4.3w low power

[5693dtcd]

Originally Posted by need_a_new_truck

Check the ignition control module.

I tried a different one and the old one was tested when it was in the shop. No difference.

[5693dtcd]

Hey there everyone-
I was unable to do an injector balance test. From some of the techs I talked to and the miles on the vehicle I went ahead and replaced the injector "spider " assy. i came away with a very slight improvement.
I hooked it up to a scanner and the timing still retards. I work at a medium duty GMC dealer. I was able to look up on the computer, per VIN, a module located inside the VCM. It is called a Knock sensor module, p/n 16208961. If this is bad- would it cause the timing to retard?
Let me know if you have ever seen this problem.
Thanks

[MT-2500]

Originally Posted by 5693dtcd

Hey there everyone-
I was unable to do an injector balance test. From some of the techs I talked to and the miles on the vehicle I went ahead and replaced the injector "spider " assy. i came away with a very slight improvement.
I hooked it up to a scanner and the timing still retards. I work at a medium duty GMC dealer. I was able to look up on the computer, per VIN, a module located inside the VCM. It is called a Knock sensor module, p/n 16208961. If this is bad- would it cause the timing to retard?
Let me know if you have ever seen this problem.
Thanks

That thing is giving you a fit.
The knock sensor module picks up the knock sensor signal and can and should retard the timing if a knock is present.
You might try and unplug the knock sensor and see what it does.
Check actual engine timing with a timing light and see if it matches the VCM/pcm readings.

The VCM gets it timing reference from the crankskaft sensor.
You might want to check it real close to.
If you have a lab scope it works goos on checking sensors.

But also remember the camshaft retard setting has nothing to do with the actual engine timing.
It only centers the dist rotor to fire direct on the plug wire terminal.

Is there any VCM reflashes out for your truck or tsb on it?
Good luck.
MT

[5693dtcd]

Originally Posted by MT-2500

That thing is giving you a fit.
The knock sensor module picks up the knock sensor signal and can and should retard the timing if a knock is present.
You might try and unplug the knock sensor and see what it does.
Check actual engine timing with a timing light and see if it matches the VCM/pcm readings.

The VCM gets it timing reference from the crankskaft sensor.
You might want to check it real close to.
If you have a lab scope it works goos on checking sensors.

But also remember the camshaft retard setting has nothing to do with the actual engine timing.
It only centers the dist rotor to fire direct on the plug wire terminal.

Is there any VCM reflashes out for your truck or tsb on it?
Good luck.
MT

I will have to check on tsb's. I finally found a knock sensor located at the back of the engine. The parts breakdown lists 2 but illustrates 1. The one I found is next to the oil pressure sensor just to the left of the distributor. The fuel supply and return pass right over it. Its is going to be difficult to even unplug. I can see it and even touch it but I cant disconnect it. Do you know where the second one is at?

The only reason I havnt looked at it before is when I checked it with the scanner the voltage fluctuated indicating it was working. If it runs great with it unplugged, do I need a new sensor or module? Other than a new computer that's the only thing that retards the timing, I think. I'll checkthe crank sensor sometime also. I'm taking it easy this weekend though. I'll check it next week. Have a good Labor Day Weekend!!

[5693dtcd]

The check engine light came on over the weekend. I have a code P0420 for a catalyst efficiency low on bank 1.
I also have a code P0304 misfire on number 4.
The code P0420 is what has me concerned. What would cause a this code?

[MT-2500]

Originally Posted by 5693dtcd

The check engine light came on over the weekend. I have a code P0420 for a catalyst efficiency low on bank 1.
I also have a code P0304 misfire on number 4.
The code P0420 is what has me concerned. What would cause a this code?

420 and 430 codes can be caused by engine running problems.
Fix missfire and engine running problems first.
Then clear codes and see if 420 or any other codes come back.
MT

[5693dtcd]

Originally Posted by MT-2500

That thing is giving you a fit.
The knock sensor module picks up the knock sensor signal and can and should retard the timing if a knock is present.
You might try and unplug the knock sensor and see what it does.
Check actual engine timing with a timing light and see if it matches the VCM/pcm readings.

The VCM gets it timing reference from the crankskaft sensor.
You might want to check it real close to.
If you have a lab scope it works goos on checking sensors.

But also remember the camshaft retard setting has nothing to do with the actual engine timing.
It only centers the dist rotor to fire direct on the plug wire terminal.


Is there any VCM reflashes out for your truck or tsb on it?
Good luck.
MT

It was so much work just to get to the knock sensor to even try to unplug it,I went ahead and replaced the knock sensor. it did not make any difference. I have checked for TSB's. I found one that has something to do with a transmission pressure switch. It bassically states that if the pressure is out of spec - to call the tech hotline. I just don't see how it has anything to do the tranny. Setting in neutral or park it still runs poorly.

