Hi, I have a frustrating problem - My 1999 Jeep GC, 4.7L V8 - On a cold morning startup the oil pressure shows 40 PSI at idle. When I accelerate the engine the oil needle drops sharply to zero and returns when I remove my foot from the accelerator. Once the engine is warm it works perfect and stays high at any RPM.

I've changed the oil pressure sending unit and the oil and filter and still the same problem. Any ideas? Could this be a weak or clogged oil pump?

Thanks in advance,

 

Update: I took it to a mechanic, and he traced it down to the oil pressure sender unit, and replaced it (again). He checked the pressure with a gauge and it was fine.

I started it the next morning and it has the same problem! Any ideas?

 

My best guess would be clogged oil pump pick up screen. I've seen that problem before in other vehicles.

 

Has your mechanic put a manual gauge on it? Thats whee I would start.

 

mine did this and I checked it with the manual gauge... the stock gauge says 5 idle, 15 load...

manual gauge says 50-55 idle... 75-85 load

 

Just a brief update: it's been awhile but the way I fixed this problem was by visiting the dealership and purchasing a Mopar OEM oil pressure sending unit. It's been running with no issues since. I'm not sure why Jeeps are so sensitive to aftermarket oil pressure sending units but that was the simple fix.

 

I can tell you why if you will listen carefully. The oil pressure sender contains a piezoelectric crystal and a printed circuit that needs quality materials and construction. I know this because I bought a NAPA Premium one for $46 that was made in Mexico. All it ever did was read 40 psi whenever the engine was running, cold, hot, didn't matter. I went and bought a Mopar for $78 and it works perfectly. 55 on cold start, 40 at idle hot, 50 down the road.

When I took the original one out, I broke it. Looking at the parts, I saw a printed circuit board inside. Huh! Interesting!

I was going to tell you somebody left the windage tray out, but that would react the same even hot.