PCM ECM ECU computer bad on 1995 SC Riviera?

Well, the 95 started acting up about a month ago.  It would start up and run fine, then "miss" here and there while running, then run for a while again to only exhibit the behavior a few days later.  Intermittent.

Then it started to exhibit this a bit more, now even at startup with the car bucking and misfiring, sometimes quitting.  Then I could start it up and it would run for a few days with no issues.

Now last week the situation has reversed itself with it rarely running and more often misfiring and bucking, dash warning lights flickering, etc and then just dying.

So I checked the normal things, fuel, fuel pressure, spark, etc and that all looked normal.  My OBD 1.5 compatible scan tool found no codes either, not stored or while running.

There hasn't been any recent work of any significance in the last couple months.  The fuel pump is 18 months new, plugs are 6 months new as are OEM GM Delco wires.

Since it didn't seem obvious at this point, and I had my arms in two other cars, I had it towed to my local mechanic who does the heavier (or less fun)  work on our cars.

We had taken it to them about a month ago to see if they could find out why it was occassionally misfiring, but at the time they couldn't replicate the problem.  This time it was clearly evident.

But they couldn't find anything wrong either, and after about a week of troubleshooting they have come to the conclusion that I likely need a new computer (PCM/ECU/ECM).  

I'm OK with replacing the PCM, but unfortunately I think they have come to that conclusion based on eliminating anything else they can think of.  I'd hate to spend $250-300 on a non-returnable PCM just to find that wasn't the problem, which they suggest is a slight risk.

So, before I call them back tomorrow, are there any suggested lines of questions I could ask to clarify it is the PCM?

Also, I know the crank position sensor can sometimes cause similar misfire/non-start issues on this and other cars/motorcycles I've had.  Do these symptoms sound like the crank position sensor could be a possible cause, and if so what could I suggest they test to identify or eliminate the crank position sensor as a suspect?

One final question:  They mentioned, and the part listings for the 95 PCM do too, that the PROM from the original PCM must get transferred to the replacement PCM.  Why?  What information is on the original PROM?

Thanks,
Al

Part information from Rockauto

1995 BUICK RIVIERA 3.8L V6 Supercharged : Electrical : Powertrain Control Module (PCM)



ACDELCO Part # 16183247      Remanufactured

MODULE,Powertrain Control; This Unit Contains A PROM That Must Be Transferred From The Old Unit To The New Unit (Only 1 Remaining)

Unit $165.79

Core $95.00

Total $260.79

Quote :

I'd hate to spend $250-300 on a non-returnable PCM just to find that wasn't the problem, which they suggest is a slight risk.

This is the most likely scenario, imo. Mechanics sometimes will blame the computer when they are stumped. It's more likely something like wiring or connectors than the actual PCM - it's the only part of the car that really shouldn't go bad.

Have they checked the coils? Maybe buy a single coil and swap it in one-at-a-time for all three.

If it were crank position sensor, I'd think a code would be thrown. At least on ODBII that's the case.

Not sure about the PROM. Maybe that's an OBDI thing?

Maybe the check valve at the brake booster making a big intermittent vac leak or the ignition module not getting ground. The 95 I have never threw a code with the bad coil or the real bad intake leaks it just ran like crap and had a surging
problem. I replaced all the lower intake and valve cover gaskets used high temp paint on the sides of the block where the intake manifold meets the heads and block to make it totally leak proof. Then after that it was just new plugs wires and the 1 coil I found that was bad. Now it runs like a bat outa hell!   I did all this work over the course of 3 weeks and during that time waiting for parts to come in the mail I cleaned all the grounds and any electrical contact I could find under the hood or any where else on the car for that matter. I also took the ecm out of the car and had a look at it to see if there was any leaking capacitors in it. Only other thing I know of that could bork the driver modules in the ecm would be if the idle control valve shorted or arcing across to the o2 sensor cus there is a plug wire right next to it. I hope this helps.

Thanks all..... I guess we'll just have to see. I'll mention the coils.

I think the PCM minus the EPROM, are identical from all 95 Rivieras. I dont think they often go bad, unless you have bad grounds. It might be worth a shot grabbing one from the junkyard for ~$10 and very little labor. Its awkward to remove the PCM but not difficult. The EPROM pops right out to switch over.

Thanks, all good info.

