ok, got a 1988 F150 4x4 with the 5.0. I just recently rebuilt the engine in it, and i have an idle prob. It will start to surge while at idle and become very severe, eventually killing it. Seems to go away somewhat when warmed up. It was just purchased not long ago, and it appears as thought someone has put a new IAC on it, along with a O2 sensor, the egr tube has been pinched off, and the AIR system is removed. I sure hope to hell the air system is not affecting this. Anyway any help would really be appreciated. Thanks

 

Could be a vacumn leak, that will make an engine "hunt"

 

You can get away with out the air injection, but a non-functioning EGR with the stock computer will do exactly what you are describing. As a matter of fact, the test procedure to check for a properly functioning EGR valve is to use a vacuum pump and apply vacuum until the motor dies. If your EGR tube is "pinched off" as you have stated, when the ECU opens the EGR to let in some inert exhaust gas, your fuel mixture will be incorrect because that inert exhaust gas is not being introduced. Sounds like your ailment. All you need to do is get the EGR system functioning, the air injection system only affects the cat in the exhaust, so if that's gone you need not worry about it. This may not be your only problem, but that's the place to start.

 

I first thought vacuum leak too, especially after the rebuild. But i can find none. It does not idle high and does not miss either. So i have decided against vacuum leak.


I almost didnt mention the EGR because it doesnt work at idle but i threw it in there. If the EGR doesnt work at idle how does it effect idle?? The EGR valves on these engines are electonic. What is pinched off is the exhaust tube going to the valve. Anyway, i just need an explanation of how the egr can effect idle.
Thanks for the replies guys

One more thing, wouldnt the o2 sensor tell the engine how to feed itself, with or without the egr valve?

 

Most likely thing would be a vacuum issue, like a leak, or a cam timing issue, or a valve issue. If the computer does not see a relatively steady level of vacuum at idle, it is always trying to compensate for the fluctuation. Make sure that the MAP sensor is hooked up to manifold vacuum. MAP (Manifold Absolute Pressure) is located on the firewall along the rear top edge of the engine compartment, just to the passenger side of center. A hose should connect it to the vacuum tree on the intake manifold. If it is not connected, it will run poorly and black smoke will be seen from the exhaust. If it is bad, it could be failing to send the proper signal to the computer, and computer has trouble controlling idle.

EGR has nothing to do with it. The computer is not supposed to command it on at idle because it will cause it to stumble. It actuates EGR only when revs are up and you won't notice it. The pinched off tube should not affect the idle at all. If anything, it would make it better (if there was a leak of exhaust gas into the intake at idle before it was pinched off).

Try monitoring the vacuum with a gauge at idle and at 1500 rpm to see if there is any steady ticking or rhythmic needle movements. There should be fairly steady vacuum if RPM is steady. A ticking needle could indicate a sticky valve that is not sealing, or a vacuum leak. The gasket between the intake upper and lower sometimes leaks on these trucks, feel around it for whistle/rushing air leak, listen for a noise change when you run your finger over the leak.

 

or get a can of starter fliud or wd 40 and spray around the gaskets . the idle will increase when you spray where a leaking gasket is . i had a simular problem and it turned out to be the MAP . if you had a major vacuum leak the idle would run high . Also have you checked the codes?

 

i will check man vac.

 

Help my 1990 ford f150 5.0 engine would not idle then if you held the throttle down it would just surge up and down. I unplgged the vacuum line to the MAP unit and it started to run alot better no surges. Is that an indication the MAP unit needs to be replaced.

I just removed the whole unit an the truck is running alot better, the check engine light is on but it idles pretty good no more surges

 

Holy thread revival batman. For the love of all that is holy, put the MAP sensor back on and fix the problem. The MAP sensor is a VERY VERY crucial sensor for the truck's computer to do it's job, nearly as important as the 02 sensor. Track down the original problem instead of just unpluging the effects from it. Check for vacuum leaks and if you find none, try a new MAP sensor, but keep your reciept just in case the problem is elsewhere.