teknics
04-21-2009, 07:59 PM
Hey guys,
Been trying this out recently while diag-ing 8's for stalling issues and/or jumpy idle.
Get in car, start, allow to fully reach operating temp (drive around, do whatever you do)
Once at oper. temp. park somewhere. Now with car in *neutral* (goes for a/t too) and clutch pressed to floor (obv. m/t only) rev the car with a quick jab of throttle to around 5-6k, let off immediately.
Now look at the tach, but rather then focus on the lines just stare at the middle so you can see the needle more then the lines. Monitor the speed it falls at, it *should* fall pretty fast. At approx 1375rpm the needle should almost bounce to a slower speed slow enough to be noticeable immediately.
I've had a few cars lately suffering IAC (idle air control) problems (IAC is within the TB, it's one unit essentially). None of these cars showed any codes, stored-pending-or otherwise. Only sign of failure was monitoring rpm vs time and comparing it to a known good rpm vs time, difference at times was noticeable. One car was easy to spot, could barely idle, very lumpy...would stall with a quick jab of the throttle, but if you eased on the gas it was fine.
Obviously car is taking in too much air at idle, with no vac leaks you assume car is expecting this extra air thats why you have no lean codes, computer is adjusting properly.
So first culprit is TB, due to internal IAC, this exact concern is typical of MPV owners, also multiple other ford models (they have external IACs however).
However, I've begun to see IAC's failing almost randomly, will allow car to idle only mildly off but will cause an intermittent stall in differing conditions.
One symptom they all showed was failure to hit the 1375rpm and slow down rate of rpm decrease, some would catch at 800 and get stumbly before leveling off, some would never level and just stall out or choke out and roughly catch itself, on some the idle was steady but low and car would catch the idle lower when revving. Which when looked at seems pretty obvious as an IAC problem.
Figured I'd get this out just almost as a poll to see how widespread this may be. This is all based on personal experience, not sure if the math and such behind it adds up, haven't looked that deeply.
All i know is cars with failing IACs have been showing up, if you're having problems why not give this quick test a shot, at the worst you waste a minute, youve wasted more then that reading this :P.
kevin.
edit: as an addin, this test isn't really conclusive of anything. Your IAC may be fine, there may be another problem being overlooked. I've simply found this symptom to correlate with bad TB's in a few recent situations, seems noticing this is also around engine replacement time not sure which comes first. I think this may cause some dead motors to seem even more dead which is why some are obvious and some aren't. I started noticing a trend, which nycgps' motor continued making it around 4/10. In all cases motors are confirmed dead, low compression. Removal TB when compression was found to be low and then re-tested showed near identical values so the TB was not hindering compression results. But then these brand new motors would be fighting to idle and no amount of memory clearing will get it to re-learn the idle. Replace with known good TB and new engine purrs. Again, personal experiences only, just throwing it out there as an *AID IN A LARGER DIAG SCOPE* ... to clarify.
Been trying this out recently while diag-ing 8's for stalling issues and/or jumpy idle.
Get in car, start, allow to fully reach operating temp (drive around, do whatever you do)
Once at oper. temp. park somewhere. Now with car in *neutral* (goes for a/t too) and clutch pressed to floor (obv. m/t only) rev the car with a quick jab of throttle to around 5-6k, let off immediately.
Now look at the tach, but rather then focus on the lines just stare at the middle so you can see the needle more then the lines. Monitor the speed it falls at, it *should* fall pretty fast. At approx 1375rpm the needle should almost bounce to a slower speed slow enough to be noticeable immediately.
I've had a few cars lately suffering IAC (idle air control) problems (IAC is within the TB, it's one unit essentially). None of these cars showed any codes, stored-pending-or otherwise. Only sign of failure was monitoring rpm vs time and comparing it to a known good rpm vs time, difference at times was noticeable. One car was easy to spot, could barely idle, very lumpy...would stall with a quick jab of the throttle, but if you eased on the gas it was fine.
Obviously car is taking in too much air at idle, with no vac leaks you assume car is expecting this extra air thats why you have no lean codes, computer is adjusting properly.
So first culprit is TB, due to internal IAC, this exact concern is typical of MPV owners, also multiple other ford models (they have external IACs however).
However, I've begun to see IAC's failing almost randomly, will allow car to idle only mildly off but will cause an intermittent stall in differing conditions.
One symptom they all showed was failure to hit the 1375rpm and slow down rate of rpm decrease, some would catch at 800 and get stumbly before leveling off, some would never level and just stall out or choke out and roughly catch itself, on some the idle was steady but low and car would catch the idle lower when revving. Which when looked at seems pretty obvious as an IAC problem.
Figured I'd get this out just almost as a poll to see how widespread this may be. This is all based on personal experience, not sure if the math and such behind it adds up, haven't looked that deeply.
All i know is cars with failing IACs have been showing up, if you're having problems why not give this quick test a shot, at the worst you waste a minute, youve wasted more then that reading this :P.
kevin.
edit: as an addin, this test isn't really conclusive of anything. Your IAC may be fine, there may be another problem being overlooked. I've simply found this symptom to correlate with bad TB's in a few recent situations, seems noticing this is also around engine replacement time not sure which comes first. I think this may cause some dead motors to seem even more dead which is why some are obvious and some aren't. I started noticing a trend, which nycgps' motor continued making it around 4/10. In all cases motors are confirmed dead, low compression. Removal TB when compression was found to be low and then re-tested showed near identical values so the TB was not hindering compression results. But then these brand new motors would be fighting to idle and no amount of memory clearing will get it to re-learn the idle. Replace with known good TB and new engine purrs. Again, personal experiences only, just throwing it out there as an *AID IN A LARGER DIAG SCOPE* ... to clarify.