But, as I suspected and was going to suggest the mechanic check first, it turned out to be the crank position sensor   

I feel much better about that, as that is a reasonable failure versus a PCM going randomly bad and a CPS is also less expensive. If it had been the PCM I would have a little concern about why, such as a mystery shorted harness somewhere, etc. Crank position sensors however do have a reputation of just going bad, and not only on this platform (my Jag and my Moto Guzzi too).

Talking to the mechanic this morning, they had tried "everything" and had one more thing to try before the last ditch PCM replacement.  Last night I wasn't sure what that "last thing" was going to be as I had that info second hand, but it turns out that last thing was going to be replacing the crank sensor this morning.

So far they say the car is running fine with a new CPS, so we'll take it home and keep an eye on it to see if that is the actual and lasting fix.  Based on how I've seen other crank sensors fail and behave, I suspect that was indeed the problem.

Thanks again for all the replies.

BTW, the car only has about 70k miles on it now, but I'm sure the crank sensor is the original 19 year old part.  So that's a pretty good service life. Although I still don't understand how these sensors go bad at all since they aren't a wear item, but they are exposed to heat and elements, so     


Al

th3fr4nchi5e wrote:

I think the PCM minus the EPROM, are identical from all 95 Rivieras. I dont think they often go bad, unless you have bad grounds. It might be worth a shot grabbing one from the junkyard for ~$10  and very little labor. Its awkward to remove the PCM but not difficult. The EPROM pops right out to switch over.

X2

When I had misfire problems I bought a PCM from the pull a part strictly as a diagnostic aid.  Not saying you couldn't get ahold of two bad ones, but that seems unlikely.  If you put the second one in and it has identical symptoms, probably not the PCM.

FWIW on my 95, it turned out to be the harmonic balancer moving enough on the crank due to a crack to pull the trigger out of the crank sensor. It also showed no codes and was intermittent.  There's a thread on here for it but I'm working from my phone and can't pull it up.   

Edit:  I see you got it figured out while my fat fingers were trying to type on my phone!  Hahaha.  

Rivillac wrote:

th3fr4nchi5e wrote:

I think the PCM minus the EPROM, are identical from all 95 Rivieras. I dont think they often go bad, unless you have bad grounds. It might be worth a shot grabbing one from the junkyard for ~$10  and very little labor. Its awkward to remove the PCM but not difficult. The EPROM pops right out to switch over.

X2

When I had misfire problems I bought a PCM from the pull a part strictly as a diagnostic aid.  Not saying you couldn't get ahold of two bad ones, but that seems unlikely.  If you put the second one in and it has identical symptoms, probably not the PCM.

FWIW on my 95, it turned out to be the harmonic balancer moving enough on the crank due to a crack to pull the trigger out of the crank sensor. It also showed no codes and was intermittent.  There's a thread on here for it but I'm working from my phone and can't pull it up.   

Edit:  I see you got it figured out while my fat fingers were trying to type on my phone!  Hahaha.  

Interesting cause/symptom with the cracked balancer. If ours acts back up after the CPS change, I'll keep that in mind.

And yeah, I was already going down the route of getting a pick/pull PCM to at least test before spending $300 on a rebuilt PCM.

Thanks for the tip!

Al

Well, the car acted back up again today, so it was apparently not the crank sensor, which is too bad.

Here's something interesting though. My wife and son report that if the car starts and is allowed to come up to operating temperature before pulling out and driving, it seems to run fine after that.

So could this be a sensor of some sort that is sensitive to temperature?

Al

Does anyone know which other Buick/GM models share the same PCM as the 1995 Riviera so I can search some of the local pick and pull yards?

Also, beyond just swapping the PROM, do we know if any special PCM software is needed to force relearning any sensors or any other programming. Or can the PCM just be swapped out.



