d3drturner

This is puzzling me so I thought I'd get some opinions from all the experts out there.

The car: 1991 C2 Tip with 153k miles.

This has happened 3 times in the past several hundred miles and now it's going to sit until I figure out how to diagnose it or I get the time to pull the engine and completely dismantle it.

The problem: After significantly warmed up, with the temp between 8 and 9 o'clock, sitting at idle, the car will start to miss, the oil pressure will drop to zero and the oil pressure light will come on. This has only happened at traffic lights at extremely busy intersections and that being the case I feel I have no choice but to get out of there and off to the side, so, I give it some gas, which is accompanied by a loud, rattling type sound from the motor which lasts for a couple of seconds, and by the time I've maneuvered out, the oil pressure and idle have returned to normal and the car runs perfectly for however long I drive it to get home. There is no smell of oil burning and the temp doesn't rise.

Last time this happened I drove straight home and idled and revved the motor in the garage for about thirty minutes, waiting for it to happen again, and it never did. Sounded perfectly normal, temp ran up to 9 o'clock, cooling fan came on and then off. Couldn't make it do anything odd.

Some background:
Took the motor out last fall and replaced the through bolt o-rings, cylinder base o-rings and head gaskets and now it's almost completely leak free except for a seeping from the center case near the rear. Tested the compression beforehand at ~160 across all 6.

Last summer I replaced the 4-6 side camshaft which had a worn lobe from a clogged cam oil-squirter tube. The last hole on the top tube was clogged with a sliver of valve cover gasket. This was a result of replacing the old covers with new Hargett billet aluminum valve covers from pelican parts. The problem with these covers is that the edges of the gasket channels are so sharp that they shave off tiny bits of gasket when tightened. The original covers have rounded edge gasket channels. The Hargett covers are seriously flawed and I wouldn't recommend them. Anyhow, that cost me a perfectly good cam. After that was replaced, it ran fine until after the through bolt o-ring replacement.

My initial inclination was to think that during the through bolt o-ring replacement some dirt got into the oiling system and now intermittently clogs an oil passage resulting in the oil pressure drop but I don't see how that would explain the missing/rough idle that occurs before the oil pressure drop.

I've checked the thermostat since I read one thread where an insufficiently opened thermostat caused a drop in oil pressure due to oil starvation at the pickup due to the constriction in the return circuit. Also, this only occurs when warmed up, so, the thermostat seemed suspect, but that doesn't seem to correlate to the missing/rough idle before the oil pressure drop. The thermostat looked clean and was easy to get out, although I didn't boil it to see if it opened all the way at the right temp.

The distributor belt is good.

Next stuff I'm going to do:
Change oil filter and oil and check oil pressure relief valves. Take the tstat back out and check operation by heating it in water or the oven. Get a real oil pressure tester and check that instead of relying on the oil pressure gauge. Do another compression test and cylinder leak down test.

BTW, oil pressure is between 1 at idle and 4 at 3k rpm when hot. Never gets to 5, which seems suspicious to me, since before the o-ring change, I seem to recall it getting to 5 when cold and on the highway. I don't hear any unusual knocking or tapping.

Misc symptom: oil pressure gauge tends to bob between 1 and 2 before completely warmed up.

Don't bother telling me to take it to a shop. Repairing the 964 motor is my fav hobby, although I would like to not have to replace the big end, which is why it's going to sit until I get this resolved.

Thanks,
DanT -- currently driving the bmw xi in the snow.

Makmov

Sure sounds like the early symptoms of an oiling problem since you have already found oil related damage, plugged oiling holes, and you never have pressure above 4 bar. I wonder if that rattle isn't the chain IIRC the tensioner is oil pressure fed.

d3drturner

That's an excellent point, Makmov--it did sound like rattling chains. There was only one plugged oil-squirter and that had a sliver of valve cover gasket sticking half out of it, which is all I saw in the oil tube when I took the end caps off and ran a wire through it. How that managed to evade the oil filter and get in there I have no idea. Beginning to suspect it's a couple of unrelated problems.

boxsey911

I don't claim to be one of the experts but this bit sounds like the same symptoms that you get with a sender that's gone bad. Mine behaved like that before I replaced it.

Also, after my engine was rebuilt I started getting an oil warning light on the odd occasion when idling. Never had it before. It happened mostly when waiting on a slight incline. It turned out to be loose hose clamp on the rubber pipe that connects to the crank case breather. I spotted it because I saw some oil traces on the right rear wheel. Once I'd tightened up the clamp, the problem disappeared.

d3drturner

Thanks Steve. I'm beginning to think I have a slew of problems that join together at times.

d3drturner

The rattling turned out to be water in the oil, I think. I drained the oil tank, in which there was significant water, changed the oil and the problem hasn't recurred. Oil pressure gauge still bobs up and down, not sure what's causing that.

Streetfighter

What kind of oil are you using?

rarebear

How long did you use the oil ? Short runs ? I changed oil last month and water came out first, but I use the P also for short distances... The rattling at start up I had , dissapeared. Previous oil change was 6 k miles ago, I use semi synth. 10/40. acea 3
Oil press at idle when hot is a little above 1.
Its a 964 C2 1989. 160 k miles

elsworg

Hi,

I've experienced this very same issue on my 993 a couple of times recently. Idle gets very lumpy then oil pressure drops off to zero on the guage. A quick blip or two on the throttle and all is well again.

Did you reach any conclusions as to what resolved the issue?