armedtrigger

I have a really odd issue that is throwing me for a loop. 96 z28 auto. Corvette heads, cam, longtubes, injectors, adjustable regulator, cold air, cat delete, egr and all other crap delete, 3"exhaust, msd coil and maybe the opti, msd wires. Shift kit torque converter. Springs pushrods and rockers. I think that's it.

On hard acceleration over 3/4 throttle once it really hits it's stride it starts to miss/buck first time it happened I thought it was the Rev limiter to give the right idea. Driving normal runs fine. New plugs and a tps. The motor is fresh so I know it's not carbon or anything internal as it does not flutter the vacuum gauge like a valve issue does. Had 40 psi fuel pressure I tried cutting it back to 20 and up to 60 and it's still there. Has new plugs on the rebuild. The computer was tuned by the previous owner the major thing we changed in the rebuild was the cam but it is close to the old one but forget trouble codes it will not store them and resets all monitors to ready on every key cycle so it will pass state.


Any help would be greatly appreciated I would like to get it running right so after break in it can go on the dyno for a fresh tune. I tried searching and it seems some people have sort of the same issue creep up but never posted the fix and didn't want to rehash a several year old thread. Thanks in advance for any help ideas.

Injuneer

Does it hold at least 40psi fuel pressure above 3,500 RPM? If it was tuned for the standard 3 bar (43.5psi) fuel pressure, do NOT alter the fuel pressure with the adjustable FPR. Raising it is going to cause it to run excessively rich in power enrichment (PE) mode (roughly 3/4 throttle and above) possibly washing down the cylinder walls.

Seems like it's acting up in PE mode, and that is very sensitive to the accuracy of the tune. What are the new cam specs vs. the previous?

You need to determine info about the disributor. AC Delco and Delphi are fairly reliable. MSD can have issues. Parts store, ebay cheapies and Chinese ripoffs, and other low cost replacements can be very unreliable.

You indicate the PCM cannot store codes.... but can it set them? Does the SES light flash to indicate the misfires? Never heard of anyone doing such a "tune". What is normally done is to tune emissions codes out in such a way to prevent the codes from setting, without causing the "system not ready" flag to set.

Is the MAF sensor stock, altered ("ported"), or aftermarket? Has it been cleaned?

Minor detail, but the only Corvette heads that were different than the F-Body LT1 heads are the 1996 LT4 heads. Is that what you have? Otherwise, all LT1 heads are identical. Have the heads been ported? Basically, the only difference between the Corvette and F-Body engines is the 4-bolt main blocks used for the Corvette. Otherwise, identical.

armedtrigger

The guy I bought the car from did not say what heads they were just from the same year corvette as the car so they may be lt4. The guy seemed to have done the research as he had all the stuff for the new motor put together to build just had not done it yet. It does not flash the MIL when it acts up however it has come on in the morning twice when it was real cold outside and then went off before I got to work and had no codes stored in it. The first time I checked at lunch and assumed it was because I turned off the car and the second I never shut it off just grabbed the scanner and plugged it in both times no codes. The way the tune is done is if I unhook the battery which it was unhooked to pull the motor we plugged the battery in and check the monitors before even pulling the car off the rack and all were set as completed. Cleared codes to see what it would do and showed all completed so I have no idea what was done to the pm for it to do that. My coworker has verified that it could be driven at WOT before the engine took a dump as he lived across the street from him before the original owner passed last month. I will try and find the cam sheet off the old cam in all the stuff I have lying around that came with the car if you think that will help.

Is there a way to tell an msd opti apart from a Delphi or Delco by looking at it the one on the car is black but don't know if that helps.

armedtrigger

Also MAF is stock as near as I can tell it still has a screen in it and looks like every other mass I have seen and ya first thing I did. I'm a Ford guy so that was the first place I went. Also it was running pretty rich I actually left the fuel pressure turned down at about 35 psi it seemed to run better at partial throttle and idle then at the higher numbers but it didn't help the miss.

Injuneer

Are you measuring the fuel pressure with or without the vacuum compensation line hoooked up.

