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Discussion Starter · #1 ·
Alright, been a while since I tore into my Trans Am to figure this out, but here is my car and situation :

1988 GTA with transplanted 1994 LT1/T56, running ****LTCC LS-Coil ignition system****. Ran fine for one year on normal opti, end of the season, original used opti I had failed, and I replaced it the following season and added LTCC and ran fine, then the issues started...two years ago. At the time I was putting it up for the winter, and it was a very fast transition from driving it every day, to it wanting to die out of no where. The car would randomly die, and if I was cruising, it would pop it's own clutch and keep going.

Getting it around to it lately, a few months ago, hooked up my laptop, got some data reports, and followed the results. I know the car was running rich for the two years, so wasn't surprised to see my O2 sensors were dirty, otherwise everything runs fine. Idles at around 750-800 rpms, throttle response is flawless, operates with an exhaust leak, but otherwise everything monitored like it was supposed to. The glitch was when it hit closed loop, it shut everything down like someone just shut the key off, no choking, no fighting to continue to run, just shut off. When it dies you do hear the fuel pump prime, again like someone flips the key off and back on. Which brings me to today.

I have tested/replace the throttle position sensor (old one was cracked badly around plug area), replaced O2s (carbon-ed up), replaced coolant temp sensor for fun, and using a sensor tester, tested the mass air flow sensor, but also tested a spare, tested map sensor, tested knock sensor. I also tested the hi and low resolution signals coming from the opti, and those are coming through clearly. Only sensor I haven't really tested was the idle air control valve, but again, it can hold a 7-800 rpm idle just fine, so I know that is working. Also replaced the fuel filter for precaution of it sitting for two years, and plan on doing oil change and filter, check tranny fluid condition/level, and if I can get my car running properly, getting an S60/drive shaft/clutch. As far as my diagnosis is going, process of elimination is leaving me the assumption my ECU is failing me, and it's the only thing I don't have a spare of to try. Can anyone elaborate any other tests I can do?
 

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Discussion Starter · #2 ·
Any suggestions? No SES light. Don't know what exactly gets "turned on" by the closed loop function so I can try and chase any specific shorts.
 

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Discussion Starter · #3 ·
bump...noone has suggestions or had a similar experience?
 

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its rare for it to just up and die when it hits closed loop.

the o2 sensors are the most important thing added to closed loop operation. nearly all the other sensors function in closed loop as well.. tps, temperature, map, the works.

the thing is, the ecm should monitor the o2 sensors for readyness BEFORE it will enter closed loop. the ecm can tell if they o2s are hot enough to read properly, and closed loop should never happen until they enter that threshold...

i suppose you could have some bad oxygen sensors or some bad wiring going on?... a realtime scanner can tell you if they're faulty by how they're switching. i'd start there.
 

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i should mention that the primary ground for the o2 sensors (which runs to the big ring termainals attached to the factory coil) is probably pretty important to the function of the o2 sensors, as is the ground strap that runs from the coil bracket to the chassis.. make sure that nut hasnt loosened off or something..
 

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Discussion Starter · #6 ·
I have checked the main engine to body ground, and also the battery to engine ground, they are connected and not loose. As far as the O2 sensors, I had the 2 year old set in there, and pulled them to install two new ones, same issue from before the replacement, and physically used my sensor tester to test all 4. New ones were perfect, the old ones needed to heat up a little longer because of all of the black crap on it, and once that burned off, they functioned flawlessly.

I did also run the scanning, and could watch the old 02s bounce around, driver side a little lean because of the extensive exhaust manifold leak it had until I just fixed it. It would bounce around until the "Closed Loop" light lit up on my lap top, and the engine just cut off like the key was turned for the split second to kill electrical, but back on to have the fuel pump priming (don't have oil pressure sending unit hooked up so it wasn't that running it).

The data log has some of it's issues, but it hasn't been tuned for the headers/3 inch collector back, and cold air intake. I have turned on the fact that it's a manual tranny, shut down all the CEL lights that didn't affect me (all the auto tranny stuff, and some others), shut off VATS, tuned in my tire sizes, the really basic stuff, haven't played with any of the tables.

At this present time I have lost my laptop data connector for the car, so I'm in search of that but again, identical results to before the testing of all the sensors, and replacement of TPS, both O2s, and CTS, and again, all the old sensors tested fine so it wasn't them from what I'm seeing. Everything works in open loop...can't understand it exactly.
 

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Discussion Starter · #7 ·
Appreciate the help btw! I'm open to test just about anything at this point. I'm ready to go to the pick and pull and get a couple ECUs...so if I can save some coin, that would be helpful.
 

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Discussion Starter · #8 ·
Also, if there was a grounding of a sensor issue, wouldn't it be similar to a defective sensor and stick it into constant open loop or fans come on...I mean is it an issue if I unplug a CTS, MAF, or O2s, and the fans don't come on with a SES? Isn't that a default?
 

