Oil Pressure Question

[jwgreen]

Hey guys,

I have a 84' GMC with a rebuilt stock 350. I'm pretty sure the place that rebuilt it didn't do a very good job, but that's another story. Currently I noticed that the oil pressure at idle in Park is about 20 and at cruise is about 40 but when the truck is in Drive at Idle the oil pressure drops to basically 0! The truck is my brothers that I work on so I don't recall it doing this before and he doesn't either. Would that be a oil pump going bad? If so, can I swap the pump out with the engine still in the truck? Any help or advice is appreciated.

 

[C10MixMaster]

Hey guys,

I have a 84' GMC with a rebuilt stock 350. I'm pretty sure the place that rebuilt it didn't do a very good job, but that's another story. Currently I noticed that the oil pressure at idle in Park is about 20 and at cruise is about 40 but when the truck is in Drive at Idle the oil pressure drops to basically 0! The truck is my brothers that I work on so I don't recall it doing this before and he doesn't either. Would that be a oil pump going bad? If so, can I swap the pump out with the engine still in the truck? Any help or advice is appreciated.

low oil pressure is a sign of excess clearance in the bearings or the pressure relief valve hanging open in the pump. If its the pressure relief valve a new pump will fix it. If its clearance issues a high volume pump or heavier weight oil might buy you time. also bumping your idle up can help keep it from dropping to zero.

 

[jwgreen]

low oil pressure is a sign of excess clearance in the bearings or the pressure relief valve hanging open in the pump. If its the pressure relief valve a new pump will fix it. If its clearance issues a high volume pump or heavier weight oil might buy you time.

With the shop that did the work there's no telling. I already had to have them replace the rings because it was smoking like crazy. But it's now out of warranty so anything that's wrong is on me. Guess I will look into replacing the oil pump first. If that doesn't work I'll add a heavier weight oil and see if that makes a difference. At least then I would know it was one or the other I guess. Thanks for the info C10, I really appreciate it!

 

[Dmack]

What are you using for a gauge, factory? If so, I would put a mechanical gauge on it to confirm that the readings are correct.

 

[jwgreen]

What are you using for a gauge, factory? If so, I would put a mechanical gauge on it to confirm that the readings are correct.

Yes a factory gauge with a electric sensor.

 

[Dmack]

Mine has the same set up and it's all over the board. Anywhere from 5-50. Probably going to put in mechanical gauges myself. I would at least put a mechanical gauge on it temporarily at least to confirm if your factory one is working properly.

 

[jwgreen]

Mine has the same set up and it's all over the board. Anywhere from 5-50. Probably going to put in mechanical gauges myself. I would at least put a mechanical gauge on it temporarily at least to confirm if your factory one is working properly.

Will do that first and see what the results are, then go from there. Thanks Dmack

 

[Matt69olds]

Cut the oil filter open, if there is bearing issues the debris will be caught in the filter.

Put a mechanical gauge on it to verify the factory gauge is accurate. You may just have a bad sending unit. If you have 20psi hot in gear, that’s plenty.

If the oil pressure is in fact low, and it’s not making any death sounds from the bottom end, a high volumn oil pump, and maybe some 15w40 diesel oil will keep it going.

Hopefully the shop that did the work just built it loose, and things didn’t loosen up due to sloppy machine work or damage.

 

[MikeB]

I had a similar problem with the 350 in a project car I bought many years ago. An oil change using 20W-50 did very little to help, so I dropped the pan and changed the oil pump and rod bearings (which were visibly worn). I pulled off a couple main caps, but the bearings looked OK.

Pressure at 600-700 RPM went from almost zero on the gauge to around 15 psi, and at higher RPMs jumped to around 40-45 from originally 25-30. (Can't remember the exact pressure numbers, but those are close.)

A few years later I sold that engine, and a year or so after that it was still running fine and making decent oil pressure!

 

[DoubleDingo]

Going to the car show on Saturday, the oil pressure was at about 50 when the truck was started. About 25-30 minutes in to the drive which put us going up the grade, it was at 20psi. I was concerned, but there wasn't a knock, nothing, and when we came to the first stop, idling was 20psi. Before leaving the car show I checked the oil, Full. Going home it was 40psi cruising, 20psi idling. I guess it's like the gas gauges, you never know what you're going to get...lol...

 

[Matt69olds]

A good rule of thumb is 10psi for every 1000 rpm engine speed. If the truck has a oil light it wouldn’t come on until about 7 psi. If your idling with 20psi hot in gear your in good shape.

 

[Shorty81]

I have a Mellings high volume oil pump and it runs 65 lbs at 2200 rpms and 35 lbs at idle (hot). That's running Schaeffer 10-30. Running mechanical gauge.

 

[jwgreen]

Thanks everyone for your replies, I have swapped out the gauge for a mech one and something I forgot to mention was that I recently installed a Vintage Air A/C system on the truck. So this was the first time I drove the truck with the a/c operational. So after some testing the pressure was within the normal ranges I was used to when the a/c was off but when the a/c was on that's when the pressure would drop and also the RPM's. So I installed the mech gauge and the bumped up the idle while in park till I got about 20 psi. Running down the road its getting 30's or better psi. I did go ahead and get a Melling HV pump but haven't decided if I'm going to put it in or not. Need to assess if I can do this without pulling the engine or not. I'd rather not pull the engine. Again, thanks for all the info gents!!

 

[Matt69olds]

Id be willing to bet the pan comes out pretty easy with the engine still in the truck.

 

[C10MixMaster]

Thanks everyone for your replies, I have swapped out the gauge for a mech one and something I forgot to mention was that I recently installed a Vintage Air A/C system on the truck. So this was the first time I drove the truck with the a/c operational. So after some testing the pressure was within the normal ranges I was used to when the a/c was off but when the a/c was on that's when the pressure would drop and also the RPM's. So I installed the mech gauge and the bumped up the idle while in park till I got about 20 psi. Running down the road its getting 30's or better psi. I did go ahead and get a Melling HV pump but haven't decided if I'm going to put it in or not. Need to assess if I can do this without pulling the engine or not. I'd rather not pull the engine. Again, thanks for all the info gents!!

i just did this in my truck. the pan came rite out no problems. you might have to turn the engine to clear the front counter weight. It can defiantly be done in the truck