Thread: muliptle misfire code when boosting

Ok, so I just bought an 02 GTP with 207,000 miles on it. Previous owner put on 50,000 of those and did no engine maintanence besides new plugs and wires and regular oil changes. Not sure how long ago though. When I test drove the car it had a full tank of gas and seemed to run perfect, bought the car and ran the rest of the gas out before refilling. Wife put 89 octane in it plus 1/2 can of seafoam, and drove it to work for a few days, she says SES light flashed at her while trying to pass and car bogged down. So in researching this site, I figure out the flashing SES light is going off because of misfiring at that moment.

So I borrowed a friends scan tool and it had three stored codes, one for low coolant temp, one for multiple misfire and one for EGR (don't remember which one now). So I notice car never gets about 165 degress while driving, bad deal in cold winters in Iowa, takes for ever to heat up cabin in car. I suspected bad thromostat. Take old thermostat out and notice the previous owner/owners drilled a small hole in a 195 degree stat. Replaced with new 195 degree stat and new gasket. Solved problem, car runs around 195 degrees, does not overheat, and have great heat in the cabin! So first code fixed.

Figured I start tackling the misfire code, so first thing I changed out was the fuel filter, its cheap and easy... Still misfires, and when I say misfires I mean at 55 to 60 Mph when it needs a little more throttle, and if push it down to WOT does it occasionally. So next up was visual inspection of wires and coil packs. Removed the aluminum heat shields from plug wires, maybe arching a spark, and also cleaned up all the coil pack to wire connectors with some fine sandpaper. One connector was rusty, cleaned up good and wire seems to fit ok. Cleaned MAF sensor with CCR MAF sensor cleaner. Not throwing any codes for MAF sensor but just some PM. Stock air cleaner, looks decent.

Suspect bad gas or at least not high enough ocatane, ran gas out to about 1/4 tank and refilled with 91 octane. Car seems somewhat better but still get the flashing SES at the stated above conditions. Oh, I cleared the codes 5 days ago and have driven it daily since, and only code I'm getting now is the multiple misfire code, the other two did not come back. Also noticed the boost was only going 1/2 to 3/4 on boost guage and was erratic at WOT. Upon further inspection, noticed the boost bypass solenoid electrical connection was unplugged. Plugged back in, guage reads 100% boost at WOT now.

Talk to previous owner last night, he never had an issue running 89 ocatane, didn't notice the boost problem (not mechanically inclined) and he didn't unhook the wire to Boost bypass solenoid. Said car has always ran fine with no problems. The car starts right up and idles and purrs like a kitten. It runs out smooth except when you want a little more boost, if you stay out of the boost it seems fine.

So where should I start? I want to methodically troubleshoot this issue, to find the exact cause so don't want to shotgun parts at it. I do not smell rotten eggs from exhaust but not sure if the CAT should be my next replacement. I have no idea when it was changed last. This car has no MODs done and I'm not interested in changing the factory setup UNTIL it is running to factory specs.

Ordering parts for LIM gasket update, exhaust elbows, new upper and lower injector orings, plugs and wires to do in a couple weeks. Should I also replace the injectors at this time with reman injectors?

Sorry this is so long but the more info is always better when diagnosing. Look forward to your suggestions and ideas on how to proceed.

Replace all the spark plugs with new Ac delco's and go from there. Its very possible the spark plugs that were put in it a long time ago are the wrong ones and have started failing. Try that first then we'll be able to try a few other possible causes.

Ok, got new plugs installed, while installing plugs I ohmed out the wires also. They all looked good except when I got to #5 plug wire, was reading an open. Upon investigation I could get a reading while touching the wire going to the clip but not the clip it's self. So may have had an issue with it arching to the coil. Got a set of new plug wires and installed. The old plugs looked like they were running hot to me, very dark brown coloration but I've never inspected plugs from a SC car before so maybe thats normal. Almost like a rust color brown on elctrode and tips, not actually rusty tho...

Test drove car and still doing the same thing, it ONLY does it when the car is adding boost, when I say adding boost I mean when I can see the boost indicator on dash start to light up. The car changes sounds and starts to run rough. If you keep it in the boost then the SES light starts to flash indicating a misfire. The car runs great until it tries to boost. I sprayed all vacumn lines with carb cleaner while idling to check for leaks and all looks good. If I floor the car the boost indicator will show 100% like it should.

Thinking maybe I should get an HEI ignition tester and check each plug wire and coil pack to make sure it is getting proper spark under load.

Thoughts?..........

Have a borescope handy?

Hearing that the previous owner ran 89 octane, I'm thinking it's time to ask him how many plugs were mangled when he replaced them. I'm thinking the low octane with boost may have chipped some pistons. However before we condemn the pistons and scare you to hell, let's talk further.

Do the coils on the car look original or are they red MSD coils? MSD coils are known to cause issues at higher rpm, stock are better.

What kind of plugs did you use? Platinum are never suggested in our cars, iridium or copper are the way to go.

Do you have a scanner capable of telling you which cylinders are misfiring when you drive it? If it was a shared coil (3/6, 1/4, 2/5) then you could replace just that coil.

That's where i'd start.

clogged cat can also cause multiple misfires. and make the sel light flash when in the gas. with over 200,000 miles thats a real possibility.

You sure could test it that way. Although I'd T in by the fuel pressure regulator or boost soleniod. Nice easy area to get to and most gauges have smaller fittings.

I don't know about the HEI deal. Never even heard of one before.

Nice price for sure.

I go old school to test packs. I hook up the HPT, take it for a spin and see the cylinders that are complaining. Then I dive bomb it and go in for the kill.