View Full Version : New 401 with strange oil pressure readings

Bulltear Ad
Bulltear Ad

06-27-2006, 10:40 PM
What I have is a fresh rebuilt 401 with Edelbrock Pro-Flo EFI in an '87 GW. The cam is an Edelbrock #2132. The timing cover is the Bulltear nickle plated item. It has new oil pump gears and I am using the NAPA Gold #1258 filter. On cold start the oil pressure goes to 65psi right away. After the engine reaches normal operating temp (198 on the EFI controller display), the pressure gradually falls to ~40psi @ ~2600rpm and 60-65mph. When I let up on the throttle to make a turn off the hiway after an 11 mile hop, the pressure drops to ~15psi. When I apply some throttle the pressure comes back to ~20psi @ ~1100rpm and ~30psi @ ~1300rpm. If the engine is restarted after a half hour, the pressure @ idle will come up to ~20psi. If the engine is restarted after a few minutes, the guage does not respond until I add some throttle. Today, the outside air temp was 79-83 depending on which bank clock you looked at. Engine temps today: hiway 198 normal and up to 203, upon startup after a couple short stops before getting home 208 and 212. The radiator is a BJ's Offroad aluminum unit and there is 18 quarts of Prestone mixed 60/40 in the system. Oil is 10w-40 Valvoline with about 250 miles on the first change following the break in oil which was changed after a little more than an hour of run time. The guage is stock with a new sending unit. It was suggested by the engine's builder to tap in a mechanical oil pressure guage to verify the readings. I will probably do that but, I need some insight as to what is going on in this engine.


zero cool
06-28-2006, 11:35 AM
When the oil is cold, it's harder to push through the passages. So you should expect higher pressure when it's cold and lower pressure when it's hot.

Also, the pump runs off the camshaft. That means as RPMs go up, the pump spins faster. That gives you higher pressure at higher RPMs and lower pressures at idle.

I'm not entirely sure there's anything wrong with your setup. But I could be wrong.

06-28-2006, 09:41 PM
I haven't seen oil pressure go that low or change as much in 150K and 175K mile worn motors that I have experience with prior to this 401. I want to make sure that I am not destroying this motor.

06-28-2006, 09:57 PM
Hard to say if the guage is reading right.

06-29-2006, 02:14 AM
I had low oil pressure in my 360 years ago after a rebuild. It would read fine cold, but as you said it was low after coming into town hot then idle in traffic pressure would drop to low to my liking.

I tried many oil brands and wieghts, I switched to Mobile 1 20W 50 Full syth. Presto!...40psi hot and cold idle.

Not good for cold weather though....AMC's are know for low idle oil pressure.

06-29-2006, 06:43 AM
I am going to tap in a test guage. I have been thinking about trying Mobil 1 15W 50.

08-09-2006, 02:12 PM
My mechanic experienced a similar situation with his 360. After trying everything else it turned out to be a deffective sending unit.

08-13-2006, 08:25 AM
Maybe I am dense.. or missing something, But those readings seem entirely normal for all the AMC's I own now or have ever driven. at least those with stock oiling systems.
Coming from the Small block Chevy world it worried me endlessly in the beginning, now unless the the lifters are tapping at an idle.. I pretty much don't worry about it.If you look at the oil pressure gauage in a wagoneer the gree line goes all the way down to 10 psi.
The 304 in the CJ has had 5 psi hot at idle for 70,000 miles and usually runs 35 to 40 psi at hiway speeds.

As to me being Dense..There is Genetic evidence to support that possibility. :oops:

Bulltear Ad