rheilman

Greetings, been searching around about this one, but most of the info Im finding is related to thermostat and the old temp gauge overshoot issue...

Maybe someone will be able to set me straight or tell me where to look.

The truck: 91 4x4 22re

Symptoms: Upon cold startup the temp gauge immediately goes all the way up! After steady highway driving the temp will drop to the middle of the gauge, but then if I slow down or stop, the temp will raise back up all the way. Using the heater does not seem to have much effect on the symptoms. The engine does not feel particularly hot to the touch when I pull over and investigate it, even with the gauge indicating full heat. Does this indicate a faulty gauge or temperature sensor?

Thermostat is less than 6 months old and temp sending unit is about a year old... Tstat was not replaced with the fancy double temp unit from toyota, just a normal one.

Any advice on where to start would be great!
Thanks
Ray

Swimmerboy2112

iTrader: (2)

Sounds like a bad sending unit or bad gauge. I doubt your temp skyrockets upon startup lol

RustBucket

iTrader: (1)

Interested to hear the responses to this too. My temp gauge is similar: upon startup it will jump back and forth from "normal" range to "full cold". After warm up it goes to the normal range and stays there. Once the engine temp rose a tad from low coolant and the gauge did indicate that, so I guess it still works...

I had replaced my t-stat too (just with a regular one, like the OP), and it made no difference. Temp sender is original though.

Quick thought, how's your coolant level? Did you burp/bleed it when you filled it up?

snobdds

Check the gage itself for loose connections or exposed wires, sometimes it will have a faulty ground and act just as you described.

rheilman

Yes, I was very much leaning towards the sending unit or the gauge, for the obvious reasons.

The sending unit is easy enough to get to, but how does one go about checking the functionality of the gauge?

Oh, and I burped it well just a moment ago just to make sure. no change there.

rheilman

ok, so maybe I jumped the gun with the Temp sending unit... From what I understand there are two Temp sending units/sensors, one for the ECU and one for the gauge. I know there is one coming out just below the throttle body, which is green in color (thats the one I changed) is that the sending unit for the gauge, or the sensor for the computer??

Can anyone tell me where the other one is located, and which sensor/sending unit is which?
Thanks

snobdds

The sending unit for the computer is located under the throttle body and is easily identified by the green color.

The sending unit for the gage is located between cylindars 2 and 3 on the intake manifold. There is a almost like a spade connector on it and it screws into the intake. Some hoses and vacuum lines will need to be removed to access it.

irab88

iTrader: (4)

'scribed
(almost) same issues here. to get the coolant temp sensor off (the green plug one), you'll need a deep ¾" socket.

sam333

Pull the wire to your temp sender( make sure its not grounded) and turn the ignition on and watch the gauge. If it goes to hot( or moves at all) then your gauge is probably shorted. Then have someone touch the temp sener wire to ground and watch the gauge if it goes full pegged to hot then its probably your sender.

irab88

iTrader: (4)

update: i replaced the temp sensor, and it seems to be working ok. i'll let y'all know later

MudHippy

Sender

Sensor of many colors

blue_CA_yota

I have the same symptoms, i replaced the gauge sending unit, located on top the motor under the intake manifold..

New sending unit didn't fix my issue at all. When I changed from my DuraTracs to the super swampers the gauge started working again.. I have concluded that I have a loose wire in the gauge cluster itself..

irab88

iTrader: (4)

sender ≠ sensor

a-ha! thanks! now to order the sender. the sensor helped my idle, but the gauge is still a bit screwy

PismoJoe

I just read a post that dealt with this issue...my temp gauge does this but if you hit the cluster (or just keep tapping the corner where the temp gauge is) it works perfectly.

I can't find the thread, I'll post it up when I find it. Anyways, the plastic-coated wiring harness for the cluster is held on by screws, which are also ground points for the gauges. These screws can loosen, or the grounds can get corroded which causes temp gauge issues.

It sounds like its worth a shot, I'm gonna dig into it this weekend. I'll post up the results

herpestes

http://board.marlincrawler.com/index.php?topic=51009.0

irab88

iTrader: (4)

^^ thanks. though, mine still is messed up and my cluster has never been messed with. but i'll still give it a look.

herpestes

had the problem were the temp gauge would act erratic at times and would either bounce around or show a read and drop to zero.
ended up removing the oil and temp side of the gauge set and replacing with another unit.

irab88

iTrader: (4)

ok, so after going through wiring diagrams and my haynes, i got some diagnostic info:

-normal operation, faulty gauge: bad sender
-erratic idle, normal gauge: bad sensor
-low engine temp, low heater temp: bad thermostat (stuck open)

i think my t-stat is stuck open. the gauge works fine, but it's cold


(please, correct me if i'm mistaken)

Buck87

Duplicate skip to the next post.Can someone delete this post? Please.

Me too I think that I probably have a ground issue that corrects it's self upon warm up.[quote=RustBucket;51614437]Interested to hear the responses to this too. My temp gauge is similar: upon startup it will jump back and forth from "normal" range to "full cold". After warm up it goes to the normal range and stays there. Once the engine temp rose a tad from low coolant and the gauge did indicate that, so I guess it still works...

I had replaced my t-stat too (just with a regular one, like the OP), and it made no difference. Temp sender is original though.

Buck87

Me too I think that I probably have a ground issue that corrects itself upon warm up. When I r&r'd the motor I broke the wire off of the sender so I replaced the sender with one from the pick and pull (no change). I think that the problem may be at the cluster.