EGT guages are NOT rpm dependent. They are strictly LOAD dependent. An engine could be run at any given rpm , put in another boat with a different load and the EGT reading will be different. Totally dependent upon the load and thus the fuel used to reach that given rpm.
I have a Hewitt dual needle unit in my boat, one for each cylinder head. My engine, the V555M , like yours is a V8 so each head/manifold needs its own thermocouple [T.C.]. On mine untill I run my engine up to about 1,800 rpm which uses about 10 -12 litres hr. the needles don't even move off the floor. If they did I would be looking for a problem, first with the guage and then the engine. At 2,200 rpm the egt will be around 650oF. BUt that same engine in a heavier boat run at the same rpms would read differently.
The coolant temps. and the EGT temps will not necessarily,in most cases, have anything to do with one another.
Agreed that each meter needs it's own T.C. How many guages and what type are they, dual needle or single needle. How many probes? two for each engine or one for each engine?
Photograph of the guages, BOTH. Does your system measure each manifold
[4] or does one T.C. measure at the join of the exhaust from the two manifolds on each engine?
Actually a good DMM should be able to read the millivolt output easily. But ask Hewitt what a good T.C. will read .
There are grounded types and ungrounded types. I don't know which Hewitt uses. Should not matter if there is not another problem.
I believe most of the meters, EGT, use a type "K" T.C.
But T.C.s can be fooled. If the leads/cable before the actual probe has suffered damage and the two wires have been shorted together the probe will be shorted out. The guage will read what the short tells it meaning the guage will show ambient temps ,not manifold temps. Seen lots of that.
That could also negate your torch test at the probe.
Unfortunately an ohmmeter test will not sort out whether you have a shorted pair of leads or a good T.C. The meter will show a reading of a few ohms in either case. Now if you get no ohms reading then there is an open or failure in the probe circuit and new one would be in order.
Pull the probe out of the manifold and use the ohmmeter to check there is continuity but that will not rule out the possibility of a short in the leads.
They can be a pain to read/test. for that reason.
Polarity matters. If the leads are reversed at the guage it will not work although the fact you did finally get a reading should mean that the polarity was correct. Check it to be sure. Memory is getting thin on the coding but I think the YELLOW lead is the POSITIVE. NOT THE RED.
Although not an expert I worked with hundreds of these things of several different types in a couple plastic injection moulding plants where T.C. were used extensively.
