1979 GMC 1/2 Ton No Oil Pressure

[bills59]

I recently bought a 1979 GMC 1/2 ton, short bed, 2wd, 350 that the previous owner parked 8 years ago due to what he thought was a collapsed lifter. I have the truck running again but there is no oil pressure at anytime. Would no oil pressure at any temp or speed mean the oil pump is bad or a bearing failed?. It does have a lifter tick but I'm sure that due to lack of oil

[Gmc.guy]

Try a filter change A few weeks ago our 2008 with a 5.3 had no oil pressure and a pile of top end noise. The filter was all collapsed and the paper element was sucked in. I switched brands and did an oil change just like new now. Maybe try a good thick oil. I wouldn't run a super thick oil daily but maybe see if your needle will jump a little with it. Try cheapie mechanical gauge too as a back up

[bills59]

I changed the oil and replaced the Fram filter with a AC Delco filter. I also added Seafoam to the oil

[Dead Parrot]

Pull one of the valve covers and see if you have oil coming out of the pushrods with the engine running. It is a bit messy but will confirm if you have oil flow or not. Gauges do go bad. Probably need new valve cover gaskets anyway.

[Matt Man]

Have you tried a different gauge? I would throw a mechanical one on there first. Does the motor knock? If it had no oil pressure, you would have damage very quickly.

[barry1982]

Im leaning toward a bad sending unit/wire to the sending unit or gauge. If your oil pump is not working or the pickup tube fell off, the engine would "knock", than seize. This doesn't take long to happen. What did the original oil look like when you drained it? I have seen guys run engines until the oil turns into grease, blinding the screen on the pickup tube. Usually even a very worn engine will show a little oil pressure cold.

[bills59]

I will pull the valve covers and check flow. I have not tried a mechanical gauge on it yet but the motor does have a ticking sound coming from the top end. The oil in it was black and looked like 20w50 or thicker

[TrailerTrash]

the current gauge may not even be hooked up

[bills59]

Would a stuck by pass valve on the oil filter housing cause zero oil pressure?

[barry1982]

I'm pretty sure the bypass valve just routes oil around a plugged filter.... shouldn't prevent oil pressure. (but I could be wrong) I have never had a filter on long enough to plug up. 79's had the "electric" sending unit. Have had trouble with these in the past. Of course its not easy to get at. Pretty well have to lay on the engine to reach.

[bills59]

Correct me if I'm wrong but if the truck had no oil pressure wouldn't the dipstick still read full with it running?

[hatzie]

Originally Posted by bills59

Correct me if I'm wrong but if the truck had no oil pressure wouldn't the dipstick still read full with it running?

Can't tell what it'll read with the engine running. The crank counterweights stir up the oil in the pan. That's why racing and modern engines have windage trays just under the crank to keep the oil from getting blown onto the crank and creating drag.

[Gmc.guy]

Originally Posted by TrailerTrash

the current gauge may not even be hooked up

On my truck it did show no oil pressure when the filter sucked up and closed. I thought that the bypass just pumped unfiltered oil to engine. I thought maybe it bypassed the sending unit too but definitely had no oiling with the noisy lifters. Wish I took a pic of the filter. It was the GKI brand.

[Gmc.guy]

Whoops quoted wrong guy

[Gmc.guy]

Originally Posted by barry1982

I'm pretty sure the bypass valve just routes oil around a plugged filter.... shouldn't prevent oil pressure. (but I could be wrong) I have never had a filter on long enough to plug up. 79's had the "electric" sending unit. Have had trouble with these in the past. Of course its not easy to get at. Pretty well have to lay on the engine to reach.

That's the right one. I am not too familiar with the Ls motors besides maintenance, I think if the bypass itself was plugged. The filter would split and blow all the oil on the ground.

[Gregski]

couple weeks ago my truck stopped showing any oil pressure yet it runs great, I also changed the oil and oil filter after this happened yet that did not fix the problem so I am wondering if the OP ever figured this out and what was it?

[bilfman]

Same happened to me, it was the sending unit.

[CSHADES]

Oil pump failure is not very common(it runs in oil) and when it does happen it usually fails by locking up. If you take off the oil filler cap when it is running you should be able to see if oil is moving in the valve cover. I would install a tempory mechanical gauge to see what the actual pressure is. If it actually has no/low pressure I would lean more towards having to much bearing clearance somewhere internally not the oil pump. If it does have real oil pressure I would remove the valve covers and inspect the rocker arms/pushrods for wear. I had a lot small blocks in the 70s and 80s that would pop a push rod through a rocker arm or a lobe on the cam worn from wear/poor quality parts (remember bad chevy cams?)

[hatzie]

Originally Posted by CSHADES

Oil pump failure is not very common(it runs in oil) and when it does happen it usually fails by locking up. If you take off the oil filler cap when it is running you should be able to see if oil is moving in the valve cover. I would install a tempory mechanical gauge to see what the actual pressure is. If it actually has no/low pressure I would lean more towards having to much bearing clearance somewhere internally not the oil pump. If it does have real oil pressure I would remove the valve covers and inspect the rocker arms/pushrods for wear. I had a lot small blocks in the 70s and 80s that would pop a push rod through a rocker arm or a lobe on the cam worn from wear/poor quality parts (remember bad chevy cams?)

Chevy, Honda, Toyota, Ford, Chrysler, VW, ... soft cams and followers are not isolated to just one manufacturer.

[CSHADES]

Originally Posted by hatzie

Chevy, Honda, Toyota, Ford, Chrysler, VW, ... soft cams and followers are not isolated to just one manufacturer.

I know they all have had problems, but gm cams in the late 70s and early 80s gave us a lot of work replacing cam, lifters and timing chains.