Hi all,

I recently moved from Massachusetts to Utah (an elevation change of more than 7000ft).

I towed my car out here on trailer and it never got started between MA and UT, so no learning.

Now, I get a really rough idle, sometimes. Sometimes it butter smooth and and other times not smooth at all.

I had a log of an idle in MA and I logged the car idling here when it was rough. The biggest difference is the timing advance.

In MA the timing bouncing between 4 to 6 degrees (I assume the timing advance column is in degrees), now it bounces between 2 and 8 degrees.

The service manual says that the base timing is 5 degree +/- 3 degrees, but at high altitudes the timing gets advanced to 10 degrees +/- 3 degrees.

Any suggestions as what could be causing this? My next step is to unplug the battery and let the ECU forget where it is and then let it re-learn at altitude.

Thanks!

 

At high altitudes the air has less mass per unit volume (density). The turbo does a great job in normalizing this effect (although it will spin faster in doing so) under boost, however, under 100 load (vacuum) you are still subject to the naturally aspirated rules on high altitude air.

Because there is much less air, and you haven't told the ECU to subtract the corresponding amount of fuel, your AFR's are probably pretty rich, and as a result, your STFT is probably considerably out of whack trying to lean the AFR's out to get to stoich.

As a start, I'd pull one whole point out of my off boost AFR's, up my idle timing advance to 11 degrees, and think about making my injector scaling numerically larger (leaning out the AFR). Then get it tuned. Your timing will also be off because you're coming from 93 octane land, and the pisstane we get up high and out west is probably giving you significant knock activity. Hope that helps.

 

before you do anything else, disconnect the battery for about ten minutes and reconnect it. Then start the car and let it idle for 15 minutes so it can relearn. If that doesn't fix it you can start messing with the tune.

 

Thanks guys for your thoughts.

Yeah, letting the ecu relearn is my next step. If things don;t improve I will start messing with the timing, injector scaling, and adjusting my AFR's.

Time to buy that wideband O2 I guess....