I am trying to figure out what is causing my 97 Expedition with 4.6 to have a low and fluctuating idle speed. Any help is appreciated.

History: Recently changed the intake gaskets due to coolant leaks causing misfires. Put new Motorcraft double platinum plugs in while apart. Also put some rebuilt injectors in at the same time. It had been idling only slightly rough and I figured with 138K the injectors could be a bit dirty. Ran great for a couple weeks, no more coolant leaks causing wet spark plugs. Then it has developed this bad idle last couple weeks. It seems worse after a longer high speed drive, such as 15 miles freeway and then get off the exit. It nearly dies and even idling in neutral it flucuates and is not a smooth idle. Works fine when cold, it is when it is fully warmed up that I have the idle troubles.

I figured it was the IAC, got a new one and replaced it. Cleaned the MAF at the same time although it looked real clean. The old IAC was dirty, but not all carboned up, I put the new one on anyway. I have not had any CEL or codes set since this idle problem started. Gas mileage even seems to be improved maybe 1 mpg with the new injectors.

Has not helped idle problems with the new IAC. Same conditions as before. The truck runs great at any throttle setting and engine speed, except at idle. So what do you have for suggestions? I need to get this working well, as it is my bad weather 4x4 vehicle, and I am in the middle of repeating snow storms lately Thanks for any advice on what to check or what to do, Terry

 

Probably won't make a difference but its worth a try to disconnect the battery for 5-10 minutes so the PCM can reset itself. While the truck is running bad try disconnecting one injector at a time. If you get a cyl that shows no difference in the idle when the injector is disconnected it may be the problem cylinder. Sometimes a low level miss will not throw a code.

 

Isn't that a coil pack and wire ignition? Your wires and coil packs may be starting to show age. I know when my V-6 started running like crap that was the problem, and it wasn't throwing any codes.

 

Yes it does have the coil packs (2) and plug wires. Not a COP engine. I have replaced the coil packs a few years ago, maybe 50K? They are not that old. That is an idea to consider, anyone else think this is it?

I am not convinced it is a miss or weak cylinder though. It runs nice and smooth once up above 800 rpm. Engine does not leak or burn any appreciable oil, it seems really good shape overall. I have had it 11 years now and since about 45K. I know all the history, it has been problem free except the coil packs I replaced a while back and the leaking intake gaskets.

 

There is this hose with the pcv valve that goes to the back of the throttle body. . Have u checked it for leaks or that it's not to smoth to were it collapses

 

if you have not recently removed and really cleaned the throttle body I would do that...you would be amazed at how nasty and clogged up those ports can get. when i cleaned mine it made a major difference in performance.

 

If the plug wires are 11 years old I'd sure change them. I'd also change the plugs if they are not Motorcraft. Other brands of plugs don't have a very good history in the Tritons.

What brand coil packs did you use? That's a possible issue too.

Hmm - roughness only at idle could be a slightly sticky EGR valve - not enough to trip a CEL but enough to allow some exhaust gas dilution at idle.

 

Thanks for all the ideas. I will see if I can do some of these over the weekend. Certainly a good cleaning of the throttle body will not hurt no matter what.

I have replaced that PCV hose, and the coil packs were from the global automotive ebay supplier same that everyone gets the COPs from. The wires could be breaking down, I will check them out.