Hilarious thermostat failure

[Kaldek]

So I've been around cars and bikes a lot but recently picked up a high mileage NJ Pajero 3 litre V6 with 373,000km on the clock. Now, it was cheap at $1,600 but she's got her fair share of issues. One thing that I just diagnosed and resolved was the hilarious behaviour of the temperature gauge.

The temperature gauge would inconsistently (but fairly quickly) climb to 3/4 on the dashboard, and then all of a sudden would rapidly drop back down to cool. It would then climb back up....and then crash back down. Repeat, ad nauseum.

I'm pretty good with cooling systems, so the first thing I did was pull the thermostat. Ummm, yeah. Looks like this thermostat hasn't been changed in all of those 373,000km. Two things about this thermostat made me laugh:

1. No matter how much you heat it, it won't open
2. It can be pushed open with your fingers

So basically the car would heat up until the pressure in the coolant inside the motor built to the point where it pushed the thermostat open. Either that or the force of the water pump (or both).

Naturally a new thermostat solved the problem but I had to laugh pretty hard over this one.

[Sean Carritt]

So I've been around cars and bikes a lot but recently picked up a high mileage NJ Pajero 3 litre V6 with 373,000km on the clock. Now, it was cheap at $1,600 but she's got her fair share of issues. One thing that I just diagnosed and resolved was the hilarious behaviour of the temperature gauge.

The temperature gauge would inconsistently (but fairly quickly) climb to 3/4 on the dashboard, and then all of a sudden would rapidly drop back down to cool. It would then climb back up....and then crash back down. Repeat, ad nauseum.

I'm pretty good with cooling systems, so the first thing I did was pull the thermostat. Ummm, yeah. Looks like this thermostat hasn't been changed in all of those 373,000km. Two things about this thermostat made me laugh:

1. No matter how much you heat it, it won't open
2. It can be pushed open with your fingers

So basically the car would heat up until the pressure in the coolant inside the motor built to the point where it pushed the thermostat open. Either that or the force of the water pump (or both).

Naturally a new thermostat solved the problem but I had to laugh pretty hard over this one.

Hi kaldek I have the same engine in my gen1 swb 1990 model 14000 on the clock. It has not done much milage as it was a backup/spare car from the person I bought it from
When I am driving the temp gets to 1/4 and that where it stays. But when I'm idling for a while esp in traffic the temp increases to 1/2. When I start moving again it drops back to 1/4.
Also I have no heat coming from the heater at all. I've checked all the pies and they all clear. I have only had the car for a couple of months. What would you reckon is the cause.

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[Jim_James]

When I am driving the temp gets to 1/4 and that where it stays. But when I'm idling for a while esp in traffic the temp increases to 1/2. When I start moving again it drops back to 1/4.

It'd be your thermostat not closing. Only when the vehicle isn't moving can the coolant warm up enough to "normal", otherwise the thermostat not closing the loop will keep it running cooler than it should.

[Sean Carritt]

Thanks so much I'l def open it up and replace the thermostat

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[Kaldek]

When I am driving the temp gets to 1/4 and that where it stays. But when I'm idling for a while esp in traffic the temp increases to 1/2. When I start moving again it drops back to 1/4.
Also I have no heat coming from the heater at all. I've checked all the pies and they all clear. I have only had the car for a couple of months. What would you reckon is the cause.

EDIT: I've just realised these motors don't use a bypass thermostat. That means that the water in the coolant jacket doesn't circulate until the car is hot. I've therefore removed what I said previously in this post and will stick to the thermostat test below.

Take the thermostat out and drop it in a pan of water. Heat the pan to boiling and see if the thermostat opens as the water gets close to boiling. You can use a temperature probe if you want to get really accurate, but if it opens soon after the water starts to simmer it's generally an OK thermostat. If the thermostat is OK, there's something else going on.

[Sean Carritt]

Thanks so much. As soon as I get home in a few weeks I'll have a look. I've also been informed it's possible the thermostat has been removed. As it takes quite a whole for the temp to reach 1/4.

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[TomW]

Hi kaldek I have the same engine in my gen1 swb 1990 model 14000 on the clock. It has not done much milage as it was a backup/spare car from the person I bought it from
When I am driving the temp gets to 1/4 and that where it stays. But when I'm idling for a while esp in traffic the temp increases to 1/2. When I start moving again it drops back to 1/4.
Also I have no heat coming from the heater at all. I've checked all the pies and they all clear. I have only had the car for a couple of months. What would you reckon is the cause.

Mine was behaving very similar to this, the gauge tended to just wander about own its own and the heater never really got warm. A new thermostat, fan clutch and after market temp gauge sees it sit around 85 degrees pretty much regardless of conditions.

[Kaldek]

Mine was behaving very similar to this, the gauge tended to just wander about own its own and the heater never really got warm. A new thermostat, fan clutch and after market temp gauge sees it sit around 85 degrees pretty much regardless of conditions.

In your case did you replace the temperature sender probe? I've also had one of these fail on another Mitsubishi of mine (Legnum VR4). In that case, the needle would climb to above half on hot days, and all it turned out to be was the sender probe was faulty! At the time, I didn't know a lot and so I replaced two thermostats, the radiator, and the radiator cap.

All to find out it was a bloody sender. Luckily I had a spare!

[TomW]

Yeah, I did a progressive replacement, thermostat first which resolved the heater issue but the gauge was still playing up, new sender which made no change, then after market gauge and matching sender. I guess the old factory gauge was just very slow to respond or had been damaged at some point or something?

After fitting the aftermarket gauge and new fan clutch it is pretty much solid at 85 which is is reassuring. I should replace the radiator at some point as well but that is pretty far down the list.