B5 S4 Thermostat stuck open? Twice?

[Lear70]

So I bought this 2000 S4 from salvage YEARS ago. I knew the thermostat was stuck open (or I thought I did), but I was in Florida so,,, whatever. I'll get around to it.

Through a long list of trials and tribulations, I finally had to take my engine out, went Stage 3 (which is awesome fun), and in the meantime brought the car up to TN last year. When I put it back together, did the Timing Belt job, all Audi OEM parts, including a new thermostat.

Once winter came, I froze. Car never came up to temp. Good coolant level, develops pressure, just never gets above 130 or so temp in the winter and I freeze to death with no heat in the car.

I have it back open now, thermostat looks fine, obviously getting coolant to that area (a little came out when I opened the cover after draining), so WTF???

Before I just throw a new OEM thermostat in there (which I purchased thinking I just had a bad one), does anyone have any thoughts?

Bueler?

[Timtheguru]

Coolant temp sensor failing and heater core clogged.

[themadscientist]

Coolant temp sensor failing and heater core clogged.

That was my first thought.

The gauges themselves can get wonky as well.

Op, I would look up the coolant temp in VCDS and confirm it with a temp gun.

[Lear70]

Thanks, gents,

The temperature gauge is correct, already confirmed through Vag Com.

How would the temperature sensor reflect the inability to get heat into the car as well during the winter? I mean I literally freeze. Below 40 outside temp you CANNOT get warm air to the vents, and on the highway... forget about it, bring a ski suit.

Does the system feed through the heater core before going on to the engine? Because the engine is running cold, too...

Would like to fix it before I button her back up.

[Zba]

Heater core

[Timtheguru]

Here you go.

[Lear70]

Thanks for the pic. One thing that bothers me - the heater core would prevent the engine from coming up to temp?

Looking at the picture, it looks like coolant flows from the radiator into the water pump, pumps through the block, comes out the back side, past both coolant temp sensors, then into the heater core.

This is what has me confused. My coolant doesn't get warm in the winter. The oil temperature barely registers on the gauge (low) and the coolant never gets much above 130/140.

If the sensors are measuring this temperature, that basically means the engine is in bypass constantly, aka thermostat open. Correct?

[a4kamila6]

If its heater core like the guys have said, you can try flushing it by taking both hoses off behind the firewall and putting two external hoses on and running hot water through it. Try back and forth meaning flush one way, then flush the other. I have heard people use CRC but to each their own. If your core is leaking, you are pulling the dashboard apart.

[Lear70]

Yeah, I was worried about that. Thanks for the heads-up on the heater core flush.

At this point I'm not convinced the heater core is the problem, seeing as my oil temp and coolant temp are both way low when it gets cold outside. This screams to me open thermostat, but with a new one, what are the odds? I just put the new one in, we'll see if it fixes the problem. If not, it's going to be a while before I do any heater core work. Just not interested. Not after pulling the engine twice in one ear. F that.

[Timtheguru]

Does the coolant temp get warm when it's warm out?

I'd still investigate swapping out the green coolant sensor on the xover pipe on the back of the engine on the passenger side. It could be failing intermittently and causing faulty readings to the gauge cluster/vagcom. If you have never replaced it, please do, it's cheap and easy.

What color is your coolant? Is it brown?

Heater core flush is the easy way to get some heat back, but the best way would be to get the all metal B5 Passat heater core.. it's a hell of a job, but the best fix. Do some research on this by googling. Lots of info out there.

[Lear70]

Thanks, Tim.

Using the yellow coolant. This engine is from Phil out at Audis4parts.com - mine bit the dust about 3 years ago due to some ass hat "Audi and Volkswagen specialist" in Orlando.

It's hard to say what the coolant does when it's warm out. It's certainly more in the appropriate range. Up to 100+ degrees and humid here in TN and the Coolant and Oil temp stay pretty close to middle of the range, even when idling in traffic, although my fan wasn't connected when I first put it back together and it got up to the top of the gauge on my first test drive sitting in traffic, had to shut down and wait for it to cool off. Found the problem - disconnected elec fan wire.

So as far as I can tell, the sensor is working right. I can replace it, but unless that sensor controls when the thermostat opens or when the heater core gets coolant, I can't imagine that's a cause of me freezing to death in the winter when I can't get heat out of the A/C system (and it blows ice cold in the summer).

Fall is ok, it gets warm enough to keep me warm, but under 40 degrees outside, forget it, you're not taking your coat off in the car.

