Tim240z Posted April 3, 2004 Share Posted April 3, 2004 OK, so I have had this problem with the engine feeling like it's dropping 2 or more cylinders and spluttering and stumbling, when warm and pulling away from a stop. If I put it in neutral (T56), it will rev fine, as soon as I put it in gear it stumbles again. So far, I have done: New O2 sensors New Opti New H2O pump New intake gaskets (fixed the rear leak) New coil (oem) New Coil Module New plugs and wires I am running full length headers, no cats, no egr equipment (disabled physically and Programming) There are no vacuum leaks that I can find---searched extensively. I have no codes...at least no light.... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 3, 2004 Share Posted April 3, 2004 Plug gap? Maybe try a little smaller... Good luck. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted April 4, 2004 Share Posted April 4, 2004 Well GM's are know for getting their injectors clogged? Maybe you could try and injector flush/EFI cleaning? And I dont mean that stuff that your poor into the fuel tank. I mean take it down to a shop and have them do a "Motor Vac EFI cleaning" of course if your like me, you wont let anyone else touch your car! Oh and make sure you have your spark plug wires routed good. If some cross you could have a cross fire condition but that usually is only a concern if you have old wires. I also kind of had the same problem and it was my IAC motor. I had it wired up wrong. As soon as I fixed that the stumble from idle went away. Is there anyway for you to check your IAC count? Guy Quote Link to comment Share on other sites More sharing options...
strotter Posted April 4, 2004 Share Posted April 4, 2004 I had a similar stumble (felt like it was ready to backfire) on tip-in and whenever I went to WOT quickly. Turned out it was a fuel-pressure problem. I did some rough calculations (given injector size & displacement) that turned out to be *way* too high. Remember that this is for a TBI system, but the calcs said I should be at about 32 psi (very high for a TBI) but eventually I ran it down to 18, less than 2/3 of the calculated value. The hint was that I had run the delta TPS and delta MAP numbers almost to 25% of their factory values (watching the 02 sensor) , which killed my high-RPM transition but made the low-RPM (tip-in) behave properly. The VE tables (and Base Pulse Width, or the equivalent on your system) have such a wide degree of tolerance that it masks some problems under most circumstances, but can't compensate under others - such as at tip in. Do you have a wide-band O2 sensor available? If you do, watch the behavior at low vs. high RPM deltas. Or, just lower your pressure and see what happens - it'll affect your entire tune, though, so you may be starting from scratch. Hope this helps. Quote Link to comment Share on other sites More sharing options...
Tim240z Posted April 4, 2004 Author Share Posted April 4, 2004 I have tested my pressure (34lbs at idle), and have also tested the pressure on the Z28 and have the same pressure. Pulling the vacuum line off the regulator, it jumps up to 45lbs, so I think the pressure is ok. The more I think aboput it, the more I think that some of the injectors are bad.... Tim Quote Link to comment Share on other sites More sharing options...
MrWOT Posted April 6, 2004 Share Posted April 6, 2004 If you have access to an oscilliscope put in on the car and have someone watch the firing line and spark line while you put a load on it. From what you describe, what I understand is that this only happens from a stop, but does it occur at high speed when you put the engine under heavy load? If it doesn't then your injectors are fine. Also, how did you disable your EGR? This kind of thing can also be caused by open EGR valve at idle. Is it blocked off? Your fuel pressure sounds good, 35psi at idle is fine, I would be highly suspect of your 'custom programming' at this point since you have eliminated all of the physical elements of the ignition system, and since you have pressure and don't have missfires at high speed and high load it very much sounds like a problem with injector drivers or ignition tables. I doubt it is a vacuum leak as that would cause high curb idle. Since it only happens in gear it is obviously load dependant, things that register off engine load are MAP sensor and Vacuum advance if you use vac advance. Check the MAP sensor and the computer fuel/spark tables. Most likely you will find your culprit there. Good luck Quote Link to comment Share on other sites More sharing options...
Tim240z Posted April 6, 2004 Author Share Posted April 6, 2004 MrWOT: The EGR is removed and blocked physically on the intake. THe PCM has been reprogrammed (by me with TunerCat) to only activate EGR at 6750 RPM (it cannot be 'switch disabled' by TunerCat, so this is the next best thing. The original MAP sensor was faulty, so I switched it with one from a 4.3 LT1, and tested with Voltmeter and got good readings. (I assume that they are the same unit) Yes, it only seems to occur in 'stop and go' type driving. Cruising on the freeway appears good. No modifications were made to any of the fuel/spark/MAP tables. Tim Quote Link to comment Share on other sites More sharing options...
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