I am posting this just in case others have an idle problem and put themselves through endless agony only to find out it's the ECU...something last on my list for an idle issue...
86 535i - idle was at 850 - 900 - tolerable to some, but not correct. All new vacuum lines, intake rubber, new CTS, FPR, plugs, freshly rebuilt and cleaned injectors, O2 sensor, cat, etc etc etc. I checked the IASV, throttle flap and the usual suspects. After deliberating and contemplating for several months - yes, the day job got the better of me, I swapped ECUs and problem was solved. I had not done this prior as PO had the ECU rebuilt by Programma about 7 years ago. So why would I suspect that? FWIW the replacement ECU is a 28 pin with a Dinan chip. It now idles at 650/700 like it's supposed to.
DJM1986-5 wrote:I am posting this just in case others have an idle problem and put themselves through endless agony only to find out it's the ECU...something last on my list for an idle issue...
86 535i - idle was at 850 - 900 - tolerable to some, but not correct. All new vacuum lines, intake rubber, new CTS, FPR, plugs, freshly rebuilt and cleaned injectors, O2 sensor, cat, etc etc etc. I checked the IASV, throttle flap and the usual suspects. After deliberating and contemplating for several months - yes, the day job got the better of me, I swapped ECUs and problem was solved. I had not done this prior as PO had the ECU rebuilt by Programma about 7 years ago. So why would I suspect that? FWIW the replacement ECU is a 28 pin with a Dinan chip. It now idles at 650/700 like it's supposed to.
The ecu does control the batch firing of the injectors.
My M20 used to idle high when it warmed up. I had all new hoses, motor was tuned up. It ran fine except the high idle. When I swapped the ECU for one with a Conforti chip in it, the idle returned down to where it should be. I don't think it was the chip, but changing the ECU itself seemed to do the trick here also.
Most high idle issues over the years have been from vacuum leaks or iasv issues...up until the ecu swap, it was the worst running 535 I had...a little embarrassing to say the least. It runs very strong now, of course the chip helps, but the mileage before the change sucked as well. Never above 24/25. I'll know by wed how the mileage is doing. That'll be the first tank on the new ecu. Not conclusive but directional.
DJM1986-5 wrote:Most high idle issues over the years have been from vacuum leaks or iasv issues...up until the ecu swap, it was the worst running 535 I had...a little embarrassing to say the least. It runs very strong now, of course the chip helps, but the mileage before the change sucked as well. Never above 24/25. I'll know by wed how the mileage is doing. That'll be the first tank on the new ecu. Not conclusive but directional.
From what I've noticed from my car and reading on the forum, 24/25 is pretty good for a m30 motor.
DJM1986-5 wrote:Most high idle issues over the years have been from vacuum leaks or iasv issues...up until the ecu swap, it was the worst running 535 I had...a little embarrassing to say the least. It runs very strong now, of course the chip helps, but the mileage before the change sucked as well. Never above 24/25. I'll know by wed how the mileage is doing. That'll be the first tank on the new ecu. Not conclusive but directional.
From what I've noticed from my car and reading on the forum, 24/25 is pretty good for a m30 motor.
Yep, unless you're real lightfooted and doing nothing more than the filthy 55 on the freeway, 25 is about as good as it gets.
DJM1986-5 wrote:Most high idle issues over the years have been from vacuum leaks or iasv issues...up until the ecu swap, it was the worst running 535 I had...a little embarrassing to say the least. It runs very strong now, of course the chip helps, but the mileage before the change sucked as well. Never above 24/25. I'll know by wed how the mileage is doing. That'll be the first tank on the new ecu. Not conclusive but directional.
From what I've noticed from my car and reading on the forum, 24/25 is pretty good for a m30 motor.
Yep, unless you're real lightfooted and doing nothing more than the filthy 55 on the freeway, 25 is about as good as it gets.
Some clarification...My back and forth to work with 10 miles of 60 mph followed by 2 miles of stop and go was only 19.5 - 20.5. On a steady cruise at 60 I couldn't get above above 22.5 - that's a consistent mechanical read, not off the computer. I could never get above 24/25 from the computer reset unless it was down hill (haha)...Usually you can get 24-26 if you reset and cruise level for a few miles at 65 - computer readout. By comparison, my other 535 was getting about 22.5 back and forth inclusive of some stop and go. And I agree, on 200 - 300 mile 65 mph trip, with my best M30/B34 E28s I never got above 24.5- 25. This one was definitely off on the averages by 1 -3 MPG. My last 800 mile round trip to Richmond she only got about 21.9 - that's a fairly level trip. We will see...
