1988 K1500 Low Oil Pressure: PROBLEM SOLVED

[Socalsteve2021]

Hopefully this will save you hours of diagnosis and digging. MY 1988 K1500 350TBI 65K miles recently started showing very low oil pressure. Typically 30-40 suddenly went to 20 with RPM, 5-0 at idle. The check gauges light would even illuminate. After changing the oil and filter and doing a lot of digging, it turns out the 1988-1991 350's had two oil sending units. If you dig through the factory repair manual you will find them.

Sensor #1: is on top of the motor, behind the distributor. It's a 2-pronged connector and a PITA to get to. This sensor feeds oil pressure data to the ECU/Fuel Injection system and will cut the fuel pump if oil pressure is lost.

Sensor #2: is above the oil filter, below the exhaust manifold. This single wire sensor sends oil pressure data to the dash gauge. In the Pic below it looks like it's been leaking for a while. Part# PS269

So - after many hours of troubleshooting, panic from the 0 oil pressure reading, etc. the mystery is solved.

 

[someotherguy]

Sensor #1: is on top of the motor, behind the distributor. It's a 2-pronged connector and a PITA to get to. This sensor feeds oil pressure data to the ECU/Fuel Injection system and will cut the fuel pump if oil pressure is lost.

This pressure switch provides power to the fuel pump circuit if oil pressure is present; it is a redundant circuit to power the pump in the event the fuel pump relay goes bad.

Richard

 

[Schurkey]

Sensor #1: is on top of the motor, behind the distributor. It's a 2-pronged connector and a PITA to get to. This sensor feeds oil pressure data to the ECU/Fuel Injection system and will cut the fuel pump if oil pressure is lost.

No. As said, it is a bypass for the fuel pump relay. It will keep the pump RUNNING if the fuel pump relay or it's associated wiring fails. It will NOT shut off the fuel pump unless the fuel pump relay has already failed.

Sensor #2: is above the oil filter, below the exhaust manifold. This single wire sensor sends oil pressure data to the dash gauge. In the Pic below it looks like it's been leaking for a while. Part# PS269

So - after many hours of troubleshooting, panic from the 0 oil pressure reading, etc. the mystery is solved.

A high-failure item. My truck has been through three of 'em, currently on the fourth. They can fail showing low oil pressure; they can fail showing erratic/fluctuating oil pressure--high or low. They can have a leaking/spraying sort of failure. Sometimes, when you unscrew the sensor, it's heavy and sloshes because the seal has failed and it's filled-up with engine oil.

BE CAREFUL with the part number. PS269 hasn't come up in my parts searches, but you didn't say what brand you were using. When I look up PS269 at O'Reillys, the photo and description appear to be incorrect for this application. PS 269 appears to be the same as NAPA OP6649. You will "have fun" getting the OEM heat shield to fit this extra-large sending unit.

I've seen several parts stores list the WRONG sending unit for this vehicle; and I think that's what happened to you. (O'Reilly's and NAPA are two.) The one you want typically has a green paint-stripe on it. Far as I can tell, it's made by one company then re-boxed to suit whatever brand-name is sold by that particular parts-store. It's smaller than the common "incorrect" unit; it actually fits the heat shield and doesn't interfere with either the exhaust or the block.

BWD S4320 seems to be the correct one.
O'Reillys (Standard) PS410
The NAPA / Echlin number is OP6729. (NOT OP6649)
AutoZone sells it as PS150.
CarQuest 57013 (years ago--doesn't seem to be a current part number.)

https://www.oreillyauto.com/detail/c/standard-ignition/standard-ignition-1-terminal-oil-pressure-switch/std0/ps410/v/a/3384/automotive-truck-1988-chevrolet-k1500-pickup-4wd?q=OP6729&pos=0

The heat shield for that sensor might be CarQuest P/N S4049S; but since that CarQuest web page has no photo, I can't confirm. OTOH, it crosses to O'Reillys (Standard) PS381 which looks reasonably close--it's just missing the spring tabs at the big end that the OEM shield has for secure fitment.

https://www.oreillyauto.com/detail/c/standard-ignition/standard-ignition-oil-pressure-switch/std0/ps381?q=OP6729&pos=0

Old, defective sender (green stripe) with OEM heat shield, compared to new, incorrect sending unit

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Old, defective sending unit on OEM pipe-thread adapter; compared to WRONG sending unit on home-made pipe thread adapter. No combination of available adapter angles could prevent larger sending unit from interfering with either the block or the exhaust.

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[AuroraGirl]

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[Schurkey]

Yeah, that's great. The big sending units DON'T FIT PROPERLY . You'd think that ACDelco would know better.

They are too damn big to fit the heat shield nicely. On my truck, I went nuts trying to make the OEM pipe adapter work with the big-asp sensor--it would interfere with the block, and when I could get it spaced out to clear the block...it hit the exhaust.

The sensor I list pops into the heat shield like it was made to do so, clears the block, clears the exhaust, and does so with the OEM 80-ish degree adapter pipe...just as if it was the correct sending unit.

The only downside is that they're an Asian-sourced component. The service life is about 60--90 thousand miles; they're a known problem for long-term reliability.

 

[AuroraGirl]

Yeah, that's great. The big sending units DON'T FIT PROPERLY . You'd think that ACDelco would know better.

They are too damn big to fit the heat shield nicely. On my truck, I went nuts trying to make the OEM pipe adapter work with the big-asp sensor--it would interfere with the block, and when I could get it spaced out to clear the block...it hit the exhaust.

