Have a 1997 Ford F-150 with a 4.2 engine. This truck when it got warmed up the oil gauge would jump all over going down the road and go to 0 when stopping and check engine light would come on, then when speeding back up, the check engine light go off and gauge goes back to mormal , but then it starts to bouce again. Put 2 new oil sending units on (truck stayed the same). The put a new oil pump on, which was defective! Replaced with another new oil pump and of curse new oil and filter. The truck is STILL doing the same way. No improvement at all. This truck has a 190,000 miles on it and does not use oil or coolant or leak anywhere. Any idea's. I am stumped as to what to try next.

 

New Gauges?

 

Am going to test gauges tonite!

 

Remember, your stock "oil pressure guage" is nothing more than a mechanical idiot light. If the sending switch sees 4-5 psi or more, it will cause the in-dash "guage" to read around mid-scale. It only has two possible readings: Zero and mid-scale.

You really need to add an external oil pressure guage to determine if your oil pressure is okay but your instrumentation is the problem, or if, in fact, your oil pressure is actually varying.

Since you have a CEL, you also need to have your codes scanned to see what the computer is complaining about.

Steve

 

Tested by using external oil gauge. at 55mph with engine temp warm got a 20 reading. at idle external gauge reads 10. Any ideas?????

 

At 190,000 miles, the rod and main bearing clearences may be getting a bit wide.
If you try some #40 oil and the running pressure increases, it's a good bet.
If so, this is one of the only reasons to run heavier oil in a worn engine to get more life out of it without rebuilding.

 

We kind of thought that this engine was about to give it up. Will try the 20 wt oil and baby it till I can get a remanufactured engine in. Thanks you so much for your help

 

There are other posts on here that mention oil pressure at idle being around 10 psi. I seem to recall that its pretty normal. Not sure I'd go running out to find a new engine quite yet.

 

My old truck which was a 1997 F150 with the 4.2 did the same thing, i replaced everything from the gauge to the switch. Took it to a REAL mechanic not the ford dealer, and the guy told me it was the bearings on the cranshafts and i trusted him, and he was very much right. So instead of buying a new engine which would take years to repay me back in driving, i bought a new truck. But that 97 was a very very nice truck, sure miss it. Got a better one anyway!

 

My 97 4.6 w/ 208k has been doing the same thing. I have put about 5k miles on it since the gage started bouncing and I have not seen any ill affects and I drive it like I stole it every day of its life.
The gage fluctuation stopped for about 1k miles but now its back. I never had a CEL only the low pressure light. I had some people on here tell me that the cable on the oil sendin unit was bad or maybe the sendin unit itself. Have you checked any of those?
I do not know anything about the oil pressure with which you reported. But if your pressure was too low wouldn't you notice a decrease of engine performance and very very quickly lose the engine?
After I realized that if I was really losing engine pressure that I would not have my engine anymore I knew I did not have a real problem and just ignore it now. If it goes it goes I got my 208k miles & counting out of it.
Good Luck w/ your truck!

 

my truck a 90 f350 bounced a bit and went to zilch, low milage but lots of ideling on it(ex city truck) drove it well and fast for a few days a couple smoke shows... till it blew up.. dropped 2 pistons broke the camshaft lost a lifter rod ....somwhere ? hah and put a big hole in the side of the oil pan... I think i did a good job on her...

so I'd get a mechanic to look into the oil pressure thing

opened her up it looked like sombody packed it full of grease......lots of sludge in there atleast a 4L pail or more could be filled....(idleing is hard on a motor)