Symptoms of a dying PCM or something else?
Symptoms of a dying PCM or something else? 1994 Mazda RX7, 2004 GTO LS1/T56 with a P59 PCM on HPTuners. Swap is over 10 years old and thousands of miles on it.
One week ago, I wanted to make some tune changes. When I turned the key to connect HPT, I noticed the fuel pump didn't prime. I tried to connect to HPT, but it couldn't. I checked my fuel pressure gauge at the rail, and as expected, no pressure. I then also realized that the CEL didn't light up, like it normally does when the key is first cycled on. Obviously, it wouldn't start. I started poking around some fuses and obvious stuff, but couldn't find anything. Maybe 25 minutes later, I just happened to try to cycle the key again, and the pump primed, CEL lit up, and I was able to connect/flash my changes. I drove the car around for a few hours, including a couple of times of shutting the engine down and starting it again. Not a single problem.
Yesterday, went to drive the car again for the first time since last week, and the pump primed, but on initial flare up it immediately sounded off, and then barely idled like it was misfiring all over the place. I gave it a real quick throttle touch, and when dropping back toward idle it wanted to die. Later, I pulled DTCs and it had a pending P0200 code. I grabbed a quick log at that same time. No obvious power/ground issues, but I still need to do some actual testing in a bit.
If the P0200 code happened by itself, I think I'd be less immediately wanting to blame the PCM, but with what happened last week with the PCM not emitting some signals, it made me question it. I've never actually had any PCM die on me, so yeah, does this ring a bell for anyone, or am I jumping to conclusions too quickly?
One week ago, I wanted to make some tune changes. When I turned the key to connect HPT, I noticed the fuel pump didn't prime. I tried to connect to HPT, but it couldn't. I checked my fuel pressure gauge at the rail, and as expected, no pressure. I then also realized that the CEL didn't light up, like it normally does when the key is first cycled on. Obviously, it wouldn't start. I started poking around some fuses and obvious stuff, but couldn't find anything. Maybe 25 minutes later, I just happened to try to cycle the key again, and the pump primed, CEL lit up, and I was able to connect/flash my changes. I drove the car around for a few hours, including a couple of times of shutting the engine down and starting it again. Not a single problem.
Yesterday, went to drive the car again for the first time since last week, and the pump primed, but on initial flare up it immediately sounded off, and then barely idled like it was misfiring all over the place. I gave it a real quick throttle touch, and when dropping back toward idle it wanted to die. Later, I pulled DTCs and it had a pending P0200 code. I grabbed a quick log at that same time. No obvious power/ground issues, but I still need to do some actual testing in a bit.
If the P0200 code happened by itself, I think I'd be less immediately wanting to blame the PCM, but with what happened last week with the PCM not emitting some signals, it made me question it. I've never actually had any PCM die on me, so yeah, does this ring a bell for anyone, or am I jumping to conclusions too quickly?
Thanks, I'll keep digging. I don't know if it's indicative of anything or not, but I did disconnect the battery for a few minutes, put it back on, and it fired right up like normal.
Going to check more thoroughly when I get some time later/tomorrow.
Going to check more thoroughly when I get some time later/tomorrow.
It is far more likely to be some corrosion or other issue with the wiring/connectors than an issue with the pcm. The P59s are one of the most reliable ECMs you can get. That doesn't mean it is impossible for one to go bad, but if your luck runs like that, I don't want to be near you in a thunderstorm.
I did some testing, based on the PCM pinout from lt1swap.com. Mostly normal, but something unusual on C1 pin 19, switched 12v IGN. It was 12v hot with the key off when first back probed. Cycled the key a couple of times, and then it began acting normally, key off 0v, key on 12v. Another time or two, I was able to get a 12v reading with the key off. I'm going to replace that relay, and I went ahead and cleaned up all of the engine bay grounds, battery ground, and the grounds near the PCM anyway.
Hopefully that's the end of it. Thanks for the sanity check.
Trending Topics
Still here, just haven't actually had time to move the car until the last few days, and wanted to give it a couple of drives before saying anything.
It fired up and ran great every time, though. No issues across maybe 6 or 7 hours worth of run time. Hoping it's good, but because of the weather I started prepping the car for winter storage today.
Since the problem was at least somewhat inconsistent before, I don't know if I can say for sure it's fixed, but I'm happy to call it done as of right now and not worry about it over the winter. If spring rolls around and it won't start, I'll complain then
Thanks everyone
It fired up and ran great every time, though. No issues across maybe 6 or 7 hours worth of run time. Hoping it's good, but because of the weather I started prepping the car for winter storage today.
