My truck is a '86 F-150. After using and reading the search results I seem to have the classic EGR issues. Runs perfect when cold, however, when it warms up the idle will deteoriorate and most noticably there is a stumble/miss almost throughout the entire operating range. It's got about 128K and thus far I have replaced the cap, rotor, plugs, plug wires, and the air/fuel filters.

I did notice tonight that once the engine is warmed up I can disconnect the vacuum line at the egr valve and there is no difference in the crappy idle (once warm). In my first post on this site, I had mentioned this problem and someone suggested changing the coil. That hasn't been done, but that post was made before any of the other new parts were added. I can drive it for days and it will be fine, and then without warning act up. Tonight, I got in after it was sitting for three days, and was fine. Drove it for almost 100 miles home, and then shut it off for about 20min and re-started and again, fine. Then...shut it off for maybe 40min and was barely driveable at part throttle.

???? Any and all help appreciated ????

 

EGR can cause idle problems if the valve is stock open; however this isn't common. When they fail, they usualy fail shut. It's rare for one to fail open, but it's not impossible.

That doesn't sound like a coil problem either. The next time it's warmed up and starts running rough, pull off the air cleaner and take a look at the top of the carburetor and see if the choke is open. This is the first place to start with your symptoms.

I will give you some friendly advice when it comes to troubleshooting - do not listen to people when they randomly say "replace this," or "replace that." If we ran through some step-by-step checks and suspected the coil and did some electrical tests that actually proved the coil is out of spec, then I'd suggest to replace it. But how can the original person you talked to know what shape your coil is in, without having mentioned testing it at all? This is why I've suggested a first thing to check, and we'll go from there

 

Thank you for the sound/intelligent advice. Yes, it can get frustrating having someone "shotgun" parts to replace. I've seen many threads on this site among others on the 'net where four different individuals will tell the origional poster to replace five different parts. Ok, enough on that.

I did pull the air cleaner off last night when it was running rough, prompting this thread. The choke was indeed fully open. Once the engine is warmed up to operating temperature and the vacum hose is removed, should there be a change in idle quality? To me semi-trained self, it almost seems to be a vacum leak when it's not right. Part throttle/light load, it will shudder/studder/miss before catching itself and then accelerating. What's most irritating as it seems to be intermittant. I drove it last night over 120 miles after it had sat for three complete days before it was acting up.

 

If you want to eliminate possible vacuum leaks, spray the lines with some carb cleaner while the engine is running. If there is a change in idle when you spray a certain area, there's a leak. Do the entire system. If there are no leaks, then lets move onto something else.

 

I agree with the above post; that's a good way to check for vacuum leaks, as they can cause the problems you are seeing. I was also going to suggest checking your vacuum advance. Pull the distributor cap off, and apply vacuum to the advance and see if the breakerplate (plate that the pickup module rests on) moves back and forth. This device is responsible for adjusting timing during the throttle range you're having problems with. Usually when it fails you simply have lack of power and mileage in this range, but it has been known to cause driveability problems as well. Won't hurt to check.

 

Update: And I did all of this before I was able to read the suggestions from above.

MAJOR vacuum leaks aplenty. The hose from the PVC to the bottom of the carb was toast. Replacing that made a big difference. In fact, it made the stumble/miss nearly go away but it was still there. While I was finishing up with that, I noticed that that the carb itself was loose on the top of the intake manifold. Turned out that there are some fasteners that had backed off and there was a large vacuum leak between the bottom of the carb where it secures to the intake and nest piece upwards. (Sorry for not knowing the exact names of the parts here). I had to loosen the carb from the intake, lift it up, and then use a ratchet bit screwdriver and tighten them back up. Screwed it back down on the intake and it's nice and snug with no movement. Still wasn't idling just right and it still was acting like a vacuum leak while on a short test drive. I adjusted the timing by ear (as I don't own a light) and was able to tell immediately a HUGE difference in idle quality! A quick test drive confirmed my ears as it was smoother, acceleration is noticeably quicker, and all-round more pleasant to drive.

I will have acess to a light this weekend and will fine-tune if necessary.

There is/was a metal line coming off of the carb at the upper left that was pulling a light vacuum. What was it and where should it have gone. The one on mine was finger loose and split off almost as soon as I touched it.

I will follow fmc400's post above tomorrow and see if that yields any results and post here.

Thanx again to all!

 

Sounds like your not afraid to get in there and see what's going on. That is what it takes. You can talk about it forever and still not find the trouble! When I don't have a timing lite, I always just look up at the hood when I am adjusting the distributor and try to get the timing where the hood is vibrating the least in connection where it sounds smoothest! Way to go!