Temperature rise after shutdown

[maxiz1]

Hello,
I've been having this problem for a long time. Can't remember when but I do recall when the car is relatively new the temperature gauge would stick at the middle after shutdown, but since then have moved up to the 1st notch after the middle tick. Even after rebuilding the engine, the problem still persist.

I never have any issues on the engine running but this is such a sore eye to watch. Does not happen in my other car (different brand). Sometime in the past I had changed the coolant flange to aluminum type and wondering could this be the cause.

I heard this is normal but if possible I wanted it to be back to the original state. Any ideas how to resolve this?

[q5 dave]

Experience the same on my b5 but it goes back to normal 30 seconds after startup. Apparently it's just heatsoak https://www.audiworld.com/forums/a4-...t-off-2757323/

[walky_talky20]

My guess is that your viscous fan clutch is worn out. The electric fan covers this up for the most part, but it has a higher temp threshold before it kicks in. Even though the gauge is in the middle, the actual temp is higher, and the amount of engine bay airflow is lower than normal. With those combined, the heat soak after shut down is now sufficient to show on the gauge.

I have 3 B5's on the road right now, and the 2 of them that had this behavior which started at about 250k miles, and the fan clutch was junk on both.

[maxiz1]

Thanks for the replies.

I see this heat soak problem is common for a lot of people. By the way, I've ruled out the lack of airflow possibility. I've changed the viscous, electric fan, thermostat and radiator during my recent rebuild. I've also knocked out a lot of the crusty stuff from inside of the block, and cleaned the head (at home though, not using specialized equipment). But this still happens.

So this morning I used a IR gun to check the head temperature when the engine is running. It turned out that the metal surface is about 15-20F above the coolant operating temperature (I hope this is the same for others), and after shutdown it did not increase further but the coolant temp equalize to it.

But still the question is.. why this did not happen when the car was relatively new?

So I thought of two possibilities:
i) My head heat transfer is no longer efficient that it runs +20F above the coolant temp
ii) I'm using the aluminum coolant flange that soak heats very quickly from the head, which in turn transfer it to the temperature sensor causing the reading to increase.

I wonder whether ii) was the reason why Audi selected plastic flange in the 1st place. The metal flange I substituted is indestructible and has been in the car for more than 7 years without any problem, unlike the plastic version that cracked almost every year. But I think I'm gonna give it a try swapping it back sometime in the future.

[strophius]

I started having this same problem a few years ago on my '01, just kind of popped up out of nowhere. Eventually what fixed it for me was replacing the viscous clutch, as Walky mentioned, the radiator (original), OEM water pump (had been running metal for years, even before the temp rise thing), coolant temp sensor, and installing a phenolic intake spacer. That was about a year ago and I've never had the temp rise since, even after moving to an area with much warmer summers. Oh, and I also have the aluminum rear coolant flange, which has been on the car for like 8 years.

All of those were done at the same time so I'm not sure which one specifically improved things. I will say that reading it with Vag-com/VCDS still shows a slight climb in coolant temp after the engine is shutoff but not enough to move the instrument needle anymore. Before, the needle would move up to the first, sometimes second, notch past the middle mark after turning off the car. Temps were always rock solid while the car was running.

[walky_talky20]

I have the metal flange on my B5 Passat 1.8T (ATW), with 230k on pretty much original everything else (as far as I know). The fan clutch is probably getting weak, but the e-fan works well. This car does not have any heat soak / temp rise issues. So I don't think a metal flange would automatically cause this problem. It may be an additional factor, but at least in my example, the metal flange did not cause any temp gauge differences.

[maxiz1]

Thanks. That rules out the metal coolant flange. So I have one final factor to consider. Oil temperature.

Walky/strophius - how much is your normal oil temperature? Should it be the same as the coolant or slightly higher? I'm running at 212F (18F higher than coolant). The thing I noticed is that if I get the coolant warmed up to temp but not the oil, this heat soak problem does not occur. Maybe an oil cooler issue? Sometime years ago I changed to hella (stamped made in china).

So this is my final guess or I just have to accept and live with it.

