D3 Idle-speed control valve, mechanical fault

[JPJ89]

Hi folks

I have just recently picked up an e46 325i 5-speed manual, unfortunately the car has already thrown me a couple of warning lights and has stored some codes. What I am experiencing is that the car starts, idles and runs perfect from a cold start, but if I am to shut the engine off hot after 10 miles, 20 miles, 30 miles or whatever and then start the engine again after 5 minutes or so the idle hunts up and down and within a few hundred yards of taking off I get 3 warning lights and the car enters limp mode. The lights are EML, DSC triangle and amber brake (!). This happens every time I start and run the car while hot, and doesn’t resolve until it’s cooled down. The codes stored in my C110 code reader are D3 idle speed control valve, mechanical fault. The first time it occurred I had to carry on for 50 miles or so which ended up also causing a check engine light with 2 other codes stored, these were: CA fuel trim bank 1 control limit, and CB fuel trim bank 2 control unit. Yesterday I removed the ICV and checked it, it was already moving freely but I thoroughly cleaned it out with carb and intake cleaner and reinstalled it, checking for leaks in the rubber boots and that the DISA valve was functioning ok while I was at it. Took the car out for a spin for half an hour and thought the issue was resolved, but went to drive off again 15 minutes later and the fault returned with the same D3 fault code stored. Any suggestions for what to try next or where to look? I don’t want to replace the ICV without being certain that’s definitely at fault. I took resistance readings from the ICV as follows 

1-2  10.6ohms

2-3  11.8ohms

1-3  22.4ohms

 

Was planning to try running the car with the MAF disconnected next just to see what happens, would this show that the MAF may be at fault?

 

[Tim-Ci]

I would be temped to believe the fault code, perhaps the ICV is not working well when hot, and might not be changing the air flow in the way the ECU wants it to. You could try a second hand one?

I've never come across an ICV fault code for an E46, so I don't think failing MAFs, O2 sensors or DISAs would cause it, as these commonly fail, but noone seems to ever have an ICV code caused by it. The ICV is used from idle up to around 2000rpm so your symptoms would suggest it could be playing up.

Mine has some jerkiness which I think might be ICV related, with a new MAF, O2 sensors, but no code logged (yet).

I would check your fuel trims on a wram (not hot) engine though, if the LTFT is within a 3 percent of 0, and the STFT is within say +/- 6 at all times, after the engine has been running for 5 minutes, I would be happy with that. If the numbers are outside this range, it MIGHT be that you have an air leak which is causing the ICV code, but this is something I've never heard of, and air leaks are very common, usually they set a fuel trim code.

Edited September 28, 2018 by Tim-Ci

[JPJ89]

It actually turned out to be the MAF at fault, there was an aftermarket one fitted by the previous owner, an OE Siemens/Bosch one sorted it out and the problem has never returned! Can only assume the aftermarket MAF was sending a false signal to the ICV or something along those lines

[Tim-Ci]

That's a new one to me, good to know, thanks.

Yes I expect the ECU was expecting a certain change in airflow reported by the MAF when it operated the ICV, and the dodgy MAF wasn't reporting the correct change in airflow. So the ECU threw a wobbler, but decided the ICV wasn't working properly, instead the MAF.

It is very common for non BMW or non genuine brand sensors to cause problems on the E46, certainly engine sensors but also things like parking sensors too. Just for info in case anyone else needs one, the genuine MAF brand is VDO/Siemens.