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Temp Gauge Problem?

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Mar 15, 2009, 2:52 PM

Post #1 of 10 (2231 views)
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I have a 2002 Toyota Avalon XLS. The car has approx. 46k miles on it. The water pump and coolant have recently been replaced, as well as the fan relays. My problem is this: the interior dash temperature gauge behaves erratically. When started and allowed to idle to operating temperature, the guage operates properly and the car does not indicate it is overheating. However, when driven to operating temperatures and then stopped (such as at a stop light or in the driveway) the needle will rapidly move from normal position (slightly below ½) all the way up to ¾ or even the red “H”. This happens far too fast for it be reflecting an actual temperature change (movement from ½ to “H” is about 2-3 seconds). Upon acceleration the needle moves back to its normal position. The temp gauge reading seems to be tied to the RPMs since when going down a long hill at a constant 60 mph w/o pressing the accelerator, the temp gauge began creeping upward even though it was fine when traveling at 60 mph over level ground. The amount of air through the radiator had not changed since the speed was the same. When at a stop light, as the car slows and the RPMs decrease, the temp needle goes up. But if I put it in neutral and rev the engine, the needle will settle back down to where it is supposed to be. Upon re-starting the car while it is still warm, the temp gauge goes all the way to “H” and then comes back down slowly. The car shows no signs of overheating at any point. The mechanic allowed it to idle in excess of an hour on a brisk day and saw nothing troubling. The radiator is topped off when cool and the overflow reservoir is within limits.

My thought is that the one-wire temp sending unit is bad. Would this cause such a problem? Or does it seem like the gauge itself is faulty?

Thank you for any insight you might offer.

Veteran / Moderator

Mar 15, 2009, 4:27 PM

Post #2 of 10 (2226 views)
Re: Temp Gauge Problem? [In reply to] Sign In

You said you replaced the fan relays, but are the fans operating properly? If the fans are not working properly, it can cause the problem you are describing. You need to be sure the temperature is not actually fluctuating before you start changing parts. See if you can find an infrared thermometer and check engine temperature when the gauge is rising. If temperature is going along with gauge, check fans or maybe a cooling system flush and fill might help.

Being way2old is why I need help from younger minds

Loren Champlain Sr
Veteran / Moderator

Mar 15, 2009, 6:58 PM

Post #3 of 10 (2224 views)
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Were you experiencing this problem prior to the water pump replacement? If not having heating problems at idle speeds, just higher rpms (regardless of actual vehicle speed), I'm wondering if the water pump belt or impeller may be slipping. Strange symptoms.
SW Washington


Mar 15, 2009, 7:10 PM

Post #4 of 10 (2220 views)
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The only problem experienced before the pump replacement was the needle going to "H" upon re-starting. The fluctuations are new, as far as I know.

Loren Champlain Sr
Veteran / Moderator

Mar 15, 2009, 7:22 PM

Post #5 of 10 (2219 views)
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When you shut off an engine warm, it is normal for it to 'build' heat and be hotter when you restart, but the temp should drop as soon as the coolant starts circulating. Are you certain that the cooling system has been completely bled of air? Does the heater work okay? As W2O mentioned, are the cooling fan(s) coming on as they should? But, what bothers me about this, once you get up to speed and get plenty of air flow through the radiator and are still having a problem.....After normal driving, immediately after shutdown, feel the radiator for cool spots. It should be even temperature all over. I wouldn't think this new of a car would have a plugged radiator, but....
SW Washington


Mar 15, 2009, 8:28 PM

Post #6 of 10 (2211 views)
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Thanks to both of you for your input. As far as I know, the fans are working correctly. This is a little difficult to determine since I don't know the actual temperature but I do believe they are operating correctly.

Heater works fine. I cannot say with certainty that there is no air bubble but it seems unlikely.

I am pretty confident this is not an actual cooling problem but rather a sensor or gauge issue. I do not believe the car would heat or cool as quickly as the gauge indicates if there was a cooling system problem.

Loren Champlain Sr
Veteran / Moderator

Mar 15, 2009, 8:40 PM

Post #7 of 10 (2209 views)
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Both the sensor and the guage can be checked, independantly. You'll need to check resistance values at varying temps for the sensor. If it is just a single wire connector, grounding the wire going to the sensor should send the temp guage full hot. I really don't think this is an issue, but if you've got a leaky head gasket leaking hot exhaust gasses into the cooling system, that can cause very quick, eratic temp changes. That can be checked with a gas analyzer or a liquid called Bloc-Check available at most part stores.
SW Washington


Mar 18, 2009, 9:06 PM

Post #8 of 10 (2199 views)
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The mechanic ran the car at idle for more than an hour. The hottest any portion of the engine got was 190 degrees.

So, he switched out the temp sending unit. I haven't gotten the opportunity to drive test the car much but will report back whether or not that was the solution.


Mar 21, 2009, 7:31 AM

Post #9 of 10 (2183 views)
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it was not the temp sending unit. At this point we've ruled out an actual mechanical cooling problem and a problem with the temp sending unit. Other ideas?

Someone mentioned the meter plate. Is this a part which can be replaced, or do you have to replace the whole instrument gauge cluster?

Tom Greenleaf
Ultimate Carjunky / Moderator

Mar 21, 2009, 9:02 AM

Post #10 of 10 (2178 views)
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>As far as I know, the fans are working correctly.<

Quote from back a few posts. Are they or aren't they working on time? Put cardboard in front of radiator (partial) CAREFULLY! and watch all temps, gauges and with infrared touchless thermo and watch the fans come on at a prescribed temp where sensed at their switch.

Some and I doubt this car will run fans if you request A/C but more and more are sensing the need for the fans and not just put them plain on anytime A/C is requested. Also - depending on location of radiator and condeser (A/C's radiator) one fan may not assist for the other.

Temp sensing shouldn't be taken randomly around the engine but rather right at the thermostat housing with a touchless thermo or tape one on it remotely. It should be close to the marked temp of the thermostat all the time after initiall opening and will rise quickly after shut down - heat rises without flow but would drop fast upon a restart. Loren mentioned that and that's normal.

Current body styles are so poor for air flow for radiator that fan(s) can be needed even at higher speeds. Far less applicable at low or idle but the air dam that gets snapped off so easy on many cars is there underside of bumper for a reason. Air flow needs to be forced thru radiator and not just "huff and puff" under there unseen! That air dam makes a slight vacuum behind it channeling the air thru, and down out bottom behind it. If that's pressure from speed the air won't flow!

More tricky things that happen: Fan(s) may be spinning backwards and that should be checked!

Noted from earlier post: Temp rises too quick for a credible reading. The sensor really could move that quickly as they are placed where they would catch the extremes. True - the whole mass of the engine isn't changing temps that fast.

More again: New water pump may not be so great depending on how much of this happened since just that replacement and what got disturbed in replacing it. Too much gasket goo or who knows are possible too.

Pay attention as you are. It's too important to ignore,

Long retired now

Temp Gauge Problem?

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