[strophius]

I think my oil temps are usually in the 220-230F range. Although they did run lower (200ish) when I had the OEM oil temp sensor, but they've been in the 220 range since I installed an aftermarket one (OEM one was leaking). This was done many years ago and way before I had the coolant temp issues, so it just may be the sensor is reading a little high.

[walky_talky20]

I definitely noticed the oil temp runs hotter as a result of a failed/weak fan clutch. Very noticeably so. Can exceed 225f on the gauge - up to nearly 250F sometimes, whereas the same car normally sit around 200F and will not exceed maybe 215F oil temp under any conditions.

[maxiz1]

Walky - is there a way to test the viscous fan? I tried to spin it by hand when it's cold and hot, but it seems to have the same amount of resistance. Should it be fully locked to the pulley when it's hot? Some video suggested to poke with a newspaper when it's running but I'm not willing to risk the plastic blade to explode. Probably using a laser tachometer to compare the speed when cold and hot?

EDIT: Nevermind. I just poked it with a newspaper and seems to be strong enough. Anyway, which thermostat are you guys running? 87C or 82C? Mine is 87C and vag shows the operating temp is 95C (slightly to the right of the middle tick)

[walky_talky20]

I currently have B5's both with and without good (used) viscous fan clutches. I should do a video comparing the behavior. And then a third with a brand new (aftermarket) unit.

Maybe I can snag a laser tachometer as well, in addition to some infrared temp readings. So much science!'

Definitely stock t-stat here. I think It's usually 90-93C in the ECU whenever I have checked it. I would think 95 would still be in the center. This thread suggests anything 80-100C shows as center of the gauge:
https://www.audizine.com/forum/showt...er-EEPROM-mods

[strophius]

For the thermostat, I'm just running an OEM 87C one.

One other thing, I didn't see if you said your electric fans were activating properly. There is a second coolant temp sensor underneath the car that is fitted into the lower radiator hose on the drivers side. I think it's what controls the electric fan activation and speed (not 100% sure). I also replaced that while doing the rest of the coolant stuff a few years ago even though my electric fan was spinning while idling.

[walky_talky20]

Yes. All B5 1.8T have a 2-stage fan switch ("dual thermal fan switch") in the lower rad hose (blue color, 4-pin, 2000+) or screwed into the end tank (gray color?, 3-pin, 97-99).

They are not really sensors, just switches. 2 switches in one package with different temperature thresholds. They trigger 2 separate relays (Low speed relay and high speed relay) for the e-fan. Low speed just runs the power through a resistor mounted under the driver's side frame rail. It is somewhat common for that resistor rust out and fail. This can short and blow the fuse, or just leave you with high speed only.

Thresholds: 84-95C (LOW) / 91-102C (HIGH). So LOW kicks in when radiator outlet reaches 95C - meaning the engine & radiator inlet is well above that. Low speed stays on until radiator outlet drops below 84C.

Notably B5 2.8's (at least the later ones) use a fancy fan control module. I much prefer the 1.8T simple temp switch and 2 relays. Very easy to troubleshoot and everything is cheap.

[walky_talky20]

I currently have B5's both with and without good (used) viscous fan clutches. I should do a video comparing the behavior. And then a third with a brand new (aftermarket) unit.

Maybe I can snag a laser tachometer as well, in addition to some infrared temp readings. So much science!

Well wouldn't you believe it, I actually made this comparison video for you all.



Knowledge morsels found in this video:
- Instrument Cluster gauge (at least in this 2001 A4), stays in the center until about 99 or 100C. 102C is only about 1 needle width past center.
- A good viscous fan clutch cannot be easily stopped at idle by a newspaper, even when cold (I try right after a cold start at 75F).

Knowledge morsels I didn't include in video form:
- Just rotating the fan by hand with engine off is not a good way to test the clutch
- There *is* a difference between the good fan and the bad one, but it is very minor, even comparing side by side.
- Best way to test: If you can stop the fan with engine up to temp, it's junk.

[maxiz1]

That's a great effort walky. Thanks! So you got 99C on vcds and the needle is almost dead centre. I think I'm going to try to replace my sensor with a genuine one and see how it goes.

