The incurable rough idle

[PAExplorerXLT]

There are so many posts here, going back a decade, about rough idle. Well, I have had it too since I came in possession of the truck. It happens starting from cold (curiously, not any cold, only when it sits overnight). Once it is warm, it purrs. I don’t work on the car myself but my mechanic has changed the idle air control valve, upper intake manifold gasket ( he calls it upper plenum or something like that), changed the fuel filter and changed one of the O2 sensors. None of them worked. He finally gave up and handed me the keys. said he doesn’t go on a fishing expedition if he doesn’t know what he is doing. Error codes are 171 and 174. I came across this video. Doubt my mechanic would miss a leaking hose.

https://www.youtube.com/watch?v=GHWucT5J36Y

 

[diggity0169]

I've got a slight rough idle and it sounds like it is missing or early detonating very intermittently. Basically it just shakes slightly in a weird way. Is that what yours does? I thought it was just spark plugs causing early detonation from too small of a gap, but I checked them and they were actually about .005 over (factory is .054 I believe). I'll be checking this thread to see what people think.

 

[PAExplorerXLT]

Once I got P0300 which is random misfire but it went away. It did shake rather violently. More often the engine simply hunts and if I let it go may die, a symptom reported by a lot of people. If the plugs were an issue why does it go away when the engine warms up? For all I know it may be on original plugs.

 

[BustedKnuckles24]

He should have replaced both the upper and lower intake manifold gaskets. Once the upper plenum is off, the lower manifold is right there. I did both on my 2000 4.0L SOHC just to not have to do the job twice. The gaskets are cheap. I'd take one more run at it, doing both this time.

 

[PAExplorerXLT]

He knew about it but somehow he didn’t. Might have thought it wasn’t needed. He said he did the spray test. My understanding is to get to the lower gasket you have to go down much deeper. Read somewhere it is a $500+ job.

 

[diggity0169]

http://engine-codes.com/p0171_ford.html

This is why he changed your intake manifold seals. I've heard that a good way to check for a vacuum leak is to spray Carb. Cleaner around seals of the intake manifold. If your engine revs up you've found generally where your leak is. I tried it once but didn't find anything

Supposedly those codes mean your engine is running lean. It doesnt mean the o2 sensor was bad but that was a cheap thing to rule out. It just means the sensor is reading too lean. Could be air flow but might not be. Maybe your fuel injection or ignition is messed up. Check air filter and oil level, pull a spark plug or two and look at them for damage or excessive carbon. Hopefully it is something easy.

 

[BustedKnuckles24]

He knew about it but somehow he didn’t. Might have thought it wasn’t needed. He said he did the spray test. My understanding is to get to the lower gasket you have to go down much deeper. Read somewhere it is a $500+ job.

Once I pulled the upper manifold, the lower was easy to do. I have no idea where he came up with an additional $500 to do it. On mine, the lower was easier to pull off than the upper manifold. It basically looked like a 2-inch thick adapter plate.