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View Full Version : 94 TBI low oil pressure when at temp and idle.


André
07-04-2009, 04:20 PM
Started last year and getting worst.After a couples of pass when dropping at each end and idling the oil pressure will fall to 5 psi.If i rev at 750 ,800 the pressure goes back up to 40 and when skiing it goes to about 60 psi.
When cold i get 60 at idle and while cruising to the course i read 80 psi at 2500 rpm.
We usually are 3 skiers and i now shut it down each time instead of idling.I use 15w40 and the oil level is fine.
Oil pump going bad? Anyting else could cause that?
Is there anything i can check?Would using 20w50 help?
I don't cruise around the lake at idle but may i hurt the engine if idling for too long?

A search here with "low oil pressure" gave not much...:(

TIA

If it's the pump it means i've got to pull the tranny and engine or just the engine?
Read that the actual pump replacement is quite easy...but that was on the Internet!;)

JLeuck64
07-04-2009, 04:29 PM
Can you double check that with a REAL pressure gauge screwed into the block in place of the sending unit? I would try this before condeming the oil pump...

André
07-04-2009, 04:42 PM
Can you double check that with a REAL pressure gauge screwed into the block in place of the sending unit? I would try this before condeming the oil pump...

I have plenty of accurate gauges.Where should i install it?
Is there any NPT tap i could use?
Thanks!

Boat is on the lift 100' from me but it's really raining buckets here now...

JimN
07-04-2009, 05:14 PM
I have plenty of accurate gauges.Where should i install it?
Is there any NPT tap i could use?
Thanks!

Boat is on the lift 100' from me but it's really raining buckets here now...

Have you recently changed oil filter brands? Some have less back pressure than others and this will show on the gauge. Also, you can remove the sender to inspect it. Sometimes, a bit of sludge will clog the little hole where the oil goes in. If the sender is good, you can connect a mechanical gauge at the plug that's just above the oil filter.

André
07-04-2009, 07:48 PM
Have you recently changed oil filter brands? Some have less back pressure than others and this will show on the gauge. Also, you can remove the sender to inspect it. Sometimes, a bit of sludge will clog the little hole where the oil goes in. If the sender is good, you can connect a mechanical gauge at the plug that's just above the oil filter.

Will do both.Thanks Jim!
Can i damage the engine idling 30 seconds with low pressure? (If the pressure check really low)

JimN
07-04-2009, 07:57 PM
Will do both.Thanks Jim!
Can i damage the engine idling 30 seconds with low pressure? (If the pressure check really low)

30 seconds? I seriously doubt it. Most of the time in extreme cold, I doubt if the oil gets to the top end in less than 20 seconds and that's on a regular basis. GM has always done destructive testing on their motors and along with running them with no coolant, they also ran them with no oil. For as long as it took for the motor to die. Then, they rebuilt it (if possible) and did it again. Then, they did it to other motors to see if they could spot any trends.

They got 250 miles out of a Cavalier with no oil when they tested it at the Desert Proving Grounds, near Phoenix, AZ.

André
05-13-2011, 02:49 PM
Well,i skied through last season with my low oil pressure at idle.When engine is cold the gauge shows 60-70 psi but at operating temp and idle i now got 0 psi of oil pressure.
At skiing speed and warm i'm now at about 45-50 psi.
When shopping for a new oil pump i asked for an 90-92 1500 GM truck with 5.7.
He had a different # for 88-91 and 92-9? # .He also said that the 5.0 GM used the same pump.The boat is a 94 TBI 5.7
Anyone knows exactly what pump i need and part # or cross reference #?
Do you need to remove the engine completely or can you just raise the back?
Do you leave the transmission attached to the engine when pullin out the engine?
Any specials advices for doing this job?

All advices and silly jokes are Welcome!!!:)

Table Rocker
05-13-2011, 04:10 PM
Have you changed the sending unit or tested with a mechanical gauge? I just don't think it is very likely that a new oil pump is going to solve the problem. An oil pump is a pretty simple device.

JimN
05-13-2011, 05:06 PM
Well,i skied through last season with my low oil pressure at idle.When engine is cold the gauge shows 60-70 psi but at operating temp and idle i now got 0 psi of oil pressure.
At skiing speed and warm i'm now at about 45-50 psi.
When shopping for a new oil pump i asked for an 90-92 1500 GM truck with 5.7.
He had a different # for 88-91 and 92-9? # .He also said that the 5.0 GM used the same pump.The boat is a 94 TBI 5.7
Anyone knows exactly what pump i need and part # or cross reference #?
Do you need to remove the engine completely or can you just raise the back?
Do you leave the transmission attached to the engine when pullin out the engine?
Any specials advices for doing this job?

