Bartman Posted October 3, 2006 Share Posted October 3, 2006 On the way home from the JCCS on Saturday my engine died on the freeway. I went to switch on the air and the engine died on me (I knew it might, because the idle speed with the A/C on is still too low), but I expected it to start again afterwords. Well it wouldn't start, and I wound up getting it towed home. I thought it would be something simple, but I still haven't figured out the problem. It will crank, but it won't fire. It's getting fuel and the Check Engine light is on when the ignition is on and when it cranks. I hooked up DataMaster to try and see what's going on and it can establish communication with the PCM when the ignition is on, but as soon as it cranks it loses communication with it. As soon as you stop cranking, DataMaster can again communicate with the PCM. I had a similar problem when I first wired in my PCM using an ignition hot wire that wasn't hot when cranking, but I fixed that a long time ago. I checked the iginition hot wire going to my PCM last night and it is still hot while cranking the engine. I'm in the process of checking all my wiring, but I thought I would see if anyone has any ideas of what could be happening. 1 Quote Link to comment Share on other sites More sharing options...
mark Posted October 3, 2006 Share Posted October 3, 2006 Do you have spark? I would imagine not but check and see. Trace all your grounds for the PCM and make sure they are OK. Is the AC wired using the PCM or is it independant of the PCM? 1 Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 3, 2006 Author Share Posted October 3, 2006 Do you have spark? I would imagine not but check and see. Trace all your grounds for the PCM and make sure they are OK. Is the AC wired using the PCM or is it independant of the PCM? I didn't check spark since the PCM doesn't have power while I'm cranking......but I will verify. I'm currently tracing through all my PCM wiring, since that seems to be a likely area for a problem. My AC is wired independant of the PCM. Quote Link to comment Share on other sites More sharing options...
mark Posted October 3, 2006 Share Posted October 3, 2006 Any time the lt-1 won't start I always think optispark. I've had several issues with mine and it gave no warning at all, just died. I've also had wiring problems from my hacked harness (I did it). Good luck, tell us what it is when you find it. Quote Link to comment Share on other sites More sharing options...
that240guy Posted October 4, 2006 Share Posted October 4, 2006 You have fuel pressure or are your injectors firing? If the check engine light is on while cranking, then your pcm is on. Also, it is normal for there to not be any live communication while cranking. I would also say optispark. Find out if your injectors are firing. If the injectors are firing, the optispark is probably okay. I say this b/c the opti controls the fuel injection timing/firing on all 94 & up lt1s. Modules can also be the culprit. The opti and module cannot be diagnosed seperately and I always replace both of them at the same time. Normally I would immediately say opti, but since the wiring harness is modified and the car died right when you turned the air on, it's probably something related to your wiring. But then again the opti is the culprit of 99% of all LT1 issues. 1 Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 4, 2006 Share Posted October 4, 2006 I would check fuses and run extra grounds from the engine to battery. I have seen some weired electrical stuff cause by poor grounds or not enough grounds. Also did you turn your a/c off and try starting it? Guy 1 Quote Link to comment Share on other sites More sharing options...
jbc3 Posted October 4, 2006 Share Posted October 4, 2006 Bart, Electrical problems suck! I don't have much to add as far as specifics, but I would look at the A/C and PCM power (constant and switched) and ground connections as well as all the fuses. I usually look at the Opti last when I hear about sudden failures. I know it can fail, even self destruct, but you usually get clues that it is on it's way out. Don't forget to check the fusable links. The SES light will stay on when the key is on and during cranking. It turns off just after the engine fires. Good luck, Jody 1 Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 4, 2006 Share Posted October 4, 2006 Bart, This might be a long shot but you might pull the PCM and have it looked at or try getting a spare unit and see if the engine fires up. For some reason I thought if the PCM dies it would go into limp mode just to go several miles but I might be wrong on that. Danno74Z 1 Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 4, 2006 Author Share Posted October 4, 2006 Thanks for all the information. It looks like DataMaster losing the connection to the PCM while cranking is normal, and not a sign of a problem. I checked my wiring to the PCM and everything looks good (constant power, switched power, and I've already run all my PCM grounds to the battery and have a good grounding strap from the engine to the chassis). The A/C has been off since the problem first started. I also checked for spark at the plug and at the coil, and although I do have spark it doesn't seem to be very strong. More information: The morning that this problem started, the CEL was on and I didn't have time to check for codes before the car show, so I don't know what code was being thrown. It ran pretty good, but it did buck a little bit at very low RPM conditions while driving in traffic. After it died I couldn't hardly even get it to crank at all. I had my cables with me and I got someone to stop and give me a jump. It would crank and it backfired but it would not start. It seemed like the timing was off or something else electrical related. When you try and start it and stop cranking the engine will continue to turn over for a short time, but very slowly (almost like it is dieseling). When I rebuilt and upgraded the top end I also upgraded to a vented opti, but I put in a used one that I got from eBay. So it could be an opti problem, but I don't want to just throw parts at the problem. If I can isolate the problem as the opti I have no problems buying a new one. Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 4, 2006 Author Share Posted October 4, 2006 I'm going to try this test tonight and see what results I get: http://shbox.com/1/4th_gen_tech2.html#opti_test Quote Link to comment Share on other sites More sharing options...
