I think I need to replace the PCM on my 2003 Suburban ??

calsdad · September 12, 2015

I've been chasing issues on my 2003 2500 4WD Suburban for weeks now.

Posted a few other threads here - and got some very helpful answers

This started a few months back when I saw "fuel supply insufficient" codes getting pulled by my Autoenginuity scan software, along with a random misfire code.

I thought the random misfire might be due to the low fuel pressure - but the truck still drove ok so I put off diving into it. Then the truck started stalling at speed on the highway (engine would just quit). At that point I figured it's time to start diving in and see what was wrong.

I started by replacing the fuel filter and the plugs and wires. The truck has 75k on it - I never replaced them before - and I bought it used so I don't know the previous maintainence schedule. That didn't fix anything - the truck was still running lousy ( it had started missing and dying under load by this point). I had found a couple of plugs that appeared to be cracked and a couple of wires looked a little corroded - and the truck might have idled a *little* better - but the main problem was still not solved.

So my next step was to pull the gas tank out - and replace the pump. I already had a problem with a leak when the fuel was completely filled so I figured I should do the pump anyway just to fix that problem. Pulled the tank - tested to see if it was leaking by turning the tank on it's side when it was out, sure enough: fuel peed out around the fuel pump when it was on it's side. So I replaced the pump with an OEM AC/Delco from the dealer - and carefully coated the top of the pump and the new lock ring so they wouldn't rust all to hell like the original one did.

Put the whole thing back together - and no more fuel supply insufficient code - and no more leakage from the tank. Two problems fixed.

But the truck still ran like crap. Still dying on the highway still missing a lot and generally running lousy. Along the way - I started seeing a camshaft position sensor failure code. So my next step was to replace the camshaft sensor. I pulled the manifold, got a sensor from the dealer - and installed it, and put the manifold back in. Ran the truck - and the code was gone. But the truck still ran lousy, and - after taking it out twice - now I get a crankshaft position sensor code. So I replaced that too.

Took the truck out for a drive - and now both codes are gone. And it's *maybe* running a little better. So a couple of days later - I took it up to HD and picked up about 3000 pounds of concrete bags. On the way back (down the highway) - it's back to running lousy : REALLY missing bad , almost bucking - etc. I managed to make it home with the thing - and pulled the codes again.

This time the cam and crank position sensor codes are back - again. So now I'm thinking - there's something else wrong.

It seemed like an ignition thing - and a couple of the coils were corroded on their contact points - so I replaced all the coils just to try to eliminate some candidates for the problem. It didn't really make it run any better though.

I posted what I'm seeing on another forum I spend some time on - and a few people suggested looking for wiring breaks, especially in and around the fusebox. So I pull the fusebox apart - and sure enough: Mouse chewed the wiring up.

I found about 12 wires that were in various stages of damage. From completely chewed thru to just having insulation missing. I soldered up (butt jointed) the ones that were bad and were either cut thru or really fraying - and the rest where it was just insulation missing, but the wiring looked ok - I carefully wrapped them with 3M automotive electrical tape (high temp).

That seemed to maybe make the thing run a little better. But it still had a miss - and would occasionally die on me. It ran well enough though - that I took the truck about 1/2 hour away to friend who offered to put the thing on his Snap-On Sonus scan tool and see what could be pulled from it.

What we found was that the injector pressure test was fine. The fuel pressure appeared to be fine. He sprayed Windex on the whole coil/wire area in pitch darkness - and neither of us could see any evidence of arcing or leakage. He also ran a test where he sprayed brake cleaner around the intake - while I watched the o2 values - and there was some evidence of a manifold vacuum leak.

I already suspected that I did something wrong - because I had re-used the original intake manifold gasket - and the holddown bolts (didn't know any better at the time). So my next step was to do that and at least fix that issue.

He also pulled the PCM out of it's mount and ran the scope on the line for the crankshaft position sensor - and he said the waveform was pretty much perfect. So we didn't really suspect fault wiring.

On the way home though - the truck ran absolutely HORRIBLE. It literally died probably 30 times. The best I could do on the highway was to maybe get it up to 55 mph going down a long hill - and the engine would keep kicking out. I think it was constantly going into the limp mode. A couple of times the engine died - and it took me 15 minutes to get it started again and keep it running.

At this point I was completely baffled. The only thing we really did that could have made it turn so suddenly worse - was that we physicaly pulled the PCM out - and I was thinking maybe we jostled the wiring somehow.

So the next day I really pulled the wiring harness apart all around the fusebox to the PCM. I found two wires with some insulation missing - they both traced out to the trailer wiring harness - and nothing else. Everything else looked physically good.

