Oil pressure driving me crazy

[xs-style]

1990 Silverado
350 gmp crate motor with 30k

New oil pump and screen
New oil pressure switch
Fresh oil and filter
Replaced one quart with Lucus stabilizer

The oil pressure is way inconsistent.

First start up after the swap it sat and idled smooth with a gauge reading of 40 psi till warm then dropped to 20, still running smoothly.

Let it sit overnight cuz it was late.

Crunk up again this morning, same result. Oil pressure holding steady about 30, engine running smoothly....all appears well.

Everything seems fine, so off to the end of the road. My road is only about a mile to the end, and I got up to about 45. That had the gauge reading at about 35. Seemed good.
So I decided to drive a loop around the neighborhood. It's maybe a 3-5 mile round trip. About halfway through the round the oil pressure dropped to zero again. Then it jumped back up. I spent the next 2 miles of the trip watching the pressure float between zero and 25 on the gauge, and feeling the truck start run rougher.

Got it back home and let it (and me....) chill. After an hour or so I went out and fired it up again. Pressure went to 30 for a few seconds then dropped to about 15. If I rev it a little I can be in the pressure up, but it's still running rough.

With a new pump, screen, switch, and fresh fluids... what's left?

Ps: checked the can bearing while I had the pan off. no play in the rods to speak of.

[Restrorob]

Check oil pressure with a manual gauge, they don't lie..... If it still reads low, it looks like you may be tear'n it back down. Rough run'n could be fuel or ignition related and nothing to do with oil pressure.

[xs-style]

Yea I may pick up the accessories for the manual gauge I have today.

It was smooth running after I got it all back together. I have an idea that while I was driving, and the oil pressure dropped the lack of fuel flow confused the computer.

Am I thinking of this right? The fuel pump runs in with the oil pressure switch . When the oil pressure drops, the computer starts to kill the fuel pump. When I rev it, it brings the oil pressure back up. I'm thinking the inconsistent pressure gives the computer to much to keep up with?

[Mr_Rich]

Originally Posted by xs-style

Yea I may pick up the accessories for the manual gauge I have today.

It was smooth running after I got it all back together. I have an idea that while I was driving, and the oil pressure dropped the lack of fuel flow confused the computer.

Am I thinking of this right? The fuel pump runs in with the oil pressure switch . When the oil pressure drops, the computer starts to kill the fuel pump. When I rev it, it brings the oil pressure back up. I'm thinking the inconsistent pressure gives the computer to much to keep up with?

Uhhhh...no! These are analog gauges and have very little influence on the PCM one way or another. I'd bet on a flaky oil pressure gauge; just like mine behaves. Once I realized it was failing I ceased fretting about it. The real culprit is the 88-91 gauge cluster that eventually starts acting up. My speedo is tied in to the PCM and when I'm going down the highway and suddenly the speedo drops to zero mph I hear the engine change tone which means it's getting a different mixture signal to the TBI. I bang on the gauge cluster with my knuckle and the speedo comes back up and the engine is happy again. There's about a ten year gap where the gauges went from analog to digital so you don't have to think in complicated terms on these years of trucks. I made a 90 ohm test cord to verify my oil pressure gauge. Mine acts up intermittently shortly after I reset it this way.
Some Ebay subs seem to do a good business selling these gauge clusters because they're going for a pretty good price for parts that are in fair condition, IMO. I've got to pull mine out...again...to try and solve the problem with the speedo and oil pressure gauge on mine.

[Marv D]

just going to throw this out there,,, because I learned a HARD lesson.

I had one of the race motors that would act funky like just like this. The motor ws a 434 SBC in my 66 and was on race #1 after being fresnened because of a incident with squirter screws jumping down a intake runner :-(
Anyways psi was screwy,, one start it would be good,, then just act 'goofy' like xs- is seeing. It was a Autometer UltaLite gauge but even good stuff can fail right! So I replaced it with another gauge and had the same psi.... so I know I had engine troubles. I pulled the motor and when the pan came off my heart sank. I had tried to save a $65 FelPro one-piece pan gasket wit a nice 'THIN' smear of 'the-right-stuff' on both sides of the used pan gasket. Well the 4" stroke rods and counterweights get real close to the pan rail even in a Little-M block.

The Right-Stuff expands more than regular silicone and the crank wiped off tiny gooey strings of silicone that had squeezed out of the pan gasket. Crap was hanging out of the oil pimp pickup When the pump sucked the hardest,, it blocked the oil even more. Cost me a set of Lunati LAD1 rods, and having to send th crank to Cali to be ground and re-nitrided.

Point of this story.. I'd make DARN sure the gauge is accurte,, then make SURE there isn't cork or silicone blocking the oil pump pickup screen. If yo know anyone with a digital boroscope it's pretty easy to get a look inside the pan through the drain plug hole.

[xs-style]

I could see where that problem could happen Mary. When I changed out the oil pump on this one, the screen was clogged with silicone. The p.o. had put a cheap chrome pan on it, and fixed it's poor fitment with what looked like a whole tube of rtv. But it's now got a new pump, Pick up tube and screen. You wouldn't think pressure would be an issue now.

