2003 Trail Blazer Reduced Engine power

owensk68

Original poster
Member
Dec 26, 2014
1
I have a 2003 trailblazer that i replaced the engine with one from a envoy The reduced engine power light keeps coming on but it only does it when it below 30 degrees outside it started doing. this before i replaced the engine but now it has started doing it again it has the 4.2l engine in it does anyone have any suggestions. Note. all the components were changed when the engine was replaced Thats what leads me to think that it may be computer related ?????
 
  • Like
Reactions: BlazingTrails

moneypit

Member
Dec 7, 2011
214
IIRC this happened once to our TB a couple yrs back & I unplugged & looked at/blew off the 2 or 3 large electrical connectors(held on by a bolt thru ea.one)at the upper left front of engine, reconnected and all was well again? Luckily I've never seen it again. Could of been a fluke...but thats what happened with ours. Make sure there's no damaged pieces & the rubber thin gaskets are seated nicely too. iirc I unplugged the black pc./connector from the gray side.
 

Attachments

  • harness connector.jpg
    harness connector.jpg
    262.1 KB · Views: 27

Mounce

Member
Mar 29, 2014
13,667
Tuscaloosa, AL
Won't the cooling fan being over speed trigger REP? Maybe when it's cold the fluid inside the clutch doesn't go back into it's resting place quick enough to please the pcm but just quick enough to prevent the pcm from setting a code.
 

bobdec

Member
Apr 19, 2013
233
Searching on 'Reduced Engine Power' and could not find a listing under what conditions the PCM will turn the indicator on in the 4.2L . Does anyone have a list or something we can reference ? It's supposed to indicate some sort of engine protection mode and I've seen TPS and Pedal position sensor DTC's will light it up, but that's all I've seen documented.... Would be nice to have a list of things to look at.
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Engine Controls - 4.2L
DTC P1221
DTC P1120
DTC P1220
DTC P1275
DTC P1280
DTC P1271
DTC P1514
DTC P1515
DTC P1516
DTC P1512
DTC P0507
DTC P1680
Engine Cooling
DTC P1484
DTC P0483 or P0493


either throttle sensor/actuator, or fan clutch

got this from a search within the service dvd under 2003 trailblazer
 
Mar 15, 2012
45
Unfortunately there is no quick answer to this. There are many reasons the REP light will come on and some are computer related and some are electrical. The codes are important as you see in the prior post, it can be a myriad of things. If you have a multimeter, you should check the voltages first make sure the 5 volt is operating from your fan clutch to the throttle body and pedal. You should at least know if something is not getting power before you have to go to higher costs of diagnostic labor.

My Envoy had no throttle response or control and started without any warning. Came out one morning to go to work and boom, it was on and running like it was on 3 cylinders. I changed the throttle body, pedal, and PCM and that did not fix the problem. I pulled harnesses and checked wires. I checked fuses and power from the fuse box to the PCM. I could not find anything that made it work. When none of those worked, I almost replaced the throttle body pigtail but could not put that kind of money into it and still not know if it was going to work. I ended up taking mine to a dealer and nearly $800 later, I had a bad (original) PCM and the throttle body pigtail needed to be replaced. I have 5500 miles on it since then and no problems. If you have someone with dealer level equipment, they may point you in the right direction if you are not electrically inclined.

There are some pretty good people here that can help walk you through some of that testing if you need the assistance. It can get rather expensive if you start throwing parts and still have to have a portion of the wiring harness replaced anyway.

Good Luck.
 

bobdec

Member
Apr 19, 2013
233
The OP (1st post) did not discuss any codes and has not come back. The P0507 in post #7 says idle speed is 200 RPM higher than desired. They are probably thinking high idle is TB related also.
Summing up it looks like what you said REP is related to TPS/APPS/Fan controls/sensors. Coincidentally as mentioned above all those circuits depend on the 5v reference from PCM.
 

IllogicTC

Member
Dec 30, 2013
3,452
Mounce said:
Won't the cooling fan being over speed trigger REP? Maybe when it's cold the fluid inside the clutch doesn't go back into it's resting place quick enough to please the pcm but just quick enough to prevent the pcm from setting a code.
I would think an issue with the fan's Hall Effect sensor or related wiring could set REP as it would affect the +5V reference, otherwise, I don't think it should. It should give you a code for being overspeed though. A lot of posts around here about clutch fans failing and defaulting to "engaged," sounding like a jet engine taking off, but no complaints of REP with them.

It should also be noted that while one of the TPS sensors shares +5V with the fan, the other does not, and determining which TPS sensor is having an issue can tell you a lot about what to inspect.
 
  • Like
Reactions: Mounce

bobdec

Member
Apr 19, 2013
233
Illogic.... Sounds like REP is not a easy as a set of DTC's. Done a lot of searching and have not found one w/o a corresponding DTC showing up eventually, unfortunately all were TPS/APPS related. Also read thread a while back, that 'implied' the 5V ref circuits are tied together back in the PCM. Do you know if that is correct.
I play a lot with my Camaro Z28 and have a spare PCM w/a bench ALDL harness and software to try things. However have not dug that far into these yet.. Darn tuning SW is expensive and really not much tuning advantage over a mail order since mine is 100% stock.
 

IllogicTC

Member
Dec 30, 2013
3,452
Yes, some circuits are tied to a common +5V supply coming from the PCM.

Throttle Position Sensor 2 shares a +5V common with Accelerator Pedal Position Sensor 1, and the Manifold Absolute Pressure Sensor.

Throttle Position Sensor 1 shares a +5V common with Accelerator Pedal Position Sensor 2, Engine Cooling Fan Speed Sensor (note that a failure of the clutch should not result in any apparent issues in this circuit, as it's just a sensor, not the actual controlling circuit), Air Conditioner Pressure Sensor, Fuel Tank Pressure Sensor, and Oil Pressure Sensor (may exclude 4.2 models with the switch rather than a proper sensor).

This is why it's important to know which TPS sensor (or if both) have DTCs stored for them. It'd eliminate some of the wiring to inspect.
 

Forum Statistics

Threads
23,273
Posts
637,499
Members
18,472
Latest member
MissCrutcher

Members Online