Running Cold - Poor Gas Mileage

[summerinmaine]

My MJ has always run a bit cold; the temp gauge rarely rises above 150 and the temp has been confirmed with an IR thermo. The only times the temp has approached 200 were on long uphill grades during warm weather. The thermostat was just replaced, with no effect. Any ideas why it should run so cold? Obviously, I don't live in a cold climate.

 

Also, the fuel economy was routinely low (i.e. 13-15mpg) but adequate (since I use it less than 2K miles/year). I've recently replaced the exhaust/cat from the manifold back (Catco Cat and AutoZone plumbing), with no seeming effect (but I'm getting ready to face the dreaded bi-annual Smog Check here in Kalifornistan). On my last trip to Baja, the fuel economy seemed normal on the trip down, but appeared to fall substantially on the trip back up (i.e. <10mpg).

 

I've always assumed that the motor (Renix FI) has never run hot enough to get out of open loop configuration, accounting for the previous mpg figures), but can't see how this might relate to the apparent change in fuel economy.

 

I'm also now concerned that my HC emissions might be whack, but am not yet ready for a Smog pre-test.

 

Any thoughts?

[Randy in Maine]

You need a thermostat that works, as yours doesn't.

 

Might have to buy one from the dealer.

[summerinmaine]

You need a thermostat that works, as yours doesn't.

 

Might have to buy one from the dealer.

 

But this is two different thermostats now. The old one (which may have been a factory original, as it was an archaic design) was clearly closed when it was removed, so it wasn't stuck open. And now the new one, with no reason to suspect that it's faulty, and no apparent effect on the operating temp.

 

Not sure how that makes sense.

[mvusse]

What temp thermostat did you buy?

 

They are cheap. Can't hurt your wallet much ($3 or $4) to buy another one, 195 degree. Do NOT buy a failsafe one. They are an expensive gimmick that only causes problems, as once they hit 210 degrees (well within operating specs) the valve gets hooked by 2 little hooks that will prevent it from ever closing again.

[BLHTAZ]

The dealership is the only place to get the correct t-stat for the 4.0L engine. It needs to have the little bypass valve/hole in it that goes in the 12 o'clock position. I have never seen an aftermarket t-stat with this feature and it will cause problems without it. The t-stat's purpose is to keep the engine warm/hot...not to cool it :thumbsup: .

[SoloCamo]

The dealership is the only place to get the correct t-stat for the 4.0L engine. It needs to have the little bypass valve/hole in it that goes in the 12 o'clock position. I have never seen an aftermarket t-stat with this feature and it will cause problems without it. The t-stat's purpose is to keep the engine warm/hot...not to cool it :thumbsup: .

 

Care to elaborate on how it will cause problems? I'm having zero problems with my non-dealer Thermo :huh???:

[BLHTAZ]

The problem is primarily with air in the system, which will cause overheating...not running cold. That's why I am not sure that the t-stat is the problem here, but it's a cheap thing to try to start.

 

The t-stat without the bypass makes it quite difficult to properly bleed the closed cooling system. If you have converted to the open system, then it's not an issue.

[mvusse]

My Valuecraft one from Autozone ($4.99) has caused me 0 problems, after somebody told me to try filling through the upper radiator hose.

[FxRacing282]

you could try a STANT thermostat. but i don't think its a t-stat problem. is your heater core hoses in the stock location?

[Eagle]

I've always assumed that the motor (Renix FI) has never run hot enough to get out of open loop configuration, accounting for the previous mpg figures), but can't see how this might relate to the apparent change in fuel economy.

 

Any thoughts?

I concur. 150 degrees is well below the temperatire at which the ECU switches to closed-loop operation. I think it's around 165 to 175 degrees.

[HOrnbrod]

I've always assumed that the motor (Renix FI) has never run hot enough to get out of open loop configuration, accounting for the previous mpg figures), but can't see how this might relate to the apparent change in fuel economy.

 

Any thoughts?

I concur. 150 degrees is well below the temperatire at which the ECU switches to closed-loop operation. I think it's around 165 to 175 degrees.

 

On my HO, it switches to closed loop as soon as the O2 sensor starts passing traffic, usually at 130 degrees or so. That's why I like the A/F meter; it tells you exactly when it goes into closed loop.