XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Aux Fan/Temp Guage Reading/Questions

Old 06-07-2019, 09:05 PM
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Default Aux Fan/Temp Guage Reading/Questions


i have a 1990 XJS, just had the waterpump replaced, some rad hoses replaced, and coolant flushed. I was hoping to get some clarification on EXACTLY how i should be reading my temperature guage. I was always under the impression that the needle should never climb above the N. now i am reading that there are different thermostats at play, and it is fine if the needle rests on the top of the N at full op temp...i am hesistant....in my case my needle sits firmly below the N under normal driving, until the car goes stationary for more than 5-7 mins, when the needle then creeps up to on the top of the N. This must be running hot? It was about a 23c day here. My auxilary fan does not kick in under any circumstance. Should it be when the car is at idle? Would this have a serious effect?Again it normally sits below the N, on the highway well below, but WILL go on top if it gets no airflow coming through rad, despite the new parts. Can a malfunctioning aux fan have this effect? What thermostats did the 1990 have, the 82 celsius or 88 celsius? What is the correct way to interpret my guage?

Any insight appreciated.

Last edited by jvickers; 06-07-2019 at 09:08 PM.
Old 06-07-2019, 10:01 PM
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The first thing to remember is that the gauges on these cars are known to be flakey. Many use a hand-held infra red thermometer at the thermostat area (and other spots as well) to get a rough idea of what the actual coolant temperature is. I did this many ago and, on my car, the bottom-middle-top of the N represented about 180-190-200F...which is perfectly acceptable. Others doing the same reported results as much as 20F degrees difference from my readings. That's a lot !

From memory the fan switch is calibrated to 94C. IF your temp gauge reads about the same as mine then I suspect your fan should've kicked in. Worth investigating. An easy first step is the jump the two wires at the switch and see if the fan works. If not, it needs to be fixed.


From what you've described I doubt you have anything to worry serious to worry about since the gauge doesn't keep going up-up-up with no end in sight. I'd be a bit concerned, though, that temp at idle speed starts to rise after just 5-7 minutes in mild ambient conditions. August in Las Vegas city traffic, sure, but that's not what you described. I'd suspect a weak fan clutch or, as touched-on earlier, an inoperative aux fan.

An 88C thermostat is the original spec for your car

Also, proper bleeding of the system is absolutely essential after repairs. Air pockets (which turn into steam pockets) will potentially do much more harm than elevated coolant temperature. Full and proper flow trumps temp readings...to a point, of course.

The following 2 users liked this post by Doug:
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Old 06-07-2019, 11:00 PM
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Doug has nailed it.

Down here, we run the 82c Stats, and the Auxillary fan switch in ALL I have been in touch with is 85C, and measures Bottom hose temp, which is way more stable than top hose temp.

Couple of papers I wrote a long time ago may ease your mind a tad. Some of the scribe does not relate, but since you are intelligent (you own a V12, nuff said), you can sort that very quickly.

The pressure cap on the header tank inside the engine bay LH side is probably getting lazy, and if its "blowing off" at less than designed, floating temps will be the signal, and mainly at idle.
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The following 2 users liked this post by Grant Francis:
Doug (06-08-2019), jvickers (06-08-2019)
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