Oil pressure gauge acting up 1988 C1500, 5.7L

[jayman4pu]

Hey all, I'd first like to say thanks for all the great advice on here I've saved a few hundred bucks on easy fixes for my truck.

Now, to the task at hand. My C1500 is acting up a bit. The oil pressure reads fine at start-up and for the first 15-20 miles driven. Usually around 40-50 psi. After that, is slowly drops to run around 15-20 psi as I drive down the road. The real problem occurs when I come to a full stop. It bounces up & down, drops to zero and the check gauges light comes on. Reading the other threads on here, it sounds like the sending unit is going bad (if I read correctly, it's the one by the oil filter, not the one by the distrib. cap).

I want to be sure I've got proper pressure, as an engine swap is not on the radar at this point. Any recommendations for a mechanical gauge? If so, where does it install?

Thanks again.

On another note, where can I go to get a factory manual for this. The Haynes manual I have is complete & utter crap. I went to alldatadiy.com, but it'd be nice to have a hard copy.

[j cAT]

Originally Posted by jayman4pu

Hey all, I'd first like to say thanks for all the great advice on here I've saved a few hundred bucks on easy fixes for my truck.

Now, to the task at hand. My C1500 is acting up a bit. The oil pressure reads fine at start-up and for the first 15-20 miles driven. Usually around 40-50 psi. After that, is slowly drops to run around 15-20 psi as I drive down the road. The real problem occurs when I come to a full stop. It bounces up & down, drops to zero and the check gauges light comes on. Reading the other threads on here, it sounds like the sending unit is going bad (if I read correctly, it's the one by the oil filter, not the one by the distrib. cap).

I want to be sure I've got proper pressure, as an engine swap is not on the radar at this point. Any recommendations for a mechanical gauge? If so, where does it install?

Thanks again.

On another note, where can I go to get a factory manual for this. The Haynes manual I have is complete & utter crap. I went to alldatadiy.com, but it'd be nice to have a hard copy.

with maximum resistance the oil guage should read max pressure. if shorted to ground should read zero. if it were sender the low pressure would have nothing to do with temp. as this is consistantly doing the same thing at every time the is in vehicle operation......your problem i believe is the engine, most likely the main bearings are worn and as oil heats up you gradually loose oil pressure. because you do not want to repair this i would use a heavy oil like 30wt. and see if it helps the pressure, if it does then you know your engine is on its last days. i know of a guy that ran on 40wt. for some period of time, and that this did work as it extened the vehicles life.........replacing the oil pump will help but will it be enough?

[jayman4pu]

Originally Posted by j cAT

with maximum resistance the oil guage should read max pressure. if shorted to ground should read zero. if it were sender the low pressure would have nothing to do with temp. as this is consistantly doing the same thing at every time the is in vehicle operation......your problem i believe is the engine, most likely the main bearings are worn and as oil heats up you gradually loose oil pressure. because you do not want to repair this i would use a heavy oil like 30wt. and see if it helps the pressure, if it does then you know your engine is on its last days. i know of a guy that ran on 40wt. for some period of time, and that this did work as it extened the vehicles life.........replacing the oil pump will help but will it be enough?

I was afraid of that. Thanks for the tips, I may try a high flow oil pump coupled with heavier oil to see if I can't limp by for 6-8 months.

[2000CAYukon]

For manuals, you can try ebay or buy new at www.helminc.com.

It would also be a good idea to use a mechanical gauge to see what your oil pressure really is doing.

//2000CAYukon

[G.A.S.]

I think the guage method will show low oil pressure.
You have a idiot light coming on with the drop in pressure but by far, check it anyway. process of elimination.
An oil pump might just do the trick.
How many miles on the engine?

[jayman4pu]

Originally Posted by G.A.S.

I think the guage method will show low oil pressure.
You have a idiot light coming on with the drop in pressure but by far, check it anyway. process of elimination.
An oil pump might just do the trick.
How many miles on the engine?

I'm hoping a pump will at the least get me by a little longer. The engine only has 117k on it. My grandfather bought the truck new in '87. He died in 2005 and that's when I got the truck. The last couple of years he was alive, it wasn't driven much due to his poor health. I just started using it for daily driving a couple months ago. It has never been used for towing, but most of the miles are not highway miles.

Before using as a daily driver, I took it to, IMHO, a very trustworthy mechanic to check over - full on inspection. Only problems he found were the brakes and some rust in the coolant. Flushed it all out, changed all the pertinent fluids, replaced brake pads/rotors/drums/cylinders and I was good to go.

This is looking down the road a bit, but where is a good place to find rebuilt engines, if that's what I do wind up having to do? I've looked online, and $1200 is about as good as I've found, with Jasper's base engine coming in at $1700.

[J-Ri]

Originally Posted by 2000CAYukon

It would also be a good idea to use a mechanical gauge to see what your oil pressure really is doing.

I second that. Sensors will read wrong after they heat up sometimes. With that few miles, I seriously doubt a mechanical problem... but who knows, that's what tests are for.

If the pressure does turn out to be low, I would switch to 15/40 and see if it goes away. You could also consider adding an oil cooler. Both are band-aids, but are cheaper than surgery.

[jayman4pu]

Originally Posted by J-Ri

I second that. Sensors will read wrong after they heat up sometimes. With that few miles, I seriously doubt a mechanical problem... but who knows, that's what tests are for.