96 Corvette was available with both LT1 and LT4 heads. Is the intake manifold natural aluminum or red powder coat?

Are you saying the engine ran fine earlier at WOT with the same parts and tune? Unclear when it "took a dump" or what you mean by that.

armedtrigger

With it hooked up. The intake is black do not know what it was originally but it has a aluminum one in the spare parts box. The car had a horrible rod knock when the car was bought had a wrist pin trying to seize up on a couple of pistons. Got new parts for that and he changed the cam while in there. It had a cam in it already but the guy I work with builds race motors in the area said the cam that was in the spare parts the original owner bought to rebuild the motor with was better. All of us at work has messed with car are stumped by it as it ran fine before the rebuild. The other guys have more experience with opti then I but both looked at the opti and said it was dry and seemed to be an aftermarket performance opti. It still has the old tune on it as we figured it would be better to have the motor broke in before letting them dog on it on a dyno but want to at least get it to run close to right before turning it over to them. To be honest it almost feels like an ignition break down but puts out good spark with a spark test but have no way to check it while driving it.

If you lay into it from a stop it won't miss but it's just smoking the tires for freaking ever the best way to make it do it is to get it over about 35 and lay in it as it won't spin the tires for as long before it hooks or to get on the highway then lay into it at 60 it will down shift then start missing as it pulls up the rpm. It will not up shift when it does it but I think that's more of a secondary symptom to the problem then a issue itself as it will shift if a lift off a little and make it stop missing.

The TPS reads from .049 volts all the way to 4.95 volts with out dropping out on the scanner has a nice ramp up and down and the mass voltage did not change after it was cleaned and never seems to drop out either. Didn't think the O2 could cause it as it is ignored at WOT and I don't trust the long term because of the way the computer acts with the tune.

Injuneer

There are only two possible Opti's that may (incorrectly) be considered "performance". The MSD is easily identifiable the red dielectric plastic used on the cap, or black dielectric on a red cap body:





The MSD has problems of it's own - like all Opti's, rotor screws fall out if you don't Loc-Tite them, the MSD has a timing adjustment, that is really only useful if you advance or retard the cam, and there is no "0" reference, so where it's set is a giant crapshoot. They have also had problems with the seal gaskets, and the optical cam position module seems prone to hiccups that cause sudden RPM surges.

Then there's the Dynaspark. Was a reliable product, made a version without the rotor for multi-coil direct fire systems. But when the company changed hands several years ago, quality suffered, and they are no longer considered anything special.

Mass air voltage won't change.... output signal is a variable frequency riding on the voltage.

TPS voltage exceeding 4.90V should set a code.

By somehow causing the PCM to not set codes at all, and not store anything, they have disabled a very useful part of the diagnostic system. There are misfire counts by individual cylinder - that will tell you if its a random breakdown, or cylinder-specific. There are codes for lean or rich operation, MAF reading rationality, etc. Basically, they've cut one arm off and sent the PCM into an MMA fight.

armedtrigger

It is not an msd looked at it it is all black however this issue is going to take a back seat as it decided to just go into neutral every time it tries to go to 3rd gear so that has become the new priority.

Chimera96

OP

the black covered Opti is not a AC Delco or Delphi. Your tune is suspect especially now with a "different" cam.

I would get a new Delphi Opti and a mail order tune based on wtf the motor is now (cam, injectors, TB, and other mods)

you have to many variables now with the tune to even be able to pull codes to point to what a problem may be

set your FP back to stock (43.5)

good idea to have a FP gauge with a hose long enough on it so you can tape the gauge to windshield and watch during "sustained" WOT if it drops...and if it does attend to the Fuel Pump and or filter to resolve.

once you get the mechanical and tune bugs sorted then put it on a dyno if you want

not sure about what you describe as the new won't shift problem

armedtrigger

It's shifts fine from 1 to 2 but when it goes to to 3rd it's like it goes I to neutral seems like these 4l60e transmissions have a 3-4 clutch issue with no prior symptoms. Drove to lunch and it was fine went to go home and 3rd suddenly acts like neutral.

Injuneer

Scan for codes. There are codes for the transmission, but they don't turn the SES light on.