

I have had some amazing advice from this forum, so wonder if some of you geniuses could help me!

Cheers

[Pondrew]

From the above, I would guess your thermostat is stuck OPEN, not closed, and the temp sensor for the gauge is faulty.
Possibly a slight airlock aswell which is preventing flow around the heater matrix.

A good way to see what is going on is to blank the radiator with a big blanket or towel over the front air grilles at tickover. Keeping a close eye on everything obviously to see what is getting hot and what is not.

It may take a while with the ambient so cold, as it will disperse heat from everywhere pretty quickly.

[guy.joseph]

That's another good thought - I guess if all the coolant was running through the radiator, the radiator could feasibly be doing an excellent job with the cold ambient temperature (and snow!) so it is circulating all the coolant and keeping it quite cool, without leaving much spare heat for the cabin?

I might give the towel idea a try, although increasingly starting to think I need to order a new thermostat!

I've also watched the "coolant temperature" on INPA (seems to be the sensor that drives the gauge in the dash) which behaves as expected, it's the "coolant output temperature" that seems too cold, although it is at the bottom of the radiator!

[colb]

I would agree, change the thermostat, sounds like its stuck open. When refilling get the front of the car as high as possible when refilling have heater controls set to Hot High setting and bleed using the bleed screw, when no air bubbles emerge do it up and test drive keeping an eye on the gauge and heater output in the car.

[MEV Z4]

Thermostat being stuck open is a possibility - but I don't think it's the issue here.

If that was the issue, the car would take Forever to reach operating temperature and the temp gauge may even sit just below it's normal middle position when it's so cold outside. I also think you'd still get a good amount of heat from the vents as the engine is still going to be 'hot' even if the thermostat was constantly open.

I'm certain the issue of poor heating is also nothing to do with the water pump, on previous cars I've had with a failing pump the symptoms are a climbing temperature needle but rev it hard and the needle goes back to the middle position as the water starts to flow. (Yes the heater will also work better when you rev a car with a failing water pump - then go back to cold when you stop).

I'm working the same bad heating issue on my 2004 Z4 2.5 Litre. The heater in it is nowhere near as hot as it should be - but everything else seems Ok.

So today I unplugged the heater core rubber hoses (On the bulkhead under the bonnet) and ran clean / hot water both ways through the heater core several times, then blew it out with my mouth (Using a clean rubber hose). Then mixed up rad flush / water - poured it in and left for 20 mins (Not how it's supposed to be done - but i did a Full Flush 2 weeks ago when I got the car). Then I flushed it both ways again with clean water and refilled the Core as much as I could before replacing the hoses. There was no evidence of blockage / resistance / sludge / grit or any problem at any part of this operation.

Driving it today (0 Degrees outside- Ie really cold outside) I stopped to examine everything. I noticed the Radiator top hose was hot, Radiator lower hose STONE COLD and the Radiator itself only warm near where the top (Hot) hose went in (Completely cold over most of it's area) and not much warmth coming out of the heater. One heater hose (Heater core Input Hose on the bulkhead) was really hot and the other hose (Heater core output hose) next to it MUCH cooler - Strange as I know there's no blockage / resistance in the heater core.

However engine seems perfectly happy - No steam / coolant loss / temp gauge reading perfect. Everything great apart from the heater....

Still sounds worrying though - Is the Coolant Circulating properly if the heater wont get fully hot and the radiator is cold?

So I got in the other car VW 2.3 Golf MKIV 2002 and did a 25 min trip (Not wasting petrol - Just made a point of using different car to compare). I I did the same examination. The air from the Heater Vents was Red Hot and heater was working perfectly as it always has done, top rad hose hot, lower radiator STONE COLD and the radiator itself only hot in the corner near where the top hose goes in - Cold over most of it's area. NOTE- I've seen videos that say a cold lower radiator hose means a stuck thermostat. Look Like That's Not true - Certainly on cold days. Furthermore, even though it was freezing outside the VW heater was giving good hot air even on idle (800rpm) so it's not true that the heater will only work with the Thermostat open. (Also - When you start any car, you get something out of the heater long before it's reached operating temperature).