This is what I gleaned from an eBay listing, but I'm not certain it is correct:

1995 Buick LeSabre Custom Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Buick LeSabre Limited Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Buick Park Avenue Base Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Buick Park Avenue Ultra Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Buick Riviera Base Coupe 2-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Buick Riviera Base Coupe 2-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Chevrolet Lumina APV Base Mini Cargo Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Chevrolet Lumina APV Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Chevrolet Lumina APV LS Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Chevrolet Lumina Van Base Mini Passenger Van 4-Door 3.8L 3800CC 232Cu. In. V6 GAS OHV Naturally Aspirated
1995 Oldsmobile 88 Royale LS Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Oldsmobile 88 Royale LSS Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Oldsmobile 88 Royale LSS Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Oldsmobile 88 Royale Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Oldsmobile 98 Regency Elite Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Oldsmobile 98 Regency Elite Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Oldsmobile Silhouette Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Pontiac Bonneville SE Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Pontiac Bonneville SSE Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1995 Pontiac Bonneville SSE Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Pontiac Bonneville SSEi Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1995 Pontiac Trans Sport Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Buick LeSabre Custom Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Buick LeSabre Limited Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Buick Park Avenue Base Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Buick Park Avenue Ultra Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1994 Chevrolet Lumina APV Base Mini Cargo Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Chevrolet Lumina APV Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Chevrolet Lumina APV LS Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Oldsmobile 88 Royale LS Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Oldsmobile 88 Royale Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Oldsmobile 98 Regency Elite Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Oldsmobile 98 Regency Elite Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1994 Oldsmobile 98 Regency Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Oldsmobile Silhouette Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Pontiac Bonneville SE Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Pontiac Bonneville SSE Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1994 Pontiac Bonneville SSEi Sedan 4-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Supercharged
1994 Pontiac Trans Sport Base Mini Cargo Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated
1994 Pontiac Trans Sport Base Mini Passenger Van 3-Door 3.8L 3800CC 231Cu. In. V6 GAS OHV Naturally Aspirated


Thanks
Al

Well the problem has returned... running fine for a day or two, then repeating its problematic performance of misfiring, stumbling and stalling.

But before we head down the PCM replacement route (I plan to get a used one from a yard to test), I've got a little more intel to share that perhaps will generate some additional theories of what's going on.

First, when the car is 100% broke, the symptoms are that the engine will turn over and fire and stumble for about a second or two, misfiring and interestingly lighting the overheat idiot light. In this condition the car will not start and idle, even if you try to feed it throttle, ultimately stumbling to a halt.

IF the car makes it through this and actually starts, and one lets the engine get to operating temp (or very close), the car will likely run fine for the drive. If one however gets it started and does not let it heat up, it will probably exhibit misfires and stumbles until it does.

Even after letting it warm up, the car is still sensitive to feeding it throttle. If gentle throttle is applied it will generally run and drive smoothly, but aggressive throttle will have it mis/stumble a bit an the overtemp idiot light in the gauge may flicker.

So far, the mechanics have verified we have fuel and spark, and have checked or replaced the following as testing continues:

-new ignition module
-crank position sensor
-coils OK
-fuel pressure and delivery OK

But when the problem exhibits itself, their diagnostic tools shows no injection pulse


I find this last item about lack of injector pulse interesting, and I wonder about the flickering overtemp light. The overtemp light may just be a red herring that is lighting because the engine is about to die, but I thought I'd mention it if it somehow is a clue.

I told my mechanics I'd ask and see if this additional info means anything to the folks here. Otherwise, other than replacing the PCM, we are at a loss.

Thanks,
Al

Have you or the mechanics tested your Coolant Temp Sensor? Though the symptoms are usually the reverse of yours and get worse as the car warms up.

Maybe throttle position sensor (TPS)? I'm not sure it would cause these symptoms, just a suggestion.

al_roethlisberger wrote:

So far, the mechanics have verified we have fuel and spark, and have checked or replaced the following as testing continues:

-new ignition module
-crank position sensor
-coils OK
-fuel pressure and delivery OK

But when the problem exhibits itself, their diagnostic tools shows no injection pulse


I find this last item about lack of injector pulse interesting, and I wonder about the flickering overtemp light.  The overtemp light may just be a red herring that is lighting because the engine is about to die, but I thought I'd mention it if it somehow is a clue.

I told my mechanics I'd ask and see if this additional info means anything to the folks here.  Otherwise, other than replacing the PCM, we are at a loss.

When this problem happens (no Injector Pulse), you need to check the INJ 2 Fuse (10A) for Power in the IP Fuse block. The PCM controls Injector Pulse via Ground Driver, not Power. Power (B+) is supplied continuously as long as the key in "ON".

This is what happened when my Ignition Switch went bad. I went through a CKP, ICM, etc before we figured it out. My Injectors weren't being powered when it failed. It was completely intermittent and lasted for weeks. Hope it helps.

Thanks for all the suggestions, sounds like I have some more testing opportunities

I'll let you know what I find.

