Chugging problem when cold

John Daniel · January 9, 2020

So I’ve had this issue for a few months when the temperature drops to -20C or so overnight. The vehicle starts strong, then after a few seconds the RPMs drop as if it is going to stall and it chugs there for a while before the idle shoots up and comes back down to normal.

Shot a video of it this morning. Replaced MAF sensor a year ago, plugs and wires are less than a year old. It uses a quart of oil every 4000km or so (once or twice per oil change).

Got a video of it here, sorry for the poor quality.

Colossus · January 9, 2020

Ok let's start with some basics-

- Is it only doing this on cold starts? (first start up of the day) or does it happen when the vehicle is warm as well?

- Did the issue come on suddenly?

-What kind of plugs and plug wires did you get and what did you gap the plugs to?

- I know you said you replaced the MAF sensor a year ago- was it brand new?

- What kind of air filter are you running?

- Have you replaced your PCV valve recently? If not, I suggest doing so. Go to NAPA to get one, (their parts are good) or get an AC/Delco one from off of amazon or your dealership parts department. Here is a youtube video:
https://www.youtube.com/watch?v=GinOcOuazB0

Lastly, What kind of oil and oil filter are you running?

Lots of questions, but your answers will help!

Go ahead and pull off the air filter box/hose and give it a good look over going up to the throttle body. Have you cleaned the throttle position sensor/throttle body as well recently? If not, it would be worth it to do so.
Here's a youtube video that might be helpful if you need it.
https://www.youtube.com/watch?v=ciiTsKISCkQ

Look forward to reading your answers.

Cheers

John Daniel · January 10, 2020

I’ll try and answer all of these. Or most anyways.

Only on cold starts. Very cold starts. Did it this morning in -20 Celsius, but didn’t do it after my 8 hour work day in -18.

Its been doing this intermittently for a bit, in cold weather, but hasn’t gotten better or worse in the time it’s been doing it.

It was a brand new Delphi MAF

Its a Fram filter, in fine condition. I swapped out the K&N filter because of the fouls MAF

PVC hasn’t been replaced but was checked a year ago, I’ll swap it out just to be sure though

I checked some PID values while running this afternoon (it wasn’t acting up at the time) short term fuel trim is way way high at idle (+25-40) but would go to zero or negative under load. This seems to resolve somewhat once the engine is warm.

John Daniel · January 10, 2020

Checked the PCV valve, still rattles though it’s a bit gummy. I’ll replace it to be sure but doubt it is the culprit. Gave the throttle body a clean while I was at it.

Colossus · January 10, 2020

We're on the right track. I forgot to ask how many miles? I seem to recall the fuel pump and fuel pressure regulator being an issue on the GMT-800s as well, as I recall, it went out on my 99 5.3L at about 80,000 miles so we put a new module in (the fuel level float was wacky as well) that helped with a bit of idle issues on mine. On my 99, the fuel filter was in-line and not in the tank. Not sure if they had it in the tank by 01 or if it was still in-line, you might check that out as well.

John Daniel · January 11, 2020

I replaced the fuel regulator the night before this happened. The pump and filter seem to be okay, there is no lack of fuel pressure on startup, no excessive turning over of the engine.

I have it going into a shop on Monday, with a cold spell coming up, I have to get it dealt with sooner than later, especially with such limited opportunities to diagnose the issue

davester · January 11, 2020

Are you running a tune? Maybe the open-loop tables are screwed up (as they are used initially when the vehicle is started, and then when all the sensors are good to go, it switches to closed-loop mode).

Check the fuel-pressure while its starting up under these conditions. It may be able to prime the system ok, but not keep it up right away.

Also check the air filter that it's relatively clean, and maybe also check the throttle plate, that it's able to move freely (as in perhaps a bit of water condenses on the plate, it freezes and prevents the plate from moving for a short bit).

swathdiver · January 11, 2020

Cold fuel? Mine does this while burning ethanol on our coldest days. The fuel in the line is quite cold and the motor stumbles burning that but once it sees fuel from the tank, all is well.