f-150 i6 idle high

[EricF]

a friend of mine came to me with a 1995 F-150 with the straight 6
has 80k on the odometer, been maintained ok.
he's been going to different mechanics off and on to try and get the engine light off. he's not rich and takes time to have stuff fixed, so he called me for help.
I opted to not want to take on the vehicle, but offered advice, and a mobile mechanic who I've known a couple years.
the guy actually turned out to be a sheister..
so now I'm back in the game.
the CEL light is now off, had the mechanic swap o2 sensors and radiator, EGR position sensor and wallah...three major issues resovled.
very shortly after, the idle started to be very low to a point where the motor would die.
So I said idle air valve, this solved the issue of the low idle. And a couple days later the idle seemed to be revving higher. the mechanic got a throttle body from a junkyard and installed it, no change in idle, it actually seems to be higher.
Now I'm involved.. I swapped the TPS and the idle seems to be lower, but it's still high. the truck has no tach.. and my old obd-i actron scantool won't talk to the truck's pcm so until I find the adapter to my more modern actron, I'm guessing the idle. But I do know it's higher than it should be.
I already the the throttle body didn't need to be replaced.
the TPS helped, but it didn't set a PCM code.

the mechanic said there was a vacuum leak at the plenum gasket, my friend verified the carb spray trick.
the mechanic started nickel & diming which ticked me off.
he charged $45 to change the IAC valve, which literally is a 20 second job.

Would a leaking plenum gasket cause the idle to be higher than normal? there is no adjustment for the idle, it's all computer controlled.

Thanks for the advice

 

[tinmanSC]

Yes, an intake leak could certainly cause a high idle condition. Is it where the intake connects by the distributor? I've seen a few posts on this at F150forum.com. You may wish to investigate.

 

[punisher]

Make sure there are not any vac line leaks under there. I forget the specific vac line/tee setup on those I6s, didn't wotk on a lot of them. Anyway, dead giveaway on intake leaks is positive fuel trims at idle, that get near normal under throttle.

Don't try to compensate for the vac leak by adjusting the hard idle stop on the throttle body (not saying you would), but I had to fix stuff like that on a regular basis when some smart shadetree decided to correct his idle speed.

Last, and least, it's "voilà" not "wallah"

 

[demarpaint]

Smoke the engine and check for vacuum leaks. Pull the IAC and make sure that it is clean along with the port it sits on, and that it is operating properly.

 

[mechtech2]

Vacuum leaks are #1 on your list.
They can be all sorts of places, not just the manifold.
I just did a manifold on one of these. Make sure you need it before you dive in!
And even if it is faulty, know that there may be other problems.

 

[artificialist]

Another thing that can cause idle to always be higher than normal is either:
A coolant temp sensor not detecting that the engine has reached operating temperature
Or a thermostat gets stuck open, not allowing the coolant temp to rise enough to allow a lower idle speed.

you would need to know the number of ohms the coolant temp sensor puts out at different temps, and point a laser thermometer near the thermostat outlet to get results.

Anyway, be prepared to discover that it isn't the only thing that is wrong. Sometimes when a vehicle goes without repair for an extended period of time, a domino effect occurs.

 

[demarpaint]

Good luck, I've had a surge problem [88 E150] during a hot engine restart for many years now. Now 5 techs have looked at it, and no one can figure it out....................We just smoked the engine for the 4th or 5th time, checked every ground, sensor, computer, IAC, TPS, advanced the timing, you name it. I was told to live with it, which is what I've been doing.

 

[EricF]

I'm gonna check the plenum gasket and verify what the mechanic claims. the vacuum lines looked ok, but I think I'll just start replacing them because well.. they're easy to get to!

the IAC valve is new, and it's motorcraft. the throttle body, I would hope is clean but looks like I'll have to pull that off too & check.

the plenum gasket looks easy to replace, but with my luck it'll be an all day job..

thanks for the advice!
This might be a good time to try and build my own smoke machine.. a guy on Garage Journal made one from a fog machine from Guitar Center..

 

[George7941]

I had a 1986 F250 with the 300 I6 and it would not pass emissions. I discovered that the intake manifold was warped and causing air to leak in.

The exhaust and intake manifolds are bolted to each other, so I pulled them off as a unit and got the manifolds machined.