Drove it on a few errand stops this morning. Ran fine until the third leg, then the engine ran so rough at idle that it would stall. If I kept the idle above 1,000 rpm, it would run. Same on the fourth & fifth legs. By then the "Check Engine" light was on. I got it home by not letting the idle drop when I hit any stop lights.


A couple of hours later, I connected a scanner to it and got the following 7 error codes --
-- Cylinder 1 misfire detected.
-- Cylinder 2 misfire detected.
-- Cylinder 3 misfire detected.
-- Cylinder 4 misfire detected.
-- Cylinder 5 misfire detected.
-- Random -- Multiple misfire detected.
-- (Pending) Misfire in any cylinder.


When I started the car at that point, the idle was normal.


Any thoughts on what the problem could be?

 

Since this is a Coil-Over-Plug (COP) system, it is unlikely that 5 coils suddenly developed faults and then faults disappeared.

Any possibility of contaminated fuel (possibly water).

Suggest driving to see if fault returns, and adding water scavenger additive to fuel (check auto parts store).

Since fault is intermittent, it may be difficult to find the fault.

good luck

 

Thank-you for responding! I had used a quarter of the tank before this happened, so it's not too clear that it would be bad gas. This evening, I put a can of SeaFoam in the tank and topped it off.


I remembered that I had this same problem two years ago. At that time, the mechanic I was using changed the spark plugs & "cleaned the throttle body" and the problem didn't reoccur until today.


If I wanted to clean the throttle body myself, what would the procedure be?

 

Cleaning throttle body is not too difficult. Remove air intake duct and open throttle plate manually and clean both sides of throttle plate and bore around throttle plate.

How many miles on plugs since replaced?

Has idle speed changed at all? An intake manifold air leak might cause random missing, but should increase idle speed.

Check vacuum lines, PCV hoses and PCV valve, and brake vacuum line. A stuck open PCV, damaged vacuum booster line, or failed vacuum booster (brake effort would increase substantially) is possible.

An advanced obd2 scanner could provide Long Term Fuel Trim which is an indicator of air/fuel mixture long term trends.

good luck

 

Thanks!

 

LTFT should be w/i +/- 10% usually, so probably not an issue.

You may need to wait until missing is sustained. Almost impossible to diagnose a condition that is not present.

Have you driven extensively since problem first occurred w/o further issues?

good luck

 

No, I haven't driven it extensively yet.


Today, I --
-- Cleaned the throttle body/plate (it had some carbon, but not a lot)
-- Replaced the air filter (it was good)
-- Replaced the PCV valve (it was gummed-up and stuck partly open.)


Thanks again for all your good advice!

 

If you can disconnect each coil, one at a time, while the engine is missing, to hopefully find the bad coil or plug (if there is one).

 

Some of the V6's in that gen had an issue with "heat soak". So when the car is at normal op temp, shut off and then try to rer-start say 15 minutes later this type of issue shows up.

This seems to show up more in the winter, when the gas stations are selling their "winter blend" of gas.