Cold start running super lean and 0 deg timing (3rzfe)

Hello all,

I have an AFR gauge on my truck so I'm able to keep an eye on it. During cold starts the initial Vroooom will be in the 12's but after about 10 seconds it'll start fluctuating from 15 to 18+ and eventually stabilize once water temp gets to 101F. The colder the morning the worst it is. Today (27F) the truck actually died and I had to hold the pedal down until it got to temp so it wouldn't die again. Once at operating temp its runs just fine.

*I think* the lean condition is being caused by something else but I dont know what it could be. The engine was completely rebuilt 3k miles ago and has been running great since. Yes I do have ONE CEL code (P0420) but I've had it for years and it hasn't caused this before. I did a thorough clean of my throttle body and it did help (there was alot of buildup) for about a week before going back to doing this.

I have a scangauge as well and have it set to my timing. During this lean running my timing will be in the single digits and sometimes even go negative 1 or 2.

Any input is appreciated.

 

bump

 

hmmm... I had a Type 1 VW that had a similar issue and while it isnt a Tacoma the issue might be the same. Only on extreme cold mornings would the motor die cold. The auto choke was closed, the throttle plate on the high idle cam... everything was correct. The issue was the throttle bore was getting iced internally. With the cold air and the pressure drop across the throttle plate the water vapor in the air would condense and freeze. I just learned to live with it.

 

check this thread- the relevant stuff relates to the coolant temp sensor on the back of the head and the table to check values. Also check your IAC sensor.

 

Possibility, but I think his truck may be starting in closed loop when it should be open loop until everything warms up?

I had a starting issue that ended up being my ECT sensor, but I had different symptoms than OP did.

 

Sounds like maybe a vacuum leak or dirty MAF, on a cold start the PCM is in open loop and not using the O2 sensors to compensate for any rich or lean condition, once it goes to closed loop it will use the O2 sensors to "compensate" for a lean or rich condition up to a certain point.
If it's a vacuum leak from a gasket or rubber O-ring those tend to be worse the colder it is since parts expand when they get warmer.
I would want to see fuel trims in closed loop, they should be +/- 10% total trim (Long Term + Short Term).

 

Reviving an old thread because we need answers!!! and I got them. I'm pretty sure it ended up being the coolant temp sensor on the back of the engine. I did replace it at some point (probably after I made this thread, but its been like 2 years and I forget easy) and I haven't had this specific issue since then.
That coolant temp sensor is also on the same circuit as the TPS and after replacing the TPS for like the 3rd time I figured the TPS probably wasn't the issue. So if your having weird starting and TPS problems you should probably replace the coolant temp sensor.

 

I had a similar issue that was resolved by replacing the ECT sensor. Which was a PITA to access, in my opinion