96 oil pressure gauge drops when shifting to park

casper7.3 · Apr 1, 2014

Hey all, I've got a weird one here and need some help!

A little background.

My buddy just bought this 1996 with about 275k miles. Oil cooler O-rings were leaking and the IPR was sticking. We replaced the O-rings, IPR, and changed oil. He drove home (about 25 miles) and when he came to a stop he noticed the oil pressure gauge flicker. He then shifted from drive to park and the gauge went to nothing for a second or so, then came back up to normal..

What we have checked/replaced/verified so far.

Hooked up a manual gauge to the hpop reservoir. hot idle there is ~10 psi in park. Shift to drive, idle goes up a bit, pressure stays the same. Now for the weird part. When you shift back to park, the rpm's drop by about 150 (~660 to ~510) and the pressure goes down to 5psi then back up to 8-10 psi. It will do this every time with the engine at operating temp. Hpop reservoir is always full.

Also checked pressure at engine block test port on left rear by oil filter. It reads the same.
Checked bypass valve in oil cooler housing, since we were just there figured maybe trash. still the same.
Replaced the check ball and spring in engine block below the hpop. Still the same.
Tried another new IPR.
No computer codes.

I just can't understand why the problem happens shifting from drive to park and also why the rpm's drop from drive to park. Normally its the other way around.

My buddy said it was fine when he picked it up from the other owner and drove it to me.

Any Idea's??
Thanks!!

Rideracelivemx7 · Apr 1, 2014

i know these motors dont have the highest oil pressure when hot but that seems a little low. At that mileage id just replace the LPOP seeing they are so cheap and you can inspect for grooves in the cover. i think i got my Melling for $80 at riffraff. Great quality and moves a little bit more oil.

zarrinvz24 · Apr 1, 2014

Rideracelivemx7 said:
i know these motors dont have the highest oil pressure when hot but that seems a little low. At that mileage id just replace the LPOP seeing they are so cheap and you can inspect for grooves in the cover. i think i got my Melling for $80 at riffraff. Great quality and moves a little bit more oil.

I would agree that changing the LPOP is probably worthwhile whether it fixes the problem or not; 200K seems to be about it for factory LPOP's. I would also consider changing the Transmission fluid/filter. At the very least have you guys pulled the A/T dipstick and looked at the condition of the fluid?

Pstroke96 · Apr 1, 2014

i would replace the pump base oil pressure should be about 40 to 70 depending on temp and rpm

MAFoElffen · Apr 2, 2014

Pstroke96 said:
i would replace the pump base oil pressure should be about 40 to 70 depending on temp and rpm

Sort of and sort of not... That is what you you see in a gaser.

I checked several Ford tech refrences. 3 sources.

The Ford first had is at these ranges, tested at different RPM's:

"The minimum engine oil pressure specifications are 82.7 kPa (12 psi) at 700 rpm, 165.5 kPa (24 psi) at 1,200 rpm and 310.3 kPa (45 psi) at 1,800 rpm with the engine at operating temperature."

The second said the normal operating specs at operating temp:
at curb idle, 10psi; at 3300 rpm, 40-70psi.

The third said the minimum is 10psi, but it didn't say at what RPM.

The fourth reference was someone who changed their LOP because the pressure was 5-7 psi at idle. Then installed a mechanical gauge at the same time to keep better track of it. He said when it's real cold, it'll idle at 60 psi, but normal operating temp and his pressure is still only 15-20psi.

So... If it's 10 psi at 700 rpm normal operating temp and the idle drops to 500 rpm... Does tha mean 7 psi might be normal for 500 rpm? <-- That is a questions that comes from logic looking at those numbers.

For me, I would think by logic, volume is more important than pressure in the low pressure oil in these motors. I would think 5 psi is low for even a 7.3l. The only physical measurement checks are done with the pump out, but if you have it out already, you might as well replace it with a Melling high volume pump while it's out. They are not hard to change. Symptoms of a bad LOP usually go along with hard starting or a hard start then dying.

