[Dollarbilly]

2002 Trailblazer LTZ I-6 4WD 143k miles. When starting after it has been sitting for a while (like overnight) the Service Engine Soon and Reduced Engine Power lights come on. While idling with the REP light on the RPM will vary back and forth from 600 rpm (idles very roughly at this point, almost to the point of stalling, and spits water from the exhaust pipe) to 1200 rpm (idles fine). If I let the truck idle for a while (anywhere from 10-20 minutes), shut it off and restart it, the REP light clears and the truck idles and drives normally, however the SES light is still on. However, after the truck has been driven some distance, when turned off and restarted, the SES light will go out. It seems that the colder the outside temp the longer it has to run to clear the REP light. When I am able to drive the truck, with the SES light on and the REP light off, the OBD2 reads P1515 and P1516 (thanks to my local Auto Zone) which are both throttle position sensor codes. The work done so far is cleaning the throttle body, replacing the throttle body/TPS sensor (which is all one unit), and replacing the accelerator position sensor. Still having the same problem. I should add that I had this problem last winter and didn't get around to fixing it. When the weather got warmer the problem went away and I was hoping that was the last of it. But now that the nights are getting colder the problem is back. Any ideas?

 

[the roadie]

The accel pedal sensor and the throttle body sensor share a reference voltage circuit with the fan clutch, which can have an internal fault or a harness wire chafing that interacts with the stuff you've changed in exactly that way. Pull the harness connector off the fan clutch near the fan shroud and see if the behavior changes. An original 2002 fan clutch isn't long for this world anyway, so you might want to run a search here and read up on how to change it.

 

[Dollarbilly]

No change

I removed the connector from the fan clutch as you suggested. No change in the problem. Still getting REP and poor idle. Something in there appears to be temperature sensitive, that's what pointed me towards a sensor problem. Would it make sense to try and read the OBD2 codes while the engine is in limp-in mode?

 

[the roadie]

You might need a dealer or independent mechanic with a high end scan tool to look at the 5V reference and the sensor data.

 

[Dollarbilly]

Other sensors?

Do you think there could be an issue with the air flow or manifold sensors?

 

[wardak33]

If you're talking about an MAF sensor, your truck doesn't have one. However, you do have an IAT sensor and an MAP sensor which can cause the symptoms you're describing but you'd also get codes that would pertain to those sensors. To make sure, your best bet is to find someone with a tech-II or a high end scanner as Roadie suggested.

 

[Ralph_T]

Do you think there could be an issue with the air flow or manifold sensors?

Has the engine been removed for any reason in the past? Any other major engine work been done? I'm thinking bad grounds.

 

[Dollarbilly]

Funny you should mention that. At 60K the timing chain let go. Tore it up bad. Got GM to pay half the cost of rebuilding the motor even though it was way past the original 3/36.

 

[Dollarbilly]

Problem Solved!

Well I finally got the truck scanned after it had been parked overnite and not run. Found internal failure of PCM and wound up replacing the PCM itself. Finally fixed the problem. Thanks to all those who took the time to comment on this thread!