bouncing idle on cold startup

[fourforty]

i have no idea what could cause this nut wen i start the car it idles very high. it slowly drops and bounces back up. it does this for quite a while. but it only happens when i start it cold. but when i start the car while the engine is still hot it tends to die because of the idle trying to rest to low. what could cause this?

 

[cheekychimp]

Idle woes are part and parcel of the initiation into the ownership of a 4G63 powered car. Pretty much anyone who has ever owned a VR4 or a DSM will have experienced them. Now that isn't to say that the idling mechanism is bad, but it does require a) correctly functioning parts capable of doing their individual job and b) proper adjustment. As with any old car, parts start to wear and people make adjustments to cater for this. Sooner or later you run out of adjustment or a part simply reaches the end of it's service life.

The bad news is that it is kind of difficult to diagnose your issue with the limited information you have given. The good news is that with a bit more information and some cleaning/adjustment/replacement of parts, a lot of issues are solved.

I'm guessing since you posted here you'd appreciate a little guidance so I'm going to briefly map out the big picture. If you are unsure of what I am saying ask and hopefully some of our technical gurus will chime in and help.

The Mitsubishi idle system is based upon a coarse adjustment (the BISS screw) and a fine adjustment (the ISC). For want of a better analogy think of a water mains valve and your household tap. If the mains pressure is too low, it doesn't matter how far you open the tap, water only dribbles out. If the mains pressure is too high however then even the tiniest adjustment sends water splashing everywhere so that opening the tap fully becomes pointless.

So it is with the Idle Speed Controller (ISC). The ISC is essentially a valve that can be adjusted by the ECU to allow more or less air to pass through and maintain a stable idle at around 750-850 rpms. To do this it requires the correct amount of air reaching it so that it can respond over it's full range of movement. If the air reaching it is too much or insufficent then it will only have a fraction of it's range in which to adjust the idle speed and consequently those adjustments will become less precise.

In addition the ECU has been programmed to compensate for the airflow based upon the 'base timing' of the vehicle being properly adjusted. If the base timing is not correctly set then the ECU cannot compensate correctly.

So to start we need to ensure the following: -

1) Base timing is set correctly;
2) The BISS screw is adjusted correctly; and
3) The ISC functions correctly and seals correctly.

To complicate matters further the stock Mitsubishi throttle body has an FIAV (Fast Idle Air Valve) which is essentially a wax plug which melts when the vehicle reaches operating temperature and diverts airflow so that the idle is raised for cold start up and then drops upon the engine reaching the correct temperature. If this valve fails or performs improperly, people tend to manually adjust the idle for cold starts and the ECU is unable to adequately compensate once the engine warms up. Consequently you need to also ensure that: -

4) The FIAV is functioning correctly.

If all of the above checks out and you still have issues then we would probably move on to checking the throttle body seals, checking for vacuum leaks, seeing if the throttle closed idle switch and throttle position sensor are properly adjusted etc.

Finally you may have heard that the ECUs in these cars are prone to acid damage, one of the symptoms of which is (you guessed it) a poor idle. So it is ALWAYS worth checking that you have a good working/functioning ECU, BEFORE you start wasting hours under the hood.

I feel for you as nothing detracts more from the enjoyment of a car than a poor idle that makes driving it unpleasant/impossible. Check out the FAQs for more information on the above and post back with questions and we'll try to help/point you in the right direction.

Paul.

 

[fourforty]

that is a lot to take in. ill try to fill in more information. so i really only experience funny idle conditions on start up and when the engine is beginning to warm up. now i feel a good jumping off point would be to check the PCM. i'm guessing that is the main ECU to check for acid damage. on a side not i am wondering how many computers are on the car. i feel that the PCM has been modified before. i bought the car with coil packs installed.so i don't know if the tune it was given was done correctly or not.

apart form that i would like to check and possibly adjust the BISS and ISC. i downloaded the shop manual but i have trouble navigating thorough it on a computer. is there page numbers where it gives me some type of guide through it.

i want to thank you for your input. i would like to take this one step at a time. so i feel one of these two places are a good point to start at. it may take me a while because i can only work on it late at night and i am daily driving it as well.

