NaeSLaS

you might want to consider fooling the TCU/ECU into thinking the engine is warmer than it really is, with some electronics.

Kiff

Reading more threads, It may be my fan that is causing the engine to run just shy of O/D temp. Is there anyway to test the clutch? when cold it does have some resistance when turning it, and when warmed up it spins freely. but when the engine is warmed up and idleing the fan does engage and spin

mwsnow

Random thoughts, and answers to some of the questions...

The transmission overdrive circuit and the temperature gauge in the instrument cluster use different water temperature sensors. The gauge has no direct connection to the overdrive. NOTE: I am reading a 1990 FSM. 1991 may be quite different, but it seems unlikely. You can use the online FSM to work through the troubleshooting steps for the overdrive unit.

The 1990 FSM shows the thermostat with the jiggle valve.

If the thermostat is working correctly, there is nothing you can do to the radiator or fan to make the engine temperature too cold. The thermostat is designed to maintain a minimum temperature by blocking some or all of the coolant flow to the radiator. When the engine is cold, the radiator is not part of the coolant flow path; all of the coolant is recirculated within the engine until heat builds up enough to open the thermostat. Even if the fan clutch were seized solid (which would be no different than nearly all cars built in the last 100 years) the thermostat would restrict flow through the radiator to maintain the minimum engine coolant temperature.

Modifying the electronics to "fool the ECU" is a bad idea. This treats the symptom instead of solving the problem. This also may cause other unanticipated problems.

Using a hotter than standard (82C/180F) thermostat is guaranteed to place additional stress on the 3VZE head gaskets by increasing the total amount of expansion of the different metals. I don't think it is necessary to launch into an extended discussion of why this is a bad idea.

Putting a tarp or something similar over the radiator is only appropriate for very cold weather, to ensure that coolant actually flows through the radiator and does not freeze from the high flow of extremely cold air. Again, the thermostat will block all coolant flow until the engine reaches the minimum temperature required, and will overheat without the cooling provided by the radiator. If the radiator never gets cold enough for the coolant to actually freeze, the thermostat will adjust until it allows just enough very cold coolant into the engine to maintain the minimum operating temperature. Covering the radiator is not a wise substitute for a correctly functioning thermostat. See the head gasket comment above.

For trucks with automatic transmissions, the transmission oil cooler is part of the radiator, so when you are blocking air flow through the radiator, you are also preventing the transmission oil cooler from doing its job. The transmission oil cooler could easily be a separate component, not connected to the radiator. The reason they are a single unit is to allow the engine coolant, which heats up quickly, to speed up the heating of the transmission fluid, which does not work correctly until it gets hot.

Toyota recommends a minimum of 50% coolant to water ratio, up to a maximum of 70%. Using higher than the commonly recommended 50/50 concentration adds more water pump lubricant and corrosion inhibitors, in addition to increased freezing and boiling protection. I've never heard anyone complain that their water pump lasted too long. Coolant is cheap, engines are expensive.

There are procedures and specs in the FSM for testing the thermostat. It is the least expensive and most failure-prone component, so I would verify that it is working correctly before doing anything else. If the thermostat is good, move on to the overdrive troubleshooting flow chart.

Kiff

Wow! That's the kind of answer I was looking for. I have tested both thermos, OEM and aftermarket and both seem to work just fine.

My question still is, why when going at higher speeds (above 70mph) my temp gauge will drop drasticly? What you are saying about the thermo closing blocking off the rad seems correct, but what if it is staying open until a lower temperature than 157F (temp needed for O/D to kick in) How can I test this?

I read one post, and I'm dreading I dont have the same thing, where there was a blockage where the temp sensor was, and during slow speeds the under hood temps would allow the gauge to read hot, but once moving down the highway, moving air through the hood, the gauge would read cold again. His solution was to take all the coolant lines off and blow out all the passage ways :S doesnt sound like a fun job at all.

mwsnow

How did you test the thermostat? The FSM lists the following specs:

- The valve should open at 80-84C (176-183F)
- The valve opening should be more than 8mm (0.31") at 95C (203F)
- The thermostat valve spring should be tight when the thermostat is fully closed. If it is not, replace the thermostat.

If the valve opens earlier than 80C, it is bad. Leave it in the hot water and watch the thermometer as the water cools. It should close again when the water temp drops below 80C. If it does not, replace the thermostat. Your problem could be caused by something as simple as a small metal burr on the thermostat that causes it to stick open as the engine cools. You need a good, accurate thermometer to do this. The type with the liquid inside of a marked glass tube is best. To me, this seems like the most likely problem. Be sure to positively eliminate the thermostat as the cause of your problem before proceeding to more expensive troubleshooting.

For the temp sensor passage possibly being plugged, it makes sense to simply pull the sensor and check the immediate area around it for built-up debris, etc. The same applies to the coolant temp sensor that feeds the transmission ECU. While you're messing with them, take a bit of sandpaper and clean any corrosion from the electrical terminal and the wire connector. Both the gauge and the transmission ECU read the total resistance of the sensor circuit. Corrosion increases the resistance, which provides a false reading.

