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.