Rich: mind explaining how you made that test cord?

[68c10airstream]

A mechanical gauge hooked up is a must, as mentioned before. Your oil pressure SENDER should be a 3 wire sender. One wire used for the gauge, and the remaining 2 wires used as a backup fuel pump 12 volt+ supply in case the fuel pump relay goes out (fails). A trouble code would be present for a failed fuel pump relay and you would detect a longer crank time to get started because oil pressure has to build up before the fuel pump would run.

If i went back into the pan of a small block i would weld the pick up tube to the pump for 2 reasons, can't move and potentially fall out and ruin the engine and do you think the press fit tube to pump is sealed 100 percent or could it possibly suck air into the pump. Easier to suck air than oil up the tube!! I believe the point where the tube meets the pump is out of the oil??

[Mr_Rich]

Originally Posted by xs-style

I could see where that problem could happen Mary. When I changed out the oil pump on this one, the screen was clogged with silicone. The p.o. had put a cheap chrome pan on it, and fixed it's poor fitment with what looked like a whole tube of rtv. But it's now got a new pump, Pick up tube and screen. You wouldn't think pressure would be an issue now.

Rich: mind explaining how you made that test cord?

Here's a pic of my test cord. I used two 180 ohm resistors in parallel to equal roughly ~90 ohms of resistance. That description of a clogged oil pump screen would concern me. Have you cut open your oil filter? I would do that to check for any debris getting through the oil galley and possibly through the bypass.

[xs-style]

Well the mechanical gauge verified that the interior gauge is accurate. I'm kinda at a loss for options right now. I'm caught between pull the engine back out and see what's going on, or just kicking this thing back to Craigslist where I found it. Anybody got any ideas before I have to pick one extreme or the other?

[sntrym95]

If the oil pickup screen was clogged pretty good initially, I would guess you have some cam bearing wear, due to lack of oil, to the point where the oil is flowing pretty good through there, but the clearances are just enough to not build pressure. You could go with a 10W40 oil which will help keep all the components lubed, but mileage will suffer and it's really only a temporary fix.

I have the same thing going on with a Vortec swap I just did and the engine guy is building me a new one to replace it.

[Marv D]

Originally Posted by xs-style

I could see where that problem could happen MarV. When I changed out the oil pump on this one, the screen was clogged with silicone. The p.o. had put a cheap chrome pan on it, and fixed it's poor fitment with what looked like a whole tube of rtv. But it's now got a new pump, Pick up tube and screen. You wouldn't think pressure would be an issue now.

Rich: mind explaining how you made that test cord?

I suspect the damage is done, and bearings are TOAST. ]The clogged screen robed the motor of oil and the poor little thing just ate it's self up. Remember the SBC oils from the TOP down,(up the back of the block, then forward along the cam / lifter annulus created when the 'cut-away' portion of the lifter is aligned with the annulus bore, then down to the top of the main saddle, and finally through the crank to the rod bearings,, but ONLY when the oil hole in the crank main journal is aligned with the groove in the upper 1/2 of the main bearing,, and finally the LAST thing to get oil is the front main and #1-2 rod journal)
Things have to be correct for all of that to stay flowing oil to the front rod bearings.
And none of that includes any of that silicone getting through the pickup and plugging passages in the block or crank.
At this point,, assuming ther eis no spun bearings, you 'MIGHT' get away with a set of bearings in the bottom end , but if you have are pulling the motor to do that FOR SURE cam bearings would be a HOGH priority in my book.

[xs-style]

The cam bearings being bad was my worst case scenario. But that still leaves me confused. The rod bearing on the bottom end should be the last step in the chain. When i had the pan off the rods were solid. You'd think they would have a good bit of play.

I should mention oil pressure holds steady at idle. It's a nice solid 35 psi. When I drop in gear is when it gets wonky. I could run a heavier weight oil, or put in a high volume pump... But I don't know how long either patch may last.

[xs-style]

While we're at it... anyone know where this plug should go?

[Mr_Rich]

Originally Posted by xs-style

While we're at it... anyone know where this plug should go?

Well...it's not tan with a black stripe so it's not ignition, at least as far as I can see in this pic. I would guess it's the plug for the underhood light (?).

[xs-style]

Silly me, should have mentioned the wire is purple

[xs-style]

Well today I did the motor medic flush. Now it's back to idling at about 15 pounds hot, but still drops when it's in gear. Haven't road tested it, but I can bring the oil pressure in gear up with the rpms.

[diveplane]

Originally Posted by xs-style

1990 Silverado
350 gmp crate motor with 30k

New oil pump and screen
New oil pressure switch
Fresh oil and filter
Replaced one quart with Lucus stabilizer

The oil pressure is way inconsistent.

First start up after the swap it sat and idled smooth with a gauge reading of 40 psi till warm then dropped to 20, still running smoothly.

Let it sit overnight cuz it was late.

Crunk up again this morning, same result. Oil pressure holding steady about 30, engine running smoothly....all appears well.

