Pontiac 400 oiling problem

[Grim]

Starting to see some weird stuff with oil pressure.

Motor is a Pontiac 400 with a WC code and 6X-4 (350 head) motor making it the High output W72 Manual Transmission motor. I am running a Manual.

Whats happening is once warm when I accelerate the oil pressure drops about 10 PSI, take foot off the gas and it goes up 10 PSI.

Normally I'd think right off the bat that it would be crank thrust bearings but touching the clutch to put some pressure against the crank does not make it change. I'm also seeing the oil pressure bounce the needle on the oil pressure gauge at steady cruising around 2600-2800 RPM. I'm running a Mechanical gauge and in fact its been changed recently to a new mechanical gauge so not a gauge issue.

Anybody seen this before and find it to be anything but a thrust bearing issue?

Could it be my transmission has some excessive end play on the input shaft and its pushing against the crank and why it doesn't change with the clutch lightly pressed? Its not popping out of gears and despite 200k on it (original to the car and I have full documentation and no mention of it being rebuilt) it feels tight and why I didn't rebuild it.

[Takid455]

short list
change filter and hook up mechanical gauge.
could recheck with priming tool by removing dizzy and spinning pump

Have seen pressure relief valves have issues in the oil pump

[Grim]

It has Mechanical gauge and I have run 2 different mechanical gauges same issue with both. Doesn't happen till fully warm and running about 2500 RPM. I dont think I can recreate it with a priming tool. :(

I will be changing the oil soon and cutting open the filter looking for any metal in it. Was hoping somebody knew of a cam walk problem or something that doesn't involve going through the bottom end. Its rebuilt and not a lot of miles on it.

[Skip Fix]

Pontiacs have a thrust plate on the cam so unless very wornit shouldn't be cam walk issues(and if so alot of metal in the oil).

Which oil pump is in it? The 78 pumps had less pressure than the Ram Air replacement ones-I could see my 78's original pump bouncing the gauge when cruising. Even the RA replacement pumps come with a weaker spring also to get close to those lower pressure ones. Sometimes a piece of trash get stuck between the bypass ball bearing and doesn't let it close completely so basically a pressure leak there-not uncommon,why some guys use a smaller ball bearing in the bypass and a 0.060 shim behind the spring to take up the difference on the spring tension.

Bearing clearances? Excessive clearance(on most of your bearings) also create kind of a "leak".

[Grim]

The 1978 W72 was the last true performance Pontiac motor (http://nyghtwyngs.com/fileup/uploads/docs/DanialsW72File.pdf). It had some extras like 91cc 6X heads, Pined mains and it came factory with a 60psi pump. That said this has been rebuilt and I didn't own it when it was rebuilt so I don't really know exactly what is in it. I was able to confirm the flat tops. I didnt record the part number of the pump when I changed pans.

\Edit I looked through my pictures and the pump has a tag M-54D. So a Melling High Pressure unit.

I am running a cheap oil gauge but the previous cheap gauge was reading the same. Running down the road at 2000 RPM its pushing about 50PSI hot. 2800 its pushing 60 PSI and its popping the bypass and staying at 60psi after that.. It will bounce 50-60 PSI at cruise above 2500RPM. At 500 RPM Idle its making about 12ish hot. 1000RPM its making 20ish. So its making good pressure overall just alarmed by this bounce. :(

[Takid455]

Throwing this out in left field....almost seems like an air pocket. Have enough oil in engine? Assuming you do, make sure the pick up is parallel with pan. Quick test is to add 1/2 qt then full qt and see if it reoccurs

[Grim]

Yeah plenty of oil.

I put a new Milodon pan on it before I installed it last year. Pick up on the pump is tacked and I measured the depth. Looks like it is about 3/8s off the bottom of the pan and its baffled. This was happening on steady hwy cruise so not a slosh problem. when I filled it the first time I poured a full 5 quarts started it to make sure the filter was full and made sure the marking on the after market dip stick was accurate..its not LOL but is no more then a half quart low when this started and I topped it off.

The only possibility there might be the lack of a windage tray and the oil is staying airborn, airated or roping on the crank.

[astroracer]

Hey Eric,
When you tacked the pickup tube did you pull the relief spring out? If not the heat may have annealed the spring a bit and that is where the "bounce" is coming from. It sounds to me like the relief spring is letting the ball bounce around on the seat. You say it happens when the motor is hot and at cruising speed so that is the first thing I would do. Replace the old spring with a new one and see what happens.
Mark

[Grim]

Hmmm now that's and Idea. I didnt install the pumps so I really don't know if the bypass spring was removed. Unfortunately its a pull the engine deal to get at. :(

[astroracer]

Hmmm now that's and Idea. I didnt install the pumps so I really don't know if the bypass spring was removed. Unfortunately its a pull the engine deal to get at. :(

Yea, bummer. I'll bet that's the problem though. Working with unknown entities can be a real pain in the kiester. That's why I went through the 509 I bought for the van. Just wasn't comfortable with the prospect of all the unknowns. I am sure glad I did! Found enough issues that it was well warrented.
Mark

[72blackbird]

If you have 200k on your 400 and the oil pressure is starting to drop, I'd say it's time for a rebuild. The only Pontiac V-8's I'd expect to have 30-40 psi at idle after 200k mi is a 350P- most of them where in grocery getters and not run hard. OTOH, most 400's were in performance cars and driven hard most of their lives.

Geno