Expert: Roland Finston - 6/13/2011

Question
Our '96 Concorde 3.5L ran into a problem recently where it would stutter when we hit the gas (you could sometimes smell unburned fuel in the cabin).  Local parts store people said to try plugs/wires and fuel filter.  We did both.  Can't say that we have the problem with accelerating any more, but the car "chugs" when it is idling.

I noticed the parts store guy gapped the plugs to 0.036 and the Haynes manual says 0.030.  I re-gapped the plugs to 0.030 and noticed that in #6, the plug had oil on it when I took it back out.

The car still idles rough, and the exhaust manifold gets red hot to the point where it smells like something is burning and smoke comes out of the side of the car.  Do you have any ideas on what to try next?  Was the 0.036 gap a better option?

Thanks,
Don

Answer
Hi Don,
I notice that the Haynes gap spec oscillates back and forth during the '93-'96 and 'later years' rather widely from 0.030 and 0.048 depending upon whether you used the RN12LYC or RC14E5. The '98 Chrysler service manual is at 0.048  with the RC12PEC5 and without any explanation for that variation you might note which plug # you have and compare to the various suggestions. I don't have a '96 factory manual so I can't give you anything 'official'.  The oil on the plug indicates either valve seal or piston ring, but it could have been 'wet' from unburned fuel as well.
When you speak of a red hot manifold I think of an exhaust system flow obstruction such as a honeycomb of the catalytic converted on that side's stream being loose and cocked at an angle such as to impede the exhaust flow through it. So you might want to have an exhaust shop check that possibility with a meter. That might cause a 'chug' or restricted output at highway speeds.
The best approach would be to try to get fault codes that are likely stored in the engine computer, either using the ignition switch or a plug-in code reader (which nationwide parts stores such as Autozone will often to for free). Use the switch this way:"On-off-on-off-on and leave on" doing that switching in 5 seconds or less elapsed time. Then watch to the odometer window to see if the mileage reading changes to to show any 4-digit numbers precedec by a P. Let me know the numbers from either way and we'll go from there. It also may be the case that the check engine light will start to flash, pause, flash, etc. Count the number of flashes before each pause. Then repeat the key switching and count again to be sure of an accurate count set. Tell me the numbers in the order of appearance.
Roland
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