Since the problem was at least somewhat inconsistent before, I don't know if I can say for sure it's fixed, but I'm happy to call it done as of right now and not worry about it over the winter. If spring rolls around and it won't start, I'll complain then
Thanks everyone
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo It is far more likely to be some corrosion or other issue with the wiring/connectors than an issue with the pcm. The P59s are one of the most reliable ECMs you can get. That doesn't mean it is impossible for one to go bad, but if your luck runs like that, I don't want to be near you in a thunderstorm.
I suppose it's possible. I didn't test upstream of the relay, but if it proves problematic come spring, that's likely where I'll move to next. If I end up in a situation where that pin is still hot with the key off, I should be able to test closer to the ignition switch to validate that it's not just the relay going wonky.
TECH Fanatic
Joined: Jul 2022
Posts: 1,036
Likes: 390
From: Alaska
GM put 3,900,000 vehicles on the road equipped with P59 ecm.
GMT800 population will wear out 40,000 fuel pumps and 87,000 4L60s before you see ONE P59 failure.
............... Check your power and ground connections
GMT800 population will wear out 40,000 fuel pumps and 87,000 4L60s before you see ONE P59 failure.
............... Check your power and ground connections
Well, I said I'd be back if it popped again, and here I am.
First start of the year was in March, and it was fine. Drove it a couple of times, no issues. Then, maybe on the 4th or 5th go a few weeks in, it did the same thing on start.
Ran through a bunch of things, some just as a sanity check. Did some simple wiggle testing on the connectors for the MAF and injectors, no change. Pulled the MAF plug, no change. The MAF circuit ground was tested ok at ~0.01v. MAF is 5v from the PCM, injectors are all a stable ~12v. After taking the PCM connectors off and inspecting a bit and reassembling, the car started right up. Ran fine for a few more weeks. Yesterday, it did something different. It started fine, drove fine for 20 or so minutes, then started sputtering like it was down a cyl. Pulled over, idled choppy, and then after maybe 40-50s it cleared up and drove home fine.
I did this last year and cleaned every ground, but I'll go through it all again when I get some more time this week.
First start of the year was in March, and it was fine. Drove it a couple of times, no issues. Then, maybe on the 4th or 5th go a few weeks in, it did the same thing on start.
Ran through a bunch of things, some just as a sanity check. Did some simple wiggle testing on the connectors for the MAF and injectors, no change. Pulled the MAF plug, no change. The MAF circuit ground was tested ok at ~0.01v. MAF is 5v from the PCM, injectors are all a stable ~12v. After taking the PCM connectors off and inspecting a bit and reassembling, the car started right up. Ran fine for a few more weeks. Yesterday, it did something different. It started fine, drove fine for 20 or so minutes, then started sputtering like it was down a cyl. Pulled over, idled choppy, and then after maybe 40-50s it cleared up and drove home fine.
Check your power and ground connections
TECH Fanatic
Joined: Nov 2011
Posts: 1,367
Likes: 144
From: Woodstock, GA
Well, I said I'd be back if it popped again, and here I am.
First start of the year was in March, and it was fine. Drove it a couple of times, no issues. Then, maybe on the 4th or 5th go a few weeks in, it did the same thing on start.
Ran through a bunch of things, some just as a sanity check. Did some simple wiggle testing on the connectors for the MAF and injectors, no change. Pulled the MAF plug, no change. The MAF circuit ground was tested ok at ~0.01v. MAF is 5v from the PCM, injectors are all a stable ~12v. After taking the PCM connectors off and inspecting a bit and reassembling, the car started right up. Ran fine for a few more weeks. Yesterday, it did something different. It started fine, drove fine for 20 or so minutes, then started sputtering like it was down a cyl. Pulled over, idled choppy, and then after maybe 40-50s it cleared up and drove home fine.
l
I did this last year and cleaned every ground, but I'll go through it all again when I get some more time this week.
First start of the year was in March, and it was fine. Drove it a couple of times, no issues. Then, maybe on the 4th or 5th go a few weeks in, it did the same thing on start.
Ran through a bunch of things, some just as a sanity check. Did some simple wiggle testing on the connectors for the MAF and injectors, no change. Pulled the MAF plug, no change. The MAF circuit ground was tested ok at ~0.01v. MAF is 5v from the PCM, injectors are all a stable ~12v. After taking the PCM connectors off and inspecting a bit and reassembling, the car started right up. Ran fine for a few more weeks. Yesterday, it did something different. It started fine, drove fine for 20 or so minutes, then started sputtering like it was down a cyl. Pulled over, idled choppy, and then after maybe 40-50s it cleared up and drove home fine.
l
I did this last year and cleaned every ground, but I'll go through it all again when I get some more time this week.
TECH Fanatic
Joined: Jul 2022
Posts: 1,036
Likes: 390
From: Alaska
It is GIGANTICLY more more likely that you have damaged pins at the PCM connector, and/ or corroded crimped ends on wire harness terminal ends ( especially the block, chassis and cylinder head GROUND points than an actual PCM going intermittent.