Can anyone confirm on what other GM models the PCM is shared with the 1995 SC Riviera?

Hoping to go to the pick/pull this weekend and pickup one or two to test.

Thanks
Al

Here is what I get from Rock Auto:

ACDELCO Part # 16183247

Buick Lesabre 1994-1995
Buick Park Avenue 1994-1995
Buick Riviera 1995
Chevrolet Lumina APV 1994-1995
Oldsmobile 88 1994-1995
Oldsmobile 98 1994-1995
Oldsmobile Silhouette 1994-1995
Pontiac Bonneville 1994-1995
Pontiac Tran Sport 1994-1995

Look correct?

Thanks
Al

One other question, if I find another PCM or buy a refurb unit and change over the PROM... will the PCM operate just fine (and the car run and operate normally) or am I going to have to have some sensors, security, other? reset by a technician with factory tech tool software?

Wasn't sure if this generation of PCM was pretty "dumb" and the PROM carried over all the vehicle specific information and settings such that just swapping it out was a simple 1:1 swap affair (hope so!)

Thanks
Al

Well, LKQ was having a 1/2 price "sale" today (funny, but true) so I got two PCMs and both happen to be refurb units so maybe that is good news.

I'll pop one in after swapping over the PROM and see what happens.

BTW, found two other threads here that may be useful for anyone else researching this issue for the SC Series 1 cars. Based on the thread below, it appears there is no programming needed, and one just pops out the PROM and installs. So I'll give that a try.

https://rivperformance.editboard.com/t6103-where-is-the-pcm-located-in-1995-buick-riviera

Another helpful thread:

https://rivperformance.editboard.com/t10487-write-up-reading-editing-and-burning-chips-for-a-series-1-l67

OK, too soon to declare victory, but the car does run and accelerates normally now.  Hopefully that is not a fluke!  


So, some answers:

1)  Yes, any of the models above exchange the same PCM as the 95 Riviera.
2)  Yes, it is simply a plug-and-play change, just swapping over the PROM.  No programming/reprogramming required.
3)  It is literally a 10 minute job of removing two screws to drop the under dash panel, unplug the three color coded harnesses, move the plastic clips to the side, and the PCM just slides out, maybe 15 if you take your time.  Of course one should disconnect the battery during this process.
4) Reverse process to reinstall (after swapping the blue PROM from inside the case)

So, some interesting discoveries.

I pulled two PCM's from the yard, one from a Lesabre and one from a Bonneville.  Both turned out to be remanufactured units.  When I pulled the PCM from the Riviera, guess what... it too was remanufactured.

After doing a little more Googling and reading about this series PCM, it seems that the 95 units (and probably the 94 too) are failing relatively commonly.  And based on my discovery of 3 for 3 being reman units, I would tend to agree.

That leads one to wonder why.  Who knows.  But it is also interesting that there are so many reman units available, and affordable. I doubt they are replacing ICs, but instead are probably replacing something like a capactor(s), resistor or cold solder work and that stuff is cheap.

So I'm going to tear apart my 'broken' one and see what I find.  It would be interesting to discover that these are failing due to something as simple as that.  It could also be a transistor, which is harder to detect to some degree of course.  But I bet it's something like that.

Anyway, if your 95 starts going wacky in an odd way with no or weird codes that the seemingly appropriate repairs don't address... the PCM may be the issue.  I'd get a cheap one from a yard for $30-50 and see if that clears it up.

We'll have to drive it for a week or two before I'm convinced of course, but so far it seems to be acting more normal than it has for a long while.

What ever happened with this issue, i have been experiencing the same problems with my 95 and it has spent the padt 6 months in and out of the shop.

ajgee2011 wrote:

What ever happened with this issue, i have been experiencing the same problems with my 95 and it has spent the past 6 months in and out of the shop.

If I were you I'd try it.

I just had to change my mass air flow sensor. It's amazing how that little part can affect how the engine runs. My car would start, but would not idle, and it would stall almost immediately. To test it, just unplug the electrical connection. If the car idles and runs, the MAF is the problem. What happens is the MAF tells the PCM there is a lot of air entering the engine, the PCM ramps up the fuel injection, and then the O2 sensor tells the PCM the engine is running rich so the PCM pulls fuel. Back and forth it goes and the engine goes schitzo.