OK I have an 88 IROC with a 355 TPI, only engine change is 1.6 roller rockers and 10:1 flat top's, I noticed last month that at highway cruising speeds my car would feel like its kinda stumbling n wants to die, n at traffic lights my car would suddenly die for no reason at all. I checked ECM error codes n it gave me ignition module, so I replaced module, put in MSD coil, new cap, new rotor. then the counterweight on the flex plate decided to fly off 2 weeks ago n the car was vibrating like crazy, didnt realize it was flex plate so went ahead n got my transmission rebuilt, which I was planning on doing soon anyway, and replaced torque converter with a 2300-2500 that footbrake's at exactly 2400. Realized it was the flex plate afterall so replaced it with a HD from Dacco.

Anyway cut to the point - now when I floor it, it revs fine through 1st gear, then in 2nd gear at around 3000rpm it bogs down, my transmission guy says it feels kinda like its missing but why only at 3000rpm? then resumes just fine after like 3200rpm, it also does it at other RPM's sometimes but its dead on at 3000rpm every time. I have a new set of plug wires sitting here that'll go on tomorrow n I'll also replace the spark plugs, but I dunno....

I thought it was the MAF cuz I was gettin MAF errors but I tried my friends MAF n that didnt help, so put mine back on and adjusted the TPS to 0.54 n now I have no ECM errors!

It's startin to bug the crap outta me, at first I thought the tranny was tryin to upshift then downshift or somethin but it's not a tranny issue as far as we can tell, any advice? I'm not running too lean, my fuel filter is 1 day old, my fuel pressure is ok, my engine runs at around 200 degrees, my smog system is entirely removed (shh), so I'm stumped.

 

If you rev fine through first gear and the stumble occurs in second gear.... Possibly the transmission.

Does it stumble in third or fourth gear? And, its not possible that the torque converter lock-up is trying to engage/disengage? You positive that the torque-converter is a lock-up one?

I'm not trying to call you a newbie, just wanna be sure you got yer ducks in order. I'm also thinking about the MSD coil you installed. I would try replacing with the old coil and see if the problem still occurs. I've seen way too many bad "NEW" parts to ever cancel them out on trust alone. I got a few more ideas, but lemme see your answer to them questions first.

 

Is this the only time it happens? In second gear @ 3000?
I find that extremely odd..
The only thing I can think of is a dead spot in your fuel map, or some other computer glich..
You need to monitor fuel pressure while this happens. Can you tape a fuel gage on the windshield?
Do you have access to a scanner? Can you take data on a run?

 

Better yet, if you have a scanner you can read the info and see what it is doing at that point. Do tape the FP gauge to the glass so you can do the run and see the pressure WHILE you are running which is going to tell us if the fuel pressure retulator is shot

 

I was considering that it may be the torque converter lockup trying to engage, but I haven't had a chance to check it out yet, I'll call Dacco Detroit in the morning to ask if its a lockup converter, I don't really care about lockup too much, but I was thinking the same thing last night.

As for fuel pressure, I'm borrowing a friends fuel pressure gauge tomorrow and mount it to my hood to see whats going on there, the fuel pressure in neutral is ok but obviously under load there's gonna be more variables.

I do have a basic scanner myself, my BLM's and INT's are all normal since I replaced the fuel filter (had a very lean idle before!) but I'll use my friends scanner tomorrow since it allows you to monitor more than one thing at the same time.

At 3000rpm in 3rd gear my accellaration is slow enough to not notice a severe stumble so if there is any problem in other gears, then I won't notice it in 3rd. I think I felt a slight bog in 1st gear earlier but I'm not too sure, I just took it for a quick test drive after changing plug wires which were long overdue anyway, but made no difference to the stumble situation.

As for the coil - I'll try the stock one to see if it goes away but I had in fact replaced the stock coil with the MSD coil BECAUSE I was having issues with the car wanting to "slow down" n bog under interstate cruise speeds. I also had an Ignition Control Module error code thrown up on the ECM so I replaced it with a stock replacement and the code went away.

I'm gonna disconnect the lockup switch later to make sure that's not the problem - then tomorrow I'll borrow my friends fuel pressure gauge n see whats happening there, and also gonna replace spark plugs since they need to be done anyway.

I find it weird also that its always at the same RPM, always right around 3000rpm, bogs down, stumbles, then picks up again. I'll keep you posted.

 

Your best bet is to give us RAW data. As in, the actual DTCs and scantool data in raw form - like the actual BLMs and INTs as well as knock counts, knock retard, etc etc when the stumble happens. Sounds like it may possibly be getting some knock (false or true) and pulling timing out for that instance. What octane fuel are you using? Does it have headers? Anything else that may cause false knock? Even roller rockers can make racket that the knock sensor circuit will pick up.......

 

Knock count was around 6 if I remember correctly, with no knock retard, BLM's and INT are around 126-130 usually around 128. No headers, does have 1.6 rockers. I use 92-93 octane gas depending whats available at the station im at.

 

I installed an exhaust cut out on weds n was headed to the track when I noticed that in overdrive, the exhaust sounded weird till i gave it a lil gas, then realized it was the lockup coming on and off intermitently, then thought, hmmm, if I floor it - then at what speed does my stumble occur? and at EXACTLY 40mph, the car stumbled at WOT, got to the track n crawled under the car n cooked my hand disconnecting the lockup switch, n voila, no more stumble!! So turns out my TCC is all screwed up n now my car's fine. I thought it may be lockup but ofcourse it had to be the last thing on my list to try - the only other thing I could think of was that my fuel pump was going out or something!