I pulled the intake manifold again - completely cleaned up the head surfaces and the manifold surfaces - and used a Fel-Pro PermaDry gasket - and new hold-down bolts and put the whole thing back together. Just because - I also replaced the fuel pressure regulator while I was at it.

Started the truck up - it runs fine in the driveway - so I took it for a drive down the street. I got about a mile or so when it started to miss, so I turned around. Got about halfway back to the house - and it completely quit. Every time I tried to start it - it would just die. Since I suspected a heat problem or heat soak problem - I just sat there and let it cool off. Took about an hour before it cooled enough so that the temp gauge was readin 165-170. It started but didn't want to - and I got it running just good enough to get it home that last half mile

Pulled the codes again - and I see a MAP sensor message. Went to check under the hood - and sure enough I had not plugged it in. I let the truck sit til about 10PM that night - and tried again (with the sensor connected) - and it started missing pretty much as soon as I pulled it out of the driveway - so I backed it right back in and parked it.

So now I'm really getting baffled. Because the thing is still throwing the crank and cam sensor codes - and it's missing like crazy and sputtering and dying.

To try and rule out some other candidates - I picked up a fuel pressure gauge myself and re-ran some tests. With the truck cold - and when I just turn the ignition key on: It spikes up to about 60psi - and then settles to about 58. With the engine running (cold) - I'm seeing about 52 steady - and the psi goes up to 60 or more if I rev it up to 1500-2000 rpm.

So I think the fuel pressure is good.

The next thing we did was I drilled two holes just before each of the cats - and put a pressure gauge on them to see if the cats were maybe plugged. The gauge reads 1-3 psi at idle. If I rev it up - the gauge(s) (we tried two different gauges) - will bounce up to maybe 4 at best.

So I don't think I have any obstruction in the cats.

Then he (my father was helping) put a timing light on one of the plug wires to see if he could figure out if there was way to check the timing by putting mark on the crank pulley or something. While doing this - he noticed that the timing light was "missing". We both watched it - and sure enough - every now and then - there would be a missing pulse. It would typically coincide with feeling a miss out of the engine too.

We tried the same thing on all 8 cylinders - and they all did the same thing.

So now we're thinking - what's common for the ignition system - on all 8 cylinders? The only thing I can think of - is the PCM.

I should add that when I had the manifold off the second time - I checked the two ground wires at the back of the cylinder heads - I pulled on them - and they were both on there tight. I also checked the ground wire that goes down from the battery to the front bumper crossmember - and that one looks tight too.

At this point my only ideas on this problem are: bad PCM - and maybe a bad ground somewhere.

Any other suggestions? What do I have to do to replace the PCM? RockAuto has them listed for not too bad money - but I know they need programming.

Doug_Scott · September 13, 2015

A couple of things, first, you know you have had a rodent problem. They don't just eat wires in one spot. You should be looking the entire harness over. Second, to check a ground connection you need to remove it and check the actual terminal on the wire. You want to see inside the connector, try bending and pulling the wire looking for any white powder coming out.

You should also be testing all those things you tested with your father when the truck is not starting. No sense testing when the truck runs.

I have never tested or found any specs for what is acceptable pressure before the cat. Generally when a cat is plugged, you will not be able to apply much more than about quarter throttle. Up to the quarter throttle it will behave pretty much normally.

calsdad · September 13, 2015

A couple of things, first, you know you have had a rodent problem. They don't just eat wires in one spot. You should be looking the entire harness over. Second, to check a ground connection you need to remove it and check the actual terminal on the wire. You want to see inside the connector, try bending and pulling the wire looking for any white powder coming out.

You should also be testing all those things you tested with your father when the truck is not starting. No sense testing when the truck runs.

I have never tested or found any specs for what is acceptable pressure before the cat. Generally when a cat is plugged, you will not be able to apply much more than about quarter throttle. Up to the quarter throttle it will behave pretty much normally.

Thanks.

I've actually pulled apart a bunch of other places on the harness - and bear in mind that the majority of it is surrounded by the corrugated loom sheath stufff - and found no visual signs of problems. It's all intact. The only mouse eaten portions were right around the fuse box area (and inside) - where a mouse had obviously taken up residence.

I've had a bunch of suggestions that the cat could be plugged - which is why I checked it. There's basically very little pressure before either one of the cats * and * the truck - even when it's running like crap - will occasionally be able to rev right up. As a for instance: I have run the crankshaft position sensor learn cycle a number of times - and it always revs right up to the 4000 rpm number with no issues. I've got to rule things out at this point - and the cats really don't look like they are the problem.

The grounds were solidly attached - with no obvious corrosion on the wires. But I didn't disconnect and clean them - I suppose I could do that to rule out that as being the cause.

