HVAC Sensor

[911/Q45]

In my quest for normal HVAC blower operation I replaced several sensors, finally achieving victory with the engine temp sensor. While replacing the ambient temp sensor I noticed something interesting. The engine air intake is located directly below the ambient temp sensor. So if you put the dreaded pop charger K&N filter on, you risk erratic HVAC operation. Without the airflow supplied by the engine intake, the sensor could read too high from radiator heat and confuse the HVAC system. Just another example of how hard it is to modify a highly integrated car like the Q.

[maxnix]

Good observation of an interdependency that no one has mentioned before.

[HeavyDuty]

That's the first I've seen of JWT POP charger & "dreaded" in the same sentence. What's the skinny with it with respect to the Q -vs- any other application they make where they instruct you to leave the IAT in the area of the filter? Is the IAT & the AMB temp sensor the same on this car?



The guy I bought mine from also made mention that Rex Birkmire told him to remove his POP for some reason, I never got to finish that conversation.

[Q45tech]

All about the detrimental effects of injesting warmer [hotter] air vs the tiny gain from the resistance to air flow thru filter and oem air box and intake.

Unfortunately [for aftermarket guys] most oem designs are quite good and offer little resistance. Just enough to meet filtration goals and sound specifications.

On the Q45 the entire air track prior to the MAF has a restriction of ONLY 5 inches of water column at 6,900 rpm.

You tap the top of the air box and insert a hose that feeds the differential manometer [Dwyer 0-30/40 inch H2O manometer] and take the engine WOT to the RPM and note that there is very little restriction till 4,000 rpm].

The V8 MAF calibration curve and the ecu software assumes that the air flowing into engine never gets above 130F whereas a Cone filter may in Summer allow 160-170-180F under hood air to suck into engine.

On the OBD2 V8, I4, and V6 there is a air temp sensor on the air box to help the ecu understand what the temp is and make downward/upward adjustments to the fuel in non standard conditions. Whereas on 90-95 Q the coolant temp sensor is used to GUESS it's warm outside along with an AC ON signal and a blend of the HVAC air temp sensor. Coolant above 194.99F starts an overheat program which enrichens the combustion [to cool the charge] and reduces ignition advance [which helps to a point but becomes self destructive when the coolant reads above 220F] producing thermal runaway.

[Jeff Williams]

That's great that you fixed your problems with a simple sensor replacement.

The K&N Pop Charger is definitely no "Cold Air Intake" system, more like a Hot Air Intake system.

The fix would be a 3" hole in the hood, and mount the filter in the vertical position, external to the engine bay. But then you would have to put in a water trap, for rainy days.

It would definitely add a "Mad MAXX" look to your luxury car.

The factory plastic intake was no thermal barrier, either. Once the engine bay heats up, the plastic transfers heat to the intake air. The only saving grace part of that, is, usually the car is in motion, and the air going into the intake is much warmer than the air in the engine bay.

I wonder if modifying the resistance to the plenum temperature sensor, would help boost power, similar to the OBDII system modifications on eBay, that modify the intake air sensor, to make it think the air is hotter, and increasing the fuel mixture.

Not that I intend to make my Q a test-bed.

I had a true cold air intake system on my '95 Nissan V-6 pickup. It really opened up that little engine (along with a Flowmaster muffler). The only problem, was, I hated driving in the rain, and was super paranoid about sucking water in the filter canister. It pulled cold air from the wheelwell.

[HeavyDuty]

Q45Tech, doesn't the nature of the velocity stack base of a JWT POP not, by design, straighten the air's entry, increase velocity & create a cooling effect by moving from a smaller diameter opening to a larger one?

The K&N Pop Charger is definitely no "Cold Air Intake" system, more like a Hot Air Intake system.

I've heard this before & I'm aware of the temperature differential under hood vs outside ambient, but when moving, IMHO, I find it hard to beleive there's that much of a difference.

The factory plastic intake was no thermal barrier, either. Once the engine bay heats up, the plastic transfers heat to the intake air. The only saving grace part of that, is, usually the car is in motion, and the air going into the intake is much warmer than the air in the engine bay.

