Have a 1990 bronco with a 351. When I start it after it has sat for some time, the engine will run just like a carbed car does with a vacuum leak, with a loping idle. After a period of time, anywhere from 5 second to 15 minutes something happens and it will suddenly run smooth as silk. It wont do it again until it has sat for a long period again. Cleaned the IAC and replaced it. No luck. Cleaned the EGR valve with no luck. Got to playin with the EGR and noticed that the egr is opening slightly when I start it cold. When I pull the vacuum hose off of the EGR valve the idle speeds up and smooths out. My understanding is that the EGR should not open at all at idle or when it is cold. Does anyone know what controls the EGR opening and what the problem with my idle could be. All the emission equipment is still intact.

 

The ECM (onboard computer) controls it along with the rest of the egine and emission control systems. Perform a KOEO and KOER test and see what codes it's throwing back at you. If you do this before you start delving into it you will save a lot of wild goose chases and a lot of needless repair/replacement. Haynes manuals and/or ford fuel injection dot com will give you details as to how to peform these two diagnostic tests using nothing more complicated than a paperclip if you want to.

 

the egr valve is typically not utilized under load or at idle, but everywhere else. its not used at idle because it produces an unstable idle. it changes the combustion temperature among other things. I wait until my egr is closed then i pull the vac line and cap it. its always given me trouble and cleaning it doesnt seem to help.

interesting thing you said. when you pull the vac line the idle smoothed out. if you pull the line your egr should be "stuck" wherever it was when you pulled the line. when you pull the line off do you cap it or leave it open?

 

EGR is an analog system. The EGR valve can be closed, to varying amounts open. The valve is opened by applying vacuum to it. Whether it is open, and if so, how far it is open, is controlled by the EEC via two electrically operated vacuum valves. One valve will be called control or vacuum, when it is open it will allow engine vacuum to go to the EGR valve's diaphragm, opening the EGR valve. The other vacuum valve is called vent. When the vent valve is open, it dumps vacuum, allowing the EGR valve to close.

So in normal running operation, above idle speed, the EEC is twitching vacuum valve to start to increase the EGR, then it closes vacuum valve, then opens vent valve to decrease EGR. By twitching these two valves, and using the stored vacuum volume in the EGR valve itself, it can set the EGR opening as per the software's parameters in the EEC for any given condition.

If you stand by the open hood when its on a roller type of emissions test, you can hear the electrically operated vac and vent valves happily clicking and clacking.

Since pulling off the vac line to the EGR "solves" the problem, that shows that the EGR valve itself is able to close completely, not jammed open by a piece of carbon. It is very doubtful that the EEC is sending an "open" signal at the wrong time to a vacuum or vent valve. I suspect that your vacuum valve is not closing tight, and it is letting some vacuum leak on through to the EGR valve. After it runs awhile, it probably seats better, and stops leaking vacuum on through when it is supposed to be closed.

That's my shot at it.