blkcamaro_z28

My car is a 97 Z28 LT1 with a OBD1 conversion, it has headers, no cats corsa exhaust, 373 gears, a tune from madz28, new plugs, new wires, but when I start and drive the car EVERYTHING is perfect couldn't ask for any better. Even when I get on it everything is fine until I get to around 4800 - 5000 RPMS and then it starts to cut in and out in and out and does this until around 5700 RPMS which is my shift point. The car does this in every gear and the tune is new but on the previous tune by pcmforless.com it also did this. Any help on what this might be? I was thinking maybe the stock valve springs might not be able to hold up, or maybe the spark plugs need to be rechecked. I'm clueless on this so far. Thanks Chris

coco95z28

my car is doing this too... its also stock (just bolt ons)

runs great till 5k rpm then starts sputtering and choking like crazy...
kinda feels like its the rev limiter)


i was about to do the opti and plugs and wires. car has 130 k now... (had car for the last 40k miles and who knows the last time they were replaced)

Fatdog2

Coil wire?Headers?O2 sensors ain't gettin hot enough maybe.My02.(somebody mentioned o2 heaters before,but I dunno.)

blkcamaro_z28

Well I have looked at A LOT of post on here trying to figure it out, I put a Granatelli MAF sensor on a few months ago and don't know if it was doing it before that, so I just cleaned my old one and I am gonna try putting it back on and give that a shot. If that doesn't work I have also heard the idle tensioner can ware out and the belt slipping at higher RPMs. I am gonna try those 2 things first and then report back. I will have to check the 02 sensors, anyone no how to check those to see if they are working properly?

blkcamaro_z28

Ok so I just changed my MAF and still no good. Guess I need to try something different lol the plugs should be good as I just put them in around 3 months ago (AC Delco) and the wires are fine with no burns or arching(MSD 8mm). I had my opti go out a few months ago and put in a used one and have had no problems there? I'm not to sure where to look next. I know my fuel pressure is good at start and idle and when I give it gas it is about 44 psi consistently. Any other suggestions?

coco95z28

u sure the opti is good? thats what im putting in next weekend. ill let u know

LearJet

opti.

charliediss

Its not throwing any codes? It sounds like you have a high RPM miss, and under wide open throttle conditions your computer no longer accepts input from your MAF or 02 sensors...so you can pretty much rule that out after you do a WOT test. Fuel is good at startup and normal driving conditions but it could be insufficient at high RPM, very possible. Did you replace your opti with an OEM unit or MSD? If not, it could be a failure...is it indexed properly? This could even be mechanical like a fatigued valve spring or a burnt valve. Really should do a leakdown compression test. Good luck.

Charlie

blkcamaro_z28

It never has a engine light come on during any of this or anytime??? I put a used OE 80,000 mile vented opti in it I bought from a well known seller on this forum who has positive feedback, so I don't think it is that (although I can't rule that out), I dunno bout the whole indexing thing though? Took out my old one and put in the new one the only way it would go in, the long pin on the camshaft had to fit inside the distributor for it to go in. I have also heard about spark plug blow out at high RPMs so I am letting my car cool off now and I am gonna check the gap in the AC Delco replacement plugs and see whats going on there. Other then the plugs I am gonnna try cleaning them with a brass brush and make sure the gap is good if not those then maybe going to get a reman'd belt tensioner to make sure the belt isn't slipping. Any other suggestions other then the opti or how can I tell if it would be the opti? or how can I read my fuel pressure at wot under load? I hate not knowing what I need to fix!!! lol

blkcamaro_z28

Ok so I just checked my plugs and wires again and everything is good, plugs are light grayish gapped around 51-52. I did notice that my injector harness 1 and 3 were swapped, the harness's are all numbered 1-8 and it was 3,1,5,7 drivers side and passenger side was 2,4,6,8 so I swapped the 1 and 3 out and took it for a spin and still no good. It runs perfect all the way till around 4800 then starts to breaks up all the way to around 5800 but keeps pulling during the stutter then shifts. I have no idea whats going on????? My car gets great gas mileage and runs great (except the stutter) so I don't think it is the O2's but I dunno. Maybe it is the valve springs, I am guessing they have never been replaced but I have no way of knowing.

Injuneer

The PCM uses the mass air data from the MAF sensor at WOT. Why would it ignore it and default to speed-density? It also uses the long term fuel corrections (if they are adding fuel) from cell 15 at WOT, and those corrections are developed based on O2 sensor readings at part throttle, so a faulty O2 sensor can affect WOT performance. The primary difference at WOT is that the PCM can no longer use the short term corrections because the O2 sensor data is not useable.

If his Opti (97/vented) was indexed incorrectly, it would be off by 120 cam degrees = 240 crank degrees. Engine wouldn't even run. Its not like the unvented/slpined Opti that can be off by as little as 20 cam degrees.

Have you actually scanned it? Not all codes turn on the SES light. Does your SES light come on for a few seconds when you turn the key to on?

Have you actually measured the fuel presure at WOT, as the engine is misfiring? Sitting in the driveway and "giving it the gas" isn't going to prove anything.

blkcamaro_z28

I have scanned it with pcm comm and free scan and it shows no trouble codes. If I turn the key to the on position the engine light does come on and stay on but the check gauges light does also because of the battery level being low I believe. I am downloading TTS Datamaster right now and I am gonna hook that up and see what it tells me. I know that it is only doing this under load so I can't rely on the fuel pressure in the driveway but I don't have a way to check it while driving. I had it checked at a local shop and there line isn't long enough the reach inside the car while driving. Are there any other ways to scan my car with the OBD2 port with a OBD1 computer installed I have the Exteme ALDL connector from moates.net that connects my OBD2 port to my USB on my laptop.

blkcamaro_z28

So I just did a run using Data Master and when the problem starts my INJ DC is going into the red hitting a max of 90.1 for the Left and a 90.5 for the Right (I know it is Computed Injector Duty Cycle, Percent @ RPM, Left and Right at least thats what the Data Master says) could that be causing my problem?. Also my spark retard hits a max of 4.0 during the same time. The L TermCounts and the S TermCounts are in the red for most of the time when just cruising but not to far under 128, they hold steady at 128 during the problem. I have the run saved and if anyone knows how to read this I will send it to you. Please give me some advice.

Injuneer

If you can post the DataMaster file on an ftp website, I'll take a look at it.

I'm not sure how reliable the DataMaster "duty cycle" calculation is. I know how it is calculated the DC, but not sure what the "BPW" pulse width number really represents - whether that includes any adjustments to the pulse width the PCM is making (e.g. - BLM's) or not. A lot of the data logs I review show the DC exceeding 100%, and the owners report that the engine shows no sign of running out of fuel.

You don't want a fuel test pressure gauge with a hose long enough to get the gauge inside the car, because putting a mechanical fuel gauge inside the passenger car is extremely unsafe, and not recommended (unless an isolator is used). You just need to be able to tape the gauge to the outside of the windshield.

z28beast

injuneer are you telling me that my unvented msd opti might be off? i replaced the timing set and just slid the new opti on....as you know it only goes in one way....how can it be up to 20*'s off? please enlighten me because now im curios. these lt1 are some crazy motors...also does the cam pin drive the opti rotor on the newer opti's? because my opti had no opening in the back of it