99 300M Very Rough Idle (almost dying) on Startup

My 99 300M 3.5L V6 for the past few months has occasionally (2 times out of
30 or so start-ups in a normal week) has a Rough Idle on startups. Doesn't
happen every time...but when it does happen, it can be felt through the
whole car as a fairly rough running engine.

Today I went on a trip, car sat for about 25 minutes..I get back in and
start it and it's running horrible. It's like it's on 2 cylinders...I gave
it some gas and it just spitted and sputtered and eventually revved up, but
never died..after about 10 seconds it cleared up. I backed out of the drive
and it started to hesitate a little as I took off and then didn't do it the
rest of the way home.

No check engine lights came on or anything..I'm not sure where to start on
this one...Like I said this is the first time that it actually sounded like
it might have actually died but didn't..other times have just been a rough
idle that smoothed out over a few seconds.

Any Ideas on what to look for?

Thanks,
Mike

 

It could be a fuel injector leaking down causing a really rich mixture. Or
just a secondary misfire be it from a plug or coil pack breaking down. A
scan tool would be needed to see what is going on when this occurs.

Glenn Beasley
Chrysler Tech

 

fuel pressure check would be first then a scantool that can look at freeze
frame data
that might show a pending fault code that does not turn on lite

 

It could be a fuel injector leaking down causing a really rich mixture. Or
just a secondary misfire be it from a plug or coil pack breaking down. A
scan tool would be needed to see what is going on when this occurs.

Glenn Beasley
Chrysler Tech

 

If there's no check-engine light, then what would a scan tool show?

If the car's computer throws a code to the scan tool, then won't it
also set the check-engine light at the same time?

How much rough idling or rough running does the engine have to do
before it sets the check-engine light?

 

A scan tool will show alot, it will show adaptive fuel cells to see if the
problem is caused by a rich or lean condition. Depending on what the fault
code set requirements are it may not set a fault code. A scan tool will
show a secondary indicator ( that is when a fault is trying to set but just
happens to fast to set a hard fault) . Also if the adaptive numerators have
been lost a cylinder misfire code will never set until those adaptives are
relearned. One reason why you DO NOT do a battery disconnect to make a
problem go away.

Glenn Beasley
Chrysler Tech

 

Oh that hurt!!

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

Being this is an intermittent issue, is this something that I should wait on
taking to the shop until, it's a more consistent issue to guarantee they'll
find the problem. Or do you guy think it's bad enough now that they can
catch it?

Right now, it seems to be a crap shoot whether or not it happens..but it
does happen at least once a week..just never know when.

Mike

 

If it were me, I'd assume a leaky injector like Glenn mentioned, and run
Techron™ or Sea Foam thru a couple of tankfuls of gas. It won't hurt
anything, and there's a good chance of it clearing up (we see it all the
time on the 300M and Dodge Intrepid forums).

If your spark plugs have more than 60k miles on them, I'd replace them
*now* (I don't care that the manual says they are good for 100k miles).
Use the Champions they list as the OEM replacement.

I'd also use a fuel-injector-safe throttle body cleaner (involves
pulling intake elbow off the throttle body - follow the directions on
the can).

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

Sorry Bill no intent!!