Idle Issue - Jim Warman

Rick, offhand I can't think of anything that would cause your concern.... perhaps an out of balance of faulty AC compressor but this would make the concern predictable.... A code scan might be in order just to see if there is something that is not critical enough to set the CEL...

If you like, PM me your VIN and I can run OASIS for you.

If you guys figure it out a friend of mine would like the answer too. He just spent $900 at a shop for manifold gaskets that they said would fix it and it's not fixed. The only thing I didn't see listed that you might replace is the O2 sensors. The code he gets is left bank lean.

There are a lot of things that can deliver a "bank lean" code... One could be a lazy O2 sensor but I find it hard to replace one without some empirical data to make me think that way (watching the O2 activity on a good scan tool helps). Flip side, some sensors/actuators can be cheaper to replace than to test.....decisions, decisions....

In retrospect, the 4.0 OHC engine did have about three different intake manifold configurations.... the only one I am familiar with is the style on my sons 97 Ranger. Later ones *may* have ported EGR flow.... I'll have to do some homework there....

Here's why ported EGR makes such a difference.... The DPFE measures only total EGR flow.... If a motor has ported EGR delivery, and if some of the EGR ports are blocked, a few cylinders will receive the bulk of EGR flow and other cylinders none at all... Result will be a very rough idle but only when EGR is commanded (and this will be decided by computer strategy). The

Since total EGR flow isn't excessive, no codes will be set.

Back to the bank lean code.... The PCM is programmed to ignore a specific number of misfires (either "dead" misfire or poor cointribution misfire - there can be a difference) in a particular time span. Anytime a cylinder doesn't consume the bulk of the oxygen it is supplied, exhaust gas oxygen content will go up. The PCM sees high oxygen content as a lean mixture... even if the mixture isn't "lean".

HTH

See if you think these symptoms add up to anything, keep in mind he just had the manifold gaskets replaced. I think it's a 98 Expedition with a V8, 180K miles. I drove to lunch with him and at times it idles properly but then while you are sitting at the light the speed will suddenly drop about 150 rpm and the idle gets rough. Often after

He fixed it himself. Don't have the exact details but something about the hose to the PCV leaking.

After you mentioned V8, we were going to head there.... There is an elbow at the back of the intake manifold that can get mushy, rip and create some grief.... This would normally affect both banks but there is a bit of a rub..... One would need a very capable scan tool to access the fuel trim PIDs to see if the other bank might be close to setting a code but hadn't yet "gotten around to it".....

My bad for not even thinking of it earlier but, in my defence, I don't work on anything with spark plugs very much. Those times I am given a gas drivability concern, I always check the 4 fuel trim PIDs..... It is one of those things that is "too" natural and gets lost in the system..... If someone were to ask me to list the first 12 things I do in the morning after getting up, I would probably forget to list "put on my socks"......

Nah.... I still had a brain fart.... glad to hear that your friend found the concern.....