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My ’89 Gmc 4.3l has been having a few problems recently. At first, it had a small vacuum leak, but i never tried to find it. Then it had low oil pressure. I know these 2 have nothing to do with one another. so I got a new oil pressure sensor, and still nothing. I stopped driving it for a while until I looked up some reasons on the internet. Matching symptoms for bad main and piston bearings all over the internet, I took my engine out and disassembled it. replaced the crankshaft, oil pump, all bearings, plastigaging all bearings, and all of them had .0015 clearance +/- a hair. After priming the oil pump for a while and everything back together I started it up. But of course only with my luck, still low oil pressure. The vacuum leak is absolutely terrible now, to the point where the engine surges near stall to over 1300 rpm.
I’m so confused.
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if you can get your hands on a smoke tester it’ll make finding your vacuum leak super easy to find. Otherwise just take some starter fluid and start spraying it around your vac lines to try and pinpoint it.
as far as your low oil pressure goes it’s a little tougher. Personally the first thing I would do is hook up a mechanical oil pressure gauge if you haven’t done so already and really verify that you have low oil pressure.
Also, since the oil pump is gear-driven off of the camshaft through the distributor, it’s possible that if you have a lower-than-normal idle due to your vac leak that it would cause a low oil pressure condition, but since I don’t actually know what actual pressures you’re seeing it’s pretty hard to call that one.
Some extra information would more than likely help a lot. Oil pressure readings, if you have a stock or aftermarket camshaft, what kind of oil pressure gauge you’re using, etc.
It also wouldn’t be a bad idea to get your hands on a junk distributor and take the cam gear off of it so that you can turn it with a drill and control the oil pressure “remotely”. This way you can send a bunch of pressure through the little passageways and such without the engine running and try to force out any kind of blockage that might be anywhere.
see if this helps.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Oil pressure factory manual:
(Minimum) at Normal Operating Temperature
6.0 psig at 1,000 engine rpm
18.0 psig at 2,000 engine rpm
24.0 psig at 4,000 engine rpmwhen i start the truck up, it has around 15 psi +/-, then after some time (maybe like 10 or 15 mins) the oil pressure drops near 0. Even when i press on the gas pedal, the pressure doesn’t rise.
I also thought since the oil pump is gear driven that could be the case. could a vacuum leak really be that bad that it causes low oil pressure? I really feel and hope that it is, because when the oil pressure drops, its usually if not always when the engine starts surging.
I don’t think i need to get a old distributor. i have the primer tool that attaches to a drill.
The crankshaft is stock, and im using a Wells 2 Terminal 1/4-18 NPTF Dryseal Thread, Normally Open, Closes 2-6 PSI switch. I’ll try to find the leaks and keep you updated.
Oil pressure is not just the clearances in the bottom end, it’s also the clearance at the cam bearings. If it’s excessive, you’ll loose oil pressure. Also the oil that you’re using makes a difference. Synthetic oils may run at a lower pressure than their conventional counterparts.
Forget about the vacuum leak for now, until you figure out the oil psi problem I don’t think it matters.
One last thing. Put a mechanical gauge on it to see if your gauge is actually reading correctly. This would be a huge waste of time if your oil psi was actually good but your gauge was reading wrong. I know you say you replaced the sending unit but if the problem is in the wiring or dash that wouldn’t have made a difference.
Keep us posted.
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