Hey everyone.... I have a problem with my 96 F350 5.8
I started idleing really high about a week ago, around 2000-3000 rpm I'm guessing. When I'm driving and there is a load on the engin is seems fine but being a stick when I press the clutch in to change gears it revs high.

I've replaced all the vacuum lines, replaced the upper and lower intake manifold gaskets, swapped out the air bypass valve and MAP sensor. The anoying thing is that sometimes it will idle normally.

At the moment I'm driving it with the air bypass valve disconnected and it runs fine. Just stalls on me sometimes at a light.

Any ideas what I should look at next? I'm thinking the ERG valve but disconnecting the air bypass valve and having it run at normal (or slightly below) idle is confusing me.

 

Sounds like you have a vacuum leak somewhere.

 

Throttle Position Sensor Bad?

 

Is the check engine light on, or any codes present? If you take a normal opperating truck and disconnect the TPS the idle will shoot waaay up. (from what iv expierenced at least) So I might check the TPS out if I were you.

 

I would check the tps they are known for causeing that problem.

 

Also, check for a faulty PCV system and oil in the intake. If oil is in the intake, it will find it's way into the TPS and mess it up.

 

Thanks for the replys... I'll check the TPS today.
The old PVC valve was clogged up so I changed that and the valve cover gromit when I had the upper intake off. I also pulled the line to it and blocked the vacumm port as a test thinking it could have been something like that too.

The check engine light was on for a while but now its off. I've looked everywhere for the OBDII connector to try and pull the codes but can't find it. It should have one beening a 96 right?

Thanks for the help so far

 

Your truck if it is over the 8500 GVW limit you will have the OBDI system not the OBDII.
Then the connector is under the hood on the driver’s fender to the rear.

 

Thats why I could find the damn thing then. I thought I was going crazy for a while there.

 

This is driving me crazy.
I tested the TPS and it seems fine. Closed is 0.86v WOT is 4.something.
About a day after that the truck just started running fine with the air by-pass plugged in for about a week.... untill yesterday. Now I'm back to the same high revs unless I unplug it.
The only thing I did was tighten the wire going to the starter, it was loose and made the solenoid freak out.

I tested the volts to the air by-pass (key on, engin off) and I'm getting 12v. Fired the engine and its stays at 12v (well between 12-14v). Should it always be at 12v or is it stuck in a loop? Computer going bad maybe?

 

Have you checked the voltage/resistance of the Engine Coolant Temp and the Intake Air Temperature sensors? Had same problem on my 351w & turned out to be out of spec ECT, and no codes were set. Resistance of these sensors decreases as air/coolant temp increase.

At coolant temp of 194 F to 248 F, ECT should indicate voltage of 0.6 to 0.27 and resistance of 2.8K to 1.18K ohms. At intake air temp of 68 F to 86 F, IAT should show voltage of 3.07 to 2.6 v and resistance of 37.30K to 24.27K ohms, according to the Ford manual.

The ECT could be telling the PCM your engine is not warmed up so better kick open the IAC & raise the RPM so it wont STALL (I hate acronyms, don't you?).

 

The ETC can cause it to high Idle if the ETC says it is hot.
This is called "over temperature idle boost" to try and cool the engine.
You have a higher idle when you first start the engine but I think it comes off that higher idle by a timer in the PCM and not the ETC sensor. This is called "cold engine fast idle".

CHECK FOR IAC SIGNAL FROM THE PCM

Key off.
Breakout box installed.
Reconnect PCM to breakout box.
Reconnect IAC solenoid.
Connect DVOM between Test Pin 21 and Test Pin 40 at the breakout box.
Start engine.
Slowly increase rpm to 3000 rpm.
Is voltage between 3.0 and 11.5 volts?
Yes
GO to «KE11».

No
REMOVE IAC solenoid and VERIFY that it is not stuck open. If OK, REPLACE PCM. REMOVE breakout box. RERUN «Quick Test».

 

When the timer takes it off the "cold engine fast idle" the idle drops to 1000 RPM and then drops again when the ECT says the temp is at 160*F.

Adaptive IAC Strategy

The Adaptive IAC strategy is designed to adjust the IAC calibration to correct for wear and aging of components. When engine conditions meet the adaptive strategy learning requirement, the strategy monitors the engine and determines the values for ideal idle calibration. The adaptive strategy stores these values for reference. These values are used by the PCM as a correction factor when controlling idle speed. The adaptive table is stored in Keep Alive Memory (KAM) and retains the learned values even after the engine is shut off.

Whenever an IAC component is replaced or cleaned or a service affecting idle is performed, it is recommended that KAM be cleared. This is necessary so the idle strategy does not use the previously learned adaptive values. To clear KAM, disconnect the battery for at least five minutes. Once KAM has been reset, let engine idle for fifteen minutes (actual time varies between strategies). Otherwise, the vehicle may exhibit idle concerns until new adaptive values are learned.

The amount of air allowed to bypass is determined by the PCM and controlled by a duty cycle signal.