SES Light

[MiDMoTSI]

Daughter came home from classes this evening and said the SES was blinking. Pulled the codes and had a P0128-coolant thermostat (coolant temp below thermostat regulating temperature) and a P0300-random misfire detected.

Last month I had a MAF issue and did pull a P0131-O2 signal low S-1/B-1. After replacing the MAF and clearing the codes, no issues until tonight.

Started the car and when shifting into reverse and drive, engine stumbled (probably the P0300), drove about a 10 mile test drive. Outside temperature was 50 and engine temperature never exceeded 150, thermostat is a 180, replaced in August, this year when I did a LIM/UIM gasket replacement and coolant flush as well as: changed the fuel filter - Wix; spark plugs - NGK TR55; wires -NAPA Belden; and new PCV. MAF (new) was changed 2 weeks ago.

Not changed are the coils, ICM, and O2 sensors. Possibility the new thermostat went bad. Prior to tonight, it usually read around 180. Coolant level is full and oil is clean.

Any suggestions besides changing the thermostat? Could that also cause the P0300 if the engine temperature doesn't exceed 150?

[DCJREDLINE]

Is the upper hose warm? If not the therm isnt opening.

[MiDMoTSI]

Barely warm, that's why I'm leaning towards a thermostat failure though it is only 2 months old. I'll swap it tomorrow, clear the codes and see if that corrects it.

I'm assuming that if the engine doesn't warm up to optimal temperature, the computer won't go into "closed loop", the fuel mixture is affected, and possibly throw the random misfire code?

[00Beast]

I don't think it would throw the misfire code, but replace the thermostat first and see what happens.

[MiDMoTSI]

Will do. I'll post the results tomorrow.

[MiDMoTSI]

Replaced the thermostat during lunch, though I haven't test drove to see if that eliminates the P0128.

I did notice that vacuum harness (with the 2 hard plastic lines) from the throttle body to the fuel regulator was cracked and dry rotted (larger diameter tube) at both ends. Definitely a vacuum leak. I pulled it off and called the local GM dealer who had one in stock, GM#24506640. Hopefully this eliminates the P0300.

[MiDMoTSI]

Vacuum harness smoothed the idle a little but, the misfire is still there. Checked fuel pressure and it showed 42 lbs-ignition on and 47 lbs-engine idling.

Ran a more detail check with another scanner. The datastream shows the misfires are occuring on cylinders 1 and 4, same coil, so I'll inspect/troubleshoot that coil. If I swap to a different location and the misfire follows, would it be safe to assume the coil is bad? After switching the coils and the misfire stays at 1 and 4, would it be a bad ICM?

[Archon]

Those would be reasonable assumptions.

[00Beast]

[MiDMoTSI]

Replaced the ICM with one from harofreak, problem went away.