AFR reading dropped?

[DavidL]

I got stuck in a hideous traffic jam coming home and whilst the cooling was fine the idle became increasingly erratic, bouncing up and down from 500 to about 1200 rpm with the oil pressure gauge matching it.
Once we finally got moving it ran ok but was quite hesitant to accelerate but did after a brief pause each time.
The only thing I noticed was that the usual running AFR had dropped from 13-14 to about 10.
So what might have caused this?

 

[blade7]

Not saying it's definitely this but my car did the same for a while when the AFM was playing up.

 

[Frenchy]

Does sound like the afm, try removing the top and cleaning the tracks, there are a number of youtube vids showing this procedure, let us know, also take a look at the idle mixture ? adjustment screw on top of the throttle body i had one playing up with erratic tickover, tweeking the screw sorted it.

 

[blade7]

The track was damaged in my AFM, I had to adjust the wiper to sweep on an undamaged part of the track.

 

[George Elliott]

Have had the same AFM trouble as described, but maybe consider the ISV too?

George
944t

 

[DavidL]

Thanks. AFM had crossed my mind and it hasn't had any attention to my knowledge. Clarks seems to suggest it needs to come out or can you clean it in situ easily enough?
I shall search youtube!

 

[George Elliott]

It will need to come out David, be careful not to adjust anything except the wiper - you will see the height mod and the bend applied to the wiper on youtube.

Good luck
George
944t

 

[DavidL]

Doesn't look too bad.
This helps
http://www.the944.com/afm.htm

the video here is ok but does jump a few vital bits!
https://www.youtube.com/watch?v=-11J8XawQtc

job for the weekend.

 

[DavidL]

As I had to come in a bit earlier today I ran the car again. Quite lumpy for the first 200 yds when stone cold but otherwise fine. AFR back to 14. I figured (if its the AFM) I could keep the revs out of the damaged track and it would be ok. Although it did idle ok at about 900 rpm but I was only stopped a traffic lights briefly.
Could the under bonnet temp have had any effect as I was stuck for quite a while.

 

[blade7]

ORIGINAL: DavidL

As I had to come in a bit earlier today I ran the car again. Quite lumpy for the first 200 yds when stone cold but otherwise fine. AFR back to 14. I figured (if its the AFM) I could keep the revs out of the damaged track and it would be ok. Although it did idle ok at about 900 rpm but I was only stopped a traffic lights briefly.
Could the under bonnet temp have had any effect as I was stuck for quite a while.

My car ran fine on and off for months, the problem started when hot, it would idle fine but struggled to get above idle. Eventually it started to cut out and wouldn't restart until it had cooled down. Tried a new water temp sensor but it wasn't that. It's not hard to remove the cover on the AFM, any scores or damage on the track it will cause running issues.

 

[George Elliott]

One extra thing I did was scan the full range of the wiper before and after while noting the meter reading. If the pre-adjustment reading has an erratic behaviour (at the 10 to 30% zone on the wiper most likely) that is a good sign.

Post adjusting, if the reading is smooth and progressive from 0 to 100% that shows the problem is fixed.

The wiper needs to be moved very slowly over its entire range using the air-flap in the AFM, and taking around 1 minute to cover the full range. The reading should be nice and predictable all the way.

Good luck

George
944t

 

[DavidL]

Thanks both.
It did get properly warm weds night and it was only really using what I assume will be the damaged track.
I'll have a look over the weekend.

 

[scam75]

Could also be the connections to the AFM. I had this issue, although it wasn't intermittent. My AFR's used to sit about 13 when idling then gradually moved downwards, 12's, 11's then finally 10's. Super rich indeed. After MUCH fannying about it turned out to be the connectors to the AFM had oxidised and were making a poor connection. The loom also bends a few inches before that plug and the wires can suffer damage at the bend. I eventually got some DeOxit and used it on the multi-plug and MAF connector (same as your AFM) and the problem went away. I also gave my K&N panel filter it's first proper clean ever and my idle AFR's are now mid 14's! Happy days.

I can virtually guarantee your fault, and the nature of it, is AFM or connections to AFM, either the plug as I described or the wiring harness at the bend, or the AFM itself. Sounds like a good time for a MAF conversion to me! []

Stuart