Kid Burok

89 V6 auto.
In the winter it takes forever to go into overdrive, but even more annoying is that when the truck cools down it will pop out while I'm driving.
For instance, yesterday as I was going down the Cajon pass and it cooled down to the point where it shifted out of overdrive and at 70 mph that sucks. It took about 10 miles of babying it in the slow lane before it finally reached it's happy temp and went back in. This has also happened to me after cresting the hill going to Vegas on the 15. This is only happening in the winter or when it's cold. This morning I drove for 20 minutes before it went into OD.
I asked for help before from a different site years ago but everyone acted like that was normal...and I'm pretty sure it's not.
No other slipping or any other transmission problems, only takes a few minutes to go into OD in the summer, and this has happened since I bought the truck in '98, just lived with it.
Thanks in advance for help guys.

DCS

It's a somewhat frequent problem on these vehicles.

The ecm won't allow overdrive until engine reaches a certain temperature threshhold. I think it's 158?F.

While many people have harmful problems with 3.0 engines overheating, you and I share the wintertime-only over-cooling problem.

There's a bank of 4 or 5 sensors on the back of the motor, mounted on a little coolant manifold. One is a coolant temp sensor that leads to the temp gauge on the dash. The other is a coolant temp sensor that leads to the ecm, and is used to determine among other things, whether OD is "allowed".

On the passenger side of that manifold is a u-shaped small coolant hose, called the bypass hose.

Your problem is either that your ecm coolant sensor is failing, or you have a failing thermostat that stays open too long and keeps your engine too cool, or a wrong thermostat (not the proper temp range) that is keeping your engine too cool, or worst of all, a blockage in a coolant passage somewhere that is preventing normal flow of coolant through the manifold, thereby "blinding" the sensor bank to the actual engine temperature.

So far I have tried two new OEM thermoststats, flushed my system, removed the bypass hose (a PITA!) and flushed again, but I still have the problem. My next attempt at a fix will be to check out the rear heater and other rearward coolant passage-related whatever's, and hope i find a blockage there. Mebbe I'll test/replace that ecm coolant sensor too.

A simpler fix if you live where it stays cold in the winter is to block off the airflow to a third or so of the face of the radiator with a piece of cardboard. That'll bring the temp up. The potential downside of that is that if your ecm coolant sensor is bad and your engine is running at normal temp, blocking the radiator will only cause you to operate at abnormal temp. Also, if you drive into a warm day and forget the cardboard, you may fry your motor.

Good luck.

Post what you learn!

mt_goat

Does your temp gauge drop at all before it falls out of OD? Maybe your t-stat is stuck open or maybe its a bad sensor.

mtnfreak

Change your thermostat. If you still have the problem, try one that opens at a slightly higher temp.
The explanation is kinda long, but this has worked for everyone I've known with this problem.

Edit - got distracted and DCS posted the longer version before me.

Kid Burok

Temp gauge never drops. That thing pretty much stays in the same spot the whole time. I've heard that Toyota makes them not move a lot so it doesn't freak people out...I read that on Land Cruiser forums though- trying to figure out something for my '97 LC.
Thanks for all that info DCM! Thank you guys for the help, I will get on this next week and post what happens!

mt_goat

Yeah my factory gauge is in the middle from about 170-235 F, I just thought if your coolant really was getting cold enough to drop you out of OD it might show a drop on the gauge. I think an aftermarket temp gauge would be a great diagnostic tool here, at least for those vehicles without OBDII. My guess is the sensor is the problem, unless its really getting cold right now where you are.

Kid Burok

I put a new thermostat in yesterday morning and that seems to have fixed this problem. I drove all the way to San Diego and back last night with no problems. Thanks so much for the help guys, I appreciate it!

elripster

I think at some point I need to put a meter on that tranny coolant temp sensor and see how it reads. I would love to defeat it so I have OD all the time. That would save me fuel since I live close enough to the freeway that I get to drive on it for a while in 3rd.

Frank