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Hello,
The car in question: Mitsubishi Mirage/Colt 1.3 GLXi 1997 M/T 140.000 miles/230.000km. And its green.
The situation. The car always starts within a second, which is nice. Then the rpm’s are around 1500, and slowly decline to 1100 orso. Drive off, so far all is well. At the first trafficlights (3 mins away), the idle drops to about 500 rpm, the lights dim a bit, vibration etc etc. Really lumpy repsons to get the engine back up to speed. If you wait long enough it will restore itself to 800 rpm (as it should be). It has never stalled, but i need to pay attention to it when driving off. After say 10 minutes all is fine, coolant has warmed up, rpm fairly stable at 800. (750 +/-50rpm is prescribed idle)
What i have done:
-clean valve in throttle body (this was quite dirty, mainly on the bottom edge and back face.)
-cleaned and checked air temp sensor in the intake manifold
-cleaned and checked PCV valve
-cleaned EGR valve (this was fairly dirty but not clogged and ever so slightly leaking cleaning fluid in to the other hole, after cleaning the fluid would be held back)
-removed idle speed controller. (to gain acces to EGR) this didn’t seem to dirty,Can / should this be cleaned further? How does this work?
-checked for vacuumleaks with various aerosol’s (not found)
-checked coolant temperature sensor operation (works)Recent work on the car:
-Front brake rotos
-Radiator (but i am quite sure i’ve filled it properly, used all the bleed screws etc)Fuel filter never been replaced by the looks of things
Air filter, old but not at all dirty.I’m considering to just order those filters new, and trow in some injector cleaner too. But that just feels like guess work :sick:
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Yeah, the way the IAC worked in Mitsubishi vehicles back then was that the ECU didn’t keep track of the position of the stepper motor. The ECU just sent signals for open or close. So, at start, the ECU would run the stepper motor all the way to the end of travel and back a certain number of steps to the start position. As you can imagine, this needed to happen quickly. Hence, the above test.
[quote=”Rob36″ post=81101]
*edit*
Another question, whilst i’m at it, in the schematic drawing in the manual, in the top left corner of the by-pass ‘chamber’ are two mostly black rectangles, what do they represent? Is there something on the inside of this valve that could be stuck/malfunctioning?[/quote]
Yeah, there’s usually a wax pellet actuated air bypass also. I think Mitsu used these up until 2004 when they decided the IAC could go it alone. I don’t recall that his was a failure prone part but anything is possible. Did you decide to loop the hoses going to the throttle body?
Ok, checked the functioning of the IAC.. and this does what it should.
Did go by the junkyard to pick up another one, they only would sell it with the intire throttle body. It also has another TPS on it, am i going to want to test that too? Got a week to return it. They both do the same thing once hooked up, the one that belongs to my car actually makes smoother/less noise while it operates.
Edit again 😛 This wax pellet by-pass system, is that somehow operated by temperature sensitive mechanisms, bi-metal or something along those lines? since i do have the whole throttle body as spare here now, i could swap the whole IAC unit (given i get those 4 philips head screws out that is)
First a wax pellet is what powers a car’s thermostat. It sounds crazy but melting wax can be made into a powerful little motor. Melted wax has greater volume so if it is in a sealed container something has to move.
Anyway, here is a quote from another forum where the wax pellet device in the throttle is being discussed: “Try checking your idle air bypass. When the thermal wax plug would go bad on my DSMs (happened on my JDM GVR4 and 95 Eclipse GS-T) the idle stepper motor made a sound just like your mentioning and wouldn’t hold an idle worth a crap. The reason it did this it was trying to compensate for the mismatch between engine coolant temp that the ECU saw and where the engine idle was. Since the wax pellet in the throttle body changed the airflow that bypassed the throttle body when it warmed up.”
I changed the wax pellet device in my ’92 thinking it was bad – it wasn’t – but I recall the fasteners being difficult to remove.
Anyway, it might be a good idea to look up the service manual procedure for adjusting the idle. On my ’92, with the engine warmed up, you shorted a plug on the firewall causing the ECU to lock the timing. This provided a steady idle. Then you adjusted the base idle screw to the prescribed setting. This was done to provide the stepper motor with sufficient travel to control idle. I doubt that your vehicle is exactly the same but a fix might be hiding there.
Good diagnosis but youre overthinking. Adding air by opening the throttle increases combustion and builds heat. This expands mechanical plungers, builds pressure and “knocks free” of build up or loosens worn/relaxed mechanical plungers and springs alike. Replace.
Little update:
Removed the IAC from the spare throttle body, penetrating oil and big locking pliers did the job. From the outside it is identical to the one on my car, but i won’t get it off without removing the throttle body from the car.
There doesn’t appear to be some wax driven system in there, what i did found was a plug with a bi metal coil and butterfly valve like plastic bit to make the opening smaller.
