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Low Oil Pressure/ 89 350 TBI

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GalDemSuga

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Folks
My oil pressure went from 20-30 psi to about 15 psi just now. I recall that over the last 2 years it's given me about 40 psi when cold and about 30 psi when warm and I'm driving.

I still got 40 psi when cold but once warm its 15 psi while driving and about 10 psi at idle. Did something go wrong in my motor?

Well the sun is rising here in Gilbert, WV, so I went to do some investigating. I popped the hood and checked the oil level it was good. I checked for leaks and dont have any. I noticed that a bolt at the #7 cyclinder exhaust manifold location is missing.

At Idle my pressure is halfway between the first and second hash marks and under load it is on/slightly above the second hash mark.
 
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If you've got factory guages, they're not known to be 100% accurate. Try to find a good mechanical guage and see if the pressure readings are any better.

When I was in high school, I had an 86 K-10 4x4 that used to run about 40psi of pressure when I was driving, but when the oil got close to the 3000 mile mark, it would drop down to about 25psi. That was my indicator to change the oil. Once I did, it would go right back up to 40psi.
 
Change your oil. I chose, rather than to buy/hookup a mechanical gauge, to just buy a new sending unit for my factory gauge. You have the pull the distributor to replace it, but thats really the hardest part. Did the whole job in an hour or 2. Just make sure you have a timing light to reset timing on hand.

Mine went from a constanat 40 -50 to just off 0 . Bad sending unit.
 
Until it starts knocking at cold startup, I wouldnt worry too much about it, the things 21 years old and probably getting a little tired. just drive nice and change the oil and you should have a few more years of reliable service left.
 
79rustyk10 said:
Until it starts knocking at cold startup, I wouldnt worry too much about it, the things 21 years old and probably getting a little tired. just drive nice and change the oil and you should have a few more years of reliable service left.
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And once it starts knocking at cold startup......ignore it, and keep driving the hell out of it, like i'm doing.
 
dhcomp said:
And once it starts knocking at cold startup......ignore it, and keep driving the hell out of it, like i'm doing.
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been there done that... used lots of Kendal 20w50.:D

I ran my 89 LD K2500 for over a year and a half with little to no oil pressure at idle. granted I beat it mercilessly the whole time I had it too. gotta love the SBC.
 
I've got good oil pressure, just some sort of knocking noise at cold startup.
 
Sure, something in your oil pump could have cracked, a bearing could have spun, lots of things can cause a sudden drop in oil pressure.

But, don't forget how the sending unit on your gauge works. Oil pressure against a diaphragm compresses a spring, which causes an arm to wipe across a coil of resistance wire.
That arm has wiped across there a LOT over the years.
It could have snagged the wire and caused it to short out or open, the diaphragm could have gotten a hole in it that let some oil get into the other side and reduce the amount of deflection that the arm makes.

Either way, if the oil is old, and you can change it, try that. If it does not help, at least you got an oil change out of the deal.

Otherwise, see if you can borrow a mechanical pressure gauge to see how much you actually have.
Putting on a new sending unit before checking it with a mechanical gauge won't tell you much.

Its almost certain to read differently from the old one. But you will not know for sure which is correct.
 
dhcomp said:
I've got good oil pressure, just some sort of knocking noise at cold startup.
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You could just have excessive piston to wall clearance and when the pistons warm up they expand and it goes away.

GalDemSuga you should hook up a mec guage and verify the pressure.If its low drop the pan and inspect the sump to see if its cloged with crud.If it checks out ok the rod bearings most likly are in need of replacement.Ive saved a few engins by just changing out the rod bearings and a new oil pump is a good idea while your there.As long as no bearing are spun and the crank looks to be on overall good shape it usualy works.
 
Folks
I drove the truck back to Atlanta yesterday. I had about an 18 hour window bedfore I was needed again.

While I was scared to drive her 475 miles, she performed like there was nothing wrong. I didnt loose any power and didnt hear any knocking the entire trip. It didnt matter if I drove 55 miles or 95 miles, that needle stayed at the second Mark during driving and never dipped below the first Mark when idling.

I've got to get back to WV today, so I'll be changing the oil and filter and resetting the ECM. I will report back. I dont have the time to trouble shoot it for another 2 weeks.
 
you've got a sending unit issue.... verify with a mech gauge..
 
Shoot my 4.3 blazer will sit at 0 when its hot outside and purr like a kitten. Has a wicked slap on startup when cold(sometimes) but its had that for like 3 years.
 
ryoken said:
you've got a sending unit issue.... verify with a mech gauge..
Click to expand...

This may be true. I changed oil and filter and reset the ECM. I replaced the missing exhaust manifold bolt too. I then fired her up with the oil cap off and revved her from under the hood to make sure oil was coming to the top and it was. The gauge still reads 15psi when driving and about 10psi at idle. I'll be leaving for WV in an hour, so lets she how she does.
 
Mabe it back to WV. No issues with performance. I drove the last 100 mile with my windows down and man do I need a valve adjustment. I cant believe I've been subjecting John Public to that chatter. It really loud:o

Pressure still reads low.
 
GalDemSuga said:
Mabe it back to WV. No issues with performance. I drove the last 100 mile with my windows down and man do I need a valve adjustment. I cant believe I've been subjecting John Public to that chatter. It really loud:o

Pressure still reads low.
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So, funny story, my valves need to be checked i think.

I've done valve jobs on solid setups (old tractors, etc.), but dont' understand the procedure on teh hydraulic lifters on these trucks.

How do you check/adj the valves?
 
dhcomp said:
So, funny story, my valves need to be checked i think.

I've done valve jobs on solid setups (old tractors, etc.), but dont' understand the procedure on teh hydraulic lifters on these trucks.

How do you check/adj the valves?
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I remove the valve covers and with the motor idling I adjust each valve. I back off the rocker nut until the valve starts to slap, then I slowly tighten until it stops slapping and at that point I give it another 3/4's of turn. You will hear and feel the motor slightly bog down and then it will readjust itself and run normal.

Repeat this for each valve. Good luck.
 
GalDemSuga said:
I remove the valve covers and with the motor idling I adjust each valve. I back off the rocker nut until the valve starts to slap, then I slowly tighten until it stops slapping and at that point I give it another 3/4's of turn. You will hear and feel the motor slightly bog down and then it will readjust itself and run normal.

Repeat this for each valve. Good luck.
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Wow, sounds pretty difficult. Guess i should check the valves. :D
 
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