• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

98 GTP popping/missfire/backfire wile under any boost! HELP!

B

Bom Ber

New member
Ka so heres the story...
I bought a 98 gtp from an auto wrecker needing an engine, had blown rods...
only replaced the block using a N/A Block with the supercharged top.
Ran like a CHAMP for a few months now im having issues i cannot figure out!!

it gradualy started doing this and got worse as time went

runs and drives smooth and beautiful under normal driving conditions, but the second i get any boost into it i start bucking/popping/missfiring/backfiring... about as bad as it can get....

ive replaced spark plugs, plug wires, all 3 coil packs, the ignion chip under coil packs even..
never changed a thing.. THEN it killed the crank sensor.. upon replacing that the car ran eff'n amazing then bucked out and died, *belts cought the wiring for crank/cam sensors,* replaced that and now back to the issue again.. :s

recently removed supercharger belt, and the car runs great/smooth fullthrottle and all.... no issue untell that supercharger belt is put back on... WTF could this be????

there is no codes, i do not have a cat converter, and the o2 sensors have been deleted from the ecm...

im stumped and sick of waisting money on this thing, tempted to send it right back to that wrecker if this issue doesnt get fixed soon :(

i was thinking either one of the knock sensors? waste gate? or possibly a weak fuel pump?

??????
 

Attachments

  • bomber gtp.jpg
    bomber gtp.jpg
    100.5 KB · Views: 19


So it has no o2's at all?

or just the rear is deleted??

How can is determine fuel if both are gone?

And, so basically you are top swapped on a stock GTP tune made for a lower compression block, there might be some problems there.
 
sorry yea just the rear o2 was deleted, and yea did a bottom end swap... but it ran like a charm/champ for the first few month *yea i deffitnly use the power of that supercharger alot* and it just slowly developed this issue started out very slight and random, to constant and cant even hit half throttle..

without the supercharger belt on its smooth runs perfect no popping... so i dont beleive its any issue with the block or the heads.. its gotta be a sensor or something.. :s
 
You realize you could blow your car up very easily with having a high compression bottom end with a sc top end? You need tuned and headers for sure.
 
I'd bet that thing is full of KR when in boost. Get it scanned before you blow it up or you will be putting another longblock in it. It's probably pulling timing to try and keep from destroying the pistons, which could have already occurred. I'd leave the S/C belt off for now and be gentle on it.
 


Well, to add to this issue, i guess i should have mentioned that when i first start it, and wile its still cold, i have full power, no lag. no popping nothing, up untell the engine gets a tiny bit of heat is when it starts. the colder the car is the better it runs :s id assume a piston is bad but its not burning oil, it makes zero nocking, and runs amazing without that belt :s !!! and as for the comp. previous owner has some kind of tuning to it, id assume he did it on his own computer at home, rev limiter hits at 4000rpms. for 2 hits then jumps to 6000rpms... i dont think thats right at all, the odd time it wont go past 4000rpms *only in park or nuetral though* and not to forget the rear o2 sensor was deleted. so maybe i should try a new computer? but being a 1998 GTP is it not 1 year only for comps?
 
Well, to add to this issue, i guess i should have mentioned that when i first start it, and wile its still cold, i have full power, no lag. no popping nothing, up untell the engine gets a tiny bit of heat is when it starts. the colder the car is the better it runs :s id assume a piston is bad but its not burning oil, it makes zero nocking, and runs amazing without that belt :s !!!  and as for the comp. previous owner has some kind of tuning to it, id assume he did it on his own computer at home, rev limiter hits at 4000rpms. for 2 hits then jumps to 6000rpms... i dont think thats right at all, the odd time it wont go past 4000rpms *only in park or nuetral though* and not to forget the rear o2 sensor was deleted. so maybe i should try a new computer?  but being a 1998 GTP is it not 1 year only for comps?
 
that's because the knock sensors don't turn on til it gets to 160* then your start pulling timing to keep from blowing your motor.
 
Hmm... well i changed the one front knock sensor today and it didnt change anything.. ill try changing out the rear one tomorrow. hopefully it works if not then next quest is to find a computer for it..
 
You have to torque them to a certain torque or they don't read right i think it's like 14 ft lbs or something like that.
 


Bom,

Changing the knock sensors wont do anything and the rear one is near impossible to reach without pulling the motor.

You have a computer/ high compression, boosted car on a stock tune problem.
 
yea i kno they need to be tourqed, and yea im aware of what the car has for tune. its the fact that when it was put together it ran drove and performed pefectly... one would think if its a tuning issue then it would have been issues like this from day one.. not? cause since it was put together i have changed nothing mechanically, *except brakes but that dont count* ha!
 
Why not pick up a scan gauge and see what its doing? Thats about the only way your gonna figure it out.
 
Chipped pistons are in his future if he doesnt park it. man listen dont be stubborn you got some issues bro. Like stated above you need to see what going on , get a scanner.
 
yea i cant just park it tho, need to get around, i just dont rough house it right now. but ka, what scanner do i need, and what should i be checking for?

and as a curiosity i wanna change the pcm.. anyone kno what years/models will fit? cause i can get a pcm from a 99 supercharged buick, wondering if thats the same or not..!! ?
Im gonna try locate a borescope and check out the pistons make sure they are not damaged aswell!
 


If you have an android phone you can download torque and get a bluetooth to obd2 adapter. If not you could buy an aeroforce gauge. You need to get your pcm tuned not just get a different one.
 
yea, it does need to be tuned. but the point of matter is the pcm thats in there ran great with this engine for 5 months, and just slowly crapped out *if thats the issue* so i want to try just anouther stock pcm and see if that smooths it out first. if one stock pcm worked then anouther one will aswell*even if just a short period* aslong as it conferms my issue, i dont want to spend $200 on a tuned pcm if thats not my problem. and yes actually i do have an andriod phone. what app(s) would i need lol
 
the app is called torque and you can get an obd2 to bluetooth adapter of ebay or amazon or something.
 
any videos on how to do the intake on a series 2 gtp and valve covers... used to have all data pro now I rely on my friend to get me print outs
and he's not helping
 
You must log in or register to reply here.
Back
Top