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.

I'd just get an aftermarket temp gauge. Every car I ever own pretty much immediatly gets fitted with a new temp gauge (mechanical type). Especially over here in oz, where high temps mean hotter running vehicles. For a few bucks i'd rather have that extra insurance that my vehicle is running cool and not boiling it's guts out and cracking heads. Just my 2 cents <img src="/forums/images/graemlins/cheers.gif" alt="" />

I don't have any insight on fixing it other than doing what you've already done - the thermostat.. Could try to get all the air out of the system...

To further diagnose you could measure the resistances at the temp sender for the gauge and the temp sender for the ECU at a known temperature, say once you're running and you're in the middle of the normal range. That temp is 190-200 degrees or so...

Next time it starts that crap, shut it down and measure the resistances at both sensor.. I bet it'll show an odd reading at the gauge temp sender (which is higher in the motor) - ECU sender will probably read right where it should..

On the 3.0 I did see an odd case where the engine temp would get hot and then suddenly cool off.. that one ended up being a going-bad water pump (it would seize, stop, and then restart) - but it doesn't apply to the 22r series.

Two things, I don't think your overheating. Like you say, at 2 am its cold outside. Your engine will not overheat in just a minute. Secondly the way you described the problem I started thinking what is he doing at 2am and not doing in the daylight hours. The only thing is the headlights. Therefore I think you have some sort of wiring problem.

Thanks for the replies. I am also thinking it has something to do with the headlights. Like I said before, I never have the problem until I am driving at 2 in the morning and the fact it was happening so fast. The guage would actually go from all the way at the top to the middle in less than 15 seconds and back up again. I can't see that engine heating up and cooling down so fast. I don't seem to have the problem any other time. Of course that never dawned on me until last night when I was driving home and I had just put in the new thermostat earlier yesterday.

As far as the mechanical guage, if I did end up buying one and installing it, where on the engine would the sending unit be placed? Also, I am assuming that the two green plugs next to the thermostat housing are the sending unit for a 22R, is this correct?

The 2 green plugs next to the thermostat are used for emissions sensors, the temperature sending unit is screwed into the head it self by the thermostat.

Another possibility:
http://www.4crawler.com/4x4/CheapTricks/index.shtml#Thermostat

Thanks 4crawler, but unless I am missing something on your website, I already adressed that particular problem. Like I mentioned in my original post, I ordered one of those thermostats you mention on your website from the local Toyota dealer (which took almost a week to get), and after installing it, I am still having the same problem. My biggest concern is the fact that I have never had this problem before. I have been driving the truck for almost 10 years in all sorts of weather (with the heater on and off) and have never had this occur. The t-stat I had in there until yesterday was in the truck for about a year with no problems at all.

By the way, you have a great website with a lot of useful info.

I bought a aftermarket guage today, I wouldn't call it mechanical as it is completly electrical(all of them in the store were fully electrical), and will install it tomorrow. Of course this brings me to another question. Since this aftermarket guage does not have a redline, at what temperature should I be concerned about overheating?

Have you checked the head gasket? I know when my HG blew on my '91 T/A that it drove perfectly fine without a miss but would overheat within a minute or two a cranking. If the HG is blown close to the sending unit it will pick up the exhaust gases and give you a false reading. Check your oil for water and vice versus.

I think I am alright on the head gasket. I installed the aftermarket guage today and took it for a test drive. The guage rose pretty slowly to about 200 degrees where it seemed to stop and remain steady (190 thermostat). Since the problem started it was never consistant, I guess I will drive it back and forth to work for a few days and see if it fixed the problem. I appreciate the answers and suggestions from you guys.