All advices and silly jokes are Welcome!!!:)

I would check the oil pressure sender before I replaced the pump. I have a '90 Chevy Silverado pickup with the 5.7 liter and the oil pressure has never been as high as I have been accustomed to seeing, but when I look into the filler cap opening with a flashlight, I can see oil flowing. Look into the filler cap holes and see if yours has any oil flowing along the lower edge of the head, below the rocker arms. I would bet the pump is fine, but the sender may be faulty.

I would also recommend having the pressure checked with a mechanical gauge.

André
05-13-2011, 07:57 PM
Could the sending unit be ok when cold and give a false reading when warm?
Was hoping to change the pump before putting the boat in but will wait and put a gauge tomorrow,weather permitting!
What does the fact that the pressure is ok when cold and at 0 when warm tell you guys?
Thanks!

Table Rocker
05-13-2011, 08:08 PM
Could the sending unit be ok when cold and give a false reading when warm?
Was hoping to change the pump before putting the boat in but will wait and put a gauge tomorrow,weather permitting!
What does the fact that the pressure is ok when cold and at 0 when warm tell you guys?
Thanks!
The sending unit has a diaphragm in the middle. When sitting, the diaphragm is in the relaxed position. As pressure comes up the diaphragm moves. If you have a hole in the diaphragm, the oil will leak to the other side of the diaphragm until the pressure will equalize and the gauge will read zero again. If you shut down, the oil can drain back out of the unit and you can start the whole process over again.

Replace your sending unit.

JimN
05-13-2011, 10:01 PM
Could the sending unit be ok when cold and give a false reading when warm?
Was hoping to change the pump before putting the boat in but will wait and put a gauge tomorrow,weather permitting!
What does the fact that the pressure is ok when cold and at 0 when warm tell you guys?
Thanks!

Being an electrical device, yes, it's definitely possible that it could have a thermal issue although the comments about how they're built probably tells the story. If the oil is cold and thick, it won't bypass the diaphragm if it's damaged. Once warm, the oil is thinner and can squeeze through, nullifying any pressure reading.

As I posted earlier, I would look inside of the valve covers first, to see if oil is running along the head. If it is, I would bet that the sender is bad but I would verify that with a mechanical gauge. Mine has shown low pressure since I got it almost 10 years ago and it still doesn't burn any oil. These engines are hard to kill and if 15K miles/year is the average for mine over its life, it has over 300K on it and I put over 130K on it, myself. I recently replaced the starter but the alternator is original and the only other work it has needed that I know of happened in '02 when a friend and I changed the valve seals and I replaced the EGR a few years ago.

Thrall
05-13-2011, 10:09 PM
Yeah Andre, unlike the in tank MC fuel pumps....... you most likely do NOT have a bad oil pump. I normally wouldn't reccomend throwing parts at something, but the oil presure sender is cheap ($20??), just throw a new one on and see what happens.
Like JimN I've run alot of small block chevys into the ground and never had an oil pump go out, but I've had plenty that showed little to no pressure on the guage.

André
05-14-2011, 09:19 AM
OK guys.I'll wait before doing anything else and will check oil flow looking by the oil cap and probably buy a sending unit since it's a cheap part.
Good thing NAPA didn't have the oil pump stock.As a matter of fact,no local car parts store had it stock on the shelf...
Thanks for the answers Gentlemens!
Will report back...if rain stop!:(

cwright
05-16-2011, 09:24 PM
I had a similar issue, and following the advice given above (replace sending unit) solved the problem. Oil pressure is always lower at idle obviously, but a new sending unit recorded less of a drop and it kept my low oil alarm from going off. My lesson learned on the project was to survey the part before removing and either have access to a hardware store handy, or pre-purchase some of the pipe fitting involved in the assembly. The pipe nipple that was screwed into the block twisted off rather than unthread. Fortunately an easy out worked on the broken off nipple, but I had to find replacements before I could run the boat. If you are in a time crunch, it might help to have a few extra fittings in your poseession when you begin. Use pipe dope or teflon tape on the treads (during re-assembly) to prevent the need of over tightening. Good luck