Sorry for the long post but this particular problem has me baffled.

I have an 86 Toy Pickup with a 22r. Recently in the last couple of weeks I have been having some problems with the readings that the temperature guage is giving me.

This problem began SUDDENLY, in the almost 10 years I have owned the truck I have never had it overheat or come even close. Anyway here is the scenario. I am currently working from 4 in the afternoon to about 2 in the morning. The reason why this is important is the fact I only have this problem late at night and it is pretty cold outside. I noticed a couple of weeks ago that when I would head home from work (2 AM) my temp guage would immediatly rise to almost the redline and then back down somewhat. I immediatly thought of the info that 4crawler has on his website ragarding the common problem that Toyotas have due to the lack of a byass etc..., so I ordered and installed one of the fancy thermostats with the secondary valve in it. Well, I am still having the problem. Tonight when I left work, I started the engine and within 30 seconds, my temp guage was halfway up. In another minute it was almost to redline. Just when I was about to pull over it began to creep down. It bounced back and forth from midway on the guage (normal op temp) to just under the redline for about 10 minutes. After that it settled down to the normal operating temp for the remainder of the 25 minute drive home. When I installed the thermostat today I took it for a 20 mile test drive and it operated perfectly. Same when I went to work, it ran great. What confuses me is the fact that I only have this problem at night, the fact that it began so sudden and the fact that it takes less than a minute for the guage to indicate I am about to overheat. I am currently thinking that this may actually be an electrical problem either with the sending unit or the guage and the fact that it only occurs when the headlights are on (remember its dark at 2 AM), the possiblity that there is a short in the wiring.

This board has always been a wonderful source of information to me and I hope that one of you Toyota gurus will have some insight on this particular problem. My next step tomorrow is to take it for a drive during the day until it warms up to operating temp and then turn on the headlights to see what happens.