1971 Mustang
Austin, it starts with the butterflies close. It actually idles very well at 900rpm after you tap the pedal. The problem is after you drive a little, let go the pedal and place it in neutral then the RPMs go down slowly to the 1,300 range and stay there. You then can tap the gas again and they go back to 900 rpm.
Quote from tony-muscle on May 8, 2019, 4:38 pmAustin, it starts with the butterflies close. It actually idles very well at 900rpm after you tap the pedal. The problem is after you drive a little, let go the pedal and place it in neutral then the RPMs go down slowly to the 1,300 range and stay there. You then can tap the gas again and they go back to 900 rpm.
What do your IAC steps say when at 1300?
IACs are at zero when the idle is high.
Quote from Austin DeHaas on May 8, 2019, 5:39 pmQuote from tony-muscle on May 8, 2019, 4:38 pmAustin, it starts with the butterflies close. It actually idles very well at 900rpm after you tap the pedal. The problem is after you drive a little, let go the pedal and place it in neutral then the RPMs go down slowly to the 1,300 range and stay there. You then can tap the gas again and they go back to 900 rpm.
What do your IAC steps say when at 1300.
IACs are at zero when the idle is high. When idling at 900rpm they are between 6 and 9.
Then the only way the engine is getting air is through the butterflies. Unless vacuum leak, but that doesn't correct by tapping the throttle.
Quote from Austin DeHaas on May 9, 2019, 10:43 amThen the only way the engine is getting air is through the butterflies. Unless vacuum leak, but that doesn't correct by tapping the throttle.
Okay, I understand the logic, but something doesn't seem right. Are you suggesting that there may be some kind of binding of the butterflies but the TPS still reads zero? The TPS reads zero when the "idle" is high and when the idle is "normal. Is the TPS measured at the primaries? if so, the issue would be with the secondaries butterflies being more open, right?
Basically, if the butterflies are shut, there are only 2 ways for the engine to get air, IAC steps, or vacuum leak. Idle TPS reading is arbitrary, since no matter where your idle screw setting is at, it will read 0 once you restart the vehicle. If your throttle isn't on the idle screw setting when the idle is high, you have a return spring issue. If you close the plates further, and your IAC steps are reasonable, and the idle still goes high, you have a vacuum leak. My thoughts.
Quote from bdhulderman on May 9, 2019, 12:20 pmBasically, if the butterflies are shut, there are only 2 ways for the engine to get air, IAC steps, or vacuum leak. Idle TPS reading is arbitrary, since no matter where your idle screw setting is at, it will read 0 once you restart the vehicle. If your throttle isn't on the idle screw setting when the idle is high, you have a return spring issue. If you close the plates further, and your IAC steps are reasonable, and the idle still goes high, you have a vacuum leak. My thoughts.
I understand the logic behind that statement, but what doesn't make sense is that, if I tap the gas pedal, the RPMs go down to the 900 rpm idles set point. At 900 rpm I still get zero TPS and the IAC go to 6-9. I am having trouble figuring out what to do next. I am suspicious that it is not a vacuum leak because it goes away after I tap the gas pedal.
I would guess a sticky throttle return. Forget about the TPS, that has little to do with throttle position at idle. Could be it is up a little bit when you start the car where TPS is reset to zero, then you tap the throttle and it goes down more, and TPS is negative, but it can't be negative, because it's limited to zero. That kinda makes sense. Interesting problem. I forget, did you say the IAC steps were higher when the RPM's are higher? Did you ever get a log file and upload it showing this?
Yes, the IACs go back to 6-9 when at the set idle of 900 rpm. I will try to take some pictures of the butterfly location before the engine starts, after, and when idle is high.