impact1223

I have a 1997 f150 4.6L 2wd and just finished replacing the timing chain cover. The truck Sat with no oil in it for a few months and now that I have everything back together and full of new 5w30 it's doing this.

It starts fine and the oil pressure reads good, a touch high but still well withing the gauge. Within 2 minutes the gauge is maxed out!

What in the world could it be? Could the sender next to the oil filter be clogged?

JCR 56

Replace the sender.

impact1223

I have replaced the sender, ran it with marvel oil and did a complete oil change with filter and it is still doing it...

Is it possible that I need to put it on the ground and drive it? Also would a bad gauge in the dash do that?

Is there anyway I can check using a DMM at the sender?

F150Torqued

From your questions, I get the sense you think the Oil Pressure sender is some smart analogue device. It's NOT. It is an OPEN/CLOSED switch that switches at about 5-7 lbs oil pressure - same as all the idiot lights of old. You can verify that with an Ohm meter or test light.


If the reading on the instrument panel changes after the initial startup reading - look elsewhere for the problem, not the sending unit.

impact1223

If I have a good understanding of the gauge, when I ground the wire going to the sending unit, the gauge should max out correct?

Mine is going up to just above half and then stopping... I also hooked up a mechanical oil pressure sensor gauge and on start up with 5w30 oil it reads 75psi, then after a while it works it way down to 40 psi after idling for a few minutes.

Is that about right?

F150Torqued

Depends on how we are going to define "max out". You did say your gauge goes up just above half way. Where ever it goes to when the contact closes is ALL the idiot gauge is ever going to do. That's "maxed out" - as far as that two position contact closure oil pressure switch is concerned. (Take no offense. Just ribbing you a little. I hate the idiot light design also.)


On more careful thought - I do believe the 'switch' is "CLOSED" when oil pressure > 5 to 7 lbs is present. So you are correct - if you short the wires together, it should read "normal" pressure - and never any more.


Your mechanical gauge readings are fine - under the conditions you describe. The 75lb reading is the result of the spring loaded 'pressure relief' valve on the bottom of the oil pump relieving and bypassing pressure back into the intake side. That is plenty for the harshest of operating conditions (high RPM, high Load). 40 is fine at idle RPM, and as the oil heats up / thins out, you can expect to see that sag further to somewhere around 17-25 lbs. Don't be alarmed at that. That is all fine for light bearing stresses at idle rpm - so long as it will increase AT LEAST 10 lbs or more for each 1000 RPM increase up to the relief pressure setting.

impact1223

By maxed out I mean like all the way up, past the red.... like a completely full tank reads on the fuel level gauge.

impact1223

by "maxed out" I mean full-out, past the red bar, can not possibly be any further over the top of the gauge, maxed out. Like when you have an insanely full tank you gas and the fuel level gauge is past full.

F150Torqued

Hope you didn't miss this part of what I said.



I just double checked my Factory wiring manual for the 04' - '06' models and the two OP sending unit wires go straight to the instrument cluster - don't pass go, stop at the PCM, or collect $200. Lol So whenever the switch closes - whatever the instrument cluster does with that switch closure is all that it can ever do. I don't think, by its design, it can ever "max out the gauge" to the end. The idiot engineers (I guess to save money) set the little OP gauge where they think the majority of users would be happy seeing it - just above midway.


I find it interesting the OP switch is NOT wired to the PCM. I know the PCM communicates with the Instrument Cluster processor on a two wire serial bus. I know (as with several other seemingly important things) that lots of operational things get 'lost' to us poor owners because of software smarts. But the only way the PCM could know if oil pressure is LOST would be via the serial bus between the Instrument cluster and the PCM. And it still could NOT know anything more than "it's > 5-7lbs, or it's < 5-7lbs". (Tolerable ONLY at Idle / no load condition. Above that it is up to us to listen for knocking or thrashing!!!)

impact1223

So how do I fix my gauge? I only have a single wire coming from the sender and I have never messed inside the dash at all.