I rebuilt my first engine, a 400, almost two years ago. I have a few thousand miles on it with no issues - well, until this week. After a spirited run on the highway and back, I noticed a quick drop in oil pressure when I came to a stoplight. Quick meaning less than 15 seconds. Drop meaning to zero. Needle buried. Normal oil pressure once back up to speed.

I built the engine. I measured the clearances. So I’m not convinced that’s my issue. I’m trying to brainstorm other possibilities:

- Sketchy oil pump. I put in a Melling pump. Standard issue.

- Clogged oil pump. Can’t say why or how yet but maybe.

- Bad oil filter. I can only hope.

- Bad gauge. Doubtful since pressure readings are only bad at a stop.

I have a mechanical gauge I plan to run to double check the stock gauge. I need to change the oil too, so that’s on the list.

Any other thoughts or similar experiences?

 

I'd get a mechanical gauge on it first to see if it's an accurate reading on your dash.

 

The gauge would be my first guess.

 

I forget, is it 2wd or 4wd and have you the correct pan and pickup strainer? I don't know if a rear sump 4wd pickup strainer will even fit in a 2wd pan with it's front sump, or vice versa ..... just thinking it out.

Once or twice I've had a flutter in pressure after punching, but as soon as I shut it off and restart the flutter was gone, I think it might have been the pressure relief passage or spring in the oil pump.

Once longer ago I had low oil pressure that a filter change cured, was not the time in 1992 I was on a trip and ended up putting new bearings in, was since then. Might have been a filter problem.

Lastly, your gauge, is it instant reading tube fed or electric using a sender? If electric still, might be momentary plugged somehow?

Melling is a great pump, standard or high volume.

My painter friend who grew up with a bunch here that sometimes hot rodded these trucks back in the '70s before I was here, said he used to run a extra quart as the HV pump he had in it would pump the oil up top faster than it returned to the pan at higher rpms. I never run mine like that, but he said that him and others did up on Rt 11 at night. In those years I was doing my own misbehaving across the mountain no where near here.

 

When the engine was revved up the pressure relief valve was pushed wide open and it stuck in that position for whatever reason. At idle it was just dumping too much and after a while it returned to normal.

 

After first fluttering needle, I studied the oil system, came to conclusion very similar, as just a "turn off / restart" and it was "no flutter" just that quick. Mine never went to zero though.

 

Probably when the engine is revved up a bit that valve is pushed all the way open. It probably just can catch a little bit because it hasn't been pushed that far open very many times. I don't think that it is really all that precise of a system.

About 10 year ago I built a 384 inch Cleveland for a customer with a Pantera. He ran it in the Silver State Open road, Sand Hill in Nebraska etc a few times. That engine has a solid roller cam and I bushed the lifter bores on the right side and restricted the left side for better oil control. With a totally stock Melling standard volume pump it would hold 95psi revved up hot. That one worried me a little bit that it might do the opposite thing and try to blow off the oil filter. It never did.

Here it is on my dyno after doing probably about 30 pulls worth of work one afternoon. Hot oil etc.

 

Hey Dave! Good to see you on this forum too.

I ran the engine pretty hard before both instances of the gauge dropping to zero. I had it wide open. So perhaps that theory tracks.

I will take it out on a stretch at normal highway speeds and see if I can reproduce the same issue without the wide open throttle.

If you have any other ideas on testing, just let me know.

 

Sounds about right. Mine does the same. Factory gauge is almost zero, but the system is at 15 psi (aftermarket gauge) and that is enough under really hot oil and low RPM. I added a high idle for when the A/C system is on, so RPM are not under 600. at 800 RPM EOP is in the 20's.

 

I add a vote to look at the pressure relief valve/spring. Broken spring or a bit of chaff stuck in the valve. IDK this engine but reading a bit, sounds like it's integral to the oil pump.

 

Had this issue and hopefully this is your problem too the wire for the oil line was melted by the exhaust when unloaded at a stop the engine dropped back to where it was supposed to be and grounded that wire making the gauge read 0

 

Always start with the easy stuff first. make sure you're actually losing pressure.

 

Long ago, maybe 1988, I ran into a Pontiac with that symptom -- OK pressure with revs, nothing at idle -- and it was a bit of blue RTV (the valve covers were over-zealously treated to the stuff) stuck in the oil pressure relief valve. One anecdata point.

 

Update. Well, sort of. New mechanical gauge and tee fitting just delivered. I think I’m going to change the oil and filter first just to do it. Assuming no change I’ll get the new gauge installed. We go from there.

 

I had that issue with my 460, it was the pressure relief valve in the oil pump. I drove it around for a couple weeks just keeping the RPMs above 1200 or so to keep the oil pressure up before I had a chance to change the pump.