Coolant gauge/light freaking out - mk1

[wolfsburged]

Not had the car running for long, but I've seen this one both of the two clusters I've had in the car.

Basically I start the car and the temp gauge just starts climbing immediately - ie. it is showing as overheating within about 2 minutes, with the light flashing and all. The coolant level is fine, and there is a new coolant level sender and coolant warning light relay. I also went through the grounding terminals under the dash and pulled every line, cleaned up, and even replaced a few connectors that were excessively corroded. This made it better (before it was also randomly flashing, light would flash with the blinkers, etc), but it still consistently pegs.

Which temp sender is the gauge reading from? There is one on the front and rear of the block. I've tried pulling and reconnecting/cleaning both terminals - no luck.

The car is not overheating - I've driven it for over an hour like this and no steam, the radiator fans come on fine, everything seems fine, just the gauge pegs. I suspect a short to ground somewhere...

Also of note is the rest of the cluster works fine - gas gauge, lights, etc. So I don't think its a cluster/voltage reg/cluster ground issue.

Any ideas?

[bajacalal]

My mk2 (85 Golf) does this too, I think it has about the same electrical system. The previous owner warned me about this and it happened the first time yesterday after months of not doing it before.
It freaked me out because I was actually going up a hill (but I was taking it easy on the accelerator) and it had to be 100+ degrees outside. However, it returned to normal a few seconds later with no sign of actually overheating.

I wish I knew what's wrong. I'll spend some time on it and if I figure it out I'll let you know.

I wonder if a pocket of air and consequent hot spot can form around the sensor bung in the engine.

[8v-of-fury]

A pocket of air could very well be what it is.. the sensor is higher in the cooling system then the front head hose is it not?

Have you replaced the sensor or gauge? They are like 25+ years old lol. Electrical *** fails man lol

[bajacalal]

Not only is it 25 years old, those 25 years were spent in the Pacific North West. The car has only recently come to see the light of California sunshine.

I already know my electrical system has issues. It's a good thing our diesels don't really need electricity to run. I have been slowly sorting it all out. I have yet to clean the connectors under the dash, that may help.

Something else happened today, not to the coolant gauge but the oil light. It started blinking. I flipped out and stopped on the side of the freeway. I cleaned the connector to the oil gauge and I added oil, so far so good. It was down about 1/2 quart from full. The dipstick was also loose. Would either of those things really cause the oil pump to suck up air bubbles? My oil pan is a bit dented which means the oil pickup thing could be damaged...

[truckoSaurus08]

If your really low on oil and you make a sharp turn you can starve the engine causing the light to come on. I did it once and checked the oil level and sure enough it was low.
Before assuming it was some electrical problem i would probably make sure it's not a head gasket issue. maybe do a compression test. Also check for leaks in the coolant system. These cooling systems seem a little more sensitive to leaks than most other cars. Even a small leak can cause the temp. to go up.