Oil Pressure Indication Drops, low speed turns

First of all, I don't hear any valve chatter or sounds indicating engine distress, so I think this may be an indication rather than a true pressure problem. Or it could be that I have several problems.

I have an 89 1500 with a 305. The oil pressure sending unit is electrical & was changed with the new motor at about 85K. It's about 170K now.

After the truck has run a while & is warmed up & idling no longer accelerated during warm-up, I notice the gauge will top out at just above 30psi at full accelleration. Normally, it's about 50 or so. When I'm in a parking lot, the indication is 10 or so & when I make a hard turn with no acceleration & get close to the steering stops (has power steering) I'll hear the usual hiss & the indication will drop to close to zero & the check gauges light comes on as it should. Of course, I could hit the gas & the indication & actual pressure will go up, extinguishing the light. Again, there is no noise from the engine.

This is intermittent. I guess it could be the sending unit, gauge or oil pump, but what is the tie-in with the power steering system. I thought that used power steering fluid only. Don't think there is a vacuum line that shares vacuum pressure with the engine. I'll check.

I also have a nagging shifting problem that shows up every 3rd day or so. Maybe it's related. The 3 speed automatic tranny shifts slowly after it is warmed up. Sometimes you have to let up on the gas like it's a manual shift to get it to shift & sometimes that works 1st to 2nd but intermittently if a all 2nd to 3rd. When it's really bad you can be driving in 3rd gear on the highway & particularly when you go up a hill, if you give it gas, it's like it tries to slip down into 2nd & the engine races. Don't have an rpm gauge, but it sure sounds like they are running away.
It'll run fine for a few days, run like it's going to explode for a day or 2 then great for a week. I can't predict when the problems will occur.

This is just weird. I've never put a vacuum gauge on this truck???

The p/s drops the idle at a full turn so the oil pressure follows suit,, but it is to low per the gage. I would put a known accurate mechanical gae on it and confirm if the pressure is really that low. If this trans is slipping as you say this is internal, for starters check and adjust the tv cable, make sure it is not binding also.

rule of thumb. 10psi per 1000 rpm. just remember 1psi is better than 0 psi. If it is reading like that I would definatley put a mechanical gauge on it. Mine reads 60 all the time and then suddenly drops to 10 or so, in all reality by my mechanical gauge it runs about 30psi.

First of all 50psi is too much. You might have it only when the engine is real cold. max is 45psi for this engine, but most sit around 30 psi when warmed up. Also from my experience the rule of tumb here doesnt apply. If you have proper viscosity oil you should be right in the middle of gage all the time no matter at idle or at 5000 rpm. max difference you should get is 5-10 psi,, more means its just very cold or your have a problem. Blocage in the oil line can cause high oil pressure or bad pump will cause like 5psi oil pressure at idle.

Connect the manual gage to it to see what it does.

You what constant 30 psi at any speed even idle(see broughy84' reply) , but
it should be 45psi at like 5000rpm cold.Or 40 psi when first started. Thats considered normal. (10psi +-) But most healthy 305s GM engines will run 30-35psi all the time with a little difference at high speeds in warm climate. ( 10w-30 oil) If you have some sorta tractor oil and your engine smokes than the numbers will be completely screwed in both directions.

I recomend Seafoam product that (people say) cleans the shit out of engines with only one oil change. ( I heard some people use tranny oil)


The tranny problem is mostly due to low level of oil or sticking valve for the third gear. It needs to be rebuilt
or the valve fitted properly. might also be a converter problem someimes or both. Its tired. Replace with a nice 700r4

I hope it helps.

Where is it written that 45 PSI is the max? so from your logic 46 would be bad and require repair? :screwy: My 97 GMC shows 55-60 going down the road at 75. I guess mine is ready to blow huh? :rofl:

Oil pressure varies with engine RPM, period. Now maybe if you have a vehicle with an "idiot" guage the pressure would stay in the same place no matter what.



First of all 50psi is too much. You might have it only when the engine is real cold. max is 45psi for this engine, but most sit around 30 psi when warmed up. Also from my experience the rule of tumb here doesnt apply. If you have proper viscosity oil you should be right in the middle of gage all the time no matter at idle or at 5000 rpm. max difference you should get is 5-10 psi,, more means its just very cold or your have a problem. Blocage in the oil line can cause high oil pressure or bad pump will cause like 5psi oil pressure at idle.

Connect the manual gage to it to see what it does.

You what constant 30 psi at any speed even idle(see broughy84' reply) , but
it should be 45psi at like 5000rpm cold.Or 40 psi when first started. Thats considered normal. (10psi +-) But most healthy 305s GM engines will run 30-35psi all the time with a little difference at high speeds in warm climate. ( 10w-30 oil) If you have some sorta tractor oil and your engine smokes than the numbers will be completely screwed in both directions.

I recomend Seafoam product that (people say) cleans the shit out of engines with only one oil change. ( I heard some people use tranny oil)


The tranny problem is mostly due to low level of oil or sticking valve for the third gear. It needs to be rebuilt
or the valve fitted properly. might also be a converter problem someimes or both. Its tired. Replace with a nice 700r4

I hope it helps.