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1997 LT1 Misfire
Ok I have searched the threads for misfires and have a weird issue that I could not find in the search. To start out when I bought the car it had a misfire that I thought was going to be easy. Upon first trouble shooting I found that one of the plug wires was not connected well and when I pulled it off the tip on the spark plug had been unscrewed and the plug wire just hanging out. Thought it was a win so bought new plugs and wires and installed them. Nope stilled had a bad misfire with the check engine light flicker. Come to find out the idiot I bought it from had ran it hot too many times and melted both heads on the lower side of 2 cylinders which in turn blew the head gasket. Lots of money and time later I found a set of heads and rebuilt the top end. Also put a new opti on it since I did not have a warm fuzy for anything that was on the car at this point. Well started the car up and it was running better but still had the mis on the #7 according to the code reader. I have done all the recomendations from other threads to trouble shoot this misfire and still cant narrow it down. Well today I went ahead and did the noid light check again and found an odd issue that i cant explain. The noid lights up as it is supposed to on all cylinders but when I pulled the connector off #5 and #7 I noticed the engine does not run any worse with them disconnected. So I put the noid light back on the connector and now it does not light up. Very confused on whey these 2 cylinders are getting a single when I start the engine but after about a minute it stops the signal but even when it is getting the signal it is not firing the injectors. I have also bought new injectors because the old ones were very dirty and again no warm fuzzy on the parts to begin with. They all ohm out good so I know the injectors are good but just not getting a signal to fire. Could this be a wiring issue at this point and should I pull the wire harness to check because even trying to shake the harness does not change the out come? I am just at a loss for the misfire cause all the trouble shooting points to plug wire issue as the computer is kicking out the code for the #7 and the flashing check engine light but have checked and triple checked the plug wires and not seeing any arcing and they are all routed correct. Thanks for any help to start with.
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Re: 1997 LT1 Misfire
There's a code for intermittent injector circuit(s), P1222. Has that ever shown up? Key on, do you have constant 12 volts on the pink wire at each injector clip? The PCM supplies a ground on the “not pink” wire on the clip to fire the injector. Intermittent circuit on the ground could be related to injector drivers in the PCM, but failure of the driver(s) is relatively rare. Download the 1996 (same as 1997) factory service manual (free) , thanks to GaryDoug: https://www.mediafire.com/?40mfgeoe4ctti ..... and take a look at the pages for P1222. |
Re: 1997 LT1 Misfire
Thanks for the quick reply. I downloaded that book when I first got the car and it has helped me alot. Unfortunately code P1222 has never shown up so that is why this is got me stumped. I will be checking the pink wire for power today to make sure I have 12 volts. I am with you that I don't think it is PCM that went bad but I am keeping that in the back of my mind cause I wouldn't put it past the last owner to have screwed it up too. On a side not is there a better scanner out there because the one I am using is just a basic code reader that doesn't show me the misfire counts? Thanks for the help it is greatly appreciated.
Mike |
Re: 1997 LT1 Misfire
You need something like the GM spec OTC Tech 2. There is actually a Chinese clone of the Tech 2, a fraction of the OTC cost, but still several hundred $$$ I believe. Member GaryDoug has one of the clones, and has found it to be reliable. He has also written (free) OBD-2 scanning software, but I can’t recall if it can access the GM Enhanced Parameter misfire count by cylinder. I'll ask GaryDoug to take a look at this thread. |
Re: 1997 LT1 Misfire
Ok rechecked all injector connectors and they all have 12 volts. Started the car and noid still shows a signal going to injectors. Also ohm'd out the injectors and all 8 show the same which I would hope they would since they are new. With car running it still has a miss and pulling connectors off 5 and 7 still does not effect the miss but pulling connectors off the others makes it run worse.
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Re: 1997 LT1 Misfire
Mike, To get the misfire counts, you need a scanner that can access the GM enhanced parameters. The readings you see in most inexpensive scanners are the ones mandated by the govt. Those cannot be copyrighted and so are available to anyone designing a scanner at no cost. Unfortunately there are only about 15-20 of those parameters and they do not include anything specific to any brand, such as the GM misfire codes, which are copyrighted.
Fortunately there are some alternatives, like buying a Chinese-made clone of the very successful Tech II , which of course has everything and sells for about $300 including shipping from China. The only drawback of this scanner is that is is very poor at datalogging. https://www.aliexpress.com/item/Tech2-diagnostic-tool-for-G-M-SAAB-OPEL-SUZUKI-ISUZU-Holden-tech-2-scanner-for-g/32893275247.html?spm=2114.10010108.1000014.5.4c871 d0fmXiHUZ&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.13338.11 2281.000000000000000&scm_id=1007.13338.112281.0000 00000000000&scm-url=1007.13338.112281.000000000000000&pvid=d9aede5 c-a0e7-4f39-a189-184e0bbb1b11 Another way is to use my free PC program (GDScan) that allows you to enter your own GM enhanced codes in a special section. The misfire codes are pre-installed for the V6 engine, but you can simply change those to the ones for the V8 and I can supply those. To use my program, you would need a pc that can be placed close to or inside the car, and an ELM327 adapter cable for OBD2 (about $30). https://www.scantool.net/obdlink-sx/ Technically, you can use a pc with Bluetooth or buy a Bluetooth USB adapter and use the adapter in the next section instead of the cable version. Another option is to use almost any Android phone with the Torque app ($5) installed. You would need an OBD2 ELM327 Bluetooth adapter for about $25. The drawback for this is that you need some talent to modify the app by including the enhanced parameters. Those are available for free in the Torquescan add-on app, but you must add and edit them manually. |
Re: 1997 LT1 Misfire
Thanks for that really good info. I will try out your program with the adapter. I am completely stumped on this problem and cant see why it should be misfiring when it is getting the signals to fire the injectors and it is getting spark. Also it is weird that it is just happening on #5 and #7.