 

[strokin4dr]

There is no wax plug.

 

[WesternMD_VR4]

welcome to the club fourforty, 328 is in the shop now b/c of this. hopefully you have better luck than me. I needed my head rebuilt as my valves were bent and the head was destroyed. Ive been told it can be a throttle body as well. you can try a idle reset but if its something in the motor then idle reset wont work.

 

[manikbastrd]

Quoting strokin4dr:
There is no wax plug.



So i have taken apart five different TBs, and looked at the FIAV, and I am really confused on this whole wax plug thing...it seemed to me to be more like a thermostat, where you have a temperature dependent spring height that can be adjusted via the screw deal under the freeze plug. Although, if trying to adjust be careful because the plastic adjuster gets brittle after a lot of heat cycles and causes it to break apart when you adjust it.

To OP:

I recently had some similar issues and I would recommend doing the easy checks first. I would check for boost leaks at your TB, any extra air getting from the turbo side of the TB to the Manifold side will cause your idle to freak out. Check for vacuum/boost leaks!!! I literally put four different TBs on my car in the process of trying to fix my idle. A BL tester, if you don't have one, can be built for about 20$. You can guarantee that it is not your TB if it tests fine. Furthermore, make sure to remove your ISC and check the gasket. This can cause your idle to get screwey as well. Pull out your trusty Voltmeter/ohmmeter and check the resistance and voltages on your IAC/ISC, TPS and the pigtails they go with. You can find the correct values by looking on tuners, reading your FSM, or just googling it. After that, if you have checked everything and set you BISS properly while fully warm, and still have issues, I would think it is your FIAV, and you can either block it off or get a rebuilt TB. I have my FIAV blocked off and the ISC does a great job of taking care of the idle when cold. If you are attempting to remove the FIAV screws on a stock TB, know that you will probably strip the heads. I used my Dewalt 20v impact/drill and got 4/5 out fine, but had to use an easy out on the last one. You may want to get some new screws in advance.

To sum it up I guess I am saying that if you don't have any codes, there are a lot more common things that will go wrong before the PCM. I would check all of those before starting at the PCM. The PCMs on our cars are pretty solid as long as the Capacitors are in good shape. If you have Link, you probably have good caps, and the ecu was tested before getting sent out from ECMlink...

hope this helps and anyone feel free to correct me if I went wrong somewhere!!!

Cheers
-Manik

 

[fourforty]

thank you for the input! i know i have a massive boost leak somewhere but i need to track it down. i think i'm going to try building my own boost leak tester like on the you tube channel jafromobile. ill post results when it is warm and dry enough to work on the car. i will also throughly inspect the throttle body. i feel it has a bit to do with the wax plug.

 

[cheekychimp]

Well it seems strokin4dr is right. There is no wax plug/pellet. I wonder who started that urban myth? As manikbastrd states that FIAV is apparently more of a thermostat and spring/plunger. I've never actually taken one apart so I didn't know that. Sorry if I misled you. Could still be worth checking out though if you don't have any luck with the other checks suggested because if it isn't working properly it does mess things up, hence the reason a lot of people block the whole FIAV off.

 

[Wookalar]

Just spray brake cleaner at the TB full blast! Lol

You get extra points if it splashes everywhere and stains your nice powder coated valve cover or AN fitttings, etc.

[/sarcasm]

 

[prove_it]

You have obviously not spent much time working on cars.

 

[Wookalar]

Yeah, havent spent much time at all working on cars. /ubbthreads/images/graemlins/rofl.gif

 

[G]

Quoting fourforty:
i have no idea what could cause this nut wen i start the car it idles very high. it slowly drops and bounces back up. it does this for quite a while. but it only happens when i start it cold. but when i start the car while the engine is still hot it tends to die because of the idle trying to rest to low. what could cause this?



This could be from an improperly bled cooling system. Did you recently service anything in regards to cooling? Do you have any coolant leaks letting air in to the system?