VERY rough idle after hard driving or turning on AC
 

If I go WOT or just drive hard (taking off quickly) and then come to a complete stop, my RPMs will go from about 800 (idle) all the way down to 100 RPM. It will go back and forth between these two values, almost wanting to die. Sometimes if I don't give it gas it will die completely. I also notice that if I drive it somewhat hard and then turn it off and turn it back on, it will also idle really really rough and want to die.

Also, if I don't drive hard at all, and turn the AC on while I'm stopped - the idle will do the same thing. If I start the truck with the AC on it will also idle very rough (the 800 down to 100 rpm bounce) and try to die.

Does anyone have any suggestions? My fuel pressure is a constant 58-60 psi. I replaced the intake manifold gasket. I also replaced oxygen sensors. I've check thoroughly for a vacuum leak also.

It comes down to the problem happening whenever the engine is at operating temp, at idle, and either after it being driven hard or under a load (like the AC being turned on)

Did you try a new Map sensor? Maybe have someone check the tuning also.

Sometimes mine will idle lower also, if I have the ac on when first starting it, but it does not go down to 100 rpm. Mine might go down to 500rpm at the most, but its fine as long as I let the truck idle for a few seconds with ac off then turn it on.

good idea. I haven't tried a new MAP sensor. I have hptuners scanner, I'll log a session while it does the rough idle and see how the MAP reacts (if it flat lines...etc..) I have taken previous recordings of the truck while it does this, logging MAP - I never noticed anything extremely obvious with it, but I will do it again and make sure I get a real close look at it.

The tuning has been checked too

I checked the map sensor very closely with hp tuner scanner, checks out to be perfectly fine.


I pulled the plugs (a couple of them were coated in oil). I'm in the process of installing new ones. Also I think one of the plugs might not have been installed all the way. All the plugs went in smoothly except one. It acts like it is fully seated when it is not. I'm buying a thread tap tomorrow to run it through the threads to hopefully clear it up and allow the spark plug to screw all the way in and seat properly.

Three of the plug wires also pulled apart while removing them... so the plug wires will be replaced also. I'll give an update tomorrow

Sounds like the idle timing needs to be adjusted..
when i did the 8.1 marine injectors i had to play with the timing to get mine to idle good with the AC on. If not it would do what yours is doin.
can you make changes to the tune?

It's a Nelson tune so it is locked, so I am unable to make adjustments. It also does it when the AC isn't on, but if driven hard. I've been in contact with Nelson, he has looked over my tune and said he sees nothing wrong with it, but still offered to take a look at it in person. San Antonio is about 5 hours away from me so I am trying to make double sure it is not a mechanical/electrical problem first. Also, it seems to be getting slightly worse each day I drive it- which sort of puts the blame away from tuning.

I chased the threads on the messed up spark plug port and got the spark plug to seat properly. So with new plugs, gapped to .025 and new wires--- the truck is still having the intermittent rough idle. I checked all the coils again also, all good.

When I hold the engine at a rev in park (1200rpm), I can hear misfires.

I unplugged the electronic throttle body and started it up and the misfires all went away (the idle automatically stays around 1200rpm when no signal from electronic throttle)

Has anyone heard of an electronic throttle body causing misfires? I wish they would have stayed with the simple throttle body w/ TPS and IAC...

I took off the TB, cleaned it real good and put it back on. Truck is still misfiring.


While driving, the volt gauge fluctuates. Not a lot, but enough to make it noticeable. Whenever the misfires start happening you can see the needle move. I'm starting to wonder if I should go ahead and replace the alternator.... I got it tested today, and the guy said it was good- but at the time he tested it, it was not idling rough..

It also fluctuates a lot while it is idling rough. The voltage doesn't drop with the rpm's, it slowly lowers and when it comes back up the engine stops running rough

watched voltage value via computer during idle/driving/rough idle condition. I did not see a correlation to the misfires. While driving it read approximately 13.5-13.6 volts. At idle it read 13.2-13.4 volts.

Tomorrow I'm going to look further in depth on the electronic throttle body.

I am surprised you can unplug the throttle body and have it not go into REP mode.

Can you check your fuel pressure, and for vacuum leaks? It sounds like something like that if it is getting worse. A vacuum leak will usually increase the idle, but it is just one of the basics to check. Just to make sure, at one time it did run properly with that tune right?

I got 90 nothing it's a small parameter somewhere in the tune that needs to be tweaked. If you can the best thing you could do is drive to sa and let nelson strap it down and show them what it's doing and probably just going to be fine tuning.