ZUK

iTrader: (1)

Seems like you have 2 probs....you had a sticking t-stat and after replacing it you said it was now "rock solid"....is the temp still dropping at 70 mph? Because if it is then you have a strange issue there.

I think you need to look at the coolant temp sensor that the tranny uses for feedback to turn on the OD circuit.

Kiff

I dont think I ever had a sticking T-stat, the gauge was always solid, just would drop really low at higer speeds. Yes it is still dropping low at extended high speeds. I think I will have a look at my senders this weekend. But if it was a corrosion/resistance issue, that would mean it would always be getting the same resistance difference, and should not cause the temp to drop too low at higher speeds.

TC22RE

One way you can check what temperature your cooling system is actually running at is to get ahold of a temperature gun and drive around till you're warmed up (as warm as it will get) then get out and check the temperature of the head close to where your temp sender is. You can get a pretty fair idea of how hot or cold your engine is actually running like that.

Just an idea.

anglr200

Has anyone figured out a resolution to this?
I have the exact same problem with a 95 3.0
Replaced thermostat twice, coolant flush, temp sensor, etc..
Thinking about installing an electronic fan setup?
What do you all think? Thanks,
-Anglr200

Jeremy Stevens

Bumping up this thread for the people who are having this issue. I fixed the overcooling issue on my 3vze and no, it wasn't the thermostat. I tested a new one and an OEM tstat in boiling water both were fine. Its not the ECT(engine coolant temp) sensor because there's no black smoke or lack of power, or check engine light. I also tested the wire that goes from the coolant temperature sender(next to the ECT sensor) with a 3.4 watt lightbulb( not your regular test light but didn't try that, took me awhile to find a full factory service manual but I found the OEM test procedure for the temperature gauge, and it's easy so you don't have to take your dashboard apart to get at the instrument cluster.) So basically I went and found a 3.4 watt light and some wiring for a ground and power, unplugged the connector from the coolant temp sender. Stuck one end of wire from the 3.4 watt light into the connector and grounded off the other side. Then turned the truck on and boom the temp gauge jumped to half way. And the little test light turned on. So that concludes the actual temp gauge in the instrument cluster is working and all the wiring leading to it. The fan clutch is fine just try to spin it by hand and it shouldn't rotate more than once. Next I thought maybe the water bypass outlet(which has your ECT sensor and the sender mounted to it) could be plugged up and giving a false reading, by the way was about a quarter of the gauge and took a long time to get there. Also maybe air in the system even though I had already flushed the coolant multiple times within the year. I thought why not I'll take it apart, take the coolant temp sender out, clean out the water bypass outlet(just ran tap water into it) flush the cooling system with distilled water and add new coolant, also make sure to burp the cooling system really good. Then this fixed my problem. Finally I had the engine warm up to normal operating temp. So what I've concluded is that the coolant may have still had some old contaminants from a recent head gasket repair and may have been making it difficult to get all the air out if the system, therefore not giving a proper reading at the temperature sender. Or it was gunked up in there. Either or, don't mix your coolant ever. ALWAYS use Toyota long life and make sure the system is clean. Park facing up hill when bleeding the system. I think I got everything covered there. Hope this helps someone!

Jeremy Stevens

Oh actually if you are getting a lower temp reading higher speeds that might mean the fan clutch( the thing the fan is mounted to) isn't working properly. because at higher speeds the fan spins faster so if it's spinning out of control it would decrease in temperature. I would try to spin it by hand it should have quite a bit of resistance by which I mean it shouldn't spin very much.

2ToyGuy

Really, if you think about it, the only way to have an "overcooling" condition is if the thermostat is stuck wide open. If the t-stat is working properly, even if the fan clutch is permanently locked up, all that will happen is that the thermostat will close down somewhat to compensate. It may be a little lagged behind what the temperature is actually doing, but not much. It may "hunt" a little, if the fan clutch is locked, before it settles on exactly the right amount of open to maintain the coolant at it's design temp, but eventually, after a few high-low throws, it will settle and stay open to the right amount to maintain the coolant at the t-stat's design temp.
After all, that's what the thermostat is for.

The worst that can happen is if the fan clutch never engages. Then it can overheat. If the thermostat is functioning correctly, though, I personally can't see a way for the system to run too cold. Maybe slightly cooler than what the thermostat is designed for, but not a lot, or it will close down, and warm the coolant up.

As has been mentioned above, the instrument cluster gauge is a totally inaccurate method of verifying the actual operating temperature of the engine. Way too many variables involved. Sender, the sender's threads being slightly corroded, the wiring, including dirt/corrosion in connectors, and so on may have an effect on the gauge's reading. The best way to see is as has been mentioned above: drive it around until the whole engine is warmed up to whatever the thermostat is going to maintain it at, about 30 minutes worth to ensure even the oil is warmed up, then jump out and measure it with a remote thermometer aimed at, or as close as you can get, to the area the thermostat is mounted in. Make note of the actual, real temp measured by the thermometer, compared with what the gauge reads at the same moment. Then you can determine just what the gauge is really telling you.

But hey, I am not a super engineer. I am frequently wrong about stuff, and cooling systems can have some peculiar quirks. If I am mistaken in this case about what's going on, then I sincerely apologize.
I wish you good fortune on this problem!
Pat☺