4.9L Intermittent PCM issue (Page 1/3)
sanderson231 JAN 14, 07:38 PM
4.9L Cadillac - I have three PCM for the car. Let's call them Abel, Baker and Charlie. Abel and Charlie have stock PROMs. Baker has a PROM reburned by Sinister Performance to disable the VATS and few other things. The car is also wired with a Baker Electronix signal generator to disable the VATs.

Abel was in the car for a couple of years with intermittent driving. I jumped in the car one day and it started then quit. Try to restart and it would fire and quit. Put a noid light on one of the injector connectors and had the wife crank the engine. The light blinked once and quit, Multiple retries and the same thing. So I say it must be a VATs problem. Could be the Baker Electronix box or the PCM. Let's give Baker a try. He has the VATs disabled. The car starts right up and I drive infrequently it for a month. This week I get about 400 yards from home and the car quits. Put it in neutral and try to resart. Fires once and quits. Fortunately, I am on a small down hill incline and so I coast in the driveway - dead stick landing. Abel is sitting in passenger seat and the console is off. So let's give him another try. Same problem, fires once and quit. Very curious two PCMs with same problem. Charlie is resting in the garage so I grab him. Swap Charlie in and the car starts right. Being the curious type I put the Baker PROM in Charlie and the car starts right up. Being even more curious, I swap Abel back in and the car starts right up. Then Baker and the car starts right up. Then Charlie and the car starts right up.

Grab an analog ohm meter and test light. Check all the ground wires on the PCM plugs and get about 0.5 ohm - doesn't sound like a ground problem. Put a test light on wires that supply power to the PCM and it lights up brightly - doesn't look like a bad connection on the power supply. For good caution I grab a can of QR electronics cleaner and spray the PCM pins and plugs and then work them in and out a few times.

The car is starting but my confidence is pretty low on whether I have found the root cause of the problem.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

La fiera JAN 14, 08:08 PM
Ditch all the ECMs and get a Microsquirt. It'll work wonders]!
Raydar JAN 14, 08:26 PM
Deja Vu!

I had the console out of my 4.9 car, to install a Double-DIN skeleton.
Shortly after putting it all back together, I noticed that it wasn't starting and idling as smoothly as it had been. Even stalled once, and was a bit "grumpy" on the restart. This started when I was looking at stuff with a scan tool. Okay... whatever. Figured it was just "a thing".
A few days later, I was going to a local event. Got about 4 miles from the house and it started sputtering and bogging, and it finally stalled. Similar to how it acted when I was scanning it.
I coasted into a driveway to ponder the situation. I disconnected the battery (front mounted... there's a knife switch) and reconnected it.
Started it back up, and it ran normally, all the way back to the house. (**More about this, in a minute.)
My EPROM is one that I programmed, to accommodate the Allante manifold, and the manual trans, but it has run just fine for a long time. It's mounted in a socket that I soldered into my "MEMCAL" in the same position as the stock EPROM.
I removed the MEMCAL, removed the chip from the socket, and reflowed all the solder connections, where the socket is soldered into the MEMCAL. Plugged the EPROM back in, and plugged the MEMCAL back into the PCM, making sure to seat it solidly. Also made sure all the PCM connectors were well seated. Tugged a few wires, looking for a loose connections. All good.
Started it back up, and it's been fine, ever since.
I'm not sure exactly where the problem was. I just hope it doesn't return.

**Bear with me...
There are five different definition files for the 4.9. They are apparently different enough that the wrong file just won't work, when programming a 4.9.
Before I made all my "Allante and manual tranny" mods, someone gave me a tune that had worked well for them. When I plugged it into my PCM, it seemed to run fine. The first time.
The second time I started it, it acted like it was in "limp home" mode. The trans started off in 2nd gear. The engine had significantly reduced power. If I floored it, it backfired.
What this leads me to believe is that, if you don't disconnect the battery, the ECM runs on the last tune that it had stored when it was shut off. Good, bad, or indifferent. The next time you start it, it will load the "new" tune.
The only way to get it to load the new tune, right out of the gate, is to disconnect the battery, and force it to load, fresh. (I had a Fiero "programmer" to tell me that I didn't need to disconnect the battery, every time I changed the chip, as long as the key was off. This was apparently the result of that. Lesson learned.

BTW... my EPROM has VATS turned off, but I also left the VATS "resistor module" plugged in. Figured I wasn't hurting anything.

Anyway... a long rambling post. But I think lots of issues can be caused by loose connections, either at the PCM, or at the EPROM/MEMCAL. But the Caddy PCM (not sure about others, but I wouldn't be surprised) can "throw you a curve", too. Doesn't make a lot of sense, just what I observed.

