O/T GMC truck

Help needed · Sep 5, 2008

I know no one really likes O/T's but anyway. My mother in law has a 1988 GMC pick up. Has about 90,000 original miles. 350 engine. Now the problem. When you start it up it has about 30psi on the oil pressure guage. Now as you drive it and the truck warms up the truck continues to hold about 30 pounds until you stop at a stop sign. When the rpms drop so does the oil pressure. The oil has been changed pretty regularly and the engine is pretty quiet while running.

So anyone got any ideas. maybe a bad sending unit? Maybe worn bearings what do you think? Thinks in advance.

JDemaris · Sep 5, 2008

Stick a gauge into it before you stop worrying. If the engine is worn, usually the rods knock a bit at startup. But, any engine with high miles can get near zero oil-pressue when at hot idle. Heck, just about every Detroit Diesel I've ever had did it since new.

With 80s GMs, the electric sending units often cause bad readings.

If it has any oil pressure at all at hot idle, and there's no noise, it's probably fine as long as the pressure comes up with engine speed.

Allan in NE · Sep 5, 2008

I'd sure check the sending unit; maybe your truck missed the recall back in the day.

Really doubt if the bearings are at fault at only 90,000 miles.

Allan

old · Sep 5, 2008

Get a machical gauge and put on it. Those gauges that are on them stock are electric gauges and they can read funny. Then go from there after you know for sure if it has a problem or not. I know my 1980 Chev has about 200.000 miles on it and holds good pressure
Hobby farm

Ray · Sep 5, 2008

The oil pressure should drop when the engine slows down.I doubt if you have more than 20 pounds at hot idle.Those gauges are not very accurate,just basically show if you have oil pressure or not.

IlliniJohn · Sep 5, 2008

Agree with the suggestions to try a mechanical gage - -but, what weight oil is in it and what temperature is the truck running at?

If, for a "far-out" instance, its got 5W20 in it on a hot summer day, the oil viscosity woul really thin out.

Might want to switch to straight 30wt in summer, and 10W30 or 5W20 in winter - -though a lot of folks run 15W40 year round

Anonymous-0 · Sep 5, 2008

The chevy engine gets its oil pressure from the cam bearings Try maniul gauge.

Mike in Ohio · Sep 5, 2008

I don't think the guages on the newer vehicles are all that accurate. I know on my 2000 expedition it reads in the same spot no matter what the rpm. It's designed to be a fancy idiot light not going down till the pressure gets below 10 pounds. I would not panic till you put a mechanical guage on it. I'm not a chevy guy but 90,000 on a 350 should just be broke in real good. Good luck, Mike

Goose · Sep 5, 2008

You might want to see if you can find out what it has in it for oil. Or just change it, put 10w30 or 10w40 in it and see what happens. Might be easier than patching in a mechanical guage. But that would be the next step.

MarkB_MI · Sep 6, 2008

Change out the sending unit. What happens is the sender can develop an internal leak. As you drive and the oil warms up, it leaks past the diaphragm in the sender until you don't register any oil pressure. Overnight the oil can leak out of the sender and the process starts over the next day. (If I recall correctly, the sender on this vehicle is mounted upside down, so oil is able to leak out of the sender.) If you look at the sender, I'll bet you'll see that it's leaking.

O/T GMC truck

Help needed

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JDemaris

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Allan in NE

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old

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Ray

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IlliniJohn

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Mike in Ohio

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