Well I've been scanning through the boards here and came up with a lot of post on oil pressue. But, here goes another one. I have a 1988 F-150, 351 engine in it, 84,500 miles on it. It's had tender loving care all of it's life. Regular oil changes and all that jazz. When I purchased it though the guy (friend of a family) said there was an oil pressure problem, the gauge would go down and the idiot light would come on. We dropped the pan and put in a brand new pump (old one looked ok), pan was clear, screen, etc. It made things better but the gauge still drops when the engine gets hot and when i'm at an idle.

Now I put on a "real" gauge and checked he actualy pressure. Same deal. Driving at like 2500 RPM things were decent, when I'd drop to an idle the pressure would drop off near zero. Throwing in a little STP at each oil change helps it for a bit but as soon as the oil wears a bit the problem arises once again. So to me it seems as though the sending unit is doing it's job. (Actually better than the analog idiot gauge it was designed to be). Someone suggested to me it could be a slightly low idle that brings the pressure down and the light on. Suggested I clean the throttle body. Did that, butterfiles close nice now. I do have an EGR valve that's not closing right according to a computer check could that have anything to do with it?

Which really brings me to the mercy of the list. I've read post that said it was a spun bearing, just getting old, but I wondered if my particular problem yields any new ideas. There is absolutely no engine noise, lifter rattle, anything! Maybe I shouldn't be worrying about it if it's not making noise but I'm in the process in restoring and I won't want to spend big bucks on a paint job if the engines gonna blow in a few years. Whatever you all can add is appreciated.

Thanks,

~Jeremy

 

A bent crankshaft or a spun bearing. In all likelyhood neither will make much noise. If you have replaced the sending unit and pressure checked it - then the crankshaft is the next place to look.

Not what you wanted to hear.

 

What weight oil are you using? If had your problem I would run either 20w50 or straight 40 weight.

Bear Tracks

 

>A bent crankshaft or a spun bearing. In all likelyhood
>neither will make much noise. If you have replaced the
>sending unit and pressure checked it - then the crankshaft
>is the next place to look.
>
>Not what you wanted to hear.


Dan,
educate me some please.....How do you know this??? What about a bent crank or spun bearing causes oil pressure to drop at low RPMs ?? I'm assuming idle speed is staying where it should be and is steady. How do you get to the crank/bearings??
Thanks in advance?
Brian

 

Dan,

Thanks for the advice. How hard is it to check out about the crankshaft and bearings. I will be taking the intake and everything off soon to replace valvecover gaskets and put new plugs and wirings on it. Am I going to be half way there already? I only want to mess with all that stuff once (at least for a while). Thanks!!

~Jeremy

P.S. Or, if anyone else has suggestions that's great as well. Thanks!

 

Excessive main and or rod bearing clearance.
Maybe this statement will help you understand why the pressure drops at idle.
In order to build pressure you must first fulfill your volume requirements.
In short at idle your pump isn't supplying enough extra oil,because of excess oil escaping between large clearances,to build good pressure.Once you spin the pump faster,through increased engine rpms, you have enough volume to build decent pressure.

 

It sounds real strange to me. It also sounds like you've been driving it round abit as well. Even with a spun bearing you usually get alittle rattle, especially when things get hot and the revs increase. I don't surpose it would be something like a blown out oil gallery plug or stuck open bypass valve?

 

I guess I should give a little update to my situation. I recently did some work like replace valvecover gaskets and intake plennum and while I was at it I decided it was time for a high volume oil pump. After much sweating and swearing I got that installed new pickup screen and all. Needless to say I get better pressure while I'm on the move and when the engine is cold, but it doesn't take long before the 'ol "Engine" light is on again and the pressure drops.

I'm not 100% familer with the pieces you mentioned. The bypass valve and gallery plug. Where are they located? Are they external on the engine like in the block or are they internal with the bearings, rods, etc? Thank you all once again!!!


~Jeremy

P.S. She is off any away getting repainted right now!! Should be a nice new beautiful color in about a week to a week and a half

 

What kind of oil filter are you using. I had a Cougar that did the same thing after changing the oil and a Fram filter. Changed the filter and the pressure was better.

 

I have always used Fram filters in it and when I changed the pump I put in a good 'ol Motorcraft and still have the problem. But I do appreciate the tip very much!

~Jeremy

 

Jeremy,
My advice to you: Go back and read the post by 4x4x4x4. I think your problem is worn rod and/or main bearings. This causes low pressure at idle, and the pressure comes up at cruising speeds. The fix? Drop the oil pan and replace the rod and main bearings. If you have never done this before, have an experienced mechanic do the job.
I've rebuilt 302 Ford v8's from the ground up, but don't know what you can expect to pay someone to do it for you.
Hope this helps. Good luck!

Bru

2002 F150 Super Crew
Black 4X4 Lariat
5.4 Engine Ford Chrome Bars
Ford Bedliner

1979 Bronco 351M 75K orig.

 

Unfortunately I believe the damage is allready done. One way to verify crank bearing damage is to install a good mechanical gauge on a warm engine, raise rpms to 2000 and release throttle. Watch for a momentary rise in oil pressure as throttle is released, crankshaft is "pulled up" against bearing shell in block momentarily lessening pressure losses due to loose bearings when under hard deceleration.
Damage was done just before noises started and "idiot lights" come on to tell you to late. Make them buy you a NEW ENGINE !
DO NOT go to "Quick Lubes".

 

Hey Jeremy. I agree with the vote on worn bearings. You replaced the oil pump? Did you use a high valume, or high pressure pump? There is a difference in the two pumps. Does the oil pressure do the same thing, hot or cold. Good luck.



Dave Powell

 

[updated:LAST EDITED ON 16-Aug-02 AT 11:08 PM (EST)]I have to apologize for the late response. Others asked about the bent crank or spun bearing. I think that 4x4x4 did a great job of explaining what is most likely the cause.

I had a problem with a 400 C that was very similiar to this. I replaced the oil pump with a high volume one to no avail. I ended up doing a complete rebuild of the engine. The reason I mentioned the bent crank is that if your clearances are loose enough and you drive her 100 mph+ than it is possible to put a small bend in the crank - at least according to the people who helped me rebuild it.

This engine had always been very well maintained and never pushed by the owner. At 115,000 miles I got the car and maintained it well but drove it 90 to 115 mph for extended periods of time. Then the light started flickering at idle. I was told to put in a high volume pump. When that didn't work they said it was most likley a bent crank.

Since the motor had been so well maintained we doubted badly worn bearings.

There is a whole lot more to this story. If you want additional details as to what was uncovered during changes and tear down I'll elaborate.