I've been told that if the cranksensor was bad, it would have a hard start or no start condition. It starts good.
Thanks.

[kb3jhp]

i would start looking at the computer if you can find a another blazer or jimmy with the same year and drive train you could try the comp out of that if you can find and owner one willing to help you

[kb3jhp]

also your readings from the scanner with the throtel at 59% eng load should be higher than 25% i think the comp uses the maf senser to no the eng load
MAF gm/sec 43.68 seems high for only 25% load

I threw a code today coming home. P0101 Mass Air circuit out of range.
I did a freeze frame on the scanner and received the following readings

Eng. load 25%
Coolant 149
ST Trim 185
ST trim % 44.5
LT Trim 138
LT Trim % 7.8
ST Trim 2 % -24.2
LT Trim 2 % 0
MAP 29.3
RPM 2306
Veh spd 52 mph
MAF gm/sec 43.68
TPS 59%

[5693dtcd]

Originally Posted by kb3jhp

also your readings from the scanner with the throtel at 59% eng load should be higher than 25% i think the comp uses the maf senser to no the eng load
MAF gm/sec 43.68 seems high for only 25% load

I threw a code today coming home. P0101 Mass Air circuit out of range.
I did a freeze frame on the scanner and received the following readings

Eng. load 25%
Coolant 149
ST Trim 185
ST trim % 44.5
LT Trim 138
LT Trim % 7.8
ST Trim 2 % -24.2
LT Trim 2 % 0
MAP 29.3
RPM 2306
Veh spd 52 mph
MAF gm/sec 43.68
TPS 59%

Do you know if the Eng. load affects the ignition timing? I have changed the MAP sensor and the knock sensor. The only other thing that I know of that controls the timing is the computer.

[5693dtcd]

Originally Posted by 5693dtcd

It was so much work just to get to the knock sensor to even try to unplug it,I went ahead and replaced the knock sensor. it did not make any difference. I have checked for TSB's. I found one that has something to do with a transmission pressure switch. It bassically states that if the pressure is out of spec - to call the tech hotline. I just don't see how it has anything to do the tranny. Setting in neutral or park it still runs poorly.

I've been told that if the cranksensor was bad, it would have a hard start or no start condition. It starts good.
Thanks.

I've changed the knock sensor- no change. Wht about the knock module in the VCM? If it has failed - will it retard it too much? If I remove it will the engine run? If it does and runs good- that might be my problem. Can the VCM be checked out somehow? I don't know of anything else that is making the timing retard so much.
Thanks

[kb3jhp]

with a scanner with live data drive and see what your knock retard is and what your ignition advance is. it should advance as far as 45 degrees at times at least thats what mine does and it runs real strong. also watch what it does when you first hit the gas you really should not see any retard i have a live data scan gauge and the only time i see it retard in the timing is when towing and pulling a hill and it starts to ping on gas post back wat you record. also have you checked your tps

[5693dtcd]

Originally Posted by kb3jhp

with a scanner with live data drive and see what your knock retard is and what your ignition advance is. it should advance as far as 45 degrees at times at least thats what mine does and it runs real strong. also watch what it does when you first hit the gas you really should not see any retard i have a live data scan gauge and the only time i see it retard in the timing is when towing and pulling a hill and it starts to ping on gas post back wat you record. also have you checked your tps

The scanner I had, I was borrowing. I have since returned it. I have never seen it advance more than 25 degrees. The timing retards to around 10 degrees with the TPS at 100%, RPM around 3000-3500. The engine is trying to get more RPM but it is missing and running rough because of the timing. I tried a new knock module(located inside the VCM), MAF sensor and an air temp sensor in the air intake. NO CHANGE!!! I talked with a GM tech who really thought it was the MAF. He thought it was leaning out the mixture too much, this would cause a ping, so it retards the timing. All this leads to low power.
I've had it to two dealers. One was wrong. The other looked at it for 3 or 4 hours. When he told me whatever it is it wont be fixed today I told him to stop what youre doing. I cant afford that kind of comment. I work at a heavy duty truck dealer so I know about diesels and am mechanically inclined. I can do research and install parts myself. This has got me though.

[5693dtcd]

Originally Posted by kb3jhp

with a scanner with live data drive and see what your knock retard is and what your ignition advance is. it should advance as far as 45 degrees at times at least thats what mine does and it runs real strong. also watch what it does when you first hit the gas you really should not see any retard i have a live data scan gauge and the only time i see it retard in the timing is when towing and pulling a hill and it starts to ping on gas post back wat you record. also have you checked your tps

While I had the scanner the TPS reading did change smoothly as I moved the pedal. Don't think I havn't thought about changing it though. This thing has got a ghost I cannot figure out.

When it's fixed I'll post what was wrong.

Thanks for reply

[Blue Bowtie]

I must have missed something... What year is the truck? That will make a difference, since the ECM/PCM would be different, and the control scheme would be different.