99 GTP still studdering in 4th gear highway driving

[Tomtopdog12]

So 3 weeks ago i posted about putting mixed gas ( oil and gas) in my tank and had a studder problem where at WOT it would lag and cut out a bit, the SES light would not flash so it wasn't misfiring. well now to present day i was on the highway coming back from a little cruze and getting onto the highway in third gear i would go half throttle and it would maybe studder a bit and jump down a few 100 rpm and then continue to rev outl, if i put it into D it would go way down in RPM like 1400 or somewhere close. it would cruze okay but if at anytime i slam the throttle a bit like half way you could feel it bog and kinda be slow to pick up, i didn't try and do a 4th gear pull as i would never do that but i know if i did it would bog and studder to much. Low RPM driving is perfectly fine and it shows no signs of bogging. At first while on the highway and jabbing the throttle it almost feels like it's a trans problem, putting it in Performance shift would take away some of that bog but it would still be noticeable. I want to replace the fuel filter as it's pretty old but i'm not sure if that would make a massive difference. Planning on getting a over kill PCM but if it keeps doing this idk if that would be worth it....


Fresh 91 non ethanol gas and a brand new race/highflow cat with a new rear O2. The car does have a leaking lower intake gasket but that will re fixed tomorrow. Not sure if that has something to do with it.


mods are as followed 3in dp catted, TR55 NGK, Front PLOG, rez del, FWI


Any help would be great, thanks.

[Tomtopdog12]

so upon scanning with my works GM tool i found that at idle the car is misfiring on Cylinder 4 and while under moderate to heavy load is misfiring on cylinder 3. a little misfire on cylinder 5 was seen but this was the extent of the problem. I changed my old (400km) TR 55 plugs and they were so bad it was crazy. one broke as i was taking it out.

all was well and i replaced them with AC delco iridium plugs and as the problem has gone somewhat down the studdering still happens, while at my work i changed a coil pack and cylender 5 and 3's spark plug wires and that didn't help. I'm stumped on ideas, i could say pquch it and buy all new coils and plugs but that's a lot of money.


What should i do?

[Tomtopdog12]

currently repacing LIM (lower intake gasket) should have the car back together later today, hopefully this will fix the studdering problem.

[Tomtopdog12]

lower intake gasket repair was done and the car has lost a lot of the studder problem, i haven't tested it enough but i believe the LIM was the problem.

[SaukRapids]

How bad were the old gaskets?

[Tomtopdog12]

Really bad, the had been colapsed in a few areas and over the 3-4 years they were bad they leaked oil all over the injectors and made a mess of everything.

[Tomtopdog12]

Problem update, the LIM didn't fix the problem but was a contributing factor.

The problem was the spark plug wires them selves were going bad and made the car misfire like crazy. picked up some napa wires in blue and the problem has since been fixed and the car runs great.