Ok, so for the past 6 months or so my car has developed a new trait:

Scenario: cruising at 75 - 85 mph in 5th gear, light throttle, closed loop operation

Result: every once in a while (consistently does this every few minutes) the car will start running VERY lean ... AFRs go from stoich (~14.7) to being between 17 and 20! Engine sounds like a lawn mower at that point.

If I let off the gas it goes away, but will come back a few minutes later. It only happens above 75 mph and only in closed loop. The only other time my car exhibits this behaviour is when the engine is warming up. It'll initially run rich for ~30 seconds and then it'll idle lean (AFR between 17 and 20) until it warms up all the way.

Any ideas? Open loop operation is flawless ... so far. I have a couple of old logs, but nothing has jumped out at me. I'm in the process of hooking up an LMA-3 and going to Logworks for all logging...

l8r)

 

wild *** guess : coolant temps settings, or a faulty coolant temp sensor?

as you coolant temp gate opens and closes your sensor is swinging full on and full off? guessing that at over 75 mph your motor is on the cusp of the thermostat opening and closing. if the thermostat sticks

i must admit it does not sound to likely to be your problem.

 

MAF acting up, mine went bad...but it would kick out of closed loop into open loop and everything would go rich 10.5~10.9 cruising at 60mph.

 

Wow, never heard of an instance of the maf actually going bad. VW issues are following you Car doing otherwise Ok?

 

You should be looking at your long term fuel trims (Mid and High) at this point..

Also log the load your hitting, compare it to the Open Loop Load tables for that RPM.. You might be crossing over into a portion of the open loop map that is a little lean.

 

Car doing fine... can you believe that!

 

I guess you have big cams and injectors ??

When you're warming up, you are running in open loop map (I assume you know that and you've set the AFR from 10-70 up to 1500rpms richer than normal?)

After 30s or the coolant temp close loop.. you switch over to closed loop.. thats where the stumble happens again .. you need to change the coolant temp switchover point higher .. or change the closed loop AFR settings

As for the cruising .. maybe its so cool that you transition in and out of close and open loop ??

 

what maps are you considering as "open loop maps"?

 

I'll double check that, although if I tip in at that speed, the AFRs immediately go rich (~12.3) and start to dip towards low 11's/high 10's as the load starts to build.

How much is a new MAF? Anyone know?

l8r)

 

new MAF= $400 shipped at mitsubishiparts.com

 

I have similar warm up issues as you, but not the cruise. Raising the crossover point from open loop to closed loop for the coolant temp. sensor has helped my warmup issue. In other words, the car doesn't go into closed loop until it's "really, really" warm.

Could very well be the coolant temp. crossover point as others suggest.

 

Maybe a possible emissions related problem.

On the topic my guess would be either run full OEM emissions or go Buschur style - none

Although I'm a novice here I do know some ugly facts on the topic of emissions; e.g., this car was engineered to protect it's catalytic converter and the content of the exhaust as a priority, even before the engine's well being.

The FSM even states that excessive missfire (increases of NOx's, CO ect) on a piston picked up by the ECU will result in respective IDC shutdown - I wonder if that can blow a tuned or modded engine up if it was loaded

 

You can usually find an 03, 04, or 05 used MAF here at FS/wanted for $80. I bought one 2 yrs ago for $60 from kwman because my honeycomb was gashed in but I dissected my original.

Unfortunately it might be tougher to find one these day's with almost a total depletion of need to go MAFless/speeddensity/AEMEMS