Reduced engine power

OVRTHEDGE11

MEMBER
2003 GMC YUKON
5.3L
4WD
AUTOMATIC
220,000 MILES

The dreaded REP for GMC vehicles. I replaced the tac module on the firewall, replaced that sensor on the throttle body with the new harness, replaced the sensor on the gas pedal. Removed throttle body and cleaned.
Removed mass air flow sensor and cleaned. Removed connections on PCM, ECM, throttle body, tac module put dielectric grease on all connections. Removed grounds on back of left cylinder head, lower radiator support on left front, on the frame under the drivers side. Cleaned all the grounds and reinstalled. Made a new ground wire from the firewall to the transmission bell housing. Made a new ground wire from the battery to the firewall.

Replaced the ECM today still same problem.

5 Replies

SATURNTECH9

MECHANIC
30,869 POSTS

What are the exact code numbers not code descriptions are setting in the computer?

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Wednesday, October 18th, 2017 AT 6:51 PM

OVRTHEDGE11

MEMBER
2 POSTS

P1518 is the code. The really weird thing is that if I take out the starter motor solenoid and replace it a few times the car starts and works for a while and then it stops working and goes back to the reduced engine power again.

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Thursday, October 19th, 2017 AT 7:39 AM

KEN L

MASTER CERTIFIED MECHANIC
48,363 POSTS

I would test the alternator to make sure you don't have any diodes leaking throwing AC into the system. Here is a guide to help you run the test

https://www.2carpros.com/articles/how-to-check-a-car-alternator

If that checks out okay check the ground strap from the engine to the frame. If missing it will cause all kinds of grounding issues.

Please get back to me. The next step is wiring troubleshooting.

Best, Ken

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Friday, October 20th, 2017 AT 1:38 PM

GER-J23

MEMBER
2 POSTS

I’m literally having the same problem with my 2003 Yukon. Did everything you’ve done too! Did you ever find out what was wrong?

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Sunday, November 7th, 2021 AT 5:30 PM

KASEKENNY

MECHANIC
18,907 POSTS

Unfortunately, it is unlikely for them to reply. However, I bet we can figure out what is going on with your vehicle and that will help others in the future.

Here is a guide that will help get us started on reduced power issues:

https://www.2carpros.com/articles/reduced-power-limp-mode

However, first and foremost we need to check for codes as there are a number of things that can cause these issues so just because you have the same symptoms doesn't mean we have the same codes.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Once we know what codes are there, we can take next steps but until we get a little more info on your situation then we would be guessing on where to start. Thanks