Mitsubishi 3000GT & Dodge Stealth Forum banner
1 - 20 of 24 Posts

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #1 ·
aight, i just replaced the battery today, cause the car died yesterday, if your interested in that story go
here

well, the car started after i put in the battery, and idled semi alright. then i drove it to the end of the street. when i put in the clutch it almost dies, then it idles at about 500rpms and it sounds weak. as long as i keep the rpms around 2-2.5k everythings peachy, but hitting hte brakes or letting the rpms fall results in the car almost dying. does this sound like an alternator issue? i would think so except that it generates enuff power when the rpms are up. any hints would be greatly appreciated.
--John
 

·
More Than Meets the Eye
Joined
·
4,632 Posts
You don't have any problem. The ECU has to re-learn. Give it 15-20 minutes of in town driving and it'll be fine.
 

·
Registered
Joined
·
552 Posts
You don't have any problem. The ECU has to re-learn. Give it 15-20 minutes of in town driving and it'll be fine.
Yep, same thing would happen if you install a UDP Pulley.
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #4 ·
wow, i hope the fix is this simple, im gonna try it right after class.
 

·
Registered
Joined
·
3,165 Posts
Hope it works out for ya bud. ;) I am resetting my ECU right now and I have one question... are our cars programmed to run 750RPM idles all the time? It seems like whenever I turn up my idle to around 800 or 900 it idles that way for about a day and then drops back to around 700. Do ours ECUs learn that our idle is set high and it brings it back down? Thanks.
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #6 ·
alright, i taught that ecu a thing or 2. it idles at 750rpms but when i turn on the ac, it stays at 750 instead of jumping to 1200 like it used to and the idle gets a little bit choppy and the headlights and interior lights dim a some. any ideas what now?
 

·
More Than Meets the Eye
Joined
·
4,632 Posts
Is that HKS BOV open air? That'll do it.

Otherwise your idle air controller may be dead.
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #8 ·
the BOV is open loop, but hasnt given me idle probs before, its been on there well over a year. $263.94 for a IAC. :( if i dont use the air conditioning how long can i do without a new one and still have the car be reliable? it seemed to be doing ok driving around today without the a/c on.
 

·
More Than Meets the Eye
Joined
·
4,632 Posts
Is the BOV adjustable? If so, make sure it's completely closed at idle.

If the IAC is dead you can always bump up the idle with the screw. Raise it to say 900 and it'll be a good compromise - a little higher than normal without AC, and a little lower than it normally is with the AC on. Either way the car should run fine.
 

·
Registered
Joined
·
10,686 Posts
Adolphus

Identical issue about idle speed not increasing when A/C is turned on.

QUICK TEST
Quick test for your IAC. If engine speed increases when you turn your wheel your IAC is probably fine.


On mine, when the A/C is initially turned on, the speed increases and then goes down to the same RPM or ~50 RPM less. Never stalls but does not bump up to 1100 RPM like it did before.

I hooked up a logger and the IAC is doing exactly what it is being told to do. Initially When the A/C is turned on, the IAC count is increased from 25 to 70 steps and then returns to about 30. From this I have determined that it is not an IAC actuator problem. The engine is doing exactly what the IAC is telling it to do.

Sometimes the idle does actually bump up and stay at 1100 (as confirmed by the ~70 steps for the IAC) higher but not all the time. Thinking it might be the A/C ON signal to the ECU. (I believe there are actually two signals. I believe one is A/C request and the other is a signal that comes from the A/C relay.)



Clint
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #11 ·
well, i think the issue is finally resolved. i had the IAC tested, which turned out to be good, then i had the throttle body cleaned. now the car idles at about 750 -800 and when i turn on the ac, it either stays solid at that level or drops a tiny bit then returns to about 750. the mechanic i ended up taking it too said this is normal, the fact that the lights dim when the a/c is on is normal too, they brighten right back up as soon as i give it a little gas. he said the only reason it idled at 1200 before was becacuse of the throttle body being dirty. i dont know if this is accurate, but as long as i dont have any further problems, im not gonna worry about it. Thanks for all the input guys.
 

·
Registered
Joined
·
10,686 Posts
Not normal but not much of a problem either. Correct behavior is that the idle should increase to 1000-1100 RPM with A/C on. I put a logger on mine and saw that when the A/C is initially turned on, the IAC is boosted up to about 70 steps and the idle begins to increase but then the IAC step count is reduced to ~40 and the idle drops down to ~700 like yours. No hint of stalling but not normal either.

If I find the cause I'll post it because it sounds like our cars have near identical symptoms.


Clint
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #13 ·
sounds good to me. Id love to get it back to the way it was.
 

·
Registered
Joined
·
10,686 Posts
Adolphus- Re-read your original post, sounds like a nightmare of car related problems across a bunch of cars.

REPLACING IAC
If I didn't have a logger to see that the ECU was sending a signal to the IAC to LOWER the idle speed after a few seconds, I too would have thought that I needed to replace the IAC.

Beginning to suspect an ECU "learning" problem. I am going to try resetting the ECU but this time I am going to make sure that I keep the A/C OFF during the initial idle training.

I can't say for sure whether the last time I reset the ECU whether the A/C was on or off. I know that resetting the ECU also resets the climate control settings to 77 degrees and I may have punched auto and turned it down to 70.

http://www.3si.org/portal/forums/showthread.php?s=&postid=998124&highlight=77+degrees#post998124

Perhaps there is a subtle software bug in the ECU where it incorrectly calculates the IAC step counts if the A/C is on during the learning mode and it never attempts to re-klearn.

There are enough cases where people have solved idle problems by resetting the ECU that it is worth another shot with very controlled conditions.

