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From Jim Stone Freelance:
http://82.221.129.208/.ui3.html
I have made a major discovery in car repair
If you have a Jatco CVT transmission that is surging when you try to keep the speed steady, there is probably nothing wrong with it. These transmissions are in lots of Chevy's All Nissans, Renault, many Dodge cars, all Mitsubishi cars, Kia, Hyundai and probably more. These trasmissions get a surging problem and everyone assumes they are bad when there is absolutely nothing wrong. No one had the answer, and I worked on one and figured it out.
First of all, I'd like to get out of the way the fact that it is known that if your fluid is old, sometimes they will do this. But lots of people change the fluid hoping for a fix and then assume they need to replace the transmission, and when they DO change the transmission, the problem goes away, and I can and will explain why that happens when replacing the transmission actually did not fix anything. But first, the fix.
This youtube video shows the exact problem I am talking about.
THE FIX:
In this case, it was to change the air filter and clean the MAF sensor. That's all I had to do on this particular car (a Mirage G4) and the problem went away. Here is why the problem went away:
Because the air filter, which had not been changed for 80,000 KM was so plugged the engine was starved for air. The MAF sensor subsequently fouled, and gave bad readings. The transmission learns throttle position when the car is new and judges it's shifting off that. When, due to lack of maintenance, the throttle needs to be in a drastically different position from what the transmission learned when all was well, it gets confused and cannot figure out where to put the pulleys, or in some CVT's, where to put first and second gear. In it's confusion it starts hunting (because it does not get the power it knows it should) and downshifts, only for there to not be enough power, so it then up shifts over and over and over again.
Changing the transmission "FIXES" THIS because the NEW transmission learns the shift points based on a screwed up system. But in this case and many other cases, all that really needed to be done was to change the air filter and clean the MAF sensor.
There are 3 other sensors that can also cause this problem. They are:
1. The throttle position sensor on the throttle body.
2. The pedal position sensor on the gas pedal.
3. The altitude sensor.
4. Already mentioned: the MAF sensor.
5. Already mentioned: The air filter.
The first three mentioned are obvious logical deduction, because those sensors can mess up the throttle feedback to the transmission. I have not seen this happen with those sensors but know enough to know they definitely could cause this.
None of those are likely to trip a code unless they are totally bad and no codes were tripped on this particular G4.
If you have any modern car exhibiting surging RPM's when the car is driving at constant speed, (regular surging rpms that hit like clockwork, not the obvious surging that is random from a transmission that is obviously bad):
Scan for codes in any of the engine sensors and if there are none, then go over the five items I listed up top, that's where the problem is, it is NOT THE TRANSMISSION.
Take that to the bank. I am a qualified mechanic that has, in a pinch, worked side by side with ASE certified mechanics, and I was the champ, considered by all of the management to be above them. I really understand the modern electronics in cars extremely well.
I hate working on cars though, ONLY IN A PINCH and I would not do that anyway, my best friend said straight to me: "Doctors would make great car mechanics, but that's not what they should do". I work on my own cars, I NEVER take them to mechanics especially after some of the crap I saw.
If you have a surging transmission, start with these two things: Change the air filter and clean the MAF sensor with legit MAF sensor cleaner from Autozone. If that was the problem, you'll notice an immediate change in the behavior of the car. The transmission will surge for a day or two (but probably not as badly) while it un-learns it's bad behavior and re-learns how to operate under conditions that are not screwed up. This Mitsubishi was VERY happy but not perfect right away, and then became perfect on the third day. Don't just look at the air filter and say "It looks OK," PUT A NEW ONE IN.
REMEMBER: This fix is ONLY for the "transmission problem" that manifests itself as perfectly rhythmic surging when the throttle is held steady, with a transmission that otherwise seems to work OK. From what I can find, there are no mechanics that know this, I had to discover this fix on my own. So if you know any mechanics, pass this along and tell them to spread the word.
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