The ICMs in these trucks are heat sensitive and can have heat related faults. That is why to really test one, you need to load it up a few times to get the temperature up there.

And monitoring the timing from a scan tool won't really tell you much without monitoring the actual spark timing with a timing light on the engine. If this problem only occurs while driving down the road, you might have a hard time using the timing light (remembers napa commercials from a few years back where the guy crawls all over the vehicle - LOL). But you may see it by getting the truck all nice and warmed up, then having a friend slowly rev the engine with the truck in reverse, parking brake set, wheel chocks in place, and the brakes held while you monitor the actual timing with the light. Don't do this too much or you may cook the transmission fluid, but enough to possible see the fault in action. If the timing from the PCM and the timing read with the light line up, the problem lies elsewhere.

I'm not sure how places diagnose injector problems now, but I know that I used to do injector load tests by monitoring the fuel pressure as I energized each injector for a specific amount of time. If the pressure drop was fairly uniform, there wasn't an injector problem.

 

Once it starts to act up I can pull over put it in park and step hard on the gas and it will only rev up to about 2000 rpms or little less. So maybe it does not really need a load on it (the engine).
If I let it cool down for about 5 or so minutes it drives OK till it warms up again. Whatever it is that is warming up (or getting hot) I don't know yet. Instead of going to the trouble of getting the ICM cooking hot and testing it at a parts store I'll just buy one. Think I should stick with ACDelco for this part? I replaced this unit acouple years ago and didn't put a ACDelco back in to it. The Acdelco's run about $100.00 as compaired to other brands maybe around $50-60 dollars.
I'm starting to notice it is not starting up right away now on the first turn of the key. Takes maybe acouple tries for it to crank over and run. Don't know if that is in anyway related or not. Mainly does it on a cold start.

Thanks,
Brian

 

I was under the impression that the computer used the signal from the ICM to determine whether the engine was running (turning over) and used that information to determine whether to pulse the injectors or not. May be different on the newer motors though, I have not played with them as much....

 

When talking about the mid-95 and earlier distributors with the ICM in the base of the distributor, then you may be correct as I believe the ICM also doubled as a CMP sensor. On the later models, the ICM was separate from the distributor and the distributor had a dedicated CMP sensor that the PCM used for injection & spark timing calculations.

 

Well, not good news I guess. Bought a new ACDelco ICM and put it in. Drove the car for about 10 or so miles and still doing the same thing. So, I guess the ICM wasn't defective after it got hot. Back to the drawing board.
Recorded some live data during the test drive. Noticed that when I floored the gas pedal when it was acting up( losing power and bucking) the TPS sensor was at 100 % and then the computer went into Open Loop. No codes though. This shows on my scanner but when I transfered the data to my computer to create a report it showed the last 2 frames as Closed looped. Maybe just an error in how it send the data to the computer. Looks like all my 02 sensors are not outputing anything or very little. I'll attach a text file here to show the captured frame data.

So far replaced Fuel pump, fuel filter,fuel pressure reg., New Air Filter, throttle body cleaned inside/out and new gasket seals, TPS sensor, Distributer cap and rotor, Ignition module, IAC valve, Cleaned EGR, New ACdelco Plugs (eliminated misfires created by Bosch plugs), New Plug wires, 2 new upstream 02 sensors (elimanated 02 sensor code). Looked at MAF sensor and wasn't really too dirty. Checked many electrical connections to sensors, Checked for worn or deteroriated vacuum lines (replace a few).
I am finding some transmission oil in the line to purge canister. Pulled the ball canister off from under the hood and it was half filled with transmission fluid. Did I over fill transmission maybe? Could this tranny fluid have gone elsewhere and screwing things up?
Have not replaced the PCV valve as I just sprayed carb cleaner in it and it rattles when shook.
Brian

 

I think I remember seeing OL operation at WOT with my Autoenginuity scanner on my Bravada as well so I'm not sure I would be too concerned with that. I don't think I recorded that particular status though and I don't have my scans at work with me.

The STFT & LTFT values have me a bit perplexed... They display as percentages... 25% is HIGH for a fuel trim, let alone STFTs of 50%...

A snap shot of your O2 sensor voltages will never look quite right and I'm not sure what your scanner is reporting on the O2 sensors as a percentage... You should be looking at your O2 sensor cross counts if it is available. That will be a direct reading of the activity of the sensor.

It is possible there is an intermittent ground somewhere in the system or that the PCM is at fault. Now, don't go running out and buying a brand new PCM to 'try out'. If you have a junkyard close by, go snag a PCM out of another '97 Blazer and drop it in. You should be able to get one from a junkyard for around $50 (going rate at the local pick-n-pull yard). If you are lucky, you can even return it if it doesn't work for a refund! LOL

Have you tried removing the O2 sensor directly ahead of the catalytic converter and driving it like that? It'll be loud, but if that cures the problem, then it is possible that something is causing the catalytic converter to plug up when really hot...