I got it all back together except a few connectors in the back I have to look up again go where (blue is to the after-run pump? Mine is deleted). Put the intercooler system back together, core support back on, hoses and wires up there, and I'll be test driving it hopefully tomorrow afternoon after I bleed the clutch.

I may try to run some air through the heater core tomorrow before I get busy with it and see if I can push coolant out the other side. 15 psi or so. If I can't, or it's obviously blocked, any ideas on how to force water through there? I don't have a source of pressure except a garden hose.

[Timtheguru]

The heatercore always has flow to it, but can be bypassed if it is clogged. The lack of heat in the car is most likely due to the heater core. Open or closed thermostat has little to do with this, it's just clogged, very common issue. (here's a flush DIY, be very careful removing the hoses.. and don't run air through it... http://www.audiworld.com/forums/a4-b...w-pics-349252/) The other thing could be the climate control module if the mixer is stuck or something (scan for codes with vagcom).

The coolant temperature not being where you think it should be could be the temp sensor, and not related to the heater core or it being 40 degrees out and you freezing in the car.

[Zba]

Yank out the thermostat and test it in some boiling water.

[MacDaddy]

^^boiling water trick FTW, but if its stuck open you will see for yourself.

A ghetto trick is put cardboard infront of your radiator so the engine runs hotter and then get to it in the summer.

Luckily you can swap out thermostats without removing the timing belt, get everything off as if you were going to do a belt and crank over the engine while walking the belt part way off the pulleys, you will end up with enough clearance to squeeze the thermostat housing out.

[Lear70]

^^boiling water trick FTW, but if its stuck open you will see for yourself.

A ghetto trick is put cardboard infront of your radiator so the engine runs hotter and then get to it in the summer.

Luckily you can swap out thermostats without removing the timing belt, get everything off as if you were going to do a belt and crank over the engine while walking the belt part way off the pulleys, you will end up with enough clearance to squeeze the thermostat housing out.

I already pulled it earlier today. Had the engine out anyway, so had it far enough apart that it only took half an hour to line up the cams, put the lock bar in, then loosen up the belt and get the thermostat housing off.

- - - Updated - - -

Yank out the thermostat and test it in some boiling water.

Going to do that right this second. It's sitting here on the table, I already put a new one in just to be sure before I buttoned the car back up.

[Lear70]

The heatercore always has flow to it, but can be bypassed if it is clogged. The lack of heat in the car is most likely due to the heater core. Open or closed thermostat has little to do with this, it's just clogged, very common issue. (here's a flush DIY, be very careful removing the hoses.. and don't run air through it... http://www.audiworld.com/forums/a4-b...w-pics-349252/) The other thing could be the climate control module if the mixer is stuck or something (scan for codes with vagcom).

The coolant temperature not being where you think it should be could be the temp sensor, and not related to the heater core or it being 40 degrees out and you freezing in the car.

Thanks for the link, I'll do that tomorrow, already have a bunch of extra hose laying around.

Quick question, are you certain about the orientation of the lines in that picture which shows zoomed in on the inlet/outlet?

It's backwards from the graphic above from Audi (left side inlet, right side outlet).

Thanks!

[themadscientist]

Can the thermostat be installed backwards on the 2.7?

[Lear70]

Thermostat that was in the car opens fine under boiling water. Definitely not it.

Wonder why the coolant temp and oil temps are so cold in the winter? They can't both be that far off...

Weird... will try to flush the heater core tomorrow.

[Lear70]

Can the thermostat be installed backwards on the 2.7?

No. Not as far as I can tell anyway.

Normal installation spring goes inwards into the block.

Turning it backwards makes the spring stick too far forward, and you can't get the cover plate on.

[Austonwerner4]

No. Not as far as I can tell anyway.

Normal installation spring goes inwards into the block.

Turning it backwards makes the spring stick too far forward, and you can't get the cover plate on.

It can be on backwards and the cover can be installed. But it will leak very slowly....

[Jism]

Did you actually check the waterpump? You should always replace that one the moment you replace belt(s).

Tecnically a clogged up heater core would not block the complete flow of coolant.

[Lear70]

Did you actually check the waterpump? You should always replace that one the moment you replace belt(s).

Tecnically a clogged up heater core would not block the complete flow of coolant.

I did when I got the engine, but I haven't since, because the engine isn't overheating.

