Bottom line up front:
- The Bentley AFM test is wrong.
- The AFM could easily be a culprit in a high rpm miss.
Background. I’ve been fighting a high rpm that “almost” always occurs at WOT and between 5k-5800 or so rpm. It seems to clean itself up just before RL. The problem does not occur at partial throttle and high rpm. I replaced and checked a helova lot of shit in this fight. I found along the way that the DME is quite sensitive to input voltage and for two events it seemed that the symptom would go away if I disconnected the TPS. That didn’t make much sense to me so I worried that it was a red herring, but for a number of sessions that TPS connected or not correctly predicted behavior.
There was some indication that it was a lean condition that was making the engine miss.
A guy PM’d be on e30tech the other day that he had a similar symptom a while back and the behavior seemed to go away when he replaced his AFM. His tale reminded me that Julio had bought new AFMs for their cars after problems bedeviled them for a while.
I spent a couple days thinking about the AFM theory but I kept coming back to the idea that the DME isn’t supposed to listen to the AFM sensor at WOT and at 4500 or so the door is supposed to be wide open anyways. That’s 2 reasons why the AFM can’t possibly be the cause of a problem that only occurs at WOT and in a specific rpm range where the AFM door doesn’t move.
On the other hand, I thought, I’ve not fooled with the AFM much in this effort, and I’m well past the point where I need to look at things that don’t make sense. So tonight pulled off my AFM and set it down next to my 4 spares and started testing them with a multimeter to see if I could find anything interesting.
The first thing I found is that the Bentley is wrong. It says the sweeper door’s potentiometer is tested with pins 7 & 8 and it’s actually 5 & 7. Also, the copy of the OEM Service Manual was no help because it shows you only how to R/R the AFM, but not how to test it.
The 2nd thing I found is that all, yes all, 5 of my AFMs had problems providing a linear increase of resistance in the last 5mm of door travel.
The way it’s supposed to work is that as the door opens the resistance increases. When the door is fully open the resistance should be highest. But what I found is that in each case the resistance was highest 4-5mm before the door opened all the way, and then the resistance dropped. So when the door was maybe 80% open the potentiometer would tell the DME that the AFM door was fully open (max resistance). Then, as the door moved thru it’s last 4-5mm of travel the resistance would drop so the AFM would be telling the DME that the door was closing. Air flow was going up but the AFM was telling the DME that air flow was going down. The AFM that I had on the car was the worst of the bunch and would have told the DME that the door was about half open, when in reality the door was fully open.
I think that the DME was reading that 1/2 open business and not letting the injectors provide enough fuel. That’s why the engine ran well at partial throttle, and that’s why the hiccuping seemed to be due to a lean condition.
The AFM I’ve been using for the last several years had a peak resistance of ~1800 ohms and when the door closed it dropped to ~600 ohms. A second one I tested showed peak 960 ohms and dropped to 525 ohms. A third went from 1070 to 930.
Solution. I figure you either have to move the circuit board a little or monkey with the sweeper arm. The sweeper arm has a little screw in it. If you look closely at the sweeper arm you can tell that it’s designed to allow adjustment in the sweep range. I loosened the screw that holds the sweeper arm in place and then shifted the sweeper arm so it didn’t sweep quite as far. So instead of sweeping beyond 1800 ohms for example, I shifted the arm so it hit it’s limit of travel while resistance was still going up.
Please god let this fix the damned thing.
LATER EDIT: The testing process above is associated with the Bentley and is probably not correct. What is probably the right answer is introduced in page 2 but understanding doesn’t really start until p3.