At Idle, I get Reduced Engine Power

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tomloans

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Only happens at Idle speed or just as I come to a stop. I have wiggled all the pertinent wires and can't get it to repeat manually. I have replaced the TB connector and dealt with all the possible ground issues and just changed out the throttle body as well. Before throttle body replacement, it would go into reduced power while driving at speed as well. Now only at idle. The idle is oddly rough as well. It will go fine for 100 or 200 miles then suddenly I will have it happen numerous times in a row then all will be fine again for a few hundred miles. P1516, P0220, P2135 are the codes. If anyone has had these same exact symptoms, please let me know. Thank you!
 

rockola1971

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Check for broken wire at TAC(Throttle Actuator Control) module wiring plug Between fender and brake booster on the firewall. The insulation of the wire itself likely is not broken, its the stranded wires inside that could be. You will have to run your fingers down each wire and if there is a break you will feel it. Corrosion at the plug also could be a culprit.

Looks like this.....
 

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tomloans

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At least all three codes are related. Looks like it's more with the pedal or TAC module, or the circuitry in between. Does your scan tool store freeze frames for the DTCs? Have you checked everything under the dash?
It is really hard to see a slight wiring disconnect. This is really tough. I haven't had this issue with this vehicle in many years. Suddenly it started again. I will dig around. By the way this is not the same vehicle that you helped me replace the cylinder heads. That one is still running great - thanks again! I mentioned I have his and hers 2003 Tahoe's. LOL Mine is having this problem ( hers did as well but some grounding solved that problem).

I am having a rough idle after warming up as well. It never does it when cold. Only after warming up and the idle then gets worse.
Check for broken wire at TAC(Throttle Actuator Control) module wiring plug Between fender and brake booster on the firewall. The insulation of the wire itself likely is not broken, its the stranded wires inside that could be. You will have to run your fingers down each wire and if there is a break you will feel it. Corrosion at the plug also could be a culprit.

Looks like this.....
Thanks, I will unplug it and and see if there is some sort of corrosion in there. I will clean it as well. Never looked at this before. I have messed with the gas pedal sensor, the three areas of grounds ( passenger side and driver side behind engine and the grounds under the drivers side seat ). I have replaced the wiring to the TB and I have replace the TB and the sensor on the foot pedal. This is over the years.
 

iamdub

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It is really hard to see a slight wiring disconnect. This is really tough. I haven't had this issue with this vehicle in many years. Suddenly it started again. I will dig around. By the way this is not the same vehicle that you helped me replace the cylinder heads. That one is still running great - thanks again! I mentioned I have his and hers 2003 Tahoe's. LOL Mine is having this problem ( hers did as well but some grounding solved that problem).

I am having a rough idle after warming up as well. It never does it when cold. Only after warming up and the idle then gets worse.

Thanks, I will unplug it and and see if there is some sort of corrosion in there. I will clean it as well. Never looked at this before. I have messed with the gas pedal sensor, the three areas of grounds ( passenger side and driver side behind engine and the grounds under the drivers side seat ). I have replaced the wiring to the TB and I have replace the TB and the sensor on the foot pedal. This is over the years.

So it intermittently has the reduced engine power issue at idle and also has a rough idle, but not when cold. Codes are all TAC/DBW related.

I'm still leaning towards something wrong with the TAC module or pedal. Maybe it's commanding more or less throttle % than needed, which is okay enough for it when cold, but not when warm. A scan tool that shows live data would show commanded and actual and, if a code is set, a freeze frame to show the status of all the sensors and actuators so you could pick out the one that is amiss.
 
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tomloans

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So it intermittently has the reduced engine power issue at idle and also has a rough idle, but not when cold. Codes are all TAC/DBW related.

I'm still leaning towards something wrong with the TAC module or pedal. Maybe it's commanding more or less throttle % than needed, which is okay enough for it when cold, but not when warm. A scan tool that shows live data would show commanded and actual and, if a code is set, a freeze frame to show the status of all the sensors and actuators so you could pick out the one that is amiss.
MY scanner is OK and will have a freeze frame but only shows so much data. But I will take look. I am going to take some time with it today. Thanks!
 
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tomloans

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tac% fluctuates at idle from 3.1% to 3.5% consistently. 3.9% from time to time. Why would it fluctuate at idle. It doesn't make sense.
 

rockola1971

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tac% fluctuates at idle from 3.1% to 3.5% consistently. 3.9% from time to time. Why would it fluctuate at idle. It doesn't make sense.
Dirty or worn spot on the potentiometer of the throttle pedal. Could also be corrosion on its wiring harness (either end) at the Throttle POT or at the TAC module. Same type of fault occurs at the POT in the TPS at the throttle body on the DBW system. The POT is just a variable resistor. Works in the same way a volume control does on an old school radio volume control. Turn the POT's shaft all the way one direction and the resistance is the max value of the POT. Turn the shaft of the POT all the way the other direction and the resistance goes to essentially zero and then all values in between zero and MAX are available and are dependent on the shaft positon, in this case your throttle pedal's position.
 
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tomloans

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So to bring some closure to this. Over the years I have had this "Reduced Engine Power" (REP) issue. So over the years I replaced the TB, the connector at the TB, foot pedal sensor, cleaned all the known ground problems, and pulled the TAC module connector to see if there was corrosion there as well. It worked until lately I started having the trouble again hence this post. Rockola, you had mentioned the TAC module up by the firewall. I couldn't remember messing with that but I finally remembered I did. So while the engine was running fine, I pressed up against the connector and it immediately went into REP Mode. I noticed immediately that the connector was moving. It turned out I never had it completely seated in there. Visually it appears I did, but the orange rubber seal was not seated and was fighting ( for lack of better words ) against the black connector clip in front. So for good measure, I took the connector apart to see if the any wires were broken. One by one I felt the wires to see if there is a break. Couldn't find one. I also pulled the TAC and just cleaned the connectors inside carefully. Instead of putting the system back together with the plastic cover, I simply plugged it in with just the orange seal and the wires straight up and down rather than bent to the side. It was then very solid. I order another from EBay for good measure and will replace it when I get it.

Before I did this on Tuesday, I drove 3 miles and had 3 episodes of REP as messing with the connector made it even worse. After the fix, yesterday I drove 250 miles with no issue at all.
So there is a moral to this story and it is pay careful attention to connectors as well! LOL Thanks for all the help once again!
You will note the culprit below and how I left it so that it stays on the TAC module.
 

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tomloans

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Alright so I carefully and methodically attached the new connector with new wiring as well. I did this just in case I am wrong about my deductions noted above and there is in fact a break in the wiring I didn't see. Remember when you do this, the wiring colors are different on the connector and some colors on the old connector are exactly the same!:oops: So I would handle each wire one at a time. I used this connector which comes with about 18" of additional wiring...


I used the red ones out of these type of connectors ( but keep reading )


These are also essential as the connectors above often will melt through and expose wire! Super important! I think I used 3/16" tubing. Just cut to size ( a little larger than the solder connector ) and run this up the line first followed by the shrink solder connector and take your time. Shrink the solder connector first. Keep the heat on the center of solder connector until you see the metal in the center get "liquidy" then work the gun to either side until you see a good shrink seal. Wait bit for it to cool off, then slide the tubing over it and shrink that into place as well. As noted above, DONT FORGET TO SLIDE TUBING ON THE LINE FIRST FOLLOWED BY THE SOLDER CONNECTOR before you apply the heat to the solder connector.


Finally I used something similar to this to shrink the connectors...


This method would also apply to the connector at the throttle body. Hope this will help someone. Thanks for all your help. You know who you are!:D
 
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