Okay here is the queen trouble shooting problem for you! I have a 99 4runner, 4wd with a 4 cylinder engine. I love it and it runs great 99% of the time. Here is the problem, When the outside temperature is around 10 degrees F or less the 4runner will start fine and idle fine, once I get in and drive it and the engine gets just a little bit warmer (full operating temperature) and once I take my foot off the gas the RPM will drop to idle (500-700) surge a few times and then die! Once the engine cools off (enters cold idle mode) it will start up and run fine until the engine gets to full operating temperature and then the same thing. I've taken it into the shops around here (Juneau, AK) and by the time they look at it the temp outside has warmed up to a balmy 15 degrees F and they can't replicate the problem. One shop replaced the Throttle Position Sensor but the next time it got below 10 degrees F (the next year) same thing!

I took it to another shop last year, they replicated the problem (again brutally cold outside and engine at full operating temp), but concluded it was a TPS as well, which had been replaced the year before!!! At my wits end here, any help from those in the cold regions would be awesome!

Thanks!

 

Your problem is definitly electrical but with out seeing the Runner I can't really tell what it is. My best advice would be to take it to a Toyota dealership and have them run a diagnostic check on the computer. Also check the idle to see if it needs to be adjusted.

 

Thanks, the two shops I've taken it too both hooked up "ECM Readers" I beleive they were Snap-on brand....not sure if that matters (both said they were top of the line units), anyway, even when they did get the engine to replicate the problem no error codes were displayed! This is so frustrating.

Thanks again for the input I do appreciate it. )

 

This is purely conjecture, but something to mull over, anyway.

TPS? Is that very sensitive to external temps? Seems to me the problem would lie in some other sensor more susceptible to the temp differences. The most upstream exposed sensor would be the MAS/MAF, whichever system is used. Its not connected directly to the engine block, so that would receive the least "warm-up" from the engine. And, if I remember correctly, don't these sensors use a heated wire or mesh to detect air flow?

 

Thanks for the idea. Yeah the TPS seemed like an odd sensor to be the cause. If it's not working when the engine is hot and outside temp is less than 10 degrees F, then why would it work when the engine is cold or when the outside air temp is over 10 degrees F? I guess anything is possible (cold sensitive short of some kind). I'll see if the mechanic checked the MAF. Thanks

 

Maybe the MAS/MAF is expecting warm air but is getting cold air and since colder air is heavier than warm air the sensor is confused and chokes out the engine by letting the A/F ratio get too rich. It would work kind of like taking the sensor off. The engine will run very badly for a minute then die even if you give it gas. I never have thought of this before. It is possible but I don't know how you would fix it. Have you always had this problem or did it start sometime after you had the truck?