Over-Heat Diagnosis

A few days ago I had a 2001 TDI brought to be with a problem of persistent over-heating. The owner replaced the thermostat but the problem persisted. We'll assume that your cooling system is properly serviced with G-12 at a minimum mix ratio of 50% and no leaks or loss of coolant.

I'll try to explain how to properly trouble-shoot this problem so less time & $ is expended.

If your TDI starts to over heat you need to understand how the cooling system functions.

There are three main components that can cause your engine to over-heat:
<ul type="square"> [*]Thermostat [*]Coolant pump [*]hoses that connect the thermostat & pump to the radiator [/list]

The first thing to check is to verifi/confirm that you can feel heat coming from the dash vents with the heat set to "high" and the fan is running.

If you can feel heat coming from your vents then your coolant pump is circulating coolant (pump is pumping).

If you can not feel heat coming from your vents then your cooling system is not circulating. Dont condem the pump (yet).

Check the lower hose coming from the radiator. This hose should feel "HOT" and firm (under moderate pressure).
If this hose is collapsed or "flat" replace the hose.

Now, that you've checked the lower radiator hose is "warm" and it feels firm with no "flat-spots".
If you have full heat coming from the dash vents your problem is a stuck thermostat.

If you have no (zero) heat coming from the dash vents, lower radiator hose is "hot & firm".
The problem is a faulty coolant pump. Most likely the impeller has seperated from the impeller/pump shaft.

Thanks Herm. As usual, your comments are clear and helpful.

Somewhat off target, but I hope it's not too far off. 2005 Jetta TDI: upon starting, coolant light blinks & remains blinking. Allowed it to run about 5 min, temperature came up to normal; no evidence of leaks; fuses seem to be okay. The fans aren't on, but it's cold (for the Oregon Coast!). What might be going on; what might be fried?

Thanks very much for any insight.

Do a search for "coolan migration"
There is a lot of info out there.

great post HT
TDICLUB - more than a forum

Great post. On my 96 Passat TDI, mechanics replaced the waterpump and thermostat while replacing the engine. I still have a overheating problem. I have heat from the vents and the top and bottom hoses are hot and firm and I have no loss of fluids. They are telling me it's not the engine and it is the radiator (and they will not cover the costs since it's not in the engine they replaced) I need help...how can I test the radiator before I buy a new one??

Update....I rechecked the heat from the vents. When sitting at idle, the car does not overheat. While I am driving it, it overheats and the vents blow cold air with the heat on? Would this be a pump problem, radiator problem or an internal engine problem....I'm desperate....help

drillsergeant said:

Great post. On my 96 Passat TDI, mechanics replaced the waterpump and thermostat while replacing the engine. I still have a overheating problem. I have heat from the vents and the top and bottom hoses are hot and firm and I have no loss of fluids. They are telling me it's not the engine and it is the radiator (and they will not cover the costs since it's not in the engine they replaced) I need help...how can I test the radiator before I buy a new one??

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Are you sure it's overheating? Is there coolant entering the round reservoir through the small hose on top connected to the this reservoir? Do the rad fans come on when it is in this overheat mode? Does the guage show way past the 190 mark? There have been cases where the head gasket leaks cylinder pressure into a water port in the head resulting in what seems to be a boil over. This boil over is actually the cylinder gasses building up and escaping through the round coolant reservoir and acting as if it's boiling over. The radiator's can be had relatively cheap so, to make the engine replacement shop happy, throw in a new one and see what happens. Later!

overtemp

Th fans and the warning light is comming on and there is coolant boiling back to the resivoir. I've called all radiator shops that I can find with no luck through their searching. The motor runs smooth and there seems to be no other problems. What is a good source for 96 TDI Parts. I've checked several links on the site but they are mostly dead links. Thanks for the help.

drillsergeant said:

Th fans and the warning light is comming on and there is coolant boiling back to the resivoir. I've called all radiator shops that I can find with no luck through their searching. The motor runs smooth and there seems to be no other problems. What is a good source for 96 TDI Parts. I've checked several links on the site but they are mostly dead links. Thanks for the help.

Click to expand...

The warning light is only for low coolant. The low coolant light could be coming on and flashing because the cylinder pressure is blowing coolant out and bringing the level below the sensing probes. What does the guage read? Later!

possibly the impeller is loose on the shaft? such that it operates at idle but not at speed?

Also check and make sure that the radiator is not plugged internaly and externaly--I had fuzzy stuff collect on the outside of mine once water hose it down

Bad radiator

I have had three instances of the radiator itself being the problem.

It looked healthy, none of the internal flat tubes were gunked up. Flushing and boiling out did not help.

The second time I took one back to the radiator shop we found the problem. The tech inadvertently scrubbed the fins agains something, and naturally tried to comb them back straight. When he did you could see the fin-to-tube joint was badly corroded *under* the paint.

From the outside the radiator looked new, but it was corroded at all of the fin joints.

