1988 F250 5.8 351

I have the exact condition you describe. I had it for at least 10 years on my original engine and now on my remanufactured engine. I've worked on my truck for months trying to eliminate the warm idle fluxuation. The truck runs fine on the road just a little rough at idle. I have 18" to 19" cold and after it warms up it always sits around 16 1/2". I've taken the lower manifold off twice and the upper three times - the last time using Ultra Black RTV everywhere. I've changed the throttle body. All sensors are new and the ECU is new. All injectors were reconditioned and the O rings are new. The compression and leakdown tests are perfect. All vacuum lines have be changed, the EGR is blocked off and nothing is leaking.

If I disconnect the IAC after the truck is warm I see 20". Hook it back up and the PSI eventually goes back to its former 16 1/2". If I disconnect the FPR vacuum line and give it 5 PSI more fuel pressure and the engine vacuum rises to 17 1/2" PSI. Reconnect the FPR line and the vacuum drops to 16 1/2". (The FPR is new too.) My theory is that the IAC (replaced once and is clean) is allowing too much air in the TB resulting in a lower PSI reading at warm idle. The computer is telling the IAC to open to the degree it does. Rather than go any more nuts I ordered the Idle Air Adjusting Kit that fmr9 wrote about from DirectFordParts.com (Ford part # F2PZ9F939A) for $29.77. If the kit will meter down the IAC air a little so it will reach 18" PSI, the idle should be fine. I post the results when I have the kit in.

 

jemini

I hope the idle kit works as well for you as it has for me. I drove myself nutty trying to get it to idle smooth. Finally read about and bought the kit. Less than 5 minutes from raising the hood to shutting it with a big smile on my face. I have only adjusted it once since then, and it was after a new distributor instal. It just idles smooth at 750rpms.
Good luck Frank

 

Was is it that we need to install the idle kit? But at some time in the trucks life it idle fine without it.I will for sure buy it if that will make it idle smooth.What does the kit do exactly? I do I know if this kit will work for me?

 

ya I'm very interested in this kit too my truck has been doing the same thing for quite a while and its driving me nuts.

 

How bad is yours?
When at a stop light, it would be so bad that people thought I wanted to race them !

 

Ok here we go again!! Today I had a little time and noticed that it smelled like raw fuel.Pulled a plug and wow black.So I changed o2 sensor,ect sensor,and now fpr and still the same thing.I am beating my head against the wall.

So my vacuum seems low at idle which would indicate a leak right? The only thing I have left to do is change the upper and lower and see if that is where the leak is.Thats all that is left. Or do you think I should bite the bullet and buy the iac kit? Help!!!

 

Did you do all the tests that Old Paint suggested to check for system component leaks?

 

I contacted one of the parts places selling the kit and ask if it would fit my 4.9 since it is sold for the 5 liter in a Mustang. The parts guy said as far as he knew it fit all size engines. Since I was very frustrated with all the work, time, money and no results, I figured another $30 for a magic kit was justified. Evidently a lot of people had problems with the idle and Ford's solution was to develop this kit.
The kit is a 1/4 inch thick aluminum plate, 2 screws, an Allen wrench, and a gasket. The plate has two screws in it along with some air passages milled into it. I pulled the IAC, discarded the two screws, put the plate on, then the gasket, then the IAC and the new longer screws. The truck was already warmed up, so I started it, applied the parking brake, and put it in drive. It dropped to its normal 500 ish rpm and rocked the entire truck. Occasionally it surged up to about 650 RPMs, and then dropped back. I started adjusting the two screws with the provided Allen wrench and in just a few seconds it was idling smooth. I checked and it was sitting at 750RPMs and rock steady. In park it idles around 1100rpms, but I am only worried about the idle speed and smoothness in drive. That has been over a year ago, and I have only touched it once since the install, and that was after changing the distributor. I can run the idle speed up and down by turning one screw. Reminds me of adjusting a carb.
Will it help your truck? I really don't know. Have you already tried everything you can think of and spent a pile of time and money? If it helps hurray, if it doesn't you are really not out much. Just assume it is snake oil and it won't work. If it does, what great surprise, and if it doesn't you have now tried everything.
Good Luck Frank

 

I did all the vacuum test over and over and I get the same result.I think I am going to pull off the intake and change the upper and lower and see how far that will get me.I am at wits end right now. Maybe the low idle is trying to be compensated by putting more fuel then less and hense I am getting the rich condition.Will post back.This site is such a big help and thanks guys.Post back if you think of any thing.Frank to you notice your truck running rich at all?

 

No signs of running rich. Plugs look good.
Frank

 

"Wits end" is a commonly felt emotion when working with these trucks.

I've had a six month stretch of "wits end". If your manifold gasket replacement works, fabulous. If it doen't, you'll know that your upper and lower are definately good. Go on from there!

This forum is absolutly a great resource.

 

I have a 95 F-150 with 351 that had very similiar idle problems, I checked IAC valve and cleaned and chased vaccum lines but in the end I ended up changing the IAC and fixed problem.

 

Actually, the first thing I did was change the Idle Air Control Valve and it did nothing for me.
Oh, well just another $80.00 for nothing !

 

Like I posted I changed every sensor including the IAC with another truck that was working fine.I tryed to get the IAC kit from ford in canada and no problem except it is 80$.SO I decided I am going to try to make one.By what I can see the plate has smaller holes that go inbetween the IAC and the intake,thats all it is.Post back if Im wrong.I let you know how that works tonight.Thanks Jim

 

By what I can see the plate has smaller holes that go inbetween the IAC and the intake,thats all it is.
I would not say that is all it is. The two screws allow you to adjust how much air goes thru. I think for my 6 they suggested starting out with one screw all the way in, off, and the other out 3 full turns or something like that. Without the ability to adjust it, it is going to be really dificult to get it right.
Perhaps another vendor could ship it to you for a more reasonable price?
DirectFordParts.com (Ford part # F2PZ9F939A) for $29.77
Good Luck Frank