Wesmaster

I don't really think it's much to worry about but thought I'd double check.

I've noticed that sometimes my EGT gauge will bounce down (not sure if it ever goes up) about 20 degrees C, and return to it's constant temp right after that. So, the needle sorta jumps and returns instantly. I've assumed that it's either a malfunction in the gauge/wiring, or worse case scenario, there is slight detonation, but I would assume that would produce higher temps, not lower...but I'm just speculating. I've only noticed this during stead operation, not WOT.

So does anyone know if the jumping has anything to do with the engine, or do you just think it's the gauge?

txst

If the needle jumps almost instantly, I wouldn't worry too much about it - especially if the temp goes down not up. All sensors/gauges have some amount of hysteresis when they are functioning correctly which prevents them from doing anything instantaneously.

Ludedude

Has to be the gauge, or more likely the temp sensor.

It might have vibrated loose and you're losing ground or the sending wire
is pinched etc. Gauges, especially of the temperature variety, don't respond instantly..also, it's not likely that your exhaust drops 20 Deg C and bounces back in the space of a few milliseconds.

RT

Wes, temp can't change that fast, too much thermal mass, got to be a loose ground or something. Another way to think of it is if a Tach did the same thing, there's no way the inertia of the moving parts can change speed that fast. Know what I mean?

Wesmaster

Cool, I just figured that if there was a misfire, or something, for that one stroke the temps would be quite different...but possibly not measured quite that fast by the gauge.

Ludedude

Heh, it would nice to see that misfire in the space of a few milliseconds, wouldn't it?

Think of how fast the exhaust gas would have to cool, given the rpm of the motor, and how fast that
"cooler" pulse is whizzing by the sensor and then think about how fast the sending unit and gauge would have to respond before going back to normal temps for the next pulse.

One way to achieve that would be with 4 sending units, one in each header pipe, to monitor each
cylinder separately, but you still wouldn't capture an isolated event of a few miliseconds duration.

Anyway...yeah, check the sending unit and/or wiring

RT

I think Wes is only checking temps from one cylinder (way upstream of the collector).

Wesmaster

Yep, #3.

RT

amazing, I still can remember some things (too bad they're obscure as hell things)