Need TPI help...anyone?

[Hogger67]

First of all i would like to thank everyone for the help i have received here in the past couple weeks. Being new to the Corvette world (after many years looking forward to it) many things are new to me and you have all been a lot of help. Here is my current dilemma. I have a 1976 Vette with a 1991 TPI 350 in the car. The engine is pretty new, only a few thousand miles on it and dressed out to show, but not running up to par. I think it is electrical somewhere but not sure. The car normally runs fine when up to speed (pretty much anything above idle), but idles like crap. The engine seems to hunt when idling, fluctuating from its normal 600-700 rpm down to as low as 450 or so. Sometimes it even drops to a level that it dies. Also, when taking off from a light, it tends to hesitate before taking off, kinda like a cough (for lack of a better description). When the engine is hunting, the ammeter also dips into the negative (discharging) instead of charging, which I am assuming also means a weaker spark contributing to the problem. Everything in the engine compartment is new. I don't know if this is a computer issue or not...thought about a new prom for it but it operates fine under throttle, which it is my understanding where the chip actually works. Could it be a timing issue or something like that. I am at a loss. Any help or tips would be greatly appreciated. When the car is running good, it is very nice. Every once in a while, it runs like there is nothing wrong.

Thank You in advance for any help on the subject.

Michael

 

[patsnitrovette]

Many things can contribute to erratic idle,idle air control could be bad,or just needs to be removed and cleaned,carbon build up is common,vacuum leaks are another one,tps misadjusted,we need more info on what you have in the motor also.

 

[Hogger67]

The car is a MAP sensor speed density TPI. The guy i bought it from said he didn't know the cam specs but that it was very close if not exact to stock. The ECU is set up for mild street. The engine has aluminum heads which shouldn't make much difference, but i think the headers and dual exhaust without catalytic converters might have minor affect, but wouldn't expect it to affect idle conditions. Other reading i have done may indicate that since i don't have an overdrive transmission and therefore the SSV isn't hooked up will probably cause these indications with the motor. Will be checking computer codes here soon. Just got home from work here and have to change out of work clothes and get out there and give it a shot. Don't think vacuum or carbon buildup would be the problem with the engine being as new as it is, but who knows how the previous owner drove it or took care of it.


Thanks again,
Michael

 

[Hogger67]

Update

From the forums I have gotten a lot of good information regarding my issues with my TPI system (described above) and have changed out plugs, rotor and checked plug wires. I also readjusted the TPS which was off a little. The fuel pressure to the injectors is fine and the vacuum on the engine doesn't seem to be the problem. I also changed over to higher octane gas which got rid of the stuttering it was having under acceleration. The engine is running a little better, but now instead of surging or hunting at idle all the time, it seems to only do it now when the ECM gets into closed loop and the engine is warmed up. My best guess of the problem now is the oxygen sensor since that is the main difference between open loop and closed loop from what I understand. Am I pretty close to a good possibility or are there other things i need to consider. I figure an oxygen sensor isn't too expensive to change out in search of the elusive problem. Any help would be greatly appreciated. Thanks all for the help I have already gotten from this board.

 

[patsnitrovette]

Do you have headers and where is the 02 sensor located at?If its down in your collector you might have to switch to a heated 3 wire o2 sensor,i did and it helped alot.

 

[Hogger67]

It is down by the collector and I do have a three wire sensor, but I am not one hundred percent sure the extra two wires are correctly hooked up, will be checking them this weekend.

 

[Vader]

FWIW, a '91 TPI should have the non-adjustable TPS. If you get really stuck, come on over to my place and post your symptoms:
ThirdGen.org

There are a lot ot TPI tuners there.