Only code on scanner is P0160, oxygen sensor heater circuit. Doesn't say anything about affecting idle. Another issue has begun now also, the truck loses electrical power completely at random times, thankfully not while I've been on the road. The engine stops completely and there are no lights or other electrical activity for a moment, then the startup dash lights come on like I had turned off the engine with the key. I have noticed that the battery indicator light on the dashboard flashes when the key is turned in the on position, not sure if that means anything. Checked battery cables, didn't find any of them loose, corroded or damaged. WTH?

 

Correct - the P0160 will not affect idle - it will affect O2 sensor readings but with that DTC set, the PCM will remain in open loop most of the time and will ignore O2 sensor signals.

I need verification of what alternator should be in your truck before I can get specific about troubleshooting the issue. During this time, GM used 4 different alternators with 2 slightly different regulators.

Look in the glove box for the RPO (Service Parts ID) label. The alternator type is the code that starts with “K” - Google the definition or post it here and I’ll confirm.

 

Thanks Gumby, Just FYI, I've driven the truck quite a bit over the last 2 days and no issues at all. The RPO code is K60, generator, 100 ampere. The alternator was replaced by the previous owner in February 2017 with a Carquest part number 8206-5AN which is a 105 amp. The truck has been driven about 30,000 miles since the replacement.

 

Service engine soon light came on this morning, hooked up code scanner. It read a P0122 code -Faulty throttle position sensor. Could this be the cause of the stalling?

 

Yes - the TPS affects all ranges of engine performance, including idle control. It’s what I call a “master sensor” in that the ECM bases almost all its control signals on TPS input.

The most likely cause of the code is a faulty sensor, however, I’d recommend verifying the wiring is in tact first, especially given the seemingly random electrical issues you just experienced - there could be a spot in the engine harness that has more than one wire with damage.

At key on, The sensor receives a 5 volt reference from the ECM on the gray wire. As the throttle is open and closed (throttle sweeping, as it’s called), the sensor returns a voltage back to the ECM on the dark blue wire.

At closed throttle, on the dark blue wire, you should see 0.4-0.6 volts. At WOT, it should be at least 4.5 volts. While sweeping the throttle, the voltage should remain steady and change accordingly (up as you open the throttle, down as you close it) - if at any point, the voltage cuts out, the sensor or wiring has a fault.

Will your scanner read live data? If so, monitor the TPS signal at key on, engine off, and wiggle test the harness, starting at the TPS and working towards the ECM. If you reach a point where the signal fluctuates, and it can be repeatedly changed, open the harness for inspection.

You can also monitor the signal at the sensor using a volt meter, or check the sensor directly using an ohm meter between the sensor terminals.

 

I'm going to do some testing tomorrow, thanks for the guidance. I may have also found the electrical issue. After some engine compartment investigating I found 2 wires just dangling down near the brake area. The insulation was cracked and missing in spots, with the engine running they had 12.9v on them, a little less with the key on only. Appears to be one red and one white wire, any suggestions as to how to figure out what they connect to. I cut them off clean and taped the ends so they can't cause any problems.Attached are a couple of pics.

Goes to a large harness at the firewall.


With engine running

 

Is this the other end?

 

Yes it is, same color wires on the connector. I'm going to get a connector and hook it back up. Looks like it connects to the vapor canister and has to do with emissions. Apparently it doesn't affect performance. Looks there are several different connectors that will fit, is there a certain one to use?

 

It’s the evaporative emissions purge valve - SMP part # S1350 should be the correct replacement for the pigtail.

It doesn’t affect performance - it’s used by the ECM to control the burning of captured vapors from the fuel tank.

Edit: It shouldn’t affect performance. If it gets stuck open it can pull excess vapor or air into the manifold which can cause relatively minor driveability problems.​​​​​​

 

Thanks, I've ordered the connector. I tested the TPS and the wiring checked out fine, got the acceptable voltage readings with no drop offs. I replaced the TPS and so far no codes and running fine. Hope that clears up the problem, thanks for the help and advice. It's been very satisfying and fun? to be able to diagnose and do my own repairs. My next project will be to change out the distributor cap, rotor, spark plugs and wires. Any advice or cautions for such a project?