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Re: 1997 LT1 Misfire
Originally Posted by Zman3030
(Post 7001215)
Thanks for that really good info. I will try out your program with the adapter. I am completely stumped on this problem and cant see why it should be misfiring when it is getting the signals to fire the injectors and it is getting spark. Also it is weird that it is just happening on #5 and #7.
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Re: 1997 LT1 Misfire
Have you checked the fuel pressure? #5 and #7 are at the end of the line... not a likely problem, pressure would have to be extremely low. Have you checked the compression.... blown head gasket between #5 and #7? Have you checked for corrosion in the Opti plug wire terminals? The #5 and #7 contact buttons in the Opti cap are right next to each other (logical because #5 and #7 fire in sequence), and susceptible to high voltage carbon tracking. Valves adjusted correctly? |
Re: 1997 LT1 Misfire
Fuel pressure is right on the money. Compression is the one thing i have not checked but as far as the head gasket I just replaced them due to the heads being bad. and know it is not blown again. Opti is brand new and the 4th one I have put on. I readjusted the valves with it running just to make sure. I have another block at the machine shop right now cause i planned on a rebuild anyway but dont want to throw a rebuilt motor in and it still have a misfire. As you can see from the pic that is what both heads looked like when i pulled them so after buying different heads and new gaskets I shouldnt have any leaks up top. As far as the rings after all that coolant it wouldn't surprise me to find low compression but would think that it would still have at least some spark. Thanks again for all the input cause I defiantly need a direction to go at this point.
https://cimg8.ibsrv.net/gimg/www.cam...35f29021f2.jpg |
Re: 1997 LT1 Misfire
The system detects misfires by comparing the crankshaft position signal to the camshaft position signal from the Opti, and compares the measured crank rotational velocity variations to the expected rotational velocity variations. The variations are due to the pulses that occur when a cylinders fires. If the pulse is not as strong as it knows it should be, the acceleration of the crank will not be as great as expected. The fact the cylinder fires is not the only factor. It's the strength of the combustion pulse that matters. Hence a misfire can be interpreted as any departure from normal, including incorrect fuel, incorrect spark timing, or incorrect compression. “Some spark” might not be enough to generate well-timed and complete combustion. The changes in crank rotational velocity are so small that a rough road, a loose crank reluctor, or the sensor contacting the reluctor can be interpreted as a misfire, and counted as a misfire. But your description indicates that you are feeling/observing actual misfires, not false indications. The current limited availability of quality Optisparks can result in them being bad right out of the box, particularly the “Made In China” junk. But it's unlikely you would get four bad ones in a row. The 5/7 relationship often points to cross-firing in the plug wires, but your new wires should have eliminated that as the problem. Were the replacement heads checked to insure the are flat? Were the valves in good condition? Was there any damage to the block where the gasket was blown and damaged the head? Do you have any indication someone might have been running nitrous, resulting in broken ring lands? And your sure it has a stock camshaft? Obviously offering more questions than answers, but trying to think of everything I've seen in the past. Once you get Gary's software running, you can run a data log and I can go through it in detail to see if it provides any clues. |
Re: 1997 LT1 Misfire
Thanks for all the good info as I am always learning about the LT1 from you guys. Yeah I can feel the engine has a mis and happens throughout entire rpm range. As far as the replacement heads they were good to go as far as being true and all seals and valves were gone through to ensure a good seal from them. Now the part about the block is the unknown. I did the best I could with the engine still in the car to check everything I could. Thoroughly cleaned it and inspected it and rotated the engine to inspect as far as i could into the cylinders and I did not see any evidence of cracks or damage. But I know that without a good magflux to check it correctly I could have damage that I cant detect with the eye. That is why I am rebuilding a known good core to replace this engine. I know that sounds like a way to just throw money at a problem and that it isn't going to fix my misfire problem but I know engines and what happens when coolant is thrown into the bearings. Also I don't know how long the previous owner ran it with the damage so more of peace of mind for myself. As far a nitrous and a cam I really don't know because the guy said everything was mostly stock. I am looking forward to Gary's software and to see if that will get me more insight into my problem. Thanks again for helping out.
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Re: 1997 LT1 Misfire
I just got the cable that Gary recommended and downloaded the GDscan. Is there any specific values or way you like to see the log? Thanks
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Re: 1997 LT1 Misfire
Gary's software produces a .csv file that I can manipulate in Excel. His standard format is fine. Don’t change anything, except the scan rate. It helps if you keep the frames per second rate high. 2 frames a second is too slow and can mask interactions between sensors or actuators. The closer you can get to 10 frames per second is best. In all honesty, I have never used Gary's software to make a data log. My LT1 engine runs on an aftermarket engine management system that has built-in data logging. However, I have reviewed hundreds of the reports, mostly for the OBD-1 version, but have worked with a few OBD-2 files. The principles are similar. Since your problem exists at idle, do a cold start, and let the engine warm up for at least 4 minutes. That should get the system into closed loop. Let it idle for 2 more minutes. Do not rev the engine in neutral.... serves no purpose. If you also feel there are misfires under load, drive it for a couple minutes, replicating the conditions under which the misfires are most obvious. Try to keep the file under 10 minutes, because it can get unwieldy at the high frames per second scan rates. |
Re: 1997 LT1 Misfire
I have to point out that the regular datalog only includes the mandated parameters on the main engine data tab ("Live Data $01"). It does not include the enhanced values on the other tab ("GM Enh"). You can however run a datalog on that section separately if you want. The "Gm Enh" tab is completely separate from the others.
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