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First, Hello to everybody. I am new to the forum and a big fan of TDI.
I am from Bulgaria and I own a 2001 VW Passat 1.9 TDI PD 130hp B5.5. I have no mods done and am not planning to do any, stock my passat is just enough of a car for me.

About my problem, recently I have done a head gasket job on the car and it works great now, but I have noticed a problem with the temp gauge that started couple of days before I did the gasket. So I start the car in the morning and drive off, I can't say if it is worming slower than before because it is winter now ( -10 C) and before it wasn't, but it does not take forever for the car to worm up to 90 degrees C (may be about 6-7 km city driving at about 2000 rpm). I do not turn the heat on until it warms up. Once its warm, no matter if I am at a traffic light or not, I turn the heat on and I see the temp gauge go down from 90C to the next line. Then drive a little more and it slowly goes up again to the 90C degrees. This same thing happens very often, like when I am driving about 80km/h down hill and let the car go on neutral for 2-3 min it goes down again, but never less than one line from the 90C. Even, before I did the gasket I drove 100 km to my town without a problem and when I stopped for a minute I noticed the temp gauge went down one line.
When I changed the HG I looked up the thermostat and tested it in hot water and it worked fine. By what I have read it could be two things the thermostat or the temp sensor. I was wondering what you guys think? Could I have put the thermostat in the wrong way (if there is a wrong way to put it), or it's most likely the temp sensor, or may be something else. Thank you in advance for the help.

vankaa
 

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Something is wrong. The warm up time sounds normal but the other problem with heat going down is not. Other than your suggestion, you could have have an air pocket which the Passat are known for. Normally it results in no heat though, not the coolant going down. I don't think you can put the thermostat reversed, maybe the temp sensor is bad.

Did you bleed out the air? Here is the article: http://www.myturbodiesel.com/1000q/b5/coolant-flush-passat.htm at the end you'll see that quite a lot of air comes out of the heater hose.
 

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I will try your suggestion and will bleed the coolant system and will change the sensor too, as soon as it gets a little warmer. Thank you much and will keep you posted.
 

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Was not the temp sensor, changed it on sunday and it still does it. I have heat in the cabin when the heater is blowing no matter if I am driving or waiting at a Light. I noticed the other day that when driving about 40 min (about 0 to 5 degrees C) through town and when I arrived home the top radiator hose was hot, the bottom was cold and the antifreeze in the overflow was cold also. Is that normal? I am not very familiar with the cooling sistem. Also, if it's not too much to ask, can someone give me the flow direction of the antifreeze throughout the the whole system. Next thing on my list is to change the thermostat and then bleed any air from the system.
 

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This is very strange. It's not the thermostat since you said it worked fine when testing it.

The coolant is not moving because the radiator hoses were hot-cold. Normally this is due to thermostat or bad water pump. It could have been due to the very cold temperatures but even still there should be enough to warm the lower hose. If the water pump fails the coolant will not circulate correctly and the engine will overheat. However, you say you have the opposite problem - the car is constantly running cold to the point where turning the heat on will cool the engine! I don't know, this is very strange.

According to the manual, if the temp gauge reads low with heater output normal, the cause is a bad instrument cluster or coolant temperature sensor. If you datalog the coolant temperature sensor you will be able to see right away the actual temp.

According to the manual, if the temp gauge is low w/poor heater output the coolant concentration is wrong, faulty thermostat, radiator fan not turning off. Another possible cause not mentioned in the manual is a failed water pump.

You're not loosing coolant and the head gasket is fresh. Why was the head gasket previously changed?
 

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Yes, I am not loosing any coolant, gasket was changed about 1000km ago due to overheating. One of the hoses of the oil cooler broke while I was driving and all the coolant came out. The car overheated, but I managed to stop right after the big STOP sign that came on the dash.

As a result, the cooling system was getting overpresurized and was pushing the coolant out of the system through the overflow on a longer travel. Car was not mixing any fluids and was not overheating and even was not pushing the coolant out much when driven in town. I gotta admit that I drove it a little over a month in that condition because it was not giving any other problems and I had no money to do the job. Throughout that time, a lot of people were telling me that it could be something else, not the HG, so I changed (and good that I can work on my car my self) the visco cluth of the fan, oil cooler, water radiator (I had a small leak anyway) and fixed the electric fan of the A/C because it was not turning on.

When I changed the gasket the only thing I think I probably did wrong is that I put the three holed gasket on, when the one on the car was two holed. I know this now, but then I did it I did not know that when shaving the head you should not put a different gasket, its only when you shave the block when you do that. I actually shaved the block a little with abrasive stone just to make sure its perfectly straight and take out any warping. I could see that it was not straight because after a few moves back and forth with the stone there were dark spots clearly visable, which the stone could not reach.

The thermostat I did not test at what temperature it opens, the only thing I tested and saw is that it clearly opens and closes in hot water. I don't know what is the effect if its faulty and it opens at a lower temp. I also changed the coolant many time after the overheat and never had a problem with air bleeding, the system seems pretty good at selfairbleeding itself.

I just want to say that when its 0C degrees out side and car warmed up to 90C, the temp goes down a little even without the heat on. I notice that between trafficlights it will be a little be under the 90C.

The problem started a week before I actually changed the bad HG. First time I noticed the temp gauge move from the 90C in any direction except when it overheated, (you should know that knowing I had a bad HG I was driving with one eye on the temp gauge at all times) was when I foolishly accelerated the car pretty fast to 100 km/h up a small hill and going down the hill to a stop light I noticed the temp gauge one line down :eek (that should be 80C by my guess).

Wow thats a lot of writing, so that is the story, I will soon try to VAG-COM the coolant temp when the car is warmed up and see if the gauge reads correctly. Thank you much for the help by now and I hope we solve this.
 

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Quick update. Finally good weather here (5oC) and I was able to replace the thermostat. The temperature now hits 90oC degrees and stays there at all times. I did manage to check the temp with VAG-COM and after a good 40 min of driving it showed 77 degrees, so I could assume that the thermostat was indeed stuck open. Thank you much for the help.
 

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Hallo. As u see I did just sign up in this forum :)
I have exactly the same issue with my car as you vankaa. So the problem for you was caused by the thermostat?
Mine was replaced for 6 month ago -approx. 8000km ago , with a brand new Wahler one. T sensor changed together with the thermostat.
If I turn the heating to position 1 temperature dorps to the 1 white line(perhaps 80 degree C) before 90 degree C , but if it is on 4 then it drops down to 1 white line after the thick white line(block) perhaps 70 degree C.
Have not yet checked to hoses, but gonna do this on Sunday.

Would appreciate your advice very much :)

Tnx!
 
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