[walky_talky20]

I will see what I can find out for FAN RPM before I swap out my bad viscous fan. I feel like that would be a much more scientific way to diagnose vs trying to jam things into a moving fan. LOL

I have a unique opportunity with so many of these darn B5's in my driveway.

[walky_talky20]

More Data Acquired.

I took the working Passat on a drive and watched the temps.
- While driving (ambient 70F): 83-85C average
- Could not get it to go higher, even running A/C and driving slow, everything just works too good.
- Notably, this Passat has the metal coolant flange.
- Temp Rise After Shutdown: ~9-12C

Short Drive, 55mph, 70F ambient. Data gathered Immediately after shutdown of Passat with good OEM fan:
- Temp is 85C
- After 1 minute = 90C
- After 2 minutes = 93C
- After 3 minutes = 94C
- After 4 minutes = 93C
- After a longer drive (20 minutes, 70mph, 75F ambient), I did see it the Passat spike as high as 97C. The gauge still showed dead center at this time.

I bought a Photo Tachometer to measure viscous clutch fan speed. Yay!
- Hot Passat (after heat soaking for 4 minutes shown above) showed a fan speed of 1180 RPM at idle. Good Fan!
- A4 with bad fan, at cold start shows fan speed of only 600RPM.
- That's half the speed!

Tomorrow I will gather more data points: Passat Cold and A4 Hot, before I finally change the junk fan clutch.

Update:
- Passat shows 1200 FAN RPM right at cold start idle (measured once idle drops to 800). Good Passat.
- Junk A4 Fan is still only 600 RPM or less even while actively overheating.

Junk A4 behavior:
- While cruising at 55mph, 75F ambient, coolant temp runs around 99-100C!
- Shut down at 100C, temp climbs to 105C after about 3 minutes. This is "half a tick" over the center position.
- This car will do much worse than this, depending on conditions. Stop & Go, hotter temps, etc.
- Winter totally covers up this problem. With ambient below ~50F, there is no issue shown on the gauge.
- The coolant temp reported by dash is about 2C higher than the ECU. Interesting.

Musings:
- I'm thinking my Passat might actually have a colder thermostat?
- Both of these cars have metal coolant flanges. I don't think this is a factor.
- Photo tachometer is super cool, and can diagnose this fan issue, if you know what it "should" be.
- I should gather fan rpm at higher engine speed. Maybe 3000 RPM, I will try to capture this.

[maxiz1]

Walky,
From your test, I can conclude that all engines experience heat soak. The only difference is for "better" engine, your operating temperature is a bit lower hence the final heat soak temperature does not exceed the temp gauge "dead zone", hence maintaining at centre. As for my car, the normal temp is 95degC, a few minutes after shutdown it rises to 108C. Kind of the same amount of temperature rise with yours.

So I think it is because there is so much scale buildup in the block that it increase slightly the normal temp. I was thinking of using a 82C thermostat but boy they are pricey. But it is something that I look forward to. With my current 87C thermostat, I got 95C hence I would assume the same temp difference when using 82C thermostat would end me up with 90C spot on. Running 5C cooler could also benefit me by having lesser knock on boost using normal pump gas.

Thanks for your effort!

[walky_talky20]

I changed the junk fan in the A4. No more overheating while I'm driving it. Yay.

I will record some new data on VCDS temps during cruise/heat soak and new fan RPMs.

[maxiz1]

Just an update of this. I think I have finally figured out the cause of heat soak despite having new radiator, thermostat and water pump. It was the airflow!!

After driving for 30 mins, vcds normally registered 95C on the coolant temp. After arriving home and while the engine was running, I switched off the AC, took out the front bumper and removed the AC radiator, letting the viscous fan doing the job without any restriction. After about 2 mins, temperature dropped to 93C. Then I shutdown the engine. Still see the VCDS heat soaking up to 99C (but not 110C before this), and the cluster needle remained in the center.

So this is due to restriction at the AC radiator. There's a lot of stones and chips trapped in the fins. which I'm going to clean it out.

Thanks for all your input and effort (esp walky)!

EDIT: Installing an EFK alone without solving the AC radiator blockage will not solve my issue. Before this, I tried to manually run the AC fan at full speed for 2 minutes before shutdown (while the AC system is OFF), but still did not fix the problem.

