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Discussion Starter #1
Hello all,

I have just joined this forum so please bare with me...

I have had my 1994 80 Tdi Avant for a few years now with no major problems but over the past 9 months or so there has been an intermittant problem with the oil pressure warning light flashing on and beeping. I did worry at first but the oil level has remained constant with no additional noises from the top end to concern me. I took it to a garage and they couldn't figure it out but basically said it was running sweet as (it has about 180k on the clock now).

This problem has occured sporadically throughout the last nine months until recently when it has been happening more often and with the oil warning light staying on for longer. Driving along at a steady 70mph at around 2500rpm it would stay on but when I dipped the clutch and dropped the revs to below 1000rpm it went off instantly - and then came back on again when I increased the revs again. I have replaced the topmost oil guage/sender unit but that hasn't cured it - I know there is another one which I am going to change when I get the chance. Does this sound like a high pressure problem or a faulty sender unit? I have lost a small amount of water from the reservoir too.

Also, just recently, I have noticed the temperature guage doesn't really get above 60 degrees! I know it is cold at the moment but surely it should still get up to temperature - especially after a 100 mile journey! I have a new thermosat to fit but I'm not so sure that's the problem as I thought that only controlled the water flow to the head and not the block.

Any thoughts on these issues would be much appreciated! Sorry for the lengthy post!

Cheers

Mike
 

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Hello welcome to the forum, I'm new too.

I think that most engines have a pressure relief valve that operates to prevent the oil pressure rising too high as the revs rise. If the spring is weak or there is dirt in the valve then the valve may be letting oil past at higher revs allowing the pressure to drop. I cannot explain why the pressure is OK at lower revs and falls at higher revs except to postulate that there may be some sort of venturi effect or other fluid dynamic phenomenon occurring. In conclusion, I would be inclined to check the pressure relief valve.

I think that your diagnosis of the thermostat being the cause of the low temperature running is probably correct, if it is stuck open the temperature will not reach the normal operating value.

Hope this is of some assistance.

Keith
 

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Discussion Starter #3
Thanks Keith, that does help.

Do you happen to know where the pressure relief valve is? I'm assuming it could be at the top of the block with an external pipe returning to the sump?

I will try the thermostat tomorrow, time permitting and post my progress after the weekend after a long journey!

Thanks again

Mike.
 

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Discussion Starter #4
Just an update on my problem.

I have changed the thermostat tonight and tested the old one in just boiled water and it seemd to open fine. So I decided to change the temp guage sender and proceeded to fill the coolant back up with an improvised header to force out any air.
I opened the bleed screw at the bulk head and there was nothing. I started the engine with the interior heater fully open and still nothing - no water from the bleed and no movement in the header. Then I decided to just check the bleed valve and it seemed to be blocked so I cleared it with a pointy instrument and voila - water came rushing out the valve.
I took the car for a run and the temp guage went up to just shy of 90 degrees and stayed there. The odd thing was that the heater was the hottest I'd ever known since owning the car! It had been a little pathetic before so I don't know if the thermostat was faulty or there was just an airlock in the system.
Needless to say, I gave the car a bit of a thrash on the way home (15 miles) and the temperature stayed constant AND I had no oil pressure light come on!! That's probably just a coincidence though - although I have to say that I had thoughts of over heating oil thinning out which was causing inaccurate pressure readings by the sensor, but I think I may be grabbing at straws there.

I still would like to find the oil pressure relief valve if anyone has any ideas....
 

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Hello Mike,

good job on the thermostat.

On the oil pressure relief valve, working from documents that I have which pertain to the VW passat of similar vintage, and which has substantially the same engine, I would suspect that the valve is part of the oil pump assembly which is probably in the sump, unfortunately. Getting to it will involve draining the oil and removing the sump, not a pleasant job really although I have done it on a couple of past cars.

Good luck.

Keith
 

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HI

Try changing the bottom (least accessable pressure switch)I had a similar problem
with the oil light coming on over 2000rpm.Apparently the reading below 2000
is taken from the top switch, above 2000 it is taken from the other one.My wire
to this was broken,you may have a similar fault.

Del
 

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On further reading I came across the following list of possible causes for oil warning light on (I've skipped the more obvious ones like wires broken and no oil):
- Oil filter clogged or bypass valve defective
- Oil pressure relief valve defective
- Oil pick up strainer clogged
- Oil pump worn
- Worn main or big-end bearings
- Engine overheating so the thermostat may have fixed the oil pressure problem afterall.

Any updates?

Keith853
 
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