Is the temperature really spiking, or do you have a faulty gauge?

Dear Car Talk

Dear Car Talk | Apr 01, 1999

Dear Tom and Ray:

Help! I've got a 1991 Plymouth Acclaim. When I start it up in the morning, the temperature gauge goes all the way to hot for a couple of minutes, then drops back down
to halfway and stays there after that. It used to never go past halfway. My mechanic (reliable and I trust him) changed the thermostat, checked my radiator fan, did a
pressure check and checked for leaks. Everything is copacetic, but the gauge still shoots all the way up when the engine is cold. Any suggestions? -- Bill

TOM: Well, either you have a temperature problem or you don't, Bill.

RAY: Oh, brilliant! Isn't he brilliant, folks? How long did it take you to come up with that little pearl of wisdom.

TOM: You didn't let me finish, piston puss. You have to find out whether the temperature is really going up that high, or whether the gauge is just SAYING that it's
going up that high. And the first, and easiest, thing to do is to try changing the temperature sending unit (TSU). That's a little sensor that screws into the cylinder head
and tells the dashboard gauge how hot the engine is. If you've led a good, clean life, Bill, the TSU will be faulty and a new one will solve your problem.

RAY: If a new TSU doesn't fix it, then it's either the engine or the gauge. So next, have your mechanic test the engine temperature using his own gauge. He can screw it
right into the TSU hole, and when he starts the car up in the morning he'll be able to see -- independent of the dashboard gauge -- whether or not the engine is really
running hot at start up.

TOM: If it is, then you've got to look for a blown head gasket, or even a cracked head. Hope that's not the case, Bill.

RAY: If the engine doesn't heat up for your mechanic, then it's just a gauge problem. Either the dashboard temperature gauge or the voltage regulator that supplies
current to the gauge is faulty.

TOM: But gauges don't go bad very often, so I'd say there's a 95 percent chance that it's going to be either the TSU or a blown head gasket. We'll say a novena for TSU,

It's amazing how many bad driving habits and wacko theories have been passed down from generation to generation! Tom and Ray set the record straight in their booklet
"Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send $3 and a stamped (55 cents), self-addressed, No. 10 envelope to Ruin, PO Box
6420, Riverton, NJ 08077-6420.

?(C) 1999 by Tom and Ray Magliozzi and Doug Berman
Distributed by King Features Syndicate, Inc.

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