[walky_talky20]

Update from my side:

-The overheating on my A4 seemed to be fixed with the new fan clutch, but I still noticed the gauge would occasionally climb after I parked it hot.
- This issue got progressively worse over a few months until it would overheat in stop and go traffic again.
- It got even worse to where it would overheat when climbing long or steep hills unless I blasted the cabin heat.
- I got very familiar with what driving styles increased or decreased engine heat!

I changed the thermostat, and this instantly fixed everything.
- The faulty thermostat was a Motorad 88C. I replaced it with a BEHR/MAHLE 87C unit (I believe this is the factory OEM unit).
- I think operating the car too long with this mild overheat condition was not good for the Coolant Temp Sensor, as it has now failed shortly after.
- I do wonder if my viscous fan was not quite as faulty as I thought. Perhaps the lazy thermostat was not putting enough heat into the radiator and thus the viscous fan would never engage fully. In any case, I was able to easily measure the RPM difference between new and old, so it was at least partially worn.

The failure of my Coolant Temp Sensor is one of the typical failures
- On cold start, ECU reads 143C when ambient is 5C. Causes a very difficult cold start (too lean).
- Dashboard Temp gauge is completely unaffected and reads perfectly accurately.
- Reseating CTS electrical connector can get ECU sensor to read correctly for a moment, but engine vibration will immediately revert it to 143C.
- Unplugging sensor (open circuit) reads -48C, so it is definitely shorting within the sensor itself.

My learnings from this:
- If you have temp gauge climbing after parking, perhaps changing the t-stat is a slightly easier and cheaper first step (on a 1.8T).

[maxiz1]

Update from my side:

-The overheating on my A4 was basically fixed with the new fan clutch, but I still noticed the gauge would occasionally climb after I parked it hot.
- This issue got progressively worse over a few months until it would overheat in stop and go traffic again.
- It got even worse to where it would overheat when climbing long or steep hills unless I blasted the cabin heat.

I changed the thermostat, and this instantly fixed everything.
- The faulty thermostat was a Motorad 88C. I replaced it with a BEHR/MAHLE 87C unit (I believe this is the factory OEM supplier).
- I think operating the car too long with this mild overheat condition was not good for the Coolant Temp Sensor, as it has now failed shortly after
- I do wonder if my viscous fan was not quite as faulty as I thought. Perhaps the lazy thermostat was not putting enough into the radiator and thus the viscous fan would not engage fully. In any case, I was able to easily measure the RPM difference between new and old, so it was at least partially worn.

The failure of my Coolant Temp Sensor is one of the typical failures
- On cold start, ECU reads 143C when ambient is 5C. Causes a very difficult cold start (too lean).
- Dashboard Temp gauge is completely unaffected and reads perfectly accurately.
- Reseating CTS electrical connector can get ECU sensor to read correctly for a moment, but engine vibration will immediately revert it to 143C.
- Unplugging sensor (open circuit) reads -48C, so it is definitely shorting within the sensor itself.

Thats correct. The CTS is a negative temp coefficient (NTC) resistor, hence the resistance decreased with increasing temp. Thats why you see the reading is high when it shorted internally.

By the way, i still havent resolved my temp climbing issue after shutdown. Even trying to run the ac fan for 2 minutes post SD does not work. But I figured out that it only happen when the oil temp reaches its setting of 100degC. The coolant temp somehow equalizes with the oil temp.

[walky_talky20]

I was mostly testing to see if there was an issue in the wire harness vs the sensor. By unplugging, and observing the reading change drastically, I knew the short was within the sensor, and not within my wiring harness. If the sensor were to fail open, the unplug test is much less useful, so that was a lucky break.

Side note: My A4 has way more horsepower now that it runs at the correct temperature. If feels like I chipped it! The ECU was definitely pulling back a lot of boost and timing when it was running too hot.

What are your coolant temps while driving?

[maxiz1]

During cruising if oil is up to temp, the lowest i could get is 91C, no matter how cool the ambient temp and even at high cruising speed.

During traffic, it can rise up to 96C.

I think at higher higher, the ECU retards slightly the timing to control detonation. Hence reducing power slightly.