SPC

Well, hopefully this won't be too crazy to someone, but I am stumped. Here's the deal.

1990 4Runner, 3VZe
Temp gauge shows no or very little temperature. ECU temp sensor also doesn't appear to be picking up temperature. Trans won't shift to overdrive. Getting about 100 miles on 40 dollars gas. Heater blows hot air, front and rear.

Installed an aftermarket temp gauge, verified it works by dropped end into hot water and watching the gauge rise. Replaced ECU temp sensor, same results. Have not checked resistance on ECU sensor under control, so I guess it's possible two sensors would be bad, but that doesn't explain why the factory temp gauge or the aftermarket aren't showing temp either. In addition after it runs long enough where it should be showing operating temperature, if you turn it off the temp gauge will rise. The only other time it showed any temperature was when it was a few liters low on coolant and we started it to check on something (in the garage being worked on, it wasn't driven or run for long that low on coolant). A few liters down the temp gauge went right up to operating temp.

Basically, it's acting as if the thermostat were stuck open, but if that were the case I would think the heater would be throwing luke warm air at best. The heater bails hot air. In the back you can't even keep your hand there for too long. When it was purchased, the head gasket was shot. Replaced that and did a coolant flush.

After reading what I just posted, it seems more and more like the thermostat is stuck, and for the cost it will be replaced. But it seems really odd to me that it would throw so much heat through the heater if the termostat were open.

Does anyone have any ideas here?

curtiswyant

Am I wrong, or isn't it normal for water to flow into your heater core when the t-stat is open? It definitely sounds like your t-stat is stuck. It's also normal for your engine temp to rise immediately after you shut off the engine. I'd make sure your coolant level is correct and make sure you're not burning any fluids...

SPC

The problem is, normally when the thermostat is stuck open (at least on everything I have ever owned) you don't get much heat from the heater core. You can drive this 4Runner in -10 weater over the highway and still get blistering warm air from the heater, yet the temp gauge sits on the peg at 0, or only slightly above this. The thermostat will be replaced tomorrow hopefully, so we will see if that fixes things. But I have my doubts.

SVTPaul

Yes it is normal for it to rise immediately after shutting it off. On this one it will slowly creep up to ~190 F after sitting for about 5 minutes. As soon as you start it back up it will slowly creep down to just barely above the peg or on the peg if you leave it idling, or drop off to no temp (on the peg) within 15-30 seconds if you drive it. However the 15-30 seconds was enough time to diagnose that the ECU temp sender sees this temperature as well since there was Overdrive until the temp went back down, and the engine ran with no hesitation (which it has at ALL other times).
All the while the air from the vents is HOT, regardless of what the gauge says. But the temp sender to the gauge (and the ECU it appears) are not picking up correct coolant temp.

DCS

I have a similar problem, which I am planning to explore today.

On the back of the motor in a location precisely designed to aggravate and frustrate, there is a coolant sensor module which has a temp sensor, a temp sender, a some other temp-related sensor-type items all in a neat row. There's four I think.

This module is a coolant pathway.

It has a U-shaped coolant hose on the exterior. The "U" is vertical.

This is a trap for engine crud. If it is blocked, here's the fix:

Remove the various heater hoses and wires above it.

Remove your hood (some say this is necessary, I'll know later today).

Drain the coolant.

Remove the thermostat housing and thermostat (front of motor).

Unhook the U-shaped hose and clean it out. Blow compressed air into the passageways.

One guy who reported this fix a year ago said that his compressor blew a huge RTV booger out the thermostat hole when he blew air into the sensor module.

Flush engine throroughly. Reassemble.

What happens is that crud gets trapped in the nooks and crannies of this module and hose, and prevents coolant flow from passing over the sensor and sendor. So your gauage reads low, and your ecm thinks the car is cold, so it locks out overdrive.

When you turn off the car, residual heat from the engine heats up the module, so it looks normal for a minute after you start again.

NOTE - IT IS ALSO POSSIBLE that an air bubble has somehow trapped itself in the sensor module, and a good burp may be all that's required to fix this problem. However, this has not worked for me. Dammit.

Before I do a total takedown, I may try to unhook the U-hose on one end only, and use a long skinny funnel to gravity feed coolant into both holes. If there's a blockage, maybe I'll identify it by seeing really slow flow in one direction. Maybe I'll dump some liquid plumber in there. Can't hurt, can it? Follow it with a little Rid-X to keep things clean.

If there's an air bubble, maybe I'll displace it.

Good luck! Report back on your findings, or Guido the yotatech thug will come and break your thumbs.

DCS

Acording to my reading, to access the u-shaped hose & module, etc, there is a lot of "laying on the engine" type work to be done. Get some blankies.

SVTPaul

Let me know if you make any discoveries. I won't get a chance to get at mine today but I will get to it over the coming week. I'm going to pick up a new thermostat and gaskets for the thermostat and sensor block on the back before I tear anything apart. Is it possible to get the coolant temp sensor block out with the upper intake manifold on there? I remember it being VERY tight back when I had the engine torn down to do the head gaskets.

DCS

Well, I never got around to Toyota time on Sunday, so I have nothing to report yet. I may have time during the evenings this week.

Dunno about sensor block removal; never done it. I don't plan to remove mine, just blow it out. I've looked at it from both sides of the car, and did not see any easily accessed bolts or attachment features. It was sort of talking to me: "Don't take me off! Don't even think about it." etc.

4RunnerUT

Hey guys, I recently bought a 90 4Runner....I noticed while on test drives that the OD was not working properly....IT was constantly shifting from 4th into 3rd....seemed like it was only during cold weather. SO I checked the forums, I found out that it could be something electrical with the tranny...Then I saw a posting about the engine temp needing to be at a certain temp for the Overdrive to work....

So I checked the Thermostat...it was stuck open, this explains why the engine temp would drop while going down hills. I also replaced the Coolant Temperature Sensor...a bit of a tedious job, took about an hour to do....Now it shifts just fine into Overdrive....

rustypigeon

I had my thermostat stick open on my 22re. I noticed when I drove home from work it would not shift into overdrive and the temp would not get as high as it should. I popped out the old thermostat and sure enough, it was stuck open. The reason mine stuck was that the jiggle valve was placed in a position that allowed it to jam in the thermostat opening. (this was the OEM toyota thermostat) When I bought a new thermostat (stant) I made sure that the valve was far enough from the opening that it would not lodge in the opening. I found it interesting that the original thermostat was designed with the jiggle valve so close.

I am not sure on your engine, but on a 22re the heat will always go through the heater core as long as you have the heater control on hot, regardless of whether or not the thermostat has opened.

When mine was stuck, the coolant got warm enough to give me heat, but just not warm enough to shift into OD.

scottmildy

I have a 90 4runner 3.0 v6. I believe I am in the same boat with these temp issues. I am experiencing the loss of OD but as well, if I accelerate from a stop the engine has a huge hesitation almost to the point of stall, once the hickup is past, it clears up. When the engine is at operating temp all is fine. I am hoping replacing my T-stat fixes the OD issue, will it fix my hesitation too??

BTW, engine blows very nice heat!!

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