Accelerating with foot off the pedal?
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Joined: Jun 2001
Posts: 509
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From: Edmonds WA
Accelerating with foot off the pedal?
I've got a ZZ4 in my 87, but the stock TPIS on top, so nothin' terribly custom (other than the long block ;-)
While driving at about 20 MPH on a relatively flat road (I was cruisin' thru a school zone), the car slowly accelerated to 30 MPH (1,000 to 1,300 RPM) over about a half-mile without my foot on the pedal. This surprised me, as I expected the car to decelerate. I was about 2 miles from home, first-thing in the morning, so it wasn't fully warmed up yet.
I experienced this kind of behavior last year, as well, but thought it might be related to an idle problem I was having. I've since cleaned up the throttle body and replaced the IAC and still notice its tendency to throttle-off accelerate.
Any ideas on why it's doing this? Anyone else experience this kind of thing?
I've also noticed that the car will accelerate a lot quicker between 1,500 and 2,500 RPM when I get into it a bit (not WOT) when it's cold, than after it's completely warmed up (when the oil temp comes up above 150). When it's all warmed up, it tends to really wake up at about 2,500 - 2,800 RPM, but is sluggish by comparison in the 1,500 - 2,500 range.
While driving at about 20 MPH on a relatively flat road (I was cruisin' thru a school zone), the car slowly accelerated to 30 MPH (1,000 to 1,300 RPM) over about a half-mile without my foot on the pedal. This surprised me, as I expected the car to decelerate. I was about 2 miles from home, first-thing in the morning, so it wasn't fully warmed up yet.
I experienced this kind of behavior last year, as well, but thought it might be related to an idle problem I was having. I've since cleaned up the throttle body and replaced the IAC and still notice its tendency to throttle-off accelerate.
Any ideas on why it's doing this? Anyone else experience this kind of thing?
I've also noticed that the car will accelerate a lot quicker between 1,500 and 2,500 RPM when I get into it a bit (not WOT) when it's cold, than after it's completely warmed up (when the oil temp comes up above 150). When it's all warmed up, it tends to really wake up at about 2,500 - 2,800 RPM, but is sluggish by comparison in the 1,500 - 2,500 range.
Instructor
Joined: Jan 2002
Posts: 111
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From: Reno NV
Re: Accelerating with foot off the pedal? (K87ZZ4)
Sounds like maybe one of the following:
(1) Your driving while the engine is still cold, and in open loop. The ECM commands a higher than normal Idle speed for cold start-up, and slowly ramps it down. My old IROC would take up to a few minutes when really cold to reach it’s normal idle speed. If you drive during this period, it can cause slight acceleration at slow speeds.
(2) It’s quite possible that your Throttle Body is worn out. The throttle blade/plates can intermittent stick upon closing. This can cause erratic idle, and intermittent elevated idle conditions. This can cause non-input acceleration to some extent.
The “it feels stronger when cold” phenomenon with TPI vehicles is due to the ECM mainly. When cold/in Open Loop mode (not in limp home mode); the ECM dials in more fuel/spark at lower RPM’s to enhance drivability. The ECM can dial in more than normal spark advance when cold, due to lack of knock/ping at cold engine temps. This (personal speculation here) does produce more low-end Hp/Tq, but only until the engine has warmed up and goes into closed loop mode. I’m sure this cold engine calibration is a trade off, the mid to high range of RPM’s has to be less than optimal.
:crazy:NanoBrain:crazy:
(1) Your driving while the engine is still cold, and in open loop. The ECM commands a higher than normal Idle speed for cold start-up, and slowly ramps it down. My old IROC would take up to a few minutes when really cold to reach it’s normal idle speed. If you drive during this period, it can cause slight acceleration at slow speeds.
(2) It’s quite possible that your Throttle Body is worn out. The throttle blade/plates can intermittent stick upon closing. This can cause erratic idle, and intermittent elevated idle conditions. This can cause non-input acceleration to some extent.
The “it feels stronger when cold” phenomenon with TPI vehicles is due to the ECM mainly. When cold/in Open Loop mode (not in limp home mode); the ECM dials in more fuel/spark at lower RPM’s to enhance drivability. The ECM can dial in more than normal spark advance when cold, due to lack of knock/ping at cold engine temps. This (personal speculation here) does produce more low-end Hp/Tq, but only until the engine has warmed up and goes into closed loop mode. I’m sure this cold engine calibration is a trade off, the mid to high range of RPM’s has to be less than optimal.
:crazy:NanoBrain:crazy:
