Knock Sensors- Engine stumble

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L8RH8R

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Greetings,
I have a 2004 Tahoe (flex fuel), 185k on the clock and have a couple of issues. Engine was replaced a few years ago and has about 10k on it. I have had this vehicle for more than 12 years and taken really good care of it.
#1 Knock sensors- So like a lot of people I too battle with these. They have been replaced with AC Delco sensors and matching wire harness. All is good for a short period of time before they cause a CEL for both sensors. I have relocated them to the head and block to no avail. The only thing I have not done is check the wires from the ECM to the sensor wiring harness for continuity. I would find it hard to believe that both could be defective. I live in Texas and don't drive this vehicle all that much. Any input is greatly appreciated.

#2. The most recent and aggravating. I noticed a few weeks ago a noticeable engine stumble that is intermittent. First, the vehicle developed a hard start condition. I checked fuel pressure and found 48 psi at idle and holds at 43 psi when off. This problem was most prevalent during cold starts. Holding the gas pedal down allowed it start and run decently after. I believe I tracked this problem down to a failing fuel pump relay (it had obvious hot spots on it as did the AC compressor relay which was having intermittent activation issues, new relays fixed both problems). Now, back to the engine stumble. This issue happens at both low and high speeds hot or cold. It feels as though the vehicle loses power for a split second before returning to normal. I noticed the RPM's will drop to just below 1K. I am not getting a CEL for this. With that being said, I was recently under the vehicle and noticed the temp sensor has a small leak and will be replaced this weekend. I did read elsewhere this could cause an engine stumble. The leak is so slight it burns off before puddling on the ground. I just drove to S. Padre for Thanksgiving. The only issue was the intermittent stumble. I have cleaned the MAF and this did not change anything. I don't want to throw the parts cannon at this thing. My next thoughts after the temp sensor replacement is the crankshaft position sensor and or camshaft position sensor.

Any help is always appreciated.
 
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L8RH8R

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How old are the spark wires? Is it fuel filter original? Check PCM Plug that knock sensors harness lead to? Check for the green/blue crusties and clean the plug contacts real good.
The plugs and wires were replaced when I replaced the engine, so about 10k (AC Delco plugs, MSD wires). Fuel filter was replaced last month (Wix). I will be checking wire continuity as well. It won’t hurt to pull the #1 plug either. Thank you for the reply.
 

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In addition to @rockola1971's recommendation to check and clean the PCM connectors, clean the throttle body. Although you don't seem to be getting a code, carefully inspect the wires back about 8" or so from the throttle body. Those wires can break under the insulation and cause issues.

Also, you might want to perform a cam-crank correlation with a capable scanner, just to rule out the crank sensor (especially if it's been replaced).
 

strutaeng

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Relocated the knock sensors to the head? I've never heard of this. Can you please explain where they were installed?

If they are relocated to the lower sides of the block, you need a knock sensor sensitivity adjustment on the ECU tune, or so I've read.

Are you getting a circuit code or an actual knock code? It could be a legit knock if it's the later.

The ECU will retard spark if a knock is detected, so perhaps these issues are related? I would say hook up a scanner and try to replicate the issue while monitoring spark advance/retard (but also look at other usual parameters like fuel trims, O2 sensor waveform, etc.)
 
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rockola1971

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Relocated the knock sensors to the head? I've never heard of this. Can you please explain where they were installed?

If they are relocated to the lower sides of the block, you need a knock sensor sensitivity adjustment on the ECU tune, or so I've read.

Are you getting a circuit code or an actual knock code? It could be a legit knock if it's the later.

The ECU will retard spark if a knock is detected, so perhaps these issues are related? I would say hook up a scanner and try to replicate the issue while monitoring spark advance/retard (but also look at other usual parameters like fuel trims, O2 sensor waveform, etc.)
The holes are already there to use on the head. Drilled and tapped from GM.
 

rockola1971

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The plugs and wires were replaced when I replaced the engine, so about 10k (AC Delco plugs, MSD wires). Fuel filter was replaced last month (Wix). I will be checking wire continuity as well. It won’t hurt to pull the #1 plug either. Thank you for the reply.
When its dark outside, open the hood and start the engine. Keep your hands outta there! Look for flashes of bright light from spark plug area all the way up the wire to and around each coil. Also listen for cracking and popping. That cracking and popping and flashes of light is ignition high voltage seeking chassis ground before it gets to the spark plug. It aint suppose to. If you find what ive described then you have to figure out is it a cracked spark plug, cracked/chaffed spark plug wire or cracked ignition coil. All of these will cause weird misfire problems on the affected cylinder. If you have access to a decent scanner it will tell you if you are getting misfires, the count and which cylinder(s).

And remember, BEFORE you go poking your hands around a running engine that if you have the problem ive described then your body becomes chassis ground while standing on earth with your hand finding that spark which is 20k volts+. You will piss your pants or worse. Your wife will ask why are you crying and make fun of you while laughing and she will tell this story for years to EVERYONE.
 

lyncht1967

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When its dark outside, open the hood and start the engine. Keep your hands outta there! Look for flashes of bright light from spark plug area all the way up the wire to and around each coil. Also listen for cracking and popping. That cracking and popping and flashes of light is ignition high voltage seeking chassis ground before it gets to the spark plug. It aint suppose to. If you find what ive described then you have to figure out is it a cracked spark plug, cracked/chaffed spark plug wire or cracked ignition coil. All of these will cause weird misfire problems on the affected cylinder. If you have access to a decent scanner it will tell you if you are getting misfires, the count and which cylinder(s).

And remember, BEFORE you go poking your hands around a running engine that if you have the problem ive described then your body becomes chassis ground while standing on earth with your hand finding that spark which is 20k volts+. You will piss your pants or worse. Your wife will ask why are you crying and make fun of you while laughing and she will tell this story for years to EVERYONE.
HAAA! sounds like you learned that lesson personally...same reason I don't let wife around when I'm working. And I'm not laughing at you, I'm laughing with you. I've gotten bit by secondary voltage and it is NOOO fun, my arm was numb up to my shoulder for the rest of the afternoon! thank god wife wasn't around. lol
 
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L8RH8R

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Well, it seems I may have fixed the stumble issue. Checked the #1 plug, looked good. Went to replace the leaking temp sensor and found it was loose, WTF. I was able to remove it without tools. Installed the new one, and cleared the knock sensor codes. The vehicle ran great with no stumble at all. If this would have not fixed the problem I would have felt the stumble in no time. Not sure if I fixed the knock sensor issue or not, only driving will tell.
Thank all of you for the input.
 
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