Dash temp gauge vs MES scan temp

[SeleMatt]

Greetings all,

So my temp gauge in my 156 JTS 2.0 has suddenly started reading no higher than approx 55 deg. Just been out driving around with MES hooked up and it says engine temp running between 74 and 78. (which is what the dash gauge historically looks like)
It's a new thermostat that's done about 400k's
My question is, has the brass sensor/sender (that I transferred from the old thermostat) on the thermostat housing let go?

All thoughts appreciated.

[bazzbazz]

Possibly, that's why I always replace both Temperature Sensor and Thermostat at the same time.

[SeleMatt]

Thank you Bazz.

I take it it that these cars must be running a 74 deg thermostat as demonstrated by MES. The car temp stopped its rising plane from start up at 74.3 degrees. 

Why is that?

In my experience with all other cars I've owned, the thermostat has always been 88 degrees to run at 90 on the temp gauge.

[bazzbazz]

Fit a new sensor, then check your readings.

Also where did you get your new thermostat from & what brand is it?

[SeleMatt]

From The Italian Job. It's a Facet and looks to be of cheaper construction than the Behr I pulled out. Dom's sent a temp sensor. I'll report back after Bathurst where I'm headed tomorrow.

[SeleMatt]

So I fitted new temp sensor this morning and drove off,
The temp rose to 49.5 on Multiecuscan and then stopped. Drove out onto the freeway and nothing changed including the car gauge which remained below 50.
Left the freeway, pulled over and disconnected Multiecuscan, then reconnected it where it then showed 66.5...car gauge about 50
Kept driving around for a bit longer where temp dropped back to early 60's...car gauge barely moved at all throughout.

So the overall result is worse than before...

What is going on here...?...

[Craig_m67]

Put the old sensor back in and see if it correlates

[SeleMatt]

Cheers Craig,

Did that and it got up to mid 60's again. It didn't seize at 49.5 like last time although it did stay on 53.7 for too long before it got going again. The temp dropped back to late 50's running downhill with the foot out of the gas. Ponced around in the traffic for a while before letting it idle and fast idle with my foot in the driveway where it got up to 78. The car gauge never exceeded 55ish...

Just noticed the car temp gauge is stuck on 55 after shutting down and extracting the key...

[bazzbazz]

Just noticed the car temp gauge is stuck on 55 after shutting down and extracting the key...

Ding Ding Ding . . .  I think we have a winner.   

Don't you just hate it when that happens.   

[SeleMatt]

Sorry Bazz but you've lost me.

What have I won?

Just been out to the car this morning and the gauge is back to zero...

I've just put the new sensor back in. Should I perform an adaptive reset before heading out again?

[bazzbazz]

Well if you've taken the key out of the ignition and the gauge is stuck at 55 deg and did not go back to zero then one might assume there is a problem with the gauge.

[SeleMatt]

That's true Bazz, but the Multiecuscan results aren't accurate either if the thermostat is an 88 degree opener given the temp has only reached mid 70's in the traffic and drops to high 60's when freewheeling at 60 and 70KPH

However

Just had a thought...

The car has an enhanced ecu upgrade which did away with the post-cat sensors (and so no more engine light ever). On the advice of the same mechanic, the mani-cats were stripped out as he said they were damaged.

My questions are these: If I have no mani-cats, how will the pre-cat sensors get hot enough to progressively lean off the fuel/air ratio as the engine warms up to 90 deg? 

If the multiscan engine temp is correct and my car is not warming up past 70 odd deg, is that because that's as high a temp as the pre-cat sensors detect (in the absence of mani-cats) and thus, I've always been running around at that temp in what must be a rich fuel/air ratio state?

Or

If I were to fit a new set of mani-cats tomorrow, would all this be resolved? 

[Craig_m67]

I think a basic description of how the temp gauge and the ECU determine temps is in order - Bazz??

As I recall, the temp gauge and the ECU source their data from different sensors?? (Never understood this myself, I may be wrong). Further the temp gauge is electronically dampened to stop it swinging wildly and freaking you out

That said, i remember chasing my tail on a temp issue, replacing several brands of thermostats and a sensors, only to realise we had a blown head gasket (diesel 156).  I now only fit OE Alfa bits (which are just OEM in fancy boxes)



If your car is not standard, missing cats and or lambda O2 probes, or has resistors (cheats) in place to help the ECU pretend everything is normal, then you're likely pushing shit up hill to diagnose stuff.  I don't say this to dissuade you, only to encourage you to be clear about what mods have been made (if you know/understand).. and what the issue is.

Is the car overheating?
Is the car running cool (not heater in the cabin)

What other symptoms are happening (dodgy idle, no heat, missing coolant, stupid rich fuel consumption). 
I take it the thermostat is new and the mileage is +400K km (that's a lot), why was it replaced?
What's a mani (mini) cat.. isn't their just the one cat and two O2 sensors (pre/post)?

[SeleMatt]

Quote Craig

Is the car overheating?

No

Is the car running cool (not heater in the cabin)

Yes, as indicated by Multiecuscan

What other symptoms are happening (dodgy idle, no heat, missing coolant, stupid rich fuel consumption). Nothing except the usual oil consumption due the 'engineered by a 5 year old' piston/ring arrangement.

I take it the thermostat is new and the mileage is +400K km (that's a lot), why was it replaced?

Brand new thermostat and sensor, K's are 140,000. I said earlier this new thermostat is about 400k's old...

What's a mani (mini) cat.. isn't their just the one cat and two O2 sensors (pre/post)?

There are 2 cats and so 4 sensors. Mani-cat is short for 'catalyser in manifold" headers such as these JTS GDI engines have

[johnl]

My questions are these: If I have no mani-cats, how will the pre-cat sensors get hot enough to progressively lean off the fuel/air ratio as the engine warms up to 90 deg? 

The pre-cat O2 sensors (ie. lambda sensors) don't 'care' whether any 'cats' exist or not, it makes no difference to operation of the O2 sensors. The sensors don't rely on the "mani-cats" to reach the temperature at which they 'come on line' (a bit over 300°C), or for any temperature input. The rate at which an O2 sensor reaches operating temperature is only due to its' internal heating coil (electically helping to heat it more rapidly), and the temperature of the exhaust gasses that it is exposed to.

If the multiscan engine temp is correct and my car is not warming up past 70 odd deg, is that because that's as high a temp as the pre-cat sensors detect (in the absence of mani-cats) and thus, I've always been running around at that temp in what must be a rich fuel/air ratio state?

The pre-cat sensors do not detect temperature at all, they only detect the % quantity of O2 in the exhaust gasses.

When coolant temperature reaches X° (as 'reported' to the ECU by the coolant temperature sensor, correctly or not) the ECU goes into 'closed loop' mode and will 'un-enrich' the AFR. If the ECU is in 'open loop' mode (which it will be if the reported coolant temperature is less than X°), then the AFR will remain rich (like having the choke on with a carburettored engine).

I don't know what coolant temperature the ECU is programmed to see as being the X° trigger point to enter closed loop mode, somebody else may know(?). At 70° the ECU may already have entered closed loop and so be adaptively fuelling the engine according to what the O2 sensors are telling it (ie. AFR detected as being either rich or lean, and correcting from there). Or, at 70° coolant temperature the ECU might not yet have entered closed loop and so may still be fuelling according to hard maps in its' programing (= rich AFR).

Or
If I were to fit a new set of mani-cats tomorrow, would all this be resolved?

No. The engine temperature isn't controlled by the catalytic convertors.

Regards,
John.