The sensor I list pops into the heat shield like it was made to do so, clears the block, clears the exhaust, and does so with the OEM 80-ish degree adapter pipe...just as if it was the correct sending unit.

The only downside is that they're an Asian-sourced component. The service life is about 60--90 thousand miles; they're a known problem for long-term reliability.

thats strange. do you think the OEM part was superseded for another fitment and they just didnt care to bother?

 

[AuroraGirl]

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it looks liek the part that preceded it was possibly the slim one you desire
10069218
can still be bought NOS, but not many

 

[Schurkey]

thats strange. do you think the OEM part was superseded for another fitment and they just didnt care to bother?

I have no inside insight. I have a working theory with no evidence.

"Somebody" saw that the sending unit for the '88 to...'91(?) trucks had a certain thread size, and drove a 0--60 psi gauge using a specific resistance range. They looked on some "master list" of sending units, found one that fit that criteria, and "poof" that's what's listed in the applications chart for those trucks.

They didn't look at the outside diameter; or scroll down far enough to see that there's more than one sending unit that will appropriately drive the '88--'91 dash gauges; and there was one that fit better.

O'Reillys, ACDelco, and NAPA don't "make" sending units, they're buying in bulk and reboxing; or they're merely selling a well-known brand that is buying in bulk and reboxing; all of this goes back to the actual manufacturer in Taiwan or S. Korea or wherever the thing is actually made. If the error in fitment is far enough up the chain of supply, everyone downstream (ACDelco, O'Reillys, NAPA, who knows who else) imports the wrong data into their computer catalog system.

I wouldn't have known about the "correct" sending unit if I hadn't gone to CarQuest. They're the ones who sold me the smaller one that fits right. From there, I just crossed the CarQuest number to NAPA and O'Reillys and Autozone, etc.; and "Guess What?" The sending units all look the same, like they were all made by the same nameless, faceless company in Asia. This was more than 20 years ago. I don't remember if CarQuest got the "right" part number out of a paper catalog; or the counterman knew from previous experience so he grabbed one off the shelf.

When I plugged the "correct" part number into the www.napaonline.com web site, it shows as fitting the LO3 in '88, but not the LO5. Go figure. Mistakes can be goofy some times.

 

[baxterday]

We had a similar issue with a knock sensor from O'Reilly's as well. The chinese made unit's threads would not work, as well as the chinese made fuel connections that pass through the back of the manifold on a 94 S10 Blazer, the were too big. I had to use the original and transfer the hoses. I avoid chinese made products at all costs. Unfortunately most of the junk in the big box auto parts stores is all sourced from china.

 

[AuroraGirl]

I have no inside insight. I have a working theory with no evidence.

"Somebody" saw that the sending unit for the '88 to...'91(?) trucks had a certain thread size, and drove a 0--60 psi gauge using a specific resistance range. They looked on some "master list" of sending units, found one that fit that criteria, and "poof" that's what's listed in the applications chart for those trucks.

They didn't look at the outside diameter; or scroll down far enough to see that there's more than one sending unit that will appropriately drive the '88--'91 dash gauges; and there was one that fit better.

O'Reillys, ACDelco, and NAPA don't "make" sending units, they're buying in bulk and reboxing; or they're merely selling a well-known brand that is buying in bulk and reboxing; all of this goes back to the actual manufacturer in Taiwan or S. Korea or wherever the thing is actually made. If the error in fitment is far enough up the chain of supply, everyone downstream (ACDelco, O'Reillys, NAPA, who knows who else) imports the wrong data into their computer catalog system.

I wouldn't have known about the "correct" sending unit if I hadn't gone to CarQuest. They're the ones who sold me the smaller one that fits right. From there, I just crossed the CarQuest number to NAPA and O'Reillys and Autozone, etc.; and "Guess What?" The sending units all look the same, like they were all made by the same nameless, faceless company in Asia. This was more than 20 years ago. I don't remember if CarQuest got the "right" part number out of a paper catalog; or the counterman knew from previous experience so he grabbed one off the shelf.

When I plugged the "correct" part number into the www.napaonline.com web site, it shows as fitting the LO3 in '88, but not the LO5. Go figure. Mistakes can be goofy some times.

its possible the intake is different in those? The LO5 im not sure what it was, but if its possibly the one in the buick roadmaster/b bodies or is itself a HO intended motor, im hinting at possible rise in the intake or possibility of a different placement in the engine bay because of frame, engine mounting/ trans selection, possible oil cooler assembly, etc..

so i looked into it, apparently lo5 is the 350 and lo3 is the 30nope(5) but the LO5/L05 whatever is also pretty undesireable and it had 2 variants. an HD for larger trucks gvw wise and lower normal trucks but still bigger than 305. im guessing this means torque so possibly manifold changes? And also the L05 i would think has a higher chance of having teh sandwich under it with a smaller filter. not sure how that would affect, but maybe something about that being there somehow change its placement. Orrr, you said interfere with manifold. maybe the exhaust is shaped and position different

also i just realized the optional larger gas tank is a thing, and I just realized my dad has complained and been happy about the fact the truck holds somehwhere close to 40 gallons, does my C1500 have a larger gas tank?

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I didnt post these for any reason, but those toolboxes are off a 67-72 F100/F250 contractor special. never thought an ad for an old ass F series would show where my dads toolboxes are from lol