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Discussion Starter · #9 ·
bump
 

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Discussion Starter · #10 ·
Interesting piece of news...my laptop had no problem connecting and reading the PCM, and my scan tool last night communicated with it and it said all was fine (code wise).

If anyone has an OBD1 computer laying around, and would like an OBD2 LT1 pcm...I have one sitting in my garage, would prefer to barter.
 

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I know very little about tuning but I am curious: What if you tuned the car to always stay in OL? Basically hook up a wideband and make sure you get the right A/F ratio at idle / cruising / WOT and never throw the car in CL mode. I wonder if this will "cheat" the car to stay running.

My car is tuned in OL and you don't really notice much except bad gas mileage around the city.
 

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Discussion Starter · #12 ·
Seems very do-able, don't have an advanced knowledge of tuning, but that is something that I will look into worst case scenerio to keep it running if I could find my damn ALDL cable for my laptop lol. None the less, I'm going to go searching for that tonight and also trace any and all wires in my harness, just to rule that out...i'm really down to the last two things, harness or computer. Computer takes about 5 minutes to swap, tune out vats, and see if it goes closed loop...harness takes much more than 5 minutes...was trying to find the easy step to cross off :-/
 

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I'm not real familiar with the ltcc system, but I would imagine it has ome sort of ignition control module also. I would suspect the icm of heating up, but that's just pulling the rabbit out of a hat.

If you have the wiring schematic for the ltcc system,and/or have a description of how the system works, let me know and I'll pm you with my e-mail address so you can send it to me. That way I can study it and see whats what.

Also, have you done the basics like taping a fuel pressure gauge to your windshield and seeing if fuel pressure drops right before the car dies.

Have you tested spark with a loading type spark tester, to see if spark is going away when engine dies.
 

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Discussion Starter · #14 ·
fuel pressure stays strong through to shutoff, can't speak of exact pressures at the moment, but its responsive to throttle, and stays solid once you shut the engine off for a long time, so I know fuel system isn't losing pressure overall.

As far as spark, no I haven't. My premise on the ltcc system is that it uses the low/hi res signal to operate, gets signal or power from where the old ignition module was wired, and ground, and if all signals present to run properly are there in open loop, there is no "brain" function in the ltcc system persay, so the difference of operation from open loop into closed loop shouldn't change how it runs unless the signal isn't getting sent to in during that transition.

I will try to find some more in-depth info on ltcc schematics.
 

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An exhaust leak before the O2s and the PCM will see a lean issue, and compensate by running rich, and can fail O2s rather quickly, especially at the manifolds cause it tends to suck air in there at the manifolds.

PCMs (ECU) rarely fail.

Swaps especially need good grounds. I would check just for farts and giggles.
PCM needs 4 good grounds. Black/white wires from the PCM.
The engine needs to be grounded with the PCM. Not just the chassis. Check your bonding cables. You can ground your engine thru your chassis from the PCM. I would add a grounding cable from those black/white wires (there will be 4) directly to the engine.

You may have a ground good enough to make readings yet not good all the time. Have you checked your readings right when engine dies?

I fought my swap dilemmas and this was a gremlin.

Fans do not always come on with the SES light. If the fans do come on immediately and no light that is cause your SES light has failed.

VATS engaged only takes the ground from the injectors.

Not saying this is your gremlin, just check for the giggles.
 

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Discussion Starter · #16 ·
going to be checking my entire harness today. It's custom and I know possible to fail. Just figured the computer was easiest to try to swap for giggles, but that will take time it appears, so harness I'm going to tackle today, and check all grounds.
 

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Even though the pcm rarely fails, I like to keep a spare, known good one around just so I can swap it as part of the troubleshooting procedure. That way you can eliminate the pcm as a suspect quickly.

And while the pcm rarely goes bad, it's wiring and connectors are another story. In the 4th gen section, the thread titled "GM service manuals", has various years of lt1 f-body service manuals. They have a handy pcm connector chart, which lists the symptoms that occur when the various pcm wires are bad. Might want to download the appropriate manual for your year of lt1 pcm and check that out.
 

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Discussion Starter · #18 ·
you know, I found a similar chart on lt1swap.com, and I ran through that, and some of the stall items don't seem to apply from what I can see...

I don't have an egr valve so those wires don't exist.
Maf sensor is working, but good point on if the ecm is seeing all the details, ill have to take a look back at my data sheets.
Iac seems to function properly

It's really odd that its during a pcm function that there is a failure, that is what has got me leaning hard on the pcm although I've never seen a pcm failure. That's why I wanted a spare pcm, but the junkyard I went to only had an obd2 computer, so I'm at a standstill on the pcm testing.
 

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Discussion Starter · #20 ·
Ive checked that website, all of them are over 3 hours from me, and I had one person respond to my thread of someone looking to swap pcms with my obd2 for an obd1, so want to see if that pans out before I throw money at another spare.
 
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