If the pressure does turn out to be low, I would switch to 15/40 and see if it goes away. You could also consider adding an oil cooler. Both are band-aids, but are cheaper than surgery.

What's odd is it just started doing that all of a sudden. Prior to last Thursday, it never dipped below 30 psi. I'm just going to pull the sensor, pop in a gauge and see what's going on. If it is low, the higher weight oil I can do. It already has an oil cooler though.


Thanks again everyone for the advice.

[G.A.S.]

Originally Posted by jayman4pu

What's odd is it just started doing that all of a sudden. Prior to last Thursday, it never dipped below 30 psi. I'm just going to pull the sensor, pop in a gauge and see what's going on. If it is low, the higher weight oil I can do. It already has an oil cooler though.


Thanks again everyone for the advice.

after hearing that, I say faulty guage on the dash

[jdr2008]

I have a 1989 k1500 and about 12 years ago I had the same problem that you have now, all of a sudden I had low oil pressure, It was fine until it warmed up, I noticed when I applied the brakes the gauge would drop to zero and the oil warning light came on, the oil level was correct, I thought to myself the oil pump is going, dropped the oil pan to replace the oil pump and there was the pick up tube laying in the bottom of the pan. I thought to myself isnt that *&^$#$ nice. I replaced the oil pump anyway, just another possible scenario that could relate to the problem your having.

[jayman4pu]

Originally Posted by jdr2008

I have a 1989 k1500 and about 12 years ago I had the same problem that you have now, all of a sudden I had low oil pressure, It was fine until it warmed up, I noticed when I applied the brakes the gauge would drop to zero and the oil warning light came on, the oil level was correct, I thought to myself the oil pump is going, dropped the oil pan to replace the oil pump and there was the pick up tube laying in the bottom of the pan. I thought to myself isnt that *&^$#$ nice. I replaced the oil pump anyway, just another possible scenario that could relate to the problem your having.

That is EXACTLY what my truck does. I was planning on dropping the pan this weekend if a mechanical gauge confirmed low pressure (replacing sending unit regardless of mech. gauge results). I'll be sure to look for that.

[jayman4pu]

Just a quick update. The truck needed an oil change anyway, so I swapped out the oil & filter. While under the truck, I located the sending unit (real PITA to get to) and disconnected the tube that it senses the oil pressure, hooked up a mechanical gauge to check it out, and what do you know, it read just fine (I let it reach operating temp). Lowest reading I got was 10 psi, which is low, but according to my manual, not out of spec at 1000 rpm. At 2000 rpm, it read 35 psi, which is well above the min. spec. of 18.

The interesting part, is when I reconnected the sending unit (took it out to inspect & make sure I bought the right one, as I had a choice of 3 different ones at the parts store) and it has worked fine since. I don't know if the electrical connector had come loose or what, but I'm going to keep an eye on it for a bit before I claim complete victory.

Thanks again for all the help.

[j cAT]

Lowest reading I got was 10 psi, which is low, but according to my manual, not out of spec at 1000 rpm. At 2000 rpm, it read 35 psi, which is well above the min. spec. of 18.


Thanks again for all the help.[/quote]
I'm sorry but I disagree with this 10psi figure at 1000rpm. The oil pressure in drive, idle at 650rpm should be 25psi min.....if at 1000rpm its 10psi this is not enough. when hot weather comes there will not be enough flow to cool down the oil . the oil will over heat and your going to create other problems in other areas, as the oil vaporizes into the combustion chamber.....

[jayman4pu]

Originally Posted by j cAT

Lowest reading I got was 10 psi, which is low, but according to my manual, not out of spec at 1000 rpm. At 2000 rpm, it read 35 psi, which is well above the min. spec. of 18.


Thanks again for all the help.

I'm sorry but I disagree with this 10psi figure at 1000rpm. The oil pressure in drive, idle at 650rpm should be 25psi min.....if at 1000rpm its 10psi this is not enough. when hot weather comes there will not be enough flow to cool down the oil . the oil will over heat and your going to create other problems in other areas, as the oil vaporizes into the combustion chamber.....[/quote]


Don't apologize, this is the kind of info I need. The manual I have (Haynes) lists factory minimums of oil pressure of 6 psi @ 1000 rpm, and 18 psi @ 2000 rpm. At time of checking things this weekend, that's all I had to go by. I knew it was to the low side of what is normal, but if it's so far to the low side that it could result in damage, well, maybe I'll be dropping the pan & putting in a new oil pump after all. Yes, I realize that it is likely a sign of bearings wearing out, but I also would like to not have to spend the money on rebuilding the engine just yet.

If 6 psi is a dangerous level, then why is it the minimum spec from the manufacturer?

I should also add that both readings were done with the truck in park. Would shifting it to drive really make much difference?

[2000CAYukon]

On my 90 K1500 5.7 4L60 with 130K on the original engine/tranny will idle around 15 when the engine is hot and 30 -35 at freeway speeds. The max pressure on the gauge is 60. When the miles were lower, it would idle around 30 PSI and around 45 at freeway.

My 5.3 Yukon on the other hand has much higher oil pressure. The TBI engines were not designed to run at 60 PSI.

I also had a leak where the oil cooler/oil filter adapter meet the block. The gasket was hard as a rock and the oil pressure did improve after replacing this gasket (probably 5 psi).

6 psi per 1000 rpm is the minimum and you are well above that. What weight oil are you using?

//2000CAYukon