Overreading Temp Gauge

Talk about the E-Type Series 3
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jagwit
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Location: George
South Africa

#1 Overreading Temp Gauge

Post by jagwit » Fri Sep 29, 2017 10:35 am

I recently rebuilt a Series 3 for a friend (mechanically only) but found that the temp gauge was reading much higher than what I would expect. At first it was sitting on the L of NORMAL and during a recent drive it moved even higher to between the L and H. New sensor made no difference.

Long story short, upon opening the gauge, I found that the wire to the right in the picture was touching the enclosure of the gauge. I believe this led to a partial short which allowed additional current to flow through the gauge, causing the gauge to display higher values that it should have.

Having moved the wire away from the enclosure, it now displays the exact same values of the gauge in my own car:
N (of Normal) = 3.95V (over the two terminals)
L = 5.6V
tn_20170927_154322.jpg
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Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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jagwit
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#2 Re: Overreading Temp Gauge

Post by jagwit » Thu Oct 05, 2017 10:26 am

This over reading of the gauge persisted even after I fitted the voltage stabiliser, gauge and sensor from my own car to the car in question. I also installed a temporary wire directly from the sensor to the gauge to eliminate possible shorts to ground on that wire.

Yesterday, I decided to remove the 82ºC thermostats and replace them with other 82ºC thermostats to which I've added a 3mm washer to the "bypass port blocking plate". I tested the ones removed and those installed and all are fully operational.

Now the temperature sits stable between R and M of NORMAL.

Conclusion:
1) The bypass port valve of the thermostat(s) did NOT close the bypass port in the thermostat housing causing a coolant "leak" via the bypass circuit and thus SOME coolant kept on recirculating through the engine and not passing through the radiator. This caused the consistently high temperatures being measured.
2) I had tested / measured those thermostats that were installed to be sure that they moved far enough to block those bypass ports but clearly they allowed enough "bypass leakage" to cause a problem.

The really interesting part is that the engine displayed no other symptoms of overheating, no smells, no dumping of coolant, the fans would rarely run for longer than 15 seconds after switching off.
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Last edited by jagwit on Mon Mar 12, 2018 6:53 am, edited 1 time in total.
Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#3 Re: Overreading Temp Gauge

Post by jagwit » Wed Nov 01, 2017 2:05 pm

Regardless of the above, I found that when driving the car for extended periods of time (on hot days), the temp would still climb to L and above.

Somewhat embarrassing to admit this but it turns out that I installed HE head gaskets into this pre-HE engine. To my defense, it never occurred to me that the gasket set, correctly ordered from SNG as for pre-HE might contain the wrong gaskets, and thus I never thought to check.

The position of coolant ports on the pre-HE heads differ from the HE heads and fitting the HE gaskets to pre-HE heads result in the coolant ports on the outside of each head being partially obstructed by the HE gaskets. This leads to reduced coolant flow and hence higher temperatures.

In the picture below you can see the HE gasket on top of the pre-HE gasket and how the coolant ports:
tn_20171031_114813_Richtone(HDR).jpg
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Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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