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I've pulled off the 2 wires off the big sensor ontop of the thermostat housing. Went for a drive and noticed loud exhaust sound when in low revs and something which sounds like a butterfly closing in the exhaust and can also hear a whining/whistle.
I'm not sure what has caused the sound but I've never experienced that noise before pulling off the wires, its if there was a switch turned on and suddenly it sounds like there's no firewall its that loud and the noise turns off after a few seconds. It sounds like if there was a hole in the exhaust its that loud.

But when on motorway or powering through a gear both noises stop then return when low revs.

Do I need to put back the sensor, is this the cold starter in the downpipe and high revs pushing it open? its friggen annoying i've never heard those sounds on my car before ever.

engine is a 4jb1-t manual 1991

I was also wondering if the early 4jb1-t engine has a different downpipe as mine goes straight down infront of oil filter or is that normal?

Seems to be a lot of room for a larger downpipe on my engine?

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It shouldnt make that much difference but removing those wires does advance the injection timing causing diesel knock. Advanced timing is good for cold starting. Driving like this is not good for a warmed up engine. It can melt pistons and overheat. Better to put the wires back.
Then we can ask why you were playing with this in near summer conditions. The 4jb1 will start without a cold start device down to near freezing

The single wire sensor on top controls the flow plugs the 2 wire 1 is a waste of time never used it, but I have no vacuum controls or anything on my engines, cold start is inside the injector pumps not electronic controlled?? With the filter facing up you can just get a 2 1/2 dump pipe on but is a nightmare best to change oil filter assembly, yours looks to be factory

How can I disable the advance timing thing? the sensor is pretty past repairing, the wires I can't even tell which one is which. Wouldn't it be advance timing continuously with out the sensor, it would always be reading 0 degrees?

I've taken off the diaphragm auto fast idler from fuel pump.

Much easier and probably cheaper to replace the sensor. You have lots of specialist isuzu wreckers up your way
Fast idle thing was only for air con

yeah getting harder to find parts for the mu's now. found second hand sensor for 40 last one connection parts had in nz and CalIsuzu had no stock in nz or japan, becoming bit of a worry.

My mechanic mate says the sensor is a breaker and the circuit is a closed loop until 5 degrees and below opening the circuit? So if I join the 2 wires on the circuit and leave the sensor unattached It should by-pass the switch and never activate the cold start/advance timing.

Does this sound right?

As long as we are talking the correct sensor yes.

It can't advance the timing as I said before only electronics on the pump is stop solenoid, rev counter sensor, tps if you have a tps, don't need that if manual either

Ok so any ideas what has happened? The only thing I've touched is that sensor on the thermo housing. I am now getting loud bore or similar to a hole in the exhaust sound at random times while driving on a cold and warm engine. The noise will completely stop like something has turned something off. I haven't noticed it on the motorway or in high revs.