I had this in a '76 chevy luv, a '95 Chevy, and an '89 F250.
So I have learned that persistent overheating problems, with the pump, fan, fan clutch, and fan controls being ok -

Replace the radiator with a *new* one, not rebuilt.

DanG

Overgheat

Harvieux said:

The warning light is only for low coolant. The low coolant light could be coming on and flashing because the cylinder pressure is blowing coolant out and bringing the level below the sensing probes. What does the guage read? Later!

Click to expand...

The gauge when operating normal reads about 190. When it starts to overheat, it quickly climbs to 240. There is no white smoke from the exhaust and I don't have a leak. I'm still trying to find a radiator and I'm looking for a site that has a waterpump. Any suggestions? Also, I have noticed that it does not have the G12 fluid, it has the green stuff. I plan on changing that also. The car had sit for almost a year while I was out of the country and I'm finding all sorts of gremlins. It did not overheat before the motor was replaced. Thanks for all the help. This is a great site and I'm glad I found it..

drillsergeant said:

Update....I rechecked the heat from the vents. When sitting at idle, the car does not overheat. While I am driving it, it overheats and the vents blow cold air with the heat on? Would this be a pump problem, radiator problem or an internal engine problem....I'm desperate....help

Click to expand...

This most likely describes a problem with the water pump.
Rule out coolant level by visual check, when cold.
Check for flow from the small hose into your coolant reservoir - on mine I have to open the top fill cap to see it well; only do this with the engine cold. If you do not have this it is an indication of a bad pump, or a clogged line.
When it is overheating does the level in the coolant reservoir get a lot higher? This indicates that a steam bubble is forming in your engine and displacing the coolant into the reservoir; a confirmation of overheating.

Are you seeing other signs of overheating - other than your temperature indicator? The temperature sensor often goes bad on these cars.

weedeater said:

possibly the impeller is loose on the shaft? such that it operates at idle but not at speed?

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Post #6 specifies new WP so, that should eliminate a loose impellar on shaft possibility. The magic words drillsergeant specifies following my questions is the fact that the guage shoots up way past the 190 mark which would lead to a radiator problemo due to both WP and T-stat being eliminated as culprits. Later!

Harvieux said:

Post #6 specifies new WP so, that should eliminate a loose impellar on shaft possibility. The magic words drillsergeant specifies following my questions is the fact that the guage shoots up way past the 190 mark which would lead to a radiator problemo due to both WP and T-stat being eliminated as culprits. Later!

Click to expand...

Post # 7 Update...." I rechecked the heat from the vents. When sitting at idle, the car does not overheat. While I am driving it, it overheats and the vents blow cold air with the heat on? Would this be a pump problem, radiator problem or an internal engine problem....I'm desperate....help" drillsergeant writes.

Harvieux,
I thought perhaps you had missed this part of the update. Can the heater vents blow cold, with plenty of coolant, for any other problem than the *new* water pump slipping on the shaft? Certainly if the only problem was the radiator the heater would be working fine with very hot fluid.
Wasn't there a recent thread where a new water pump was doing exactly this?

My original post about radiators was just to pass on information, which seemed to be the intent of the Original Poster. Then I became concerned that drillsergeant might think I was replying to him. I think weedeater has a good point.

DanG

drillsergeant said:

I'm still trying to find a radiator and I'm looking for a site that has a waterpump. Any suggestions? .

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Sarge, one stop shopping. Free shipping with order over $50.

https://www.autohausaz.com/search/group.aspx?sid=lz0bjr55ebn54jblbjuti445&makeid=800026@VW&modelid=1360213@PASSAT%20TDI%20&year=1996&cid=16@Cooling%20System

Thanks for all the help. One forumshad something about air in the system. When I went to put fluid in the car, alot of air came out the Resivoir. I am going to follow the steps on getting that out and see what happens. I am ordering another water pump and radiator Monday. Hopefully that will fix the problem since I do not have a trusted TDI mechanis within 2 hours of me. Anyone want to come to Fort Benning Ga? lol Thanks again

Herm TDI said:

Check the lower hose coming from the radiator. This hose should feel "HOT" and firm (under moderate pressure).
If this hose is collapsed or "flat" replace the hose.

Now, that you've checked the lower radiator hose is "warm" and it feels firm with no "flat-spots".
If you have full heat coming from the dash vents your problem is a stuck thermostat.

Click to expand...

Thanks Herm, I had an issue which was hopefully resolved today after careful research on the forum.

My 2002 Golf, ALH, manual transmission decided for the first time in 6 years and 207,000 miles to pop over 190 degrees while driving.

Possible arena of reasons:

1. A bad fan sensor relay [location: lower driver side radiator] MKIV that controlls fans [low and high speed]
2. Dual Fan Modual computer [location: under battery] MKIV
3. Thermostate [location: front right passenger side block] MKIV
4. Or ...sadly ...a clogged or corroded radiator [reading posts in this thread]

My thermostat seized closed. I've read on the board that when a thermostat on an MKIV fails, usually it will fail open. Not in my case.