The only difference was that on the VW the heater core input & output hoses were BOTH really hot. (Z4 one hose hot / other hose MUCH cooler). This seems to be the CRUCIAL difference.

With that in mind and after a lot of internet searching and thinking I came up with these results / ideas -

i) BMW Expansion tank thermostat stuck shut stopping heater output going down to radiator? No expansion tank only has a thermostat on Automatics and it only controls when the Automatic Transmission fluid - not the coolant / water.

ii) Coolant Air Bubble? No I've bled it good and proper from the BMW Bleeder on top of the Radiator, squeezed all the hoses and done enough miles to get all air out - Also this would not explain why the heater's output hose is cold.

iii) BMW Blocked Radiator? No - The VW is doing the same, it appears that on a cold day the thermostat hardly opens. Also had the same poor heating issue on my E36 2.5 ages ago -Took out the radiator and it was completely clean & flowed perfectly, put it back in and then realised the problem was low coolant because head gasket was leaking.

iv) Blocked Pollen Filter? Took it out - Made no difference to the Z4 heater.

v) Rad Cap. No it's not leaking / steaming so it's OK. Even if it wasn't pressurising the only effect would be slightly worse engine cooling / hotter engine which is not the issue here.

I think the following may be the answer -

The heater control valve (Located under / near rear of the manifold on Z4) opens and closes in response to a signal / voltage from your dashboard switch. The signal is delivered via a Temperature Sensor located on the heater unit. This sensor feeds(?) / regulates (?) what the heater control valve does after factoring in the air temperature (Is that right?).

I refer you to this video on Youtube where they're fixing an E46 and explain it well with pictures.

BMW No Heat Fix – Thermostat, Heater Core Sensor and Valve DIY FIXED

I suspect that sensor he's talking about is a common fail. If that sensor gave a lower voltage than it should to the Heater Valve then the valve may only open half way and give you only warm heater output. The heater valve itself will appear to work as you can still vary the temperature between cold and 'not hot.'

This would also explain why the Heater core input hose is hot and the output hose is MUCH cooler. The reduced flow will still make the Input hose hot, but this reduced flow also means the water spends much more time in the core and by the time it comes out it's substantially cooled.

The suggested remedy is replace the Temperature Sensor on the Heater Unit - or just unplug the feed to the Heater Valve (Under rear of Manifold) and apparently it defaults to fully open (That may have an implication about your Air Con working properly, but mine appears not to work any way - A Faulty sensor might also explain that as a bonus).

This may explain why on older models like the E36 (Which have dual climate control) you can get hot air out of one side and not the other. I assume there's a separate sensor for each side and one could fail before the other.

From about 1998 with the end of the E36 they dropped the separate passenger / driver climate control.

Anyway I'll unplug the Heater Control Valve on the Z4 tomorrow and see what happens.....

[MEV Z4]

Follow Up - Couldn't find the Heater Control Valve on the Z4.
Correction- it's not under the manifold and maybe it doesn't have one.

Replaced the Heater Temperature sensor (Twist in type) which you access by removing the radio / air vents & plastic panel just in front of the gearstick - You'll see the Sensor there (In the Top right area of where you can see) with a Blue & Yellow wire coming of fit. Made no difference - Still a poor heater.

Least of my worries as now the Timing chain /guides appear to have jumped / broken.
Car starts but wont idle and cuts itself off, if you rev it then it will splutter up to speed but every 20 seconds makes a noise like a 10p in a washing machine. (Chain slapping something?)

I suspect also the oil pump / strainer is blocked by the remains of a plastic chain guide as the lifters all of a sudden became more noisey / tickey.

I wont give up on this - but I'll have to leave it for a month or two.