mark Posted October 4, 2006 Share Posted October 4, 2006 My money is on the opti spark. Hope you find the problem. Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 5, 2006 Author Share Posted October 5, 2006 Well surprisingly enough, my opti and spark checked out okay with the shbox tests. I'm kinda stumped now. I still think the spark looks a little on the weak side...any suggestions? Quote Link to comment Share on other sites More sharing options...
mark Posted October 5, 2006 Share Posted October 5, 2006 If the car has spark and fuel, not many PCM things will keep it from at least starting. I've disconnected most of my sensors and the car will at least start. Not many electrical problems with the pcm will cause a non start issue. Disconnect a sensor or two and see if you get a code. Any chance you have mechanical problems? Compression OK? My money is still on the optispark. Quote Link to comment Share on other sites More sharing options...
gexgexgexgex Posted October 5, 2006 Share Posted October 5, 2006 i know this kinda sound dumb but when my 94 trans am was having a similar problem starting all it was was a bad battery. So maybe check and make sure you have a strong battery, these lt1 motors do not like to start with a bad battery. Another easy problem i had was spark plug wires, it gets very hot in the engine bay and the wires and even the couls just get so hot they break, well atleast on my TA. Hope this helps Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 5, 2006 Author Share Posted October 5, 2006 If the car has spark and fuel, not many PCM things will keep it from at least starting. I've disconnected most of my sensors and the car will at least start. Not many electrical problems with the pcm will cause a non start issue. Disconnect a sensor or two and see if you get a code. Any chance you have mechanical problems? Compression OK? My money is still on the optispark. The sytem isn't throwing any codes right now, but can I get it to throw any without first starting? Can the opti still be bad even though it passed the shbox tests? I'm not convinced that its getting a strong enough spark, so I still need to figure out how I can test that.i know this kinda sound dumb but when my 94 trans am was having a similar problem starting all it was was a bad battery. So maybe check and make sure you have a strong battery, these lt1 motors do not like to start with a bad battery. Another easy problem i had was spark plug wires, it gets very hot in the engine bay and the wires and even the couls just get so hot they break, well atleast on my TA. Hope this helps The battery didn't work all that well when it first broke down, that's why I had to get a jump (which is strange, because it was working fine until then). I've been jumping it while I'm trying to diagnose the problem as well. The wires are fairly new as well, but you never know. Quote Link to comment Share on other sites More sharing options...
mark Posted October 5, 2006 Share Posted October 5, 2006 Don't jump it. Put a battery in it even if you have to swap out one from another car. The PCM will not let the car start unless it sees a certaian voltage, even if the car will crank. This is on my 93 PCM. I assume yours is the same. gex may be on the right track. If a known good battery will not start it, get an opti. It never hurts to have a spare. Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 5, 2006 Share Posted October 5, 2006 Had a web site problem and I think you touched on the fact you don't have any codes now but something must be stored in the PCM when the engine last failed. Can you pull anything from the PCM? Bart, You said: The morning that this problem started, the CEL was on and I didn't have time to check for codes before the car show, so I don't know what code was being thrown. It ran pretty good, but it did buck a little bit at very low RPM conditions while driving in traffic. A trouble code should be stored. It is normal for the CEL to be on then go out after the engine starts. Even with the engine off or not working you should be able to decode the PCM DTC. Something to try. A defective MAF sensor will prevent a no start condition. Disconnect the MAF sensor. The PCM will default to speed density and the engine should start IF the MAF is bad. Danno74Z Quote Link to comment Share on other sites More sharing options...
mark Posted October 5, 2006 Share Posted October 5, 2006 Danno, Good point, Mine is speed density and I didn't think about the MAF. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 6, 2006 Share Posted October 6, 2006 Usually a MAF going bad will give you some kind of warning, like running rough at certain RPMS. That's what I've seen anyways. I would try the SD mode like stated above too. Does your LT1 have a crank sensor? I dont think they put them in till 96' but thats a possible cause of a no start. How do you know your getting fuel? Not fuel pressure but actual fuel out of the injectors.... Guy Quote Link to comment Share on other sites More sharing options...
that240guy Posted October 6, 2006 Share Posted October 6, 2006 obd1 lt1 ecus don't store codes unless it is a continuous problem (hard fault). The opti can't throw hard faults when the engine isn't running because the computer can't check the opti during cranking or at KOEO. At this point I would put an Opti and a module in the car if you have communication with the PCM, have fuel pressure, injectors are pulsing and there are no mechanical problems. Backfiring is also an opti issue. Double check to make sure all of your Pids are normal with the koeo. Make sure coolant temp is not reading -40, make sure the tps goes through its 5 volt range, pull the map off and suck on it and see if you can get a reaction. If everything looks good, IMO replace the opti. And get a new one this time. Summit has their own brand opti now that is only 249.99. But I have a friend running the accel opti and he has a custom grind isky cam and his motor frequently sees 6800 rpm. The accel is holding up good and is only 299.99. Quote Link to comment Share on other sites More sharing options...
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