Everything seems fine, so off to the end of the road. My road is only about a mile to the end, and I got up to about 45. That had the gauge reading at about 35. Seemed good.
So I decided to drive a loop around the neighborhood. It's maybe a 3-5 mile round trip. About halfway through the round the oil pressure dropped to zero again. Then it jumped back up. I spent the next 2 miles of the trip watching the pressure float between zero and 25 on the gauge, and feeling the truck start run rougher.

Got it back home and let it (and me....) chill. After an hour or so I went out and fired it up again. Pressure went to 30 for a few seconds then dropped to about 15. If I rev it a little I can be in the pressure up, but it's still running rough.

With a new pump, screen, switch, and fresh fluids... what's left?

Ps: checked the can bearing while I had the pan off. no play in the rods to speak of.

oil pressure sending unit? , or dash temp stepper motor?.

[xs-style]

This setup doesn't have the sending unit down by the oil filter. Would a 90 have been wired up for one? I was of the impression that the sending units by the filter didn't start till a little later. The original motor I pulled out was just plugged off above the filter, and I didn't notice an extra wire on that side.

[Mr_Rich]

Originally Posted by xs-style

This setup doesn't have the sending unit down by the oil filter. Would a 90 have been wired up for one? I was of the impression that the sending units by the filter didn't start till a little later. The original motor I pulled out was just plugged off above the filter, and I didn't notice an extra wire on that side.

I'm positive that it would have to be. A 90 isn't much different than an 88 or 89. Now the engines that came out of cars had this plug. The 350 that's in my truck now came out of a '90 Cadillac. It's the exact same block as the engine I pulled three years ago with the exception that the one I have now is roller-cammed.

[68c10airstream]

Spent about 1/2 hour on alldata pro and found conflicting info. So to help out i looked into fuel pump system and oil pressure sender and finally found the 3 wire electrical diagram which contains your oil pressure sender and your backup fuel pressure switch all combined into one plug in at one unit. Tried to find an illustration but no luck.

I remember an argument with our parts manager about trying to get the right part to fix a gmc of about your vintage. After a lot of looking i took the part off and ran the part number through out parts system and i believe the part was under the fuel system, not oil system.

Anyway from my old man memory look at the base of your distributor and look for a long silver cylindrical unit, about 4" tall, 1" diameter, installed horizontally or vertically with a 3 wire plugin containing a tan wire amongst the wires. I remember having to buy a specific snapon socket to fit in there to remove it.

[Mr_Rich]

Let's get the terms straightened out here. The oil pressure sender goes to the gauge. The oil pressure indicator, and I quote from my GM Electrical and Wiring Diagnosis and Wiring Diagrams; "The Oil Pressure Indicator comes on to warn the driver when the engine oil pressure is low. Battery voltage is applied to one side of the bulb. A ground path is provided by the Oil Pressure Switch. It is closed when the oil pressure is below 27 kPa (4PSI). This tests the bulb when the Ignition Switch is turned on to start the engine. After the engine starts and normal oil pressure builds up, the Oil Pressure Switch opens. The Oil Pressure Indicator goes out." The indicator/ switch is the plug near the distributor at the back of the engine. The sender is under the exhaust manifold on the left side.

[68c10airstream]

In my post above (# 20) i was remembering customers coming in with a dash oil pressure gauge reading incorrectly and changing the silver tall can near the distributor. I,ve swapped a few of these tbi motors and agree about some switch?? down by the oil filter boss. When i looked in alldata circuit #31 looked to be shown for 2 different circuits, so i assumed the base cheap truck would use a switch for a bulb on the dash, and a upscale deluxe truck with full gauge package (oil pressure gauge) would use a sender, not realizing that these trucks used both if i understand correctly. I'm not here to upset anyone just trying to help, and through the years using alldata has been hit and miss, especially with wiring.

[speedygonzales]

Correct me if I'm wrong but didn't you say a mechanical gauge has already confirmed your dash gauge was working correctly? So why are chasing down a way to make the dash gauge read differently?

I had the exact same problem you had and the lower end was wiped out. Guess what I noticed. The rods were "tight" but the bearings and crank were wiped out.

I have never heard anyone say their gauge was doing like yours and it turned out to be either the sensor or the gauge. Really low pressure when the idle speed decreases is a bottom end problem. Period.

You bought an engine on Craigslist? SMH

Pull a main cap.

[Marv D]

Yup Speedy G, I said that in post 11. I don't care what you make any electrical gauge read,, Not gonna change the damage done in the bearings.
The pickup was plugged with silicone and took out the bottom end. End of story.

Originally Posted by xs-style

Well the mechanical gauge verified that the interior gauge is accurate. I'm kinda at a loss for options right now......

and.

When I changed out the oil pump on this one, the screen was clogged with silicone. The p.o. had put a cheap chrome pan on it, and fixed it's poor fitment with what looked like a whole tube of rtv.

[xs-style]

I acknowledged in post #12 it could be those bearings. Honestly it looks like thats the only option left.

The point here had been to make sure I've tried everything else I could before going there. I also wanted to make sure I hadn't missed any electrical connections, since I haven't messed with tbi motors very often. Unfortunately it looks like onward and inward for this one.