The first thing I tested today - was the fuel pressure. It was fine - (as detailed above) - and the truck actually did start running like crap - and even died shortly after that - the gauge still read good. So I don't think the fuel pressure - is a problem either.

If I can get it back into the state where it's not starting at all - or running extremely lousy - I can do the timing light test on it to see if I'm getting a consistent strobe out of it.

Doug_Scott · September 13, 2015

Something I have seen before is a plugged cat. And it would rev to redline in park, provided you did not floor the gas pedal. With no load on engine, it will rev to redline at quarter throttle, even with a plugged cat. But, if you floor the gas pedal, it will not rev to redline. It will likely not even get past 2500rpm under WOT.

Many years ago I had a car come in that was stored in a barn for 2 years. It was a mid 60's Dodge Polara. Nothing fancy, just a nice clean original car that got parked because the husband died unexpectedly. The car would start, but would not drive. The woman's kid tried to fix it, doing all the common things that he thought it was. In this case it was a mouse nest inside the muffler. The woman got lucky, mice usually prefer to nest inside the car, and just ruin any fabric they come in contact with.

Later that summer there was a 6 month old commercial Dodge van towed into the dealership. The back bumper and doors were buckled up. Owner was at his cottage over the weekend. He parked his van where he normally does late Friday night. Sunday night he got back in the van, fired it up, and put it in reverse to back out of his spot. Van was on fast idle, and this was before we had the apply brake before shifter will move, the van shot backwards, he applied the brakes only to find the pedal go straight to the floor. Van backed into a tree, that actually stopped the van from going down a steep embankment to the lake. He had no idea what was wrong, and since it was under warranty he had it towed in to the dealership. I got the job to check it out. All three flex hoses were gone. The fittings were still there, and connected, but, the hose part was completely missing on all three lines. Later in the week they found a dead skunk under the deck. Apparently it had eaten the hoses. Total damages were just over $1k, and that was in 1978. Had the tree not been there, the van would likely have gone into the lake, and would have been written off.

calsdad · September 13, 2015

Something I have seen before is a plugged cat. And it would rev to redline in park, provided you did not floor the gas pedal. With no load on engine, it will rev to redline at quarter throttle, even with a plugged cat. But, if you floor the gas pedal, it will not rev to redline. It will likely not even get past 2500rpm under WOT.

Many years ago I had a car come in that was stored in a barn for 2 years. It was a mid 60's Dodge Polara. Nothing fancy, just a nice clean original car that got parked because the husband died unexpectedly. The car would start, but would not drive. The woman's kid tried to fix it, doing all the common things that he thought it was. In this case it was a mouse nest inside the muffler. The woman got lucky, mice usually prefer to nest inside the car, and just ruin any fabric they come in contact with.

Later that summer there was a 6 month old commercial Dodge van towed into the dealership. The back bumper and doors were buckled up. Owner was at his cottage over the weekend. He parked his van where he normally does late Friday night. Sunday night he got back in the van, fired it up, and put it in reverse to back out of his spot. Van was on fast idle, and this was before we had the apply brake before shifter will move, the van shot backwards, he applied the brakes only to find the pedal go straight to the floor. Van backed into a tree, that actually stopped the van from going down a steep embankment to the lake. He had no idea what was wrong, and since it was under warranty he had it towed in to the dealership. I got the job to check it out. All three flex hoses were gone. The fittings were still there, and connected, but, the hose part was completely missing on all three lines. Later in the week they found a dead skunk under the deck. Apparently it had eaten the hoses. Total damages were just over $1k, and that was in 1978. Had the tree not been there, the van would likely have gone into the lake, and would have been written off.

I checked the cats. Drilled holes in the pipe just before each one and put a gauge on them. Then started the engine up - at idle there's little to no pressure. If I rev it up to say 3000 rpm - the needle bounces back and forth between vacuum and say 3-4 psi. Both cats act the same. I can put the brakes on (parking and push my foot down on the regular brakes) - put the thing in drive - and rev it up to 2500 rpm or so - and I see the same behavior out of the cats.

I've run the crank relearn process probably a dozen times - I can get it to rev to 4000 rpm with no problem. One time I even screwed up the relearn process (no parking brake) - and it revved up to like 5500.

I really don't believe the cats are plugged.

bagger97 · September 13, 2015

GM used to use a double wall cross over pipe, not sure if they still do, and if the inner pipe collaspes it will act like a plugged muffler.

calsdad · September 13, 2015

GM used to use a double wall cross over pipe, not sure if they still do, and if the inner pipe collaspes it will act like a plugged muffler.

It really doesn't look like any sort of exhaust problem. I can see misses if I stick a timing light on it and just watch the strobe.The

I put a gauge in the pipe before each one of the cats - and there was almost no backpressure. I'm also pretty sure the pipe that is in there is a single wall.