Altitude & D-Jet Adjustments Options

[ThinAir]

I've had some issues lately with very low & rough idle as well as some hunting behavior. As part of preparation I've been reading Brad Anders' tretis on D-Jet Idle Stability (http://members.rennlist.com/pbanders/idle.htm). I know that hunting is an indication of a vacuum leak, but I'm wondering about proper settings once that is solved.

In his last paragraph he mentions that since the D-Jet doesn't do any altitude mixture correction, you should expect additional instability in the mountains. Well, I live and drive at 7,000 feet so there are few places with thinner air for driving than Flagstaff. Although I used to have a car that idled fine by my standards, I'm wondering now about how adjustments are affected by altitude.

If I were to take my car down to Tempe (1,000 feet) for work at a shop that I trust to set CO and such, would I be wasting my time and $$$ because of the big difference in altitude? I don't know of a shop in Flagstaff that knows 914s well enough to do this.

[SLITS]

Ernie, I disagree about D-Jet altitude compensation. The MPS is reactive to both atmospheric pressure and vacuum in that it constantly compares the two in operation.

I do remember years ago a discussion of a module that was available that plugged into the harness for altitude compensation when the vehicles constantly were driven at high altitude though I do not remember what the spec was for "high altitude operation". It apparently was very rare.

Also, there is a discussion of the system on Shoptalk forum wherein the voltage to the system and RF interference is discussed along with valve adjustments, tune and worn engine low vacuum having a great effect on the D-Jet system.

I would tune it to where you live, just as you would a carburetor as there is significant difference in air density at altitude and temperature affecting the A/F ratio.

[TheCabinetmaker]

I've driven my djet at altitudes greater than 7000 feet. Including flagstaff and the canyon a few times. I have never experienced any problems from my mps. Hunting at idle is usually a lean mixture caused by an improperly adjusted tps, or the mixture knob on the ecu.

[Dave_Darling]

The barometer (basically) that was used for altitude compensation was on the L-jet systems, not the D-jet ones.

In theory, since D-jet measures pressure relative to the ambient air pressure in the engine bay, it is not affected by altitude.

In practice, it does seem to be--at least to some extent. I would bet that a 6000-foot change in altitude would make the finer adjustments valueless. It might be cool for large-scale changes, though.

Tune at the altitudes you drive in. Use a wide-band O2 meter to make sure the mixture is reasonable.

And Curt is right about the idle hunting; it is usually caused by a lean mixture.

--DD

[Drums66]

Ernie, I disagree about D-Jet altitude compensation. The MPS is reactive to both atmospheric pressure and vacuum in that it constantly compares the two in operation.

I do remember years ago a discussion of a module that was available that plugged into the harness for altitude compensation when the vehicles constantly were driven at high altitude though I do not remember what the spec was for "high altitude operation". It apparently was very rare.

Also, there is a discussion of the system on Shoptalk forum wherein the voltage to the system and RF interference is discussed along with valve adjustments, tune and worn engine low vacuum having a great effect on the D-Jet system.

I would tune it to where you live, just as you would a carburetor as there is significant difference in air density at altitude and temperature affecting the A/F ratio.

......The last sentence,...is the way to start.....always has worked!
(Drums66 & I approve yhis message)
(DRUM66 & I approve this message

[ThinAir]

Thanks, guys. My ECU was adjusted a few years ago when Joe Sharp brought a tailpipe analyzer to RRC in Moab (6,000 feet), but I understand that things can change over time and need readjusting.

I'm not up to speed on wide-band analyzers and what's on the market. Any recommended models & sources?