Hello, I am new to this forum and need help! I have a 2001 Montana with the 3.4 engine. The car runs and then just dies, won't start sometimes and will idle for hours at other times. I have changed the fuel pump and filter, plugs and wires, all 3 coils and ignition module, replaces PCM, every sensor on the engine, both crank sensors, cam sensor, throttle position sensor, air intake sensor, Map, MAF, EGR Valve, everything on the engine I know of besides the knock sensor. Also have all new fuel injectors.

The engine can be running fine and then just cut out or start to judder like I am out of fuel, other times like it has just turned off clean, the tach will go to 0 and then without changing the key position it will look like I restarted the car with all the dash lights flashing. I am beginning to wonder if it could possibly be the ignition switch with the key sensor being intermittent?

I have checked ground connections on most sensors and the 5V power circuits. Cleaned the ground connections I have found but nothing has really been very dirty at all. The car has about 220K on it but it is the second engine and it was a crate engine from GM, so it is only about 65Kmiles on the new engine. The original engine did the same thing but almost always if it had rained or when it was very high humidity. It would go away then if I could get the engine to warm up somehow, but now it has nothing to do with water, I am in California and it not wet anywhere or even damp when it happens. Oh yes the car would show no dtc codes related to this. It would never throw a code or show the check engine light.

I have a Actron CP9580 scanner and was watching it run and every sensor seemed to be working correctly from the list it will show and then it just cuts out.

Just for those who wonder I also have a 1967 Firebird. I wish the Montana was anything like working on the old bird which anyone can fix.

Steven

By the way I was a smog and front end mechanic when I was much younger and know my way around a car, but this one is really got me stumped. Done everything on a car except for upholstery and that includes body work and custom body work and building kit cars and tooling for them years ago.

I think you could be on the right track with the ign switch or possibly a ignition relay. If you lose communication with the PCM when this happens you know it'* a power or ground issue.

i was kind of thinking something with bcm, does the security light do anything? not sure that would have anything to do with the flickering. unless a bad ground is also the ground for the bcm

Today I will be looking into the BCM and the ignition switch PassKey module. I'm not sure yet how you would test either one but I do have the factory manual. I never really paid any attention to the security light which is on top of the dash by the window, I'll have to see what it is doing? Not even sure what normal is as I have never really looked at this before in all the time I have had the van.

I have seen some people posting on the net about bypassing the ignition switch by cutting the wires but I am not about to do that. Is there any tests for the PassKey system besides seeing if the light is going on? Since this is an intermittent problem it has been a bear to find. What other ground points should I look at?

I am not getting the oft mentioned security light, I checked that out and it is functioning properly, goes off when the key is installed and never comes back on after that. Moved a few fuses around inside the car just to see if they had dirty connections but didn't change anything. I had already put new relays into the underhood fuse block.

How would I know if the PCM is loosing communication, any ideas on that? The car started right up cold, ran for awhile and then after about 5 minutes as it was warming up just shut down and wouldn't start again. I know this isn't the old starter problems we used to have on older style Chevrolet'* with hot starter wires, I am running out of ideas what to try next.

I got the car to run long enough to use my Actron CP9580 scanner and capture a bunch of frames of data. Everything looks good and normal. I can copy the result if someone wants to look at them. Not sure how to save that data to a file directly from the scanner? Maybe I can figure that out. It almost appears that when it dies it pulls out all the timing but I'm not sure if that is just because it is shutting off, dying? Here are the measured outputs from one of the frames the scanner recorded.

Mil status off
ABSLT TPS (%) 4.3
Eng. Speed 2590
Calc. Load (%) 9.4
MAF (LB/M) 1.84
MAP ("HG) 7.7
Coolant (*F) 163
IAT (*F) 72
Ign. Adv. 37.5
Second Air Atmos
Short Term Fuel(%) 0.8
Long Term Fuel(%) 5.5
Fuel Sys. 1 CLSD
O2S11 (V) 0.160
ST FT RM11 (%) -2.3
o2S12 (V) 0.415

fyi not your issue if the light isnt on but normal operation of the system is to start for a few seconds then shut off and blink the light and shuts the injectors down for a period of time after.

but i had a 03 monte that had intermittent starting issues and it ended up being the socket the relay or the ign relay was loose had to carefully tighten socket with a little screwdriver

Justin,
Ill check out the tension on the relays as I did think they weren't that tight when I put in the new relays under the hood. After reading the manual more thoroughly I am starting to suspect the PCM when it is going from the closed mode into the open mode. Once it meets the conditions to go from one mode to the other is when I think it is shutting down, I'm not sure yet how to confirm this but I will be looking into that. Anyone know how they actually test the pcm for operation off the car? At this point I don't think it is any sensor that is causing the problem, it is a communications problem from what I am seeing.

if you unplug the maf or something it will stay in open loop, it thought replaced the pcm already?

The PCM was replaced by the mechanic who had tried to fix the van before me but it is a rebuilt unit, wondering if they missed something. You know how rebuilders are, they do the minimum to see if something works and sell it again. I'll try and unplug the maf and see if it keeps running with it off. Thanks for the suggestion.

Steven