If I weren't getting any coolant flow, I'd overheat the engine in a New York second in the 105 degree TN summer.

I'm definitely getting coolant movement, I'm just not getting the engine to operate where I think it should when it's cold (mid gauges) and not getting heat to the A/C system.

Will see what a coolant flush looks like tomorrow.

[themadscientist]

Something's definitely screwy here. I have personally experienced repeat failures of OEM parts, right out of the box. Rare, and usually relegated to electronic components, but it does happen. Bad batches. Once you get that thermostat out, you could do the hot water test, but by slowly upping the temperature, confirmed with a thermometer of some kind, to see when and if it's opening early. It sounds like the most plausible thing at this point. Heater core clogged or not will not make a difference in operating temperature. That's all up to the thermostat and fans.

[MacDaddy]

Just dont re-use the timing belt once it has been uninstalled.

[Lear70]

Something's definitely screwy here. I have personally experienced repeat failures of OEM parts, right out of the box. Rare, and usually relegated to electronic components, but it does happen. Bad batches. Once you get that thermostat out, you could do the hot water test, but by slowly upping the temperature, confirmed with a thermometer of some kind, to see when and if it's opening early. It sounds like the most plausible thing at this point. Heater core clogged or not will not make a difference in operating temperature. That's all up to the thermostat and fans.

I did that last night.

It opened smoothly and, once removed from hot water closed smoothly. Put it back in, opened smoothly, then closed smoothly when pulled. It was a brand new OEM part. Have no reason to believe it was faulty.

We'll see what the heater core flush does today. May also see if I can find a replacement coolant temp sensor today while I'm out and about.

[christianb5s4]

I had this same exact issue when I bought my car, only exception being I didn't have the no heater issue. Turned out to be the non-OEM green coolant temp sensor, the car hit consistent temps but would not get up to full operating temp and the issue would get worse in colder weather.

That sensor eventually took a dump and I replaced with a new OEM sensor and the issue was gone.

[Lear70]

I'll pick one of those up to, just to eliminate it from the possibilities.

[Timtheguru]

Grab extra O-rings and plastic U clips for it, I usually keep 5 or so of each on hand, but then again I have multiple B5/B6/C5 cars I own and work on.

[AudiTechS4]

buy a cheap laser temp gun and verify temps and radiator inlet and outlet and check temps at heater core hoses this should tell you your issue

[Lear70]

buy a cheap laser temp gun and verify temps and radiator inlet and outlet and check temps at heater core hoses this should tell you your issue

Thanks,

That's what I'll have to do if flushing the radiator and replacing the thermostat and sensor I just got don't work.

We'll know hopefully tomorrow afternoon, although it won't cool off enough to really know what I'm dealing with for another month.

[Jism]

Grab extra O-rings and plastic U clips for it, I usually keep 5 or so of each on hand, but then again I have multiple B5/B6/C5 cars I own and work on.

The plastic clip breaks off with the least resistance. Ask the dealer or shop for a steel one, that should fit exactly as the plastic one. Trust me, i've bin there (Coolant sensor green left-back engine).

[Lear70]

OK, done for the day. Busy day...

Besides finishing all the electrical connections and such, I replaced the coolant temp sensor (it was nasty, looked burnt, and probably original), and flushed the heater core.

With just a small amount of water, I got very little sludge out of it, it ran orange-ish for about 5 seconds then clean water. Ran it both ways for 2 minutes, and didn't see any kind of resistance to water flow or reduced flow. Nice and steady, so I don't suspect a clogged heater core.

I'll be putting the radiator core support on tomorrow and servicing it with coolant. Is there a recommended procedure to make sure I don't get any air in the system and it flows properly?

Thanks,

[christianb5s4]

Here's a kind of DIY on getting the air completely bled from the system, the heater core is the highest point so you'll want to do a final bleed at the bleed point by the heater core:

http://www.audizine.com/forum/showth...cooling-system

[Timtheguru]

Be sure to open the bleed bolt/valve below the intake manifold, next to the throttle body, when you are filling the expansion tank. Once coolant comes out of the bleeder the engine and radiator should be bled, close the bleeder, then bleed at the heater core. You may want to try and prefill the heater core with your premixed solution before you add all that plain water into the mix, this may be overkill.

Our cars are self bleeding with the expansion tank, so after enough driving all the air should work itself out.

Let us know how it goes.

[Lear70]

Great, thanks!

[eplgh]

I'm having similar problems, did you find the fault in your car?