car is a 1986 trans am, 305 tpi. has a aftermarket cam, bigger injectors, was built for a paxton supercharger which is not on the engine anymore.

when i floor the car, it slowly picks up then at about 5-10 mph it will pick up fast. it will not spin tires at all. at idle it runs choppy, im not sure if thats the cam doing it or not. it shakes at idle also. i replaced the plugs 2 months ago, the wires look new, accell supercoil is new.

wierd part is, i guessed the tps voltage before, and it burned out fine and was fine off the line. after i fixed the iac and set the tps to fac specs with a code scanner, it will no longer burnout. the tires will both spin, but it wont go above 2000RPM and it will just sit there and spin slowly. im thinking either a missfire or a bad sensor, but im not sure where to start.

one more thing, when i floor it from a moving start (about 10-15mph) it will pick up fine and launch the car.

 

How much vacuum are you pulling in at idle? How big is the cam. Most cams with a bigger then stock profile will "shake" a little unless you have a really dead on tune. Even then, it can't be helped if you go too big. Thats really weird, I have the opposite problem, my car spins all of 1st and into second, I have never once been able to hook up. But after that, its real slow, lol. Hope you get it all figured out!

Will

 

yep sounds like a vacume problem to me too. When i was a newbee to the TPI motor i tore it down to a cam and intake on it. Well when i was finshed it did the exact same thing as yours. It took me a copple of hours to fiqure what i did wrong, turned out to be i hooked the breather hose to the wrong port on the throttle body. lol it ran like sh*t. oh well lesson learned.

 

i was thinking it was a vacuum problem. where should i start to check for leaks? i found one leak when i first got the car, it was on the left side of the tpi, right under the TPS sensor. there are 3 places that looks like a vacuum hose should be connected. the first is capped off, because i have no emissions stuff. the other 2 are still open, but i dont feel any air sucking through them. should they still be capped?

also: when i put it from park to drive, it likes to rev down to about 200RPM sometimes, then jumps back up. sometimes it will just stall

 

I would start off by buying a can of brake cleaner. Start up your engine and spray all around the vacum lines, and intake gaskets. If the motor revs up when you are spraying the cleaner thats a leak.

 

You can't have those inlets on your TB open. The top one goes to a breather on your passenger side valve cover. I would suggest hooking it back up that way. The secound inlet is for smog, cap it if you are not running smog stuff.

 

Sounds like it might be timing related, either its a tad quick or to slow. Advance the timing and see if the idle smooths out. Some people will keep it a tad retared so its sounds rumpity. You may also want to look into what type of cam you have, some SC cams are ground to run good only under boost but usually not to the point where idle is effected since there isnt usually boost @ idle.

 

My 88' IROC TPI throttle body has two ports on the TPS side and one underneath.

Side ports:
Top - to passenger side valve cover.
Bottom - to gas vapor recirculation canister; it affects gas mileage, how much I don't know.

Underneath:
To EGR Solenoid, and valve on vacuum line to bottom side port.

 

i checked for vacuum leaks yesterday, couldnt find any. for the first open spot on side of the tpi, im not sure where on the passenger valve cover to hook it up to. i have a breather oil cap, and it connects from the pass side valve cover, to the driver side. i capped it off for now. on the second open spot i hooked up one of the vacuum lines to it. that stopped the leak there, but still same motor problems. i noticed that if im under the hood and i hold the throttle open, the rpms increase without me moving it. i will hold it steady at about 2000 rpm, then it will increase on its own. i tried unplugging the tps, and it still did this. the engine visibly shakes at idle in drive (700rpm).

i have no idea what kind of cam it has, the guy never told me anything about it. all that i know is it was built for the paxton supercharger setup.

 

well i think i made some progress. i found 2 vacuum leaks. one was under the tpi, a red plastic hose that was broken. i capped that off. the other one was directly under the iac. same kind of red hose, one part was just cut and the other one was plugged into the bottom of the throttle body where the intake connects. this stopped my shaking at idle, and its raining out so i cant really test the hesitiation off the line. i will update later when its dry out.

 

Guaranteed your car will run much better now. Should all but eliminate that off-idle issue. Probably got rid of your "park-drive stall/RPM dive". The RPMs should drop a few hundred RPM when going into drive, but it shouldn't go below 500RPM or so.

 

yea the car runs better, still slow off the line though and will not burnout. the shaking at idle is alot better, but still happens sometimes. any other places i should start to look for more problems? btw, thanks for all the help so far.

 

Dirty IAC or mis-adjusted TPS?

 

I had shaking at idle, wasn't the engine though, it was the motor mounts, installed energy suspension Polys, and the shakes were gone.