Oil Pressure Problem?

[joeyb2]

I have a 1991 Ranger with a 4.0. My 'problem' is that when I let off on the throttle to shift the 'dummy' gauge drops to 0. Is there a problem here? I also started getting 'the knock' this week. Is this something to worry about? Are these related? I searched, but I can't find anything that addresses this. If I missed something can someone just point me to a thread that addresses this? Thanks.

 

[adsm08]

I would say that is probably something to at least investigate, but a drop in pressure when engine speed comes down is something to worry about. My dad's truck did something similar once upon a time. It wasn't too much longer after that started that it spun the thrust bearing.

 

[RonD]

Yes, "knock" and low oil pressure are often related, one can cause the other, both ways.
The knock could be the result of low oil pressure or the cause of it.

Oil pressure is the back pressure created by the crank and rod bearings plus other oil passages.
The oil pump sends X amount of oil to the oil filter, it is then sent to the oil passages and bearings, because good bearings are a tight fit all the oil the oil pump sends can't be "pushed" out, so a back pressure builds up, that's the oil pressure of an engine.
If a bearing starts to fail it's gap widens and more oil can flow through, so oil pressure drops for the whole system, back pressure is less, bearing can also start to knock.

If less oil is coming from the oil pump or filter then pressure drops, less back pressure.
Less oil pressure means bearings at the farther away points of the oil passages get less oil so can heat up and wear more, causing a loose bearing which reduces pressure even more and will start to produce a knocking noise.

Oil pressure at idle should run 6-8psi, at 3,000rpm about 30psi, higher rpm means more oil flow from the oil pump, so higher back pressure.
It reads like the oil pump is ok.

I would try a different sender and change the oil filter, then investigate the knock.
I wouldn't run the engine too much until you have confirmed at least some oil pressure at idle.

 

[joeyb2]

According to my 'dummy' gauge I have plenty of pressure at idle, on the 'm' but almost to the 'a' in 'normal'. It only drops if I'm on the throttle and let off, then it hits 0 and takes 3 seconds to climb back up.

 

[Surrey]

Mine has no pressure at idle, but I suspect that is due to leaking valve covers. Might get around to replacing those today if Im not too busy with my calculus homework...

 

[RonD]

Valve cover gasket leak and oil pressure are unrelated.
However low oil pressure would mean less oil would be going to the cam/rockers, so valve cover leak would be less

 

[RonD]

According to my 'dummy' gauge I have plenty of pressure at idle, on the 'm' but almost to the 'a' in 'normal'. It only drops if I'm on the throttle and let off, then it hits 0 and takes 3 seconds to climb back up.

I would check the sending units wire, reads like it is loose

 

[Surrey]

Valve cover gasket leak and oil pressure are unrelated.
However low oil pressure would mean less oil would be going to the cam/rockers, so valve cover leak would be less

really? damn it. its leaking a lot of oil from there (I think its from there?) and was thinking the two were related...

I am exhibiting similar symptoms as Joeyb2, although my oil pressure is low whenever its idling, and goes up when you give it gas.

It has a tick, that gets louder as oil level drops. My brother borrowed the truck and ran it til the oil light came on at least once...

 

[pcollins]

We've got a tractor that acts sort of similar. When its cold it reads lots of oil pressure, once its up to operating temp and the oil thins out the oil pressure reads basically 0. Been like that for years and the engines fine.

 

[RonD]

On most engines you only need oil pressure of about 5psi, most "idiot lights" don't come on until oil pressure is down to 4psi.
Flat head engines don't need a lot of oil pressure, all the oil passages were almost the same level as the oil pump, they could basically use gravity to feed the bearings oil, lol.

If you actually had an oil pressure of 0psi then engine would start knocking and rod bearings would break, or a main bearing would spin and engine would seize.

The "ticking" noise when oil pressure is low is usually the lifters and rockers, these are at the top of the engine, when oil pressure is low not as much oil can be pumped to the top of the engine, so lifters and rockers get metal to metal contact, tick, tick, tick.

 

[joeyb2]

I changed my oil to 10w-30 today (it was 5w-30 for our wonderful Canadian winter)and the 'dummy' gauge started reading slightly higher and stopped dropping, but the knocking continued. Is the damage to the lifters done? Getting a real gauge this week hopfully.

 

[joeyb2]

Had my dad listen to the truck last night and he said the tick was injectors. Gonna run some cleaner through my next tank. The pressure problem also seems to be a faulty sender. Thanks for the help! TRS rocks!