Did you not mean "air in the intake is *cooler* than the bay?"

I wonder if modifying the resistance to the plenum temperature sensor, would help boost power, similar to the OBDII system modifications on eBay, that modify the intake air sensor, to make it think the air is hotter, and increasing the fuel mixture.

On Honda/Acura apps, Jackson Racing uses an additional MAP sensor in their s/c attached to a relay that switches to a 10k ohm resistor installed inline on the IAT to trick the ecu into thinking the air is *cooler* outside, in an attempt to richen the mixture when under boost. Unfortunately, it doesn't work the way they designed it, so even though it's still in their kits, it's defeated within the relay, extra work, more wiring, useless. But that's honda/acura, and speed density, not Nissan & mass air.

If the extrapolation between OBD I coolant temp sensor & HVAC reference lets the ecu "guess" what the ambient temp is, would it not also richen the mixture if the ambient temp was cold, or would it excusively rely on the voltage in the MAF?

Thanks for the response, guys, this is interesting.

[Jeff Williams]

Cooler = Warmer. Yes, I have my terms mixed up. Thanks, HD.

The cooler the air entering the engine, the richer the mixture can be, and the worse gas mileage you get, But you net a few more ponys.

The richer mixture will also have a cooling effect on the engine exhasut. A richer fuel mixture will produce a cooler exhasut temperature. This would be reflected in the coolant temperature.

This is essentially how a simple airplane engine is adjusted in flight. A desired exhaust temperature, and RPM (based on elevation) are dialed in with fuel mixture adjustment and prop pitch adjustment. The throttle is usually set, for a desired airspeed, then small adjustments are made in mixture, for the most efficient fuel burn. But, these are not water cooled engines, but air cooled engines, and I might be way of base.

But the discussion is great, is it not?

[maxnix]

The cooler the air entering the engine, the richer the mixture can be, and the worse gas mileage you get, But you net a few more ponys.

But on OBDI, there is no intake air temperature sensor so there is no richness compensation for hotter than ambient intake air.

All air is assumed to be the external ambient temperature, roughly between 0° and 100° F.__________________Brian1995 Q45 & Q45t & 2000 Q45

Discover the power of the button!

[Jeff Williams]

But on OBDI, there is no intake air temperature sensor so there is no richness compensation for cold or hat intake air.

All air is assumed to be the external ambient temperature, roughly between 0° and 100° F.

Do you think it uses the ambient sensor for the OAT temp?

[maxnix]

Don't know without looking at the wiring diagram in the FSM.

Better idea, why don't you do it? You are probably more skilled than I in reading them. __________________Brian1995 Q45 & Q45t & 2000 Q45

Discover the power of the button!

[Jeff Williams]

I'll add it to my holiday reading list.

I guess I better install that CD...

[Q45tech]

The MAF is the primary control as the SAE uses 68F as the design point [where HP is speced] for all engines. On OBD2 the IAT allows the ecu to better fine tune [mistune] the mixture for preignition goals.

Don't think that the HVAC external temperature sensor is read in fine increments, but in broad strokes coupled with whether the AC compressor is on or not.....Something like : Cold, Normal [50-75F], Hot outside.The coolant temp sensor is used to reduce ignition advance by 1 degree per every 5F above the 190F point: 195,200,205,210,215F.........reduce 1 thru 5 degreesThe ecu has a limit of responding to 222-225F............since a new engine coolant system was tested to stay in the 170-184F range.

10F of do nothing range to allow for system aging and another 20F of self correcting range..........beyond that the knock sensors have another 5-10 degrees of timing reduction [of additional protection].

Not turning the AC on in Summer will allow more advance.Nor letting the engine coolant temp rise above 190F will allow more advance.

The HVAC ambient outside sensor reads: 6190 ohms at 32F, 2650 at 68F,1270 at 104F, and 660 ohms at 140F.........doubt you could fool the system [much] and still get full AC control range.

Whether a 10F reduction in ambient say from 104F down to 94F or even down to 85F would do much good as the AC on signal is probably more important than the actual outside temperature.