New Oil Pump, Same Low Pressure

[old_silverado]

88 K1500 stock 350 TBI/700R4 282k miles (original engine - original owner (yes, 36 years)) Last week the oil pressure suddenly went from 40 psi cruising/ 25 psi stopped (hot) to 20 psi cruising/5 psi stopped (in gear.) I swapped the oil pressure switch (third one) with no change. I plumbed a mechanical gauge temporarily in place of the oil pressure switch and over 2 days consistently got 25 psi cruising/ 8 - 10 psi stopped. Today I swapped out the oil pump for a new one. The oil pressure is identical to before. There was only a trace of oil film in the pan and oil filter adapter, no solids whatsoever, nothing shiny, nothing sticking to a magnet (rather pleased as the pan has never been off.) The crank, mains, rods, cylinders and pistons look great (from underneath.) No signs of bearings heading south (no noises, vibrations, nothing upon visual inspection from beneath.) I'll see next if I can plumb in a mechanical gauge in place of the oil sensor but being downstream it shouldn't be any better. The only recent change was another distributor (new) about a month ago. There never was an oil cooler on this one. What's missing here?

 

[Caman96]

Clogged oil filter?

 

[old_silverado]

Oil and filter were changed before I changed the oil pressure switch (looked normal.). They got changed again during the oil pump replacement.

 

[Caman96]

Oil and filter were changed before I changed the oil pressure switch (looked normal.). They got changed again during the oil pump replacement.

Just asking since it wasn’t mentioned.

 

[termite]

Oil and filter were changed before I changed the oil pressure switch (looked normal.). They got changed again during the oil pump replacement.

Did you cut the first filter (one in use when pressure dropped) open for a look inside for glitter? And subsequent filter(s)?

Same brand of oil filter? Maybe swap to a different brand filter for the joy of it. 8-10 psi hot idle is still with GM spec if I'm not mistaken. No lifter clatter, I wouldn't sweat it too much.

 

[PlayingWithTBI]

88 K1500 stock 350 TBI/700R4 282k miles (original engine - original owner (yes, 36 years))

Congrats! I've only owned my 88 for 22years, LOL. What oil are you using? Maybe a heavier weight will help. I've been running Mobil1 0W-40 for years in Ca and AZ (let's not get into another discussion on who's oil is the best )

I swapped the oil pressure switch (third one) with no change.

Which "Switch"? There's an Oil Pressure Switch on the China Rail behind the distributor which turns on the fuel pump when it senses ~4+ PSI. The Oil Pressure Sender is on the left hand side of the block above the oil filter which feeds the gauge.

I plumbed a mechanical gauge temporarily in place of the oil pressure switch and over 2 days consistently got 25 psi cruising/ 8 - 10 psi stopped.

That's not really that bad for an original engine, the rule of thumb is 10 PSI/1000 RPM. What pump did you use? A Melling 99-HVS high volume pump with an ARP 230-7003 stud kit for a 454 will help/ Just needs a little trimming on the windage tray.

What's missing here?

Nothing other than old age. The cam bearings and or/cam will lose a lot of pressure with little wear. Send an oil sample to your favorite lab for analysis for a better idea with what's going on

HTH

 

[Drunkcanuk]

I've had my 91 for 28 years.
I would try a thicker oil like @PlayingWithTBI said. That's a lot of miles, things wear out inside there. It's probably the cheapest diagnosis you can run. And can't hurt anything.

 

[Schurkey]

PlayingwithTBI beat me to a couple of crucial points.

The sending unit that drives the gauge on the instrument cluster is a high-failure item. They can cause a too-high oil pressure reading, a too-low oil pressure reading, or just plain too-variable readings.

Many "computerized" parts catalogs list the WRONG sending unit for the '88--'89 trucks. The incorrect replacement is too damn big. The heat shield has to be forced over it, which means the heat shield is usually left off. It's a mess.

I put a big, long thread about this on another forum. I'm too lazy to re-post it all here. Problem is, that other forum has been "down" long enough that I wonder if they pulled the plug. All the part numbers and prices were in that thread, and I didn't have it stored on my computer.

Anyway, you're looking for a sending unit spaced out from the oil filter area by a brass ~80-degree elbow. The sending unit should have green paint on the wrenching surface. This sending unit is sold under various part numbers by all the usual parts stores--O'Reillys, Advance, AutoZone, NAPA--but so far as I can tell it's made by one company in Taiwan and then re-boxed by everyone else.


OP6649 is the WRONG sending unit.

You must be registered for see images attach

The 90-degree elbow and wrong sending unit still does not fit. It just interferes somewhere else. I tried everything to get that sending unit to work, and nothing helped.

You must be registered for see images attach

I'll re-check that other forum in a few days--please remind me--and if it comes back to life, maybe I'll work up the ambition to move that info here.

Also...I'm "done" with normal SBC oil pumps. I'm moving to the M99HV-S "big block" oil pump kit from Melling. Bolts to the rear cap exactly like a small-block pump, requires some (easy) trimming/modification of the windage tray on our engines, and comes with an acceptable pickup tube/screen for stock pans.
www.summitracing.com/parts/mel-m99hvs

Melling M-99HV-S Melling High-Volume Oil Pumps | Summit Racing

Free Shipping - Melling High-Volume Oil Pumps with qualifying orders of $109. Shop Oil Pumps at Summit Racing.

www.summitracing.com

 

[old_silverado]

Thanks for quick replies. The replacement oil pump is the stock Melling (M55 I believe.). I didn't go with higher pressure as the engine is older (so are the seals.). I didn't think it needed more but I also thought after 282k miles the original pump was starting to fail. Apparently wrong as the oil pressure was the same before and after.
I used to run the 10w30 that was spec but swapped years ago when 10w40 kind of replaced 10w30 on the market. It's had the Havoline Synthetic Blend in it for decades. I probably will put something thicker in if nothing else comes up. I ran 30HD in everything back in the 80s and 90s with no problems (deep South.)
There are two oil pressure sensors. The one that reads for the dash gauge is known in parts catalogs as the oil pressure switch, not sure why as it's not a switch. Easy to distinguish as it's the single wire. The double wire one behind the distributor is the oil pressure sensor. I learned years ago to make sure of the correct part as most that are claimed as correct are too large for the heat shield. Anyway I checked the pressure with a mechanical gauge. I wouldn't think much of this if it had happened gradually but it was one day to the next with the across the board pressure drop.
I didn't cut open the old filter as I didn't find anything at all in the pan or else where. I might dig it out of the trash can just to see. I've always used AC Delco filters with no issues. I did put a Fram on this truck once. I always lightly oil the seal before screwing them on. After 5k miles the Fram was stuck so bad I had to put a screwdriver through it to get it back off.

 

[evilunclegrimace]

Friends don't let friends run Fram oil filters,