P300 code - skips at idle misfire when accel

[jhall]

Hi all,

Been a while.

Boths cars were running great until the 2002 DTS decide to pitch a fit on Thursday.

Idle had intermittent miss and feels like a cylinder is dropped when accelerating.

The P300 code showed up. Car has 180K miles. 4.6 liter

Here's what I have done

• Replaced plugs
• Swapped out both ignition packs from my 2000 DTS/ Did not find anythig wrong with the ones that were in the 2002. This effort appears to have saved me $250 in an incorrect diagnosis on the coils.
• Cleaned throttle body and checked the air plenum gasket.

I pulled the injector electrical connectors one at a time with car in 'drive' at an elevated idle and to observe changes in engine speed. Injectors appear to be working OK - at least putting fuel in. I was not able to pinpoint any one cylinder with a problem. I can hear the misfire at the tailpipe with everything connected and the skip can be felt inside the car. Engine also twitches.

After exchanging the back coil (second swap effort), the engine had cooled. On start-up, it ran smooth. Thought I had it nailed....but it started skipping again in less than 5 min.

After cleaning the throttle body, it seemed to improve a little, but still has a distinct miss. P300 code is still thrown when hitting the road and you can feel the miss.

Questions:

Is it possible an injector could be causing this symptom? I checked for air leaks where I could see hoses. Asked the wife when / where she filled up last (Walmart). Seems the problem started a day before, so does not appear to be water, but I put a bottle of STP moisture chaser in the tank just in case.

Is it possible to pull the injectors and observe injector spray patterns while cranking (disable ignition and just crank engine)?

Ideas / comments appreciated.

P.S. - Hoping to avoid a trip to the dealer on the 2002. Just bought a 2009 DTS that was intended to replace the 2002 later this year, but not right now!.....................sorry for incorrect spelling of "idle" in title - don't know how to edit it.................

[Göran W]

_{No other code?}

[jhall]

Thanks Göran,

An MSM 1327 and IRC U1064 were in history, but these do not appear to be related.

Cranked car this morning and and it feels like cyclinder is not firing - especiallt when accelerating. Starting to look more closely at injectors.

[Logan]

"Cleaned throttle body and checked the air plenum gasket.".....are you checking the right gasket? The one that fails is between the intake and TB. Always fails on the bottom. It's very difficult to actually see the failure...Can almost look like new on the top-side.

Intake has to come off to replace.

[jhall]

Hi Logan,

Good to hear from you.

I replaced the TB gasket a few years ago (5?). Did a check by spraying fluid around the gasket. Does not appear to be here. Last time it failed, I believe I got a 'lean mixture' code.

One thing I did notice was when the car is turned off, I can hear what appears to be a vacuum-type sucking sound immediately after shut-down. It lasts 2-3 seconds My 2000 does not make this same noise. Is it possible that the backfire protector valve at the end of the air plenum could be leaking - or maybe the plenum is leaking at the engine interface gasket?

I'm really stumped on this one right now.

[jhall]

Just occurred to me - What if a spark plug boot is bad? This may be the reason for the intermittent miss at idle and then full misfire under acceleration load. Has anyone seen a boot fail? Would the system be able to identify the bad cylinder from a misfire in this area? They looked OK when I installed new plugs, but maybe I missed something.

[BodybyFisher]

If you can hook up a reader/analyzer that can read misfire counts you can determine which cylinder the miss is occurring in, if indeed it is specific to a cylinder.

I don't like that you are hearing a vacuum leak after you turn your engine off, that may be the clue, and I would focus on that. Have someone turn the engine off while your head is under the hood to more specifically locate the vacuum leak sound.

The anti-pressurization valve in the intake usually whistles when bad but it could be what you are hearing on turn off, look at brake booster, plenum, PCV, and gently torque down your intake to spec.

[jhall]

Considered checking crank sensors, but finally had to throw in the towel today Not enough time and need to get it on the road.

Dropped it at dealer today. They called back and commented that there is random misfire and they think its the plug boots. Told them to go ahead and replace them. Other than an external visual check and a look down the tube, it's hard to test these for leaks. Hope they are right in the diagnosis. Will keep you posted.

[jhall]

Updating - Dealer installed new plug boots and misfire problem is gone. An expensive lesson, but doubt I would have found it. Old boots looked OK.

[BodybyFisher]

Thanks for the info great to know