Oil pressure light flashes when idling

I have a 1981 diesel rabbit pickup. Recently the oil pressure light has been flashing on and off. It seems to light up when I idle and then go off again as soon as I accelerate. The first thing I checked was the oil dipstick. It was a bit little low so I added two quarts of new oil. This did not change anything. What's might cause this?

little low and adding 2 quarts?
little low to me is 1/2 quart.

Bad sensor, bad filter, bad grounds, bad connection on the sender. it is the one on the side of the head.
improper sender on the head is another possibility.

I guess 2 quarts is about 2l it's only about 1 between minimum and maximum running that low or that high on oil is not good… where is the level on the dipstick when cold?

I overfilled it a bit. I think I probably added ~1/2 quart more than necessary. It was a bit below the low mark on the stick and I misjudged how much the gap represented.

My dipstick is hard to read because it's behind a couple hoses, so when I pull it out it tends to smear against the sides. In retrospect I was probably a half quart low and now I'm a half quart high.

Briano1234 Sounds like you're thinking it might be a false indicator. Is that theory consistent with the fact that it happens pretty predictably, only when it's idling and warm? I would think a loose sense or something would be more random.

Not sure what type of oil pressure sensor you have on a diesel but on petrol cars they are cheap to buy, might be worth trying a new sensor and see if that helps?

I can't remember the exact pressures, but I know if you don't use a diesel gauge off the head you will blow the innards out of a Gassers mech gauge.

The only time my light would glow was when it was about 1 qt low, and I could only see it at night at idle….
I did have to replace the sensor a few times over 350Kmiles (I think I replaced it twice) as they will leak oil and that can spew it out if not caught early….

IIRC the cold pressure (oil is cold is over 90psi.)

 

I had my mechanic replace the oil pressure sensor. Still does the same thing. It's strange… it ONLY does if after it's warm. No light while it's warming up or driving. ONLY idling when it's heated up. Now what?

Sounds like the system is working correctly. As the engine heats up the oil gets thinner giving less pressure. Increasing RPM pumps more oil so that even the hot-less-viscous oil produces enough pressure to extinguish the warning light. A hot engine at idle will give the lowest pressure which is what the light is showing.

Symptoms of wrong/old/insufficient oil or a worn engine and/or oil pump. Could possible be a duff oil pressure regulator/relief valve being stuck open but I believe they usually fail closed increasing oil pressure and blowing oil out of crank seals etc.

Is the oil and filter fresh and upto the correct level?

I'd try a thicker oil or one designed for older high mileage used engines. When it was designed in the 70's the engine oil would have been a 20w50 oil, then VW moved to 10/15w40 and now many modern cars use 0w/30.  What have you got in it now? 

20w50 isn't going to work, but Shell Rotella has a CC/CD rated 10w40.

I used 10w30 cc/cd in my 81 Diesel rabbit for 20 years, and only had the blinky light when I was low on oil.

Try cleaning or replacing the main grounds Battery to frame and frame to engine.  

Also use a good quality filter, not a generic storebranded  Frame or Bosch stay with a wix Mann or Mahl, Genuine VW.  As the Generic/Storebranded/Bosch/Fram don't have the inbuilt anti-drainback valve that can cause the blinky light from hades at idle.

The Pressure relief valve can stick partially open as I have had that on one of my Cabriolets….

mine did this, oil pump was faulty and was only working properly when loading engine

The oil was just changed, and I'm assuming the filter was too because I had my mechanic do it while it was at the shop for something else. I didn't ask which brand of oil they used, but I'm confident it was an appropriate choice because it's an old school VW shop. (They're usually lecturing me about the right brand/weight of oil, filters, etc) I'll confirm all this this next time I pass by that shop.

Assuming the oil and the filter are correct, it sounds like the oil pump is the main suspect. Is there any easy way to inspect/diagnose that? Or is it the kind of thing where the test is just putting on a new one?

Also, if it is the pump, should I be worried that it may fail entirely at some point? I'm guessing they don't get BETTER over time ;-)

If it's wrong oil you gonna have to bin it anyways, use the recommended oil for the golf, can't remeber now what brand but its german and euro car parts do it so catch it when they have sale on (almost always) 

Take off sump once drained, drop out oil pump and see how easy the crank drive spins. If it's got any resistance bin it but I was in slight doubt and for £20 I just replaced it

Quantum synta

The worst part in changing the oil pump is the front 2 and rear two bolts as they screw in o aluminum seal carriers.  

The rear two between the bellhousing and the engine are a pain and you can strip them easily, when returning those 4 bolts the key is to use a wobble socket extension and hand start them and hand tighten them. They cross thread really really easily, do not use a ratchet on them hand thread them with a screw driver handle only. until the final torque and that torque is 20NM no more no less. That is the Torque of all the oil pan bolts.  I have had the windage trays break on me so when you order a pump you might as well order a new windage tray, and the Rubber Gaskets are better than the cork but either way no sealant at all on them.

The flange on the oil pump was gnarled. AND we found troublesome wear patterns/scratching on the half-cylinder bearings when we took the pan off. Looks like the previous owner might have run the oil dry at one point because we also saw a lot of blueing and discoloration of the metals that clamp around the cam. We're replacing the bearings but there's a bit of scarring on the cam, which will probably hurt the life expectancy of the engine. It's disappointing but I'm going to drive it for a while and see how it feels… and keep my eyes open for another diesel rabbit project down the road. ;-)

Thanks for everyone's help getting to the bottom of this!