It appears to be shut more at lower temperatures, got it sitting above the heater now to see what happens when ‘the engine’ and the whole assembly heat up.Seems to be the other way around actually, hadn’t put it back properly. The one on my car could have been stuck in the hot position since last winter, collect dirt for possibly 15 000 miles, and now give me issues.
I might actually be able to check the functioning of this valve without removal of the IAC ‘body’, the plug is somewhat accesable. First thing saturday morning i’m going to check this out.
Hm the forum ate my reply it seems.
Well, cleaned, tested (freezer/oven), put it all back, had the engine relearn its settings for 10 minutes, let it cool down, and test drove it. No luck, problem remains. I’ve tested it several times, with a bit more or less opening on the valve by default (the bi metal coil is screwed in place with a little adjustment slot) this seems to have some marignal effect on how the engine idles when cold, but the problem is still there.
Any other things i should be looking at / testing? I really hoped this was it, so i’m kind of clueless now. Vacuum leaks that i might have missed keeps springing back to mind. I’ve seen ppl check for this with cigar smoke, is that worht a try? Only tried by ear/eye and with some earosols so far.
🙁
Scheduled a visit to the garage for the old box. The garage i want to take it to doesn’t has a very fancy building and deal with older japanese cars a lot, they don’t have time till friday or somewhere next week. Can’t really take a day off anymore so they get it in my vacation, the two days before xmas i don’t need it.
The guys first respons to the story was ‘sounds like some temperature senitive valve or sensor’. Told him i had checked air temp sensor, coolant temp sensor, ISC stepper, TPS/idle switch and cleaned the throttle body, ISC body + bi-metal mechanism and the EGR valve.
I guess they are just gonna have to go and take a look/test drive for themselfs, atleast i won’t have to pay them for cleaning stuff.
2 weeks left to sort it myself and save a bit of money 😉
And it became a crank, no start today at a few degree’s below freezing. #$%#
Bad judgement call having the garage visit 2 weeks away..
To me it sounds like your idle control valve is fine. I think you’ve determined that by your posts. That said, you may not be getting enough air into the engine during warm idle. This could be because the computer isn’t commanding a high enough idle. Either due to poor information that it’s getting from say a lazy coolant temp sensor, or some other sensor input. It could also be a mechanical issue. While reading your post, I kept wondering if there was an idle adjustment screw somewhere. If there is, you might try adjusting it a little to see if you can bring the idle up a couple of hundred rpm. If you can, this might be your solution. If not, then we may have to look into the computer controls a bit more closely.
It wouldn’t be a bad idea to check for vacuum leaks as you suggested. However, I would think if you did have an issue with a vacuum leak it would effect other areas of engine performance. It’s a tough call really and I’m sorry to say that I have little experience with your type of vehicle.
Good luck, and please keep us posted on your progress.
I would have sworn i typed up quite a long reply here yesterday.. seems not to have made it.
It has been sorted that afternoon by my regular garage (yearly check-up’s and jobs i don’t want to attempt myself) for less then a full tank of fuel costs me. Thats the good news. The bad news is that the issue is a result of a job i’ve neglected for too long. (some excusses for this, mostly not very good) which was adjusting/checking the valve clearance.
Results were: tight exhaust valves + excessive heat which probably caused some tiny cracks in one of the sparkplugs. Both of those problems seemed to cause little trouble when everything was hot and expanded..
There is a few lessons to be learned in this.. don’t be an idiot and neglect a part of you maintenance, specially if you don’t know how long its been since the work has been done on the car (if you recently got it) kind of let this one slip my mind while at the same time taking up a job far further away.. And lastly, don’t get side tracked / focussed on one area too much.
While doing all this cleaning work i did notice my sweating valve cover gasket a lot.. figured i’d get round to installing a new one, and checking the valves whilst doing that, after i would have this issue sorted. Yes, I feel properly stupid now.
But thanks for the help everyone, hope this at least provides help to someone else..
First, you’re not stupid. We all do dumb things from time to time, including me. That said, I don’t think not adjusting the valves was your problem. In fact, in most cases you’re better off leaving them alone. The important thing is to make sure you get them right. It’s actually valves that are too tight that are the biggest problem. Loose just makes noise, tight burns them up. The only way they get tight, is if someone puts them that way. So don’t beat yourself up too much.
As always, keep us posted on your progress.
I’m glad it is sorted out one way or another, thats for sure.
What confuses me a bit is that i always thought the clearance changed because the valves would ‘sink’ deeper in to the valve seat, and have the stem come up closer towards the rocker/cam lobe..?
Also i’ve since found out the inspection schedule recommends clearance checks every 60 000 miles, thats a LOT longer interval then i expected. On my motorcycle a check is recommended every 6 000 miles for example.. Only thing i know for sure is that these valves haven’t been touched for at least 27 000 miles and weren’t giving me this sort of trouble last winter.
Some jobs i’d like to get done during my vacation, but thats all quite straight forwards and non engine related. Considering also to give it a gearbox oil change, by the book that should happen about now (or 8 years ago).
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