I HAVE READ EVERY POST ABOUT IDLE SURGE IN THIS FORUM INCLUDING THE ONE ABOVE. NO LIE. CAN THE IDLE THESE TRUCKS HAVE EVER BE SOLVED?


I'm having this exact same problem as the fella above plus many others. My Motor is one year old. I do not get any codes except pass codes. I cleaned out the IAC and throttle body too.My egr is new and so is O2 sensor. The MAP sensor and TPS sensor pass the multimeter test. New plugs, new cap, new rotor.

Guess I'll check for vacuum leaks next--use soapy water, the best way?--??

Someone told me the IAC's are such a problem that Ford sells a manual adjusment plate to go between the IAC and throttle body , has anyone used this with success?

If I have no vacuum related leaks what do I troubleshoot next? I am getting frustrated and don't know what to check next?

 

I'm having some idling problems with the '92 5.0 F-150 as well. It idles high and then drops down. So I figured it was the IAC, cleaned IAC out it was fine, then jumped back up again after a cruise. Went out and bought a new one. Still does it..Sometimes when I'm driving this truck it acts like it wants to buck/stall, its very strange! Need to put the clutch in and rev it cause I get so scared its going to stall on me while driving.

 

For what it's worth, my 5.0 Mustang has started idiling badly too. When I'm stopped the RPM goes up and down, almost stalling unless I give it a little gas. I haven't tried to fix it yet, I only drive it every few weeks.

 

Check tps voltage at idle first off, and also, unplug the IAC electrically and set idle speed to the correct rpm (warmed up, all accessories turned off, timing set correctly). Re-check tps voltage - you want to have it right around .9V at closed throttle. Plug the IAC back in and go from there.

 

So if its not the IAC and base idle thats giving us the problems, then its gotta be the TPS Voltage, is this correct?

 

Possibly. I suppose it could be an incorrect MAP sensor output, but that is pretty unlikely, since MAP sensors don't usually go bad. There really isn't that many things it could be - the idle surges because for some reason, the computer is probably getting mixed signals (idle from one sensor, not idle from another, and so I think it may be getting confused and the idle surges).

 

One thing you might want to check is the voltage at the computer....there could be a bad wire between the sensor(s) and the computer...so the sensor is putting out .9V but the computer is getting something different....

 

Thanks for the replys guys. I'm heading up there now to take a look at it. Hopefully I'll bring this bad boy home tonight I'll keep you posted.

 

Once the IAC is unplugged, where is the adjustment screw located to make the idle adjusment?

Have you heard of the manual adjusment plate Ford sells that sandwiches between the IAC and throttle body. They said its about 30 dollars CDN. Seems tempting to try it out for that price.

 

i feel yalls pain, my 87 5.8 was doin that for a while, and it is carburated, with no computer, did the exact same thing, did it for a while, and stopped, then went a little while, and started doin it again. then it started buckin real hard at full throttle, and i could never figure either one of em out, timing set right, no leaks, clean filters, and good fuel, all ignition parts short of the distributor were replaced, and even the carb, and intake gaskets, still same thing, i know it is a little different, but just my .02, hope yall figure it out, dont have the problem anymore, motor finally died, and the 500 hp stroker replica will be here in a week! BEST OF LUCK!

 

the screw is the stop screw, it has no head to it, you need to rotate it with a set of plyers. To set the throttle plates, shut off the engine, rotate the screw until the plates just start to bind in the bore, then take the screw back in another 1/2 turn. The IAC will take it from there to set correct idle, but the IAC is not the problem here, it is recieving the signal to make a correction in the idle and doing so, I have found the black wire in the single connector at times to be the problem here behind the battery, not always, battery terminals needing cleaned, I have never replaced an IAC valve for an unstable idle problem in 12 years. Idle with the IAC wiring pulled off should be low, about 500-600 rpm. If it is 800-900 rpm and you adjusted the throttle screw already, look for a vacuum leak, pulling the MAP sensor hose off will enrichen the fuel control, but see if it stabilizes the idle. If you have a nice multimeter, the signal at the MAP KOEO will be 156 HZ depending on the elevation you are at and 108 HZ running (approx) up to 1000 ft above sea level, it must also read 50% duty cycle and be steady no matter the elevation. This is measured on the center wire at the MAP sensor. TPS voltage running center wire .95 vdc. Fuel pressure 35-45 psi. Possible clogged filters in the back of the fuel injectors. Try some of these findings. Broken Wire

 

I have one on my truck that the previous owner installed. I want to get rid of it though.

The truck should idle normally (~650 rpm) with the IAC unplugged (with the engine fully warmed up). If it doesn't then turn the stop screw out to increase the opening of the throttle plates. Once you have the correct idle speed, check the tps for the .9V, and adjust accordingly.

I agree with broken wire - the IAC is not the problem here; it's doing what the computer is telling it to. It's just a matter of figuring out why the computer is toying with the idle.

 

Found the problem with the truck, it was the TPS. Truck wount even start now. When the TPS is saying WOT the computer will not allow the injectors to spray, from what I hear.

 

I ran a code check again tonight and received pass codes for KOEO and KOER.

I listened for and physically looked for any vacuum leaks. One vacuum line on the manifold"tree" was nicked so I replaced that. It was only feeding the cruise control though.

The TPS reference voltage was 4.6 volts.

At idle the TPS reads.9volts and progresses to around 4.6 at WOT

The TPS resistance test did not seem right.
With throttle closed I got 0.78 K
At WOT I got 3.23 K
The service manual's numbers are much different.
Could this be my problem part?

 

I'm fighting this too. I have been trying for weeks to fix this.
I spent a fair amount of time on it yesterday. My idle surges between 1000 and 1200 rpm. When you hit the gas it hangs at 1500 rpm then slowly drops to 1000-1200 surging.

I did notice that with the new or old IAC unplugged, I idle at ~900 rpm. I have never adjusted the idle stop screw. I have 2 TPS sensors, and old one and a new one, both are NAPA replacemnts. If I plug in a TPS that is not connected to the TB and in its lowest (closed position) the truck will idle at about 750-rpm which seems to be where I always remember it being. However, the voltage at idle is only .48 volts.

I checked for vacuum leaks with propane. The only place I could find was the EGR valve. If I waved the propane by the vent holes on the lower half of the EGR valve, the idle would go up. The EGR will pull and hold vacuum though. Is this normal to have a vacuum leak here?

I also notice a air leak or sucking sound at part throttle loads when driving. Sounds like it comes from the passengers side of the engine compartment. I cannot hear or find it under the hood when there is no load on the engine.

Fuel milage seems to be really crappy now and the power seems to be down.

Any ideas here?

Thanks,
Ed