[This message has been edited by Raydar (edited 01-14-2023).]

sanderson231 JAN 14, 11:14 PM
Raydar,

Sounds like if it happens again I should disconnect the battery and see if that cures the problem.

Before I bought the Baker Electronix black box, I had someone burn a PROM with the VATS disabled. Got it back and the car ran in limp home mode. Didn't know there was such a thing as limp home mode until that happened. I then bought the Baker Electronix box and installed a PCM (Abel) with a stock PROM. The car ran reliably for several years but with not a lot of miles on it. For lack of a better hypothesis I'm leaning towards iffy electrical connections.

All three of the PCMs are 16132240's. As far as I know any 161632240 should be able to run any of the PROM's that are intended for the 16132240. Tunercat shows 16 different PROM's (broadcast codes) for the 16132240 and as you said 5 different definition files (I now own three). But I think as long as the correct definition file is used for the broadcast code then you shouldn't get in trouble. Most of the time the PROM won't read if the definition file is wrong. I have my car wired per the 92 Deville factory service manual. Maybe there are some small wiring difference if using a PROM from a different year that could create trouble. Or maybe there are wiring differences between a 92 Deville and 92 Fleetwood.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 01-14-2023).]

sanderson231 JAN 14, 11:58 PM

quote
Originally posted by La fiera:

Ditch all the ECMs and get a Microsquirt. It'll work wonders]!



Way too many other nagging Fiero issues to even think about it . Like where is the small vacuum leak on my 3800 S/C that makes the car lean at idle despite the integrator going to 182.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

Raydar JAN 15, 03:54 PM
The PROM can sometimes be read with the wrong definition file. But it may load non-sensical data into the fields. I've seen that.
I have speculated that the main differences between the different definition files (and its associated broadcast codes) is automatic transmission wiring, and maybe gear ratios.
The 2240 should be able to run any broadcast code written for it. But that's not to say that all of the data is handled the same, or is input/output on the same pins.

In your case, I would suspect an intermittent wiring or connector issue. This, since your issues seemed to occur across Abel, Baker, and Charlie.
Followed by a bad connection at the EPROM/MEMCAL. (That would be an odd coincidence, however, for the same reason.)
fieroguru JAN 15, 05:01 PM
What is the tach doing during all of this? Reason I ask is that the injectors are triggered by the ignition pulses, so it might be an ignition related issue. That would help with the issue being random and across 3 separate PCMs.
sanderson231 JAN 16, 12:01 AM

quote
Originally posted by fieroguru:

What is the tach doing during all of this? Reason I ask is that the injectors are triggered by the ignition pulses, so it might be an ignition related issue. That would help with the issue being random and across 3 separate PCMs.



Since the would fire a quit I was thinking fuel but I like where you are headed with an ignition problem

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

87GTBrad JAN 30, 06:05 PM
Hi slightly off topic Does anyone have contact information for someone that still is programing the obid 1 chips to run the 4.9 ? I had Westers in Alberta put a tune in for my 4.9 swap ( came from a 94 Deville stock 5 speed trans and hooking up the ecm to speedo killed the output to the dash). It runs but not great, looking for a tuner with similar Fiero 4.9 swap experience. Local speed gurus are saying to dump the gm ecu and replace it with a link system $$$$. I appreciate help I live in BC Canada.
sanderson231 JAN 30, 07:01 PM

quote
Originally posted by 87GTBrad:

Hi slightly off topic Does anyone have contact information for someone that still is programing the obid 1 chips to run the 4.9 ? I had Westers in Alberta put a tune in for my 4.9 swap ( came from a 94 Deville stock 5 speed trans and hooking up the ecm to speedo killed the output to the dash). It runs but not great, looking for a tuner with similar Fiero 4.9 swap experience. Local speed gurus are saying to dump the gm ecu and replace it with a link system $$$$. I appreciate help I live in BC Canada.



The VSS that came in the 4T60E auto that came with the Cadillac has an entirely output than the VSS from a stock Fiero 5 speed. Do not connect the output of the Fiero VSS to the 4.9L PSM. I would run it to the dash through the C-203 connector the same as in the Fiero. If the 4.9L has not been modded, I doubt there is anything to be gained in terms of max power by a custom tune. There are perhaps some parameters to that would help driveability with a manual transmission. We need Raydar to chime as he has done a 4.9L w/ 5 speed. Do not spend money on an aftermarket ECU - entirely not necessary. We'll get this worked out.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged