No oil pressure at low rpm

[GatorBate]

Can anyone tell me why I keep loosing oil pressure when my motor drops below 700 rpm.:dunno: I replaced the sending unit, the gauge, and rebuilt the oil pump. When I first start the motor it shows 50 psi. Once it warms up it drops to 20 to 25 psi. If I let it idle down or lug it a bit to about 6 or 7 hundred rpms the oil pressure goes to zero. I was running 5-30 in it, just change oil and put in 10-40. It didn't make any difference.

 

[jasonmark1993]

Id put a mechanical gauge on there to see what you actually have.

Its not uncommon to only see a few psi at very low rpm; maybe the gauge/sender just can't read that low.

My brand new engine is identical to yours with the 50-60 cold, but mine stays at 25 at idle when warm. If you see 50 when cold and can't imagine you having none at idle when warm.

 

[Purple77CJ7]

How many miles are on your engine GatorBate? AMC v8's will lose a lot of oil pressure with warn cam bearings. Oil flows through the cam first then to everything else, rod and mains last. I'm dealing with the same thing, babying it along, prolonging the inevitable rebuild. Cold I have about 50 psi, hot it will drop down to 10 at idle or less if really hot. I replaced rod and main bearings last spring, made no difference in pressure.

 

[derf]

The pump housing may be gouged and bypassing. You may need to replace some or all of it. I'd start with a pump mid plate from bulltear.com

 

[IOPort51]

I recommend go mechanical on the gauge.

I hate sounding 1/2 axed but if the lifters are not clattering at start up I do not think I would loose sleep. I would plan a new oil pump as AMC v8 have a history of wearing them out.

would a gouged pump be a problem only when hot??

 

[GatorBate]

The pump housing may be gouged and bypassing. You may need to replace some or all of it. I'd start with a pump mid plate from bulltear.com

I replaced the pump gears yesterday, that gave me 5 lbs of pressure. However I did notice that the filter adapter plate is gouged. I'll try the pump mid plate. I don't know the history behind this motor, but it has silicone oozing out of every joint and it leaks oil. It burns a little oil and the compression runs between 110 to 120 so it is showing signs of ware. Maybe one of these days I'll do a valve job on it and replace the cam and its bearings. hopefully the bottom is OK.

 

[derf]

I recommend go mechanical on the gauge.

I hate sounding 1/2 axed but if the lifters are not clattering at start up I do not think I would loose sleep. I would plan a new oil pump as AMC v8 have a history of wearing them out.

would a gouged pump be a problem only when hot??

A gouged pump is a problem no matter the temp. When the engine is cold, you'll get slightly better pressure but that doesn't last very long. Once you're up to operating temp, you've got problems.

Because the walls of the factory pump are just aluminum, they wear out far too soon. Contaminants will wear grooves in the pump housing which lets oil bypass the gears. This kills your oil pressure at all temps and that's a bad thing.


The typical first failure point from low oil is the #7/8 journal which usually ends up causing a throw rod. This is because that journal is pretty much the last one in line and it gets starved the most often, wearing it out faster than the rest of them.

That being said, I had a 401 that had pretty even bearing failure through the whole engine. When I pulled the cam, the inner layer of the front bearing came with it. There were pieces of bearing in the bottom of the pan and stuck in the oil pickup tube.


On a tired old engine, I'd seriously look at when, not if, it needs to be rebuilt. I'd spend an afternoon with the oil pan off checking crank and rod bearings (at least check the end play). I'd get the oil pump apart and check not only the face of the filter adapter but the walls of the pump. New gears are great but if the pump housing is shot, it will need to be replaced.

I have a Grand Wagoneer with a 360 that has 170K+ miles on it which is unusual for an AMC engine, unless there was a rebuild in that time. Once I get my truck back up and running and I get my CJ closer to being done, I'll be tearing that engine completely apart and rebuilding it because it's got basically zero oil pressure and has for a while. I just don't trust it.

 

[poppatello]

I'm no engine expert @ any level. I'm going by with what I've been told over the years. If you do the top end you should plan on doing the bottom end @ the same time while the engine is out. If not u'll put more stress on the lower end that is worn and cause a failure that could be prevented.

Also, my experience with lower oil pressure @ warm up temps is also a sign of bad valves or rings. When the engine warms up the valves or rings could expand cause a blow by which results in low oil pressure.

You stated you have oil leaking so if that is the case I would say you are in need of a complete rebuild. I don't think you can install new rings with out removing the lower half. Again I could be wrong but going by what I've been told...

 

[jasonmark1993]

Rings and valves aren't really able to affect the oil pressure.

You're right about the rebuild. Unless you just have some valve seals and you know the bottom end is good you should do the whole thing.

I was thinking of just putting a manual gauge on to see where its at at idle, not replacing the whole gauge.

 

[GatorBate]

It makes more sense to me that low oil pressure at operating temp would be a bearing problem. Bad rings and valve would cause blow by which would burn oil. Worn bearing would allow more oil to pass between them and the cam or crank surface when they warmed up and expanded a little.

I've gone through the rear axle, the Transfer Case , the Transmission , why stop there? LOL

I did not see or feel any gouges on the walls of the oil pump when I put the new gears in. Just gouging on the filter plate. So I've ordered the mid plate from bulltear.com as derf suggested. Hopefully that will help with the total loss of oil pressure at low idle.

A temporary manual pressure gauge is a good idea. Then I would know exactly what is going on with the pressure. However, I've had those lines break and pump the oil out. Had a neighbor pump all his oil on the ground and burn up his engine.

I think a engine rebuild is in my future. Dose anyone know a good place to get a short block? I know a guy that has a 360 I could pickup cheep. Maybe I will will get that and rebuild it to replace my AMC 304 .

 

[coldair]

Baker machine in Logan Utah does a great job. I can get you the number if you want.

 

[GatorBate]

So here is an update.

I installed the pump mid plate from bulltear and that helped a little. Today I hooked up a manual oil pressure gauge warmed up the engine and compared it to the stock gauge. At a low idle (600 rpm I think) the electric gauge dropped to 0 lbs. However, the manual gauge would read 10 lbs. At normal idle (800 rpm ?)I get 20 lbs on both gauges and at around 2500 rpm I'm getting 60 lbs on the manual gauge 40 on the stock gauge. So I'm guessing i am OK.

I question my rpm because I did not have a working tack hooked up tonight.

 

[jasonmark1993]

Thats more than enough. You don't need much pressure at idle as there isn't much of a load on the bearings.

 

[Posi]

10 at idle is good seeing a fresh AMC 304 is 13

 

[Flex BT]

Would it make a difference if the oil pressure gauge was installed without using teflon tape? I know my fuel gauge didn't read properly until I wrapped it. Didn't leak, just read wrong.

 

[IOPort51]

If the lifters start to rattle, you have a problem. It is a dead give away.

The FSM is of the opinion that you should not just set and idle for more than a couple of minutes with out revving the engine for a few seconds.