Well here's the situation. My 94 z28 is still having problems. It will run for a bout 10 minutes of road driving and then start fading out. I say of road driving because it seems it will idle longer than it will actually run on the road. By fading out i mean it will run strong for about ten minutes and then it starts cutting out eventually dying. After dying it won't start back right away but will start back some time later say an hour or so.

I have replaced the optispark, module, and coil recently. Also i have replaced the plugs and wires, although i dont suspect those. And a new alternator was also installed.

I took it to the nearest GM dealership to have it checked out and they said they could get the right reading from the computer, saying it had been modified. But they suggested that the computer was flooding out the engine with fuel. Their technician said it needed different O2 sensors than i have.

Any help with this problem would be helpful. Just a point in the right direction with be of great appreciation!

 

Get a manual fuel pressure gage and tape it to the windshield. Take and drive it till it acts up then notice your fuel pressure. If it's below 35/37lbs it is the pump.

 

what he said.. fuel pump is a possibility, but due to the whole only doing it in closed loop thing, it is also possible that your o2 sensors are actually going bad.. at least one of them.. or their wiring.. not that dealerships are great, but o2s are a logical thing to check (not that you need a different kind, but maybe just that the ones u have went bad). i see that you changed everything else, but you don't seem to have taken them seriously on that one?

i recommend checking your fuel injectors, fuel pressure and o2 sensors. if it is a single fuel injector that is clogged or something, that could be the most likely cause for the symptoms your seeing... when the car brought the o2s into the loop, they'd detect that a bank was more lean than it should be (because that one cyl was really lean), and they'd try to add more fuel.. this would make all the other banks flood with gas, without helping the lean condition at all (because if an injector isn't firing, the air from that cyl isn't gonna be burned no matter how much fuel is in all the other cyls)... thus eventually the computer hypercorrects the bank so much that it runs like crap (on a v8 engine, 1 cyl not firing anyway cause it has no gas + 3 others flooded with gas = almost no power from that bank = a 4 cyl engine dragging the 4 other dead cyls along for the ride)..

injectors would be my bet

 

note that a search for something like car running like crap after 10 minutes (or more accurately: running like crap in closed loop... actually u could prolly search for: [closed loop {bad OR evil OR crap OR hesitation OR stuble etc etc etc}]) would have probably brought up the answer i just gave you somewhere, as I've given it out a few times before to a few people who were experiencing very similar problems to what you are experiencing... try to search first ask later in the future

 

this similar problem happened to me on my 94'. It ran 100% until about 7 min in , after that it ran like it was on 3 cylinders. let it cool down and then it would run perfect for the same amount of time. I noticed i after it started cutting out, i.e. closed loop, i was losing the high resolution pulse from the optispark while the low resolution pulse was fine. thats why it would still run, but act like the timing was off. removed the BRAND NEW AC delco optispark and opened it up to find that the silver timing disc had overheated and was warped and eventually melted a part of the laser. replaced the faulty opti and has run perfect ever since.

 

but you threw a code.. i think he mentioned that he didn't throw any opti codes.. if he does throw dtc 36.. he should check/replace his opti.

 

Thanks for the help. The thing is i recently just replaced the O2 sensors so is could possibly be the wiring. But yeah i didnt throw any codes. How would i go about checking the injectors?

 

I had something similar and i changed the ICM and that cleared it right up.

 

Were I in your shoes, the first thing I would do is order a copy of TTS Datamaster and a cable. All's you can do is throw parts at it and hope for the best unless you can see what the computer sees and logically troubleshoot from there. Could be fuel pump, could be injectors, could be opti, could be O2s, etc., etc., etc. Too many possibilities that each require lots of time and parts if you try a process of elimination approach...

 

That is pretty much my situation. My uncle's mechanic has the newest version of Snap-On's scan tool. Can this be of any help to me?

 

you can use the scan tool and monitor the o2 sensor voltage...... it should read anywhere from 1mV-1000mV. if it reads only from 3-400mV to 6-700mV then its a good chance its bad.

 

No, like I said, he can test the injectors, which are more logical than the fuel pump, for the aforementioned reasons. Having a scan tool won't help him diagnose the problem if it is the injectors, either. Scan tools are great, but they are only one of many parts of a logical diagnostic process.

Anyway, do a search for testing fuel injectors, because I don't feel like re-writing the howto on that right now, as it is late. It won't be very hard, pulling the fuel rail is easy as pie on our cars.

As for testing the O2 sensors, if they are good, and your car is running correctly, in closed loop, they will oscillate between low voltage and higher voltage, (as the A/F ratio goes above and below stoich). Since your A/F ratio may be pegged at rich or at lean, it may be harder to tell whether or not they are working correctly. When unplugged, I believe they read something near 450 on my 95. One way to test whether or not they work is to record the readings for each side, then swap the sensors, and re-record the readings. Assuming that your A/F ratio stays fairly constant, and is different on each bank, you should be able to see if they both read nearly the same thing. If they both read something clearly wrong, no matter which bank they're on, that'd be a good sign they're bad. There are still other ways to tell... like if they read 0, they'd be bad.. You should also be able to search on diagnostic symptoms of bad o2 sensors.

Anyway, happy diagnosing, let us know how your fuel injectors are. And or if you find anything else out.

 

Well my uncle's mechanic said this diagnostic tool was able to test the injectors. I dunno he just mentioned that when he looked at it before but i will get it scanned monday since he is gone for the weekend. Another question, as of the moment im running open headers since the car was much easier to work on with them this way, how much is this hurting me?

 

open headers helps performance.

noids are the lights he's talking about. they just tell you if your injectors are working electrically, not if they are working mechanically.

 

Ok just checking.