I live on a rather large hill. It is about ¾ a mile down. In my first gen I can just put it in fourth and let the backpressure hold my speed all the way down without any issues. Well yesterday I took my FD out and was driving down that hill. I usually pull it out of gear and use the brakes to save on gas and let the motor temp come down a little. But this time I was practicing double clutching, motor was running cool, so I left it in fourth using the motor to hold my speed while I went down the hill. All was fine until I came to a stop at the bottom. As the car started to idle it was very rough and my air/fuel gauge (autometer, yes cheesy, but it gives me a general idea) was showing off the map lean. I had to hold my foot on the gas to keep the car running. It almost sounded as if I lost a coil and one rotor was not firing. I figure that could have happened but most likely not the case. The light turned green and I slowly started to pull away. I then looked at the gauges again. All looked normal except the air/fuel gauge, still reading below normal and very lean. I stayed out of the boost and started to look for a good place to pull over. Right then the car jumped back to a “normal” sound, state and the gauge read rich as hell like normal. Everything was fine from then on out.

My thoughts:
I know my car runs way too rich all the time, so under a long deceleration could I have fouled my O2 sensor or spark plugs for a short time?
Otherwise nothing else is coming to mind.

Also, what goes on under a deceleration like that? I can see my air/fuel gauge (on all my cars) read off the charts lean when under a deceleration condition. After talking with a few people they say that is normal. I also notice (on FD only due to the PFC) that injector duty cycles are 0% under deceleration. Is there any fuel being injected into the engine under this condition. If not that throws out my thoughts from above.

Just a weird situation and wondering what you guys think or have ever seen this?

Thanks

 

Under deceleration (your foot completely off the gas) the engine goes deep into vaccum and does not inject any fuel, as evidenced with your 0% injector duty cycle. But if you look into the rev/idle screen on your commander there is:

F/C AE
F/C E/L
F/C A/C

These are the rpm settings for your fuel recovery, so as you come down in speed your injectors come back on at that set rpm level to hold an idle. So no you could not have fouled your O2 sensor or plugs coasting down a hill in gear.

 

So, I guess (from mjw's des.) it means the car had just got the fuel back when it started running rich again?

POM HB

 

It acted like that for almost two minutes before it came back. This was with the idle bouncing from 500 to 1500 from my foot being on the gas to keep it running. It sounded real sick durnig this time too. I thought it would recover as soon as it hit the set RPM? I will check my fuel recovery RPM settings.

What are your recovery RPMs set at just for reference?

Thanks

 

Doesn't sound like your fuel recovery is the problem to me, i would look elsewhere (though I don't know where) and leave those alone.
Bob

 

Yeah, I was going to see what other peoples are before I changed mine. Although mine are all set (without looking) around 800 all the way up to 1100 for fuel cut recovery.
Also I didn't figure coasting down a hill at 2kRPM would cause a condition where my recovery settings would come into play. But I will look into that area.

More ideas please, and thanks for all the input already.

 

coasting down the hill as we know your engine vacuum would be as high as it might ever get. If you have a vacuum line on the fence conduction wise it might have collapsed and took time to recover, as well as your map sensor could have reacted that way as well. The fuel should have been leaned out by the ecu and since it showed lean on your display and ran poor it likely was lean. You may have just pulled a higher vacuum for a long period of time and reveled a weakness somewhere.

 

That was condition and revealed

 

I agree with J.S.J, Its seems likely that a flimsy vac. hose could collapse under that much vacuum...

 

probably vac line to your map sensor.
Check for crimps and deterioration.
Especially if your car was hot, and you are using thin vac hose. It will collapse and stick together, then take 2 minutes to "unstick" itself.
You can test this by simply squeezing a piece of vac hose between your fingers and hold it tightly for a couple of seconds. Let go and see if it springs back or kind of peels itself apart.

 

I tried to reproduce this problem at night when the traffic was less and I could not. Yes the car was running cooler this time. I will be installing a GM 3Bar MAP sensor soon, so I will replace/check those hoses. Tracking down an intermittent vacuum hose may be tuff. Thanks for the suggestions I am looking into this.

 

Will do, Thanks