I'm posting this to help someone else out who will have the same problem. My car developed a miss under part to mid throttle and idle surge when the engine reached operating temperature. I spent 1 1/2 hours reading searches to try and find out what causes this and didn't get a clear answer. I was convinced that it wasn't the opti because I don't believe that the ignition system can cause a surge. The opti tends to get blamed for alot of stuff. I cleaned the MAF and the IAC. The IAC is just a needle & seat type setup and cleaning doesn't do much good unless you have an abnormal amont of carbon buildup. If your IAC module is malfunctioning, you should just replace it. Those two things did not fix the car and it was the same. I decided to replace the IAT even though it showed to be good, and presto no more problem. The surge was gone and 95% of the miss was gone as well. My car has 128K and I've had it for a year and never looked at the plugs or wires so it may need a tune up. This is a cheap $9.00 and no tools required fix. Hope this will be of help to someone who searches in the future.
Bill

 

You're talking about the intake air temp. sensor on the rubber elbow before the throttle body, right? I'm having a problem that sounds exactly like yours. Maybe I'll try that on Monday when the dealer opens.

Andy

 

Yes it is the sensor on the intake elbow.
I got mine at Autozone for $9.00

 

I posted a bit on this subject as well.
I have changed:
plugs/wires no help
IAT like you, helped for a day
O2 sensors, no help
MAP sensor & TPS sensor, no help
Just got done taking apart my throttle body and cleaned it
along with the IAC which was pretty gunked up.
Cleaned MAF today as well.
Even though the car is more responsive, at operating temp. she surges at idle and is still sluggish/hesitant up untill 2000k rpm.

I'm glad your IAT fix worked for you & hope you get more then a day out of like I did.

I also agree the opti gets over blamed as well, and still belive this is a sensor/computer problem.

I hope someone else has another idea for me.

For now still surging...

 

The IAT is nothing more than a temperature regulated switch which through the PCM alters injector pulse and mixture as the density of incoming air changes. Failure of this sensor would account for both problems being hesitation and surging idle. Since it's function is temperature related, it also fits the situation mold.

If it helped cure the problem for a day before it went out, it's possible that you got a defective part or the PCM is burning it out. A PCM problem might also explain a lack of code.

I will let you know in a few days if mine has the same result.

 

I have the aluminum intake elbow. I checked the IAT sensor and it was rading as if the air was 200deg F, when it was only 70 degrees outside. I unplugged the IAT and the car runs great. No more surging. I ordered a new part, it should be here tomorrow. I am going to put it in the plastic part of my intake. I think that since the metal intake elbow gets much hotter than the plastic one, it is transfering the heat to the IAT causing a high temp. reading, which is in turn causeing my car to run extremely lean.

I let you know what happens with the new one installed.

Thanks everyone for the advice.

 

Might check those wires going to the IAT as well.They do become crispy critters as time goes by.My02

 

Could not find this thread earlier, so I started a new one, du should have put on my glasses. Anyway to sum it up, new IAT cured my stumble/hesitation. The old IAT tested good a couple of times. After chasing this for 2 plus years I am very pleased and thankful.

93 M6 Red 59K
Off Road Pipe, EGR, TBB, B&M Shifter.

 

Okay, screw it, I'm going to buy one tomorrow...

 

Your old IAT tested good and it solved your problem when you replaced it? Was it the wires or what? Thats strange... Now you guys are getting me all paranoid about mine .. which seems to read just fine on the scanner

 

Well mine took a dump after one day like the other guy's. I have to believe it's a PCM problem. I'm going to check around with local dealer techs to see if they've seen this problem before. I'll try unplugging it when the problem starts today just to see what it does.

 

and it made no difference whatsoever, the hesitation is still there.

 

I put in a new IAT from autozone yesterday (13.16 w/tax) and it has cured my idle surging and occasional part throttle hesitation. I am so happy for now anyways , until the next part breaks, which is on schedule to be next week....

 

Hey Chicken, let us know if it stay's fixed.
Bill

 

Guess what?
93's don't have IAT sensors, went to AutoZone & Pep Boys, they both said the same thing. 94's and up, so that's kinda trippy...I asked the guy @AutoZone what sensor is on my intake elbow, he didn't know, it's not on their parts list, he figured that it's called a air temp sensor, and on their computer it says it's not available, I would have to get it from the dealer...
So that's pretty interesting...
I did buy a new TPS, I needed to change that and the IAC valve, both are factory installed, 173K miles on both of them.
I'll let you know tomorrow how it runs.

So far I've changed the:

Opti
Coil
Plugs
Wires
WP
160 stat
EGR
MAP
TPS
O2's
EVAP cannister
Fuel filter
Cleaned TB & IAC
Cleaned K&N FIPK

Need to:
Replace fuel pump
Replace IAC valve
Replace coolant temp sensor, the one on the WP.

Can y'all think of anything else?
I have a 93, so I don't have an MAF, and apparently not an IAT sensor, I guess it's got to do with having Speed Density instead.