One other thing I found on 7.3l's was that there was a problem with the design of the sump pickup, that sometimes causes the pickup tube to crack. If you block the front wheels and jack up the rear end (or park it on an incline, nose down), and the pressure raises, then the pickup tube is most likely cracked. Raising it that way would raise the level at the sump above where it usually cracks. Checking and ruling that out might be a good idea...

casper7.3 · Apr 2, 2014

Thanks for all the replies!! It has a rebuilt trans, fluid is ok. We are really on the fence about the pump and cracked pick up because the truck starts right up cold. It does takes about 4 seconds for the gauge to register (dash and mechanical). The truck will idle hot all day long at 650 rpm and 8-10 psi oil pressure. Once you shift to drive and back to park is when the pressure drops...I'll talk with him today about the pump. I think ya'll are right about it not being an unjust replacement.

Thanks again. I'll try to keep this updated and any other thoughts/suggestions are welcome!

MAFoElffen · Apr 2, 2014

When you talk to him... So you get an idea of the actually work that is involved. There is actually less work invovled in replacing the Low Pressure pump than it would be to replace the water pump:
http://www.ford-trucks.com/forums/985332-melling-lpop-install.html
There is only 8 bolts (if you include the fan clutch as one) and it's out. Realistically, in a shop, knowing that it only takes 2 different sockets, air tools, a fan clutch wrench set, 2 pullers (Damper and housing) and an installation bolt with nut and fender washer (to install the damper) ... You're talking about 45 minutes if you took your sweet time and cleaned everything up as you went. 2 hours max if you used hand tools, had to search for tools, pullers and really had no ideal of what you were doing.

My gosh, it's just behind the vibration damper. Unbolt the fan shroud then... Getting the fan clutch off or that dampener on and off is a toss up whether which one is the hardest part. From there on it's just a cake walk. 4 bolts hold the housing on to the front of the timing cover. Another puller pops that right off. The gears pull off from there.

Strokersace · Apr 2, 2014

I haven't read all the replies on this yet. But... Mine doesn the same thing occasionally. Except it has 180k on the motor, BTS hpops, new motorcraft ipr's, new melling lpop, and a BTS transmission that's in great shape with clean fluid and no restrictions. Oil is changed regularly too.

casper7.3 · Apr 2, 2014

MAFoElffen said:
When you talk to him... So you get an idea of the actually work that is involved. There is actually less work invovled in replacing the Low Pressure pump than it would be to replace the water pump:
http://www.ford-trucks.com/forums/985332-melling-lpop-install.html
There is only 8 bolts (if you include the fan clutch as one) and it's out. Realistically, in a shop, knowing that it only takes 2 different sockets, air tools, a fan clutch wrench set, 2 pullers (Damper and housing) and an installation bolt with nut and fender washer (to install the damper) ... You're talking about 45 minutes if you took your sweet time and cleaned everything up as you went. 2 hours max if you used hand tools, had to search for tools, pullers and really had no ideal of what you were doing.

My gosh, it's just behind the vibration damper. Unbolt the fan shroud then... Getting the fan clutch off or that dampener on and off is a toss up whether which one is the hardest part. From there on it's just a cake walk. 4 bolts hold the housing on to the front of the timing cover. Another puller pops that right off. The gears pull off from there.

Thanks I guess.

I am aware of the replacement procedure. We are on the fence about it because I have done a lot of research on this. What I found is of all the people who responded back that they had tried a lpop, they still had the same concern. Yes now there is a new $80 melling pump and they will never need another pump but, it's still not fixed. Most of the concerns were low pressure at idle. I have low pressure when shifting from drive to park. At idle, in drive, or in park, I have 8-10 psi=normal.

I'm just trying to gather as much info as possible before opening the wallet more. $80 bucks is $80 plus, it's not my money..

casper7.3 · Apr 10, 2014

Update====

Truck is fixed. The oil pressure relief on these trucks are kept in place with dimples in the housing, the dimples failed and were not holding relief valve in place.. I guess with the gasket installed and the relief against the block, it allowed enough of a gap to bleed off oil pressure between the housing and relief body. Installed a new oil cooler to filter adapter(which has the relief held in with a snap ring) and all is good.

Thanks for all the replies!