1. Car warm
2. Lights OFF
3. Environment controls OFF
4. Reset ECU
5. Let car idle for 10 minutes
6. Drive around town for 15 minutes
7. Let car idle for another 10 minutes
8. Turn off car
9. Start car
10. Check idle with / without A/C

TIME CONSTANT OF ECU/IAC CONTROL
I don't know why the ECU has such a long time constant on modifying the idle speed - If the idle is supposed to be 750 with accessories off, then that is what it should control to. Likewise, with the A/C ON, if it is supposed to be 1000 then it ought to control to 1000 (within the control limits of the IAC.) Perhaps this was done to tolerate other controls that affect that affect idle speed (dashpot / cold engine idle boost) that are not directly controlled by the ECU.

I may play around with this some more to see if I can better characterize the time constant of ECU/IAC idle control.

It still seems like there are a bunch of extra control that make the idle control MUCH more complicated than it needs to be. I wonder if 2002+ cars still have separate mechanical devices to control idle speed or if the engine sensors and IAC take control of all of these functions?

DominickGT - regarding idle speed
Short answer to your question is YES. The ECU tries to maintain the idle at 750 RPM's and uses the IAC to increase or decrease the amount of air that flows through the throttle bypass.

If you open up the base idle screw far enough, the idle will no longer be in "closed loop control" because the idle will be too high even with the IAC at 0 steps. (This is why there is a procedure to set the base idle speed at a nominal 750 RPM with the IAC at a baseline of ~20 counts.)


Clint
 

·
Registered
Joined
·
10,686 Posts
Adolphus

I think I have found the "bug" in the IAC / A/C control.

THEORY
There are multiple signals fed to the ECU that are used to turn on the A/C compressor. My theory is that the IAC A/C idle setpoint compensation routine is only active when the climate control is calling for A/C to cool the car - not when the A/C has been enabled for defrost.

Once the ECU determines a good idle setpoint for the IAC with A/C load on, it may use it for both defrost as well as cooling.

Subtle bug that would go unnoticed during the summer (probably the most common time to notice A/C behavior) but would cause the same symptoms for anyone that resets their ECU during the winter.

TRY THIS
Try warming up the inside of your car to 75 degrees and then lowering the temperature setting while in AUTO mode to 65 degrees. What I observed was that as soon as the climate control began requesting A/C, the idle speed jumped up to 1000 RPM just like it is supposed to.

However, if the A/C compressor is only on to dry the air for the defrost setting and not being requested to cool the car, the engine idle speed does not bump up correctly (yet!)


NOTE: In all of the posts where I said my A/C was not causing the idle speed to increase, I was enabling the defrost not lowering the climate control temperature to enable the A/C.


Clint
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #16 ·
good call, i was also using the defrost feature since avg temp was 40 or less during my resets. perhaps ill try to reset again when i get back to FL in 3 weeks--its still warm there. good thought, hope this works
 

·
Registered
Joined
·
10,686 Posts
Confident that this is the issue with my car, only remains to be seen whether this is chronic behavior on all cars when using defrost (with A/C clutch activated) during the winter or if the car can "learn" a correct compensation setting.

When the inside of the car is 75 degrees and then lower the climate control to 65, the idle pops up to 1000 just like it should.

This could also be a simple case of changing conditions (getting colder) making it hard to isolate a problem. I cannot say for sure that this has not always been the case - until I had the problem with the idle screw self-tightening, I had not thought even once in 10 years about the idle speed of my car.

QUESTION TO GROUP
When it is 1) butt cold outside, 2) your engine is warm and 3) your climate control is set to auto and calling for heat...

Q1 - What is your idle speed?

Q2 - Does your idle increase to 1000 RPM (or drop a little) when you press DEFROST?

Q3 - What year/model car? (Perhaps they fixed this in second generation.)


Clint

PS: I can only imagine what a dealer would have replaced and charged to "solve" this problem.

Wish we could create a better FAQ for the site. Searching is OK (and often entertaining!) but IMHO it would be better to create a knowledge base and not just random opinions.

Symptom-> Test-> Isolate Problem -> Solution

makes more sense than

Symptom-> Guess -> Replace -> Guess -> Replace
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #18 ·
in given situation stated in above post,

Q1- base idle tends to be ~750

Q2- press defrost idle drops about 100rpms and then it might regain 50rpms or it just stays at 650

Q3- 1992 3000gt vr4

I havent had a chance to test your theory of increasing the cabin temp then turning on cold air. ill try to get that done today and get results back to you.

as far as what a dealer charges you to fix this problem--about 75 dollars for cleaning your throttle body and resetting your base idle, then they tell you that everything is normal when the a/c doesnt cause the idle to rise. at least thats what they told me...
 

·
Registered
Joined
·
10,686 Posts
Sounds like you found an honest dealer.

If they said this was "common" when the A/C compressor was turned on when the climate control was not requesting cold air, I would have to agree.

My bet is that your car will behave EXACTLY the same way.

If this behavior was documented in a FAQ I would have saved a lot of time/frustration.



Clint
 

·
rock climbing addict
Joined
·
225 Posts
Discussion Starter · #20 ·
alright i finally got around to testing your theory. i heated up the car to a decent temperature then turned on the a/c to a level where it was blowing cold air. the idle raised to a hair under a grand and stuck there. congrats on figuring out the secret. i never really thought about the temperature of the air being key. im hoping this problem is over, now to figure out why my speedometer doesnt start working until my car is completely warmed up. if i dont let it warm up for about 10 minutes it sits pegged on zero then at almost exactly the 10 minute mark it jumps up to where its supposed to be. its only started doing this since it started to drop below 40 degrees at night and only for the first drive of the day. weird but not debilitating so im just gonna deal. anyway, good work cbatters
--John
 
1 - 20 of 24 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top