 

Dude, like I said before its the cat.. demand that the dealer replace it.. its probably still under warranty...

 

Longterm (25%)and shortterm (50%)fuel trims are pegged at max I guess from what I read elsewhere. Yet O2 sensor voltages have dropped to near 0 volts output. This is when it is the most serious. Something really wrong here.
I just replace my Ignition Coil with a new ACdelco one last night. It was one of my last hopes. Well this morning same thing after about 15 miles it lost most of it's power. Couldn't even make it up an off ramp on the thruway. Shut it down for about 2 to 3 minutes, restarted, then it had much power and I made up the off ramp and to work. Was really bad also on the way home from work last night. Lucky I was able to get it up to 10 mph.

Engine ground? Well I don't know where they all are, but would that effect everything? All my sensor readings? For instance my TPS sensor gets 10 volts in, and a ground, with a variable voltage out. Seems that is working correctly according to my scanner. No noisy voltage spikes or dropouts.

The PCM/ECM possible problem if it is bad. But to turn car off for acouple minutes when acting up, and back on, then works normally for awhile seems strange to me. As if there was something overheating in the computer. But 2 to 3 minutes wouldn't seem like enough time for any electronic part to cool down enough to start working normally again. After restarting it this morning on that exit ramp I must have driven it for about 5 more minutes and didn't act up really bad where I couldn't accelerate. Still not leaving that option out though.

The same would go for the CAT gets plugged but 2 to 3 minutes to cool down makes it unplugged? Humm?

So, I read more and more on this forum about people with similar problems. The MAF sensor is now coming to mind as it can effect fuel trim maybe by alot if faulty, or really dirty. I looked at it didn't look dirty to me but maybe I have no idea what a clean one really looks like either. I'm not getting any MAF sensor fault codes.

I'm going to unplug the the MAF sensor before I leave for home tonight and watch my fuel trims and O2s and see what happens.

Suppose I could stop by a parts store and pick up a cleaning spray for it anyways.

Also wondering about a broken clip that hold the plastic cover on to my throttle body. The clip broke off and the cover is still held on with a plastic lug like thing. So, I suppect I'm getting alittle more air into the throttle body then what is just passing through the MAF sensor because the cover is not fully tight on the throttlebody. Don't know how much a difference that would make as problems should arise even when the after driving just acouple miles.

Guess I should focus on these excessive fuel trim values as a clue to what is going on.

Thanks,
Brian

 

A leak behind the MAF will cause problems all of the time if it is a constant leak.

The MAF is constantly being read by the PCM for use in determining the operating characteristics of the motor. Unplugging it will determine if it is possibly contributing to the problem.

One thing that does come to mind is the switch from open loop to closed loop. When you restart the vehicle, there will be a period of time when the engine will be in open loop operation while the PCM monitors sensors to verify that conditions are right to go to closed loop. This shouldn't be 2-3 minutes though...

 

Just can't help think that something is overheating somewhere. As for last night I lost alot of power. It was pretty bad. I pulled over into a parking lot and sat there for about 5 minutes with the engine off. Restarted it and was able to drive it maybe about another 7 miles without a major problem. It did start to get bad (slightly noticeable)during that time but I kept the speedat about 45mphand made it home. So, I'll do the MAF thing (unplugging it).
Thought about it being a timing issue. Timing belt/chain but thinking about that didn't seem to make any sense unless the belt is expanding due to heat and throwing off the timing (if it's a belt and not a chain). But then again shutting down and restarting shouldn't temporary cure that problem.
I donotice the OpenLoop operation for acouple minutes until the ECM checks everything then switches over to closedafter runningfor a minute or two. Last night I took more data as it was acting up but it remained in closed loop operation the whole time.
Fuel trims were jumping high and O2s were barely showing any output.
Here is just one frame of this data.
STFT B1 % 35.1
LTFT B1 % 25.0
STFT B2 % 37.5
LTFT B2 % 25.0
MAP (inHg) 17.4
RPMs 1295
Speed (mph) 20 <Ready to race now
Spark Advance(degrees Cyl #1) 30
IAT (F degrees) 64
MAF (lb/min) 2.846
TPS (%) 10.9
O2S B1 S1 (v) 0.0 <Nothing?
STFT B1 S1 (%) 49.4
O2S B1 S2 (v) 0.015
STFT B1 S2 (%) NA
O2S B1 S3 (v) .110
STFT B1 S3 (%) NA
O2S B2 S1 (v) 0.0 <again
STFT B2 S1 (%) 50
FUEL Sys 1&2 in Closed Loop
Calc Load (%) 10.5
ECT (degree F) 199

Not normal is O2 sensors and Fuel trim. With upstream O2 sensors being at 0.0 does it mean it sees perfect conditions and is not affecting the fuel trim? Think maybe I should get a better look at the O2 sensor wiring harness? Possible break somewhere? Yet fuel trim is way high.


Brian