I'm wondering if the old ECU wasn't monitoring the idle switch. A quick way to tell is by letting the throttle up while in gear and going downhill, then taking a look at the fuel econometer (assuming it works).
If the needle buries itself all the way to the left (better fuel economy) then the ECU sees the idle switch. What's happening is that the ECU turns off the fuel injectors if the engine in going too fast while idling. If you put the car out of gear and coast, you'll notice the indicated economy actually gets worse.
RonW wrote:I'm wondering if the old ECU wasn't monitoring the idle switch. A quick way to tell is by letting the throttle up while in gear and going downhill, then taking a look at the fuel econometer (assuming it works).
If the needle buries itself all the way to the left (better fuel economy) then the ECU sees the idle switch. What's happening is that the ECU turns off the fuel injectors if the engine in going too fast while idling. If you put the car out of gear and coast, you'll notice the indicated economy actually gets worse.
I had given some thought to the idle switch, but not that much and wish for the sake of knowing I had tested the way you described.... When I punched the accelerator in neutral, the idle would settle at 700 and then rise quickly. On a cold start, it would stay at 700 for 5 to 10 seconds and then climb to 950. I have duplicates and triplicates of everything so I had lots of fun before I decided the ECU must be the culprit...But I never tested it when rolling...
RonW wrote:I'm wondering if the old ECU wasn't monitoring the idle switch. A quick way to tell is by letting the throttle up while in gear and going downhill, then taking a look at the fuel econometer (assuming it works).
If the needle buries itself all the way to the left (better fuel economy) then the ECU sees the idle switch. What's happening is that the ECU turns off the fuel injectors if the engine in going too fast while idling. If you put the car out of gear and coast, you'll notice the indicated economy actually gets worse.
DJM1986-5 wrote:I had given some thought to the idle switch, but not that much and wish for the sake of knowing I had tested the way you described.... When I punched the accelerator in neutral, the idle would settle at 700 and then rise quickly. On a cold start, it would stay at 700 for 5 to 10 seconds and then climb to 950. I have duplicates and triplicates of everything so I had lots of fun before I decided the ECU must be the culprit...But I never tested it when rolling...
I'm not saying the idle switch was the culprit. I'm saying the ECU might not have been reading the switch correctly, maybe due to an internal fault, or maybe a bad connection at the ECU's connector.
I seem to be having the opposite problem. Right now, I have a 24 pin Dinan chip in, car idles at 850-900, all vacuum lines are hooked up, smoke tested for leaks, all clear. Car runs great other than the higher idle.
Swap in an ecu with the stock chip 28 pin...car idles dead at 750, but idle is a bit more rough, noticeable lack of power throughout the powerband. Don't really know what's going on, I was expecting the Dinan to only make changes at WOT.
RonW wrote:I'm wondering if the old ECU wasn't monitoring the idle switch. A quick way to tell is by letting the throttle up while in gear and going downhill, then taking a look at the fuel econometer (assuming it works).
If the needle buries itself all the way to the left (better fuel economy) then the ECU sees the idle switch. What's happening is that the ECU turns off the fuel injectors if the engine in going too fast while idling. If you put the car out of gear and coast, you'll notice the indicated economy actually gets worse.
DJM1986-5 wrote:I had given some thought to the idle switch, but not that much and wish for the sake of knowing I had tested the way you described.... When I punched the accelerator in neutral, the idle would settle at 700 and then rise quickly. On a cold start, it would stay at 700 for 5 to 10 seconds and then climb to 950. I have duplicates and triplicates of everything so I had lots of fun before I decided the ECU must be the culprit...But I never tested it when rolling...
I'm not saying the idle switch was the culprit. I'm saying the ECU might not have been reading the switch correctly, maybe due to an internal fault, or maybe a bad connection at the ECU's connector.
That must have been the case - ECU and internal fault. I have not opened it up to see if there are any readily apparent issues - doubt I could tell other than say a burnt connection. I know the car was running rich...and I just replaced the O2 to rule that out. It was time and it didn't change the idle. As Fox above says, the car seemed sluggish for sure. By comparison this morning, the engine is now much more responsive - throttle is lighter under foot. I'll report back mileage on Wed.
