Just bought a 98 4.2L 4X4 from a local dealer, great price, I think I'm starting to see why. 103,000 on her, once it warms up the oil pressure drops to nothing when I hit the brake. Pressure is fine when idling or when my foot is on the gas, but when I slow down for a light or sign, it drops down far enough so the light on the dash comes on. Sending unit is fine, replaced the oil pump and still have the same problem. Starting to get nervous , anyone have any other ideas? Willing to try anything. REALLY hoping I don't need a new engine. Thanks

 

You may have a wiring problem.
Have some one sit in the truck (engine running) while you wiggle the wire looms under the hood.
Usually, the main engine harness shorts out on the EGR tube.

Good luck.

 

Those senders will put the guage at full reading at 6 psi, if you have it drop, that means less than 6 psi of oil pressure.

 

thanks for the replies, not really sure what the 6 psi signifies. Based on other threads it sounds like the gauge reads all or nothing, so what could be causing that much nothing? Fine at idle and driving, but when I'm braking it gradually drops down to nothing. Needle bounces all the way down, so it's trying to keep pressure up, but it eventually drops all the way down by the time I stop. Maybe the RPM's drop too low when braking? If so, is there any way to adjust that? Not much of a mechanic, learning mostly from replies, but I know low low oil pressure is bad. Any suggestions appreciated

Duffer

 

you can try cleaning the IAC valve or replacing it. you would really have to hook an oil pressure guage to it and see if its really doing what the dash gauge says.mines a 97 4.2l 116k miles still no noise just oil pressure guage bounces at idle sometimes when hot. but an actuall oil pressure gauge did show my pressure drop all the way to 10 psi when hot in idle. they say the light comes on at 5 psi. i also replaced the sender didnt help it. have an oil pump but havent had time to replace it. think it will still do it afterwards.

that leaves 2 things in my case. 1. pressure releif valve in front cover is sticking. 2. main bearings are worn out. so now i might just wait till it blows then go buy another F-150 but i will keep this one and replace the motor because the truck still looks as good as the day i bought it.

 

You need new main and rod bearings ! Drop the pan and put em in don't let it blow that's silly they only cost about a 100 bucks.

 

do you have to pull the engine to get to the pan? i.e. $100 in parts and $800 in labor?

 

another question. If the main and rod bearings are shot, would I see lower oil pressure all the time?

Duffer

 

Just as you described low oil pressure at idle. Above idle oil pump it putting out more than it's losing at the bearings so you get pressure.

If you can get the pan off the truck with the engine in it should be relatively easy. I've never done a f150 but plenty of other cars.

Drop the pan loosen the main bearings let the crank drop a little slide new bearings in. Tighten, take caps off rods push piston up a little new bearing tighten 8 times and your done put pan back on tah dah new motor.

 

thanks for the advice, hopefully this works out, I'll let you all know, if it does this web page saved a really nice truck

 

Check your ground wires. I had a 92 F150 that the ground wires for the gauges ( on the firewall) worked loose and caused the gauges...oil and temp to fluctuate....solution was simple as tightening a nut...good luck!

 

Is there a possiblity that everyone is missing. During braking the oil is forced to the front of the oil pan and if the oil pick up tube is uncovered the pump goes to zero output until the oil is back where it can be picked up. The problem could be the pick up tube has come off/ has a hole in it/ not fitting into the pump properly. Something I'd check before doing a bottom end overhaul !

I had a dodge van that would do just what you are saying when it was 1.5 guarts low, top off the oil and it was fine.

Good luck

Gene

 

problem here is to check the pick up tube you have to drop pan, to replace bearings you have to drop pan, to drop pan in a 2wd you have to pull motor. if you have a 4wd you just have to remove the front diff or even maybe just drop it down alittle.

and guess what i got a 2wd.

 

Take the truck back to the dealer immediately and hand him the keys! Don't do any work on it. Many States have used vehicle warranty laws. Do the research and demand a refund. The dealer probably knew it had problems. At the bare minimum, speak to the sales rep. who sold it to you.

I hate to hear of dealer sales reps. hiding the solami to an unsuspecting buyer.

Jeff

 

i agree with OE812 take the truck back and either demand it fixed or demand your money back...when i say demand you have to "demand" people this day and age will try to screw you anyway they can but most are chickens...you get up in their face a little and they'll mostly crumble...push your weight around and you should be able to get this fixed or get your money back