Car almost dies when i get off the gas with A/C on

[dmj]

I have been noticing that whenever I am coming to a quick stop my car almost dies, the rpm drops to almost 500 before settling back to normal idle. What should I be looking for vaccum or boost leaks ,timing problems etc.

 

[ktmrider]

No idle drop issues with A/C off?
If not then check your ISC, mine did that and I found one of the four coils bad/open.

 

[dmj]

ISC checks out good. I think I am going to do a boost /vacuum leak test and see what happens.

 

[toybreaker]

When you turn the a/c on does the iac count increase?

Log your vehicle speed sensor and/or iac count as you roll up to a stop.

The vss should show some activity and the iac will show some extra counts when the throttle is closed and the vehicle is moving.

(On trailing throttle, the ecu uses the vss input to run the iac out a few extra counts to act like an electronic dashpot to make up for the vaccuum loss when the brakes are applied.)

No vss signal = no iac increase = rpm sag/stalling when rolling up to stops.

The engine rpm should be 1000rpm (plus or minus ) and then it'll come down to 850 (ish) a couple of seconds after you come to a complete stop. Roll the car forward a bit and then stop again. The iac will increase a few counts, causing the rpm will jump up to 1k for a sec or two, then the iac will trickle back down and the engine will return to the base idle setting.

You can also verify the ecu/iac loop is working by turning the steering wheel back and forth a scosche at idle. The iac should jump a few counts when the steering wheel is turned to account for the load of the pump.



If the iac zero's out at any time, follow the v-faq on setting your base idle set screw.

 

[dmj]

Ok The car is fully warmed up If I put the car in Neutral and car is not moving and tap the accelerator pedal with the ac off it returns to normal idle if i put the ac on and tap the pedal and release the car stumbles idle drops to almost below 500 rpms.this removes the VSS as a factor. Thanks for your reply Toybreaker ,I am new at logging and dont know how to log the vss or iac if you could walk me through it would help a lot. Thanks in advance Dexter.

 

[toybreaker]

^^^ If that's the case, follow Mike's sage advice and check the iac coils.

Quoting ktmrider:
No idle drop issues with A/C off?
If not then check your ISC, mine did that and I found one of the four coils bad/open.

 

[dmj]

I will check the IAC again as soon as the engine cools down.

 

[toybreaker]

Just caught your edit.

No worries, I get's lots of practice troubleshooting idle issues on these cars /ubbthreads/images/graemlins/smile.gif

I have also had some trouble reading the vss on a logger. There are several different software variations in the programs available for datalogging, and some of them might not support this function. I know the vaule is under vss, and you;re just looking for any activity, not a specific number.

I usually just use my snap-on mt2500 scanner (the brick) for this particular data point, as it will show the actual vehicle speed.

I think Mike's got your fix. /ubbthreads/images/graemlins/cool.gif

 

[stevep]

What MMCd logs as VSS isn't the DSM/GVR4 VSS. It's another of those hijacked commands Cbatters stuck in the MMCd code to support his hardware that he never sold. The real VSS is at 0xc6 in the E931 code and I think at the same place in E943 but I'm too lazy to go check. I'm also too lazy right now to document how to log random variables in MMCD right now but you can depending on where in memory on the ECU the variable is.

 

[dmj]

Just checked the IAC coils all ohm are within spec. I have 2 different IAC's on hand a spare grey one all coils ohm out at 31 but causes major idle surge. The black one that is currently on the car has 2 coils @41.6 ohms and the other 2 @41.5ohm the car idles great except for the above mentioned problem with the ac on. This IAC is less than 4000 miles old and worked fine until a recent throttle body rebuild.Part # on the black IAC is E9T15292C with a Mitsu Logo

 

Does your logger show the ISC steps increase when you turn on the A/C? When sitting still at idle, does the idle RPM increase when you turn on the A/C? If I recall, on my GVR-4's the idle was ~100 rpm higher with the A/C on.

 

[dmj]

When i turn on the ac idle goes down by 100 rpms then slowly comes up to idle rpms Idle was set at 850 in neutral with ac off when I turn on the ac idle drops to 750 then slowly comes up to 850.

 

What are the ISC steps on your logger doing?

 

[dmj]

Jeff I have never logged the isc before I dont know which abbreviation to use for the isc is it plen or acle Thanks for the reply.

 

[dmj]

Found ISC on page 2of logger, boy do I feel dumb.I will post isc counts later today .Well I logged the ISC when the ac is turned on isc counts jump to 41, when i tap the throttle the counts jump to over 60 to compensate for the low idle then return to 38-41 at idle.On a side note with the ac off the car idles @750-843 rpm according to the logger,when I tried to adjust the idle as soon as I ground the terminal at the timing connector the idle drops by almost 200 rpms whether i ground the obd1 connector or have the logger plugged in.I also did a boost/ vac leak this morning and found none..What would cause the drop in idle when the timing connector is grounded?

 

[stevep]

Grounding the timing adjust connector with nothing attached to the diagnostic connector set the timing to it's base 5* BTDC advance so you can sync the mechanical to electrical by turning the CAS. If you have the mode pin (10) grounded either by having the datalogger connected or with a jumper at the same time as the timing connector it puts the ECU into BISS adjustment mode setting the ISC to it's default position so you can adjust the idle speed.

Make sure your IPS is working before you adjust the BISS.

 

[dmj]

Anyone in the Orlando Area that has an good IAC to lend for a quick test? I need to solve this problem.

 

[marui]

i have the same problem here, haven't done anything yet but i noticed that after changing to N/A TB this kind of problem started. i will monitor this thread for any updates, please help guys.