1999 Gmc Sonoma 2 2l 4 Cylinder Cold Outside 32 Degrees Below Truck Starts

Well it sounds like you might have a bad O2 sensor or maybe an egr valve or cat converter. Do you have a digital multi meter? If so we can run some tests to see what you need to replace.

yes, I have a digital multi meter. I also need to add a symptom that I just checked this morning. I changed the plugs about 2 or 3 months ago for the first time on this truck which I bought 6 months ago (CIL) was on when I bought it, but got it really cheap. Anyway, #4 plug was covered in ash deposits back then and when I pulled it this morning had the same thing but not as severe. I also note that truck never warms up to 195 degrees that the truck is supposed to run. It seems to stay at about 180 degrees per my Scangauge II. Also, I cleared code with Scangauge and it hasnt come back on yet but gauge says "not ready". Sorry about all the new info but all these little things are coming back to me now that I think about them.

okay, here is update. Changed thermostat and plugged a hole that I found in the intake resonator box, there was a chunk broken off where it goes into gasket at the throttle body. Engine runs a little hotter (190degF) now . Scanguage ii says "no code" and "ready" so I guess it ran all its tests and found nothing. Drove it again yesterday to work, lunch and home it was snowy and cold all day. The hesitation problem still exists when engine us under load. Engine warming up didnt seem to make it go away this time (this is unusual/new). Maybe because it never really warmed up outside. Aaargh this is frustrating issue.

Air you able to check the mass airflow sensor? It sounds like it may be malfunctioning. How is the mpg? Is it about normal? Have you replaced plug wires or filters. How old is the fuel filter? Next time you have a chance ohm the coolant temp sensor (unplug it) ohm the sensor not the harness when it is cold like in the morning. Note the air temp and get back to me with the temp and the ohms of the sensor ok. Sorry it took so long to get back to you have been very busy.

thanks for the reply, yes I can check the mass airflow sensor. Mpg is about normal 20 to 23 combination of city highway miles. I have replaced wires, plugs, air filter, thermostat and fuel filter. I'll try to check the temp sensor tomorrow and clean inspect the MAS. Thank you very much for your time, I understand you must be busy so dont worry.

update. No MAS on this engine, that i could find. Unless it is hidden somewhere that I cant see. I took the resonator box off and cleaned the IAC sensor, the MAP sensor, throttle body and replaced the TPS. It wasnt so cold here this morning so it didnt hesitate when I took it for a short drive. Will try tomorrow to take out the Coolant temp sensor to check on ohm meter.

Ok let me know what you find and we will go from there.