Missing help needed

[ThomasJ2352]

I have a 2005 GP 3.8SC with 144k that is missing badly. For many months I didn't have the upstream O2 sensor hooked up...went to have it smogged and of course it bombed out. Found and hooked it back up then started to have Cat efficiency codes come up along with cylinder 4 misfiring codes. Replaced both O2 sensors and the 4th plug and all seemed better though still had the CAT code. All about 3 weeks back.

It sounds like it has an exhaust manifold leak on the side of the firewall; I replaced the manifold on the front last year to get it to pass smog...it was leaking badly.

So just this weekend I replaced all the plugs and wires, cleaned the MAF and now its only throwing the cylinder 4 misfiring code. Swapped coils and its still there. Its idling but quite poorly...the lights oscillate in time with the misfiring (one way to read RPM I suppose). A can of Seafoam and 91 octane didn't help either.

I'm ready to punt as I'm out of ideas....anyone have a suggestion besides run it off a cliff or trade it in; neither of which the wife is excited about?

Thanks
Tom

[KeithGTP03]

A can of Seafoam and 91 octane didn't help either.

Do you usually run under 91?

[ThomasJ2352]

yea...usually run 87

[Slick2500]

Change out your cat, its probably plugged. And do your engine a favor and run at least 91.

[ThomasJ2352]

Would that fix the missing? Suppose if the back-pressure is high enough.

[Slick2500]

Yes a clogged cat can cause misfires and damage your engine if not taken care of.

[KeithGTP03]

A clogged cat will cause misfires in all cylinders...not just one specific cylinder. Running 87 in a car that requires 91 or above prolly caused knock and chipped piston #4

[Slick2500]

Most likely he did Keith.

[ThomasJ2352]

Wow...I hope not...I checked out the manual this afternoon and it says they recommend 91 but could run mid grade or regular unleaded rated at 87 but acceleration would be degraded. It does say not to use below 87. Would a compression test on #4 show the chip if its there?

[Slick2500]

It could, try to find a borescope and look in there.

[Tuner-Extraordinaire]

Wow...I hope not...I checked out the manual this afternoon and it says they recommend 91 but could run mid grade or regular unleaded rated at 87 but acceleration would be degraded. It does say not to use below 87. Would a compression test on #4 show the chip if its there?

Running 87 in a 04+ is one of those "no station has 89+ available" kinda things. If it's available to you, run the highest octane possible.

[98L36]

Injector maybe? If I've got my head right misfires are hopefully one of 3 things: plugs, wires or injectors. Just my opinion but I'd rather switch an injector around for free and see if the problem moves rather than spend $$ when you really don't know 100% for sure whats wrong.

[KeithGTP03]

Wow...I hope not...I checked out the manual this afternoon and it says they recommend 91 but could run mid grade or regular unleaded rated at 87 but acceleration would be degraded. It does say not to use below 87. Would a compression test on #4 show the chip if its there?

Yes...cause if the piston chipped, it will most likely damage the valves and they wont seal. You should see a noticeable drop in pressure, if thats the case.

[ThomasJ2352]

Ok...just went out to do the compression test on #4...when I tried to remove the plug it was loose Been changing plugs for 40 years and rarely forget to tighten one...cant remember the last time if ever. When I got it out a chunk of the tip was gone as was the center electrode...dropped a replacement plug in and its now back to being the rocket in acceleration but it still idles badly...now I'm getting multiple cylinder misfires...it doesn't feel like its misfiring on acceleration but it sure feels like it at idle.
Now maybe the clogged CAT? Any way to test it before plunking down $600 for a new one?

Any more ideas?

BTW, really appreciate your suggestions.
Tom

[KeithGTP03]

When I got it out a chunk of the tip was gone as was the center electrode...

Did it look like this?



This was mine...I figured it was a chipped piston, but after a borescope session, piston was fine. You didnt say if the compression was good.

[Tuner-Extraordinaire]

$600 for a cat? Where the hell you getting a cat?

[KeithGTP03]

$600 for a cat? Where the hell you getting a cat?

The safari?

[ThomasJ2352]

Your holding my plug! I was so anxious to try it with a new plug I forgot to give the compression a go...in retrospect I need to do that.
Regarding a CAT...I can only get it from a dealer here in Calif...not allowed to
buy a used one and auto-parts stores don't carry em for that new of a car.

[KeithGTP03]

I would do the comp test or drop a scope down there to be sure. And who's gonna know if the cat was bought aftermarket?

[ThomasJ2352]

I'll give the comp a go tomorrow...regarding the aftermarket...here is what
the online outfits are indicating:"49-state legal; Not for sale or use in California;
Cannot be shipped to California addresses"