Click images to enlarge




The Alert on the dash


The problem


The remedy



I have a question on the housing for the thermostat. How are you to install it correctly? Are those two plastic prongs designed to grasp the top hoop shaft of the thermostat itself allowing easier installation? As you can see in the photo, the prongs broke off on the hosing. Luckily I had another one handy.

BrianCT

My 04 Jetta just began running over 190 while on the move and yesterday the high temp alert began flashing for the first time. I pulled off the hway and let it idle for 10-15 minutes until the temp went down to almost 190. Ran the heater full, blowing hot air and this helped to keep the temp from red lining as I made my way home. The temp needle bounced between slightly above 190 and about 1/4 inch from red lining depending if I was on level or down hill surface to climbing up an incline..... Coolant reservoir was full but it did not look like there was coolant spilling into it while I was letting the Jetta idle after the warning light flashed on. I just had water pump changed and coolant flushed and changed this past April when I had timing belt changed..... Jetta now sitting at my local VW garage waiting for the Dr to look her over.
I was not running the A/C when the temp began running hot, use A/C hardly ever. Like the windows down and sunroof wide open..... Any suggestions?

This is something my 2000 TDI Beetle (5 speed manual) just recently began doing! 246,000 Miles and its finally overheating I havnt had the time to trouble shoot the components but everything being said here seems to be what I thought I should check. Also my A/C clutch went sometime last year. The engine is getting super hot like even after driving a few miles, the fans are not coming on but I think that the issue with my buggy is the coolant system namely the thermostat water pump etc. Tomorrow I plan on taking a look after I drive it up and down the street a few times. I have not checked the Heat because here in MA its been crazy humid. And with no A/C i just ride with the windows down anyways (still have to fiddle with that window switch sometimes) I'll try to give you guys some more information tomorrow and a more detailed and thorough explanation.

Oh Great now my Red Coolant light is flashing after I drove it to Manhattan KS and back. I don't remember if the O-Ring is installed backwards or correctly, after installing a new thermostat OEM. Would that make a difference? I will check the coolant level and check for leaks tomorrow morning. What with the VAG-Com do I need to check it with? All this after installing a new thermostat on my 04 NB TDI PD BEW

You probably just need to add some more coolant. It often goes down on the first heat cycle or two after you replace the thermostat or otherwise drain the system.

In fact it can go down a LOT on the first run, especially if you do not pull a slight vacuum on your vent hose (until fluid comes out) while you are refilling the system. Gravity filling does not work well at all.

One thing I have begun noticing more and more often; In rust belt states, the radiators are failing because the tank crimps are rusting out. They leak slowly at first, sometimes with little evidence and then all of a sudden they begin pouring out. Also on many of these cars there are a ton of leaves and debris in between the radiator and condenser. This can compromise cooling as well.

Good points.

In rust belt states or seacoast areas, I have seen apparently healthy looking radiators that simply could not reject the heat. I found that the fin to tube joints were corroded and that the corroded area would simply not conduct heat as well.

They were very normal looking, with paint covering the corrosion. The corrosion was not apparent until you ran your finger over the fins, and they broke away from the tubes.

what hevster said ... those VW plastic/aluminum radiators will not seal well after years and they will start seeping coolant for the sides

EDIT: Thanks for the replies going right now to check it out and add some G12 Coolant. Did add G12 Coolant because the bubble tank was low.

Thanks Weirdajs/Aj/Andy

My 09 Jetta Sprotwagen TDI. Like the sealed battery system the cooling system can lose coolent. In Texas we have had many 100 plus days and my warning light came on while on a trip through west Texas. I was lucky that a VW dealer was fairly close. They checked out the system and said the coolent droped below the sensor in the reservoir. They added a small amout of coolent. I guess it is like a sealed battery. Often we have trouble with our batteries in the summer time here in this heat. I now carry a qt of coolent already mix with distilled water in case this happens again. This was six weeks ago and no more warning light. Oh, the heating gauge still showed normal with all this.

Overheating, '02 Golf TDI, thremostat issue

Resurrecting this helpful thread...

Reading numerous thread on the issue, mine is the same as this one, posted above by winvavw. The coolant expansion tank does have a pressure relief valve built in that vents to atmosphere (dribbles down the passenger's side of the engine compartment) when the car overheats. This overflow worked like it should when the car overheated a couple of days ago and expelled a bunch of coolant.

The expansion tank, cap, etc... all test out fine. They hold and relieve pressure as they should. I've yet to check lines to see if air passes through them, but will try that next. Perhaps something is clogged. All of the observable indicators/symptoms as originally written up by Herm_TDI tell me that the thermostat is done. A new one, with housing, is en route.

To my main question. Many other people on the forum state that the thermostat is designed to fail in such a way that the car never overheats. They state that it actually never gets warm. That's a nicely engineered set-up, but is it the only way it can fail?

Can the thermostat "stick" in such a way that the car overheats?