Porsche Enthusiasts Club Forum

The car is a 1986 924S, non-o2 sensor. It has no running problems, no hunting, no bogging down under load; but the CO is 5.1 and it has failed the MOT as a result. The HC is also a bit high at 480 and it smells rich. All ignition components are new.

I'm led to believe you adjust the bypass on the MAF to lower CO, but the garage (non porsche specialist) reckon this isn't working.

Do all roads lead to the MAF under these sorts of circumstances?

Clark's Garage can't help me, as their pages only describe o2 sensor models.

Cheers

1. Clean and test Idle Control Valve
2. Clean AFM
3. clean throttle body

Then check the fuel rail vacuum lines and for any other vacuum leaks, however these are buried in part under the intake manifold.

Something is telling the DME (ecu) that it's getting air flow than it is, hence the over fuelling.

Thanks. The breather system associated with the ICV is contaminated with oil residue, is this suspicious? The throttle body gets cleaned regularly because it gets fouled up with oil, it probably isn't that tbh.

When you say clean the AFM, do you mean internally by removing the black cap? If i can get the starting value down to as few ohms as possible by cleaning the wiper and strip, then this should signal less fuel to the DME...right?

I've never seen a test procedure for the thermistor in the AFM either. If it's knackered the DME might always think the engine is stone cold.

Does the 2.5 8v have a temp sensor on the engine coolant that can cause over fuelling like on the bigger engines?

I think it does

Simply because there is no choke or cold start valve like on 924,

Think there are 2 temp sensors, one is directly connected to the gauge, the other to the dme, they are in a similar position on the block under the intake manifold.

Bottle it and send it to the beer industry.

I had the same last MOT - and has always smelt a little rich.

There is a CO adjustment screw (5mm Allen key head iirc) on the outlet side of the AFM. Turning this anticlockwise will reduce the CO to a small extent - it takes lots of turns. When mine failed on CO we left the machine measuring and spent a few minutes adjusting the screw and re-checking. Brought it within the limit eventually.

The garage are saying it had no effect. Maybe it's corroded a bit.

I'm still in the process of data acquisition on this job, i'm generating a punchlist of all possible electrical components that can be subjected to test and inspection. Once this is done work will begin with the most easily testable components first on the list, Critical Point Analysis to be exact.

Punchlist so far:

DME temp sender
Throttle switch
Injectors
AFM

DME Temp sender- R between Bn wire on loom plug and chassis- 0.1ohm
R at Amb.- 1.56k
R at 80 DegC- 348ohm
Status- PASS

Throttle switch- R between 2/18(closed)- 0.2ohm, (open)- OL
R between 3/18(closed)- OL, (open)- 1.8ohm switching before fully open
Status- PASS

Injectors- R accross coil- all 2.6ohm

AFM- V between 9/6- 5.06v
V between 7/6(closed)- 254mV, (open)- 4.66V
R accross Thermistor temp sensor at Amb. 22/6- 2.5k
Status- PASS

Everything seems to be within specifications. I cleaned the AFM with IPA, which was a bit gunky inside, and left the bypass screw all the way up after cleaning it up a bit. It's very difficult to access the airway on the bypass. I took the lid off the AFM and cleaned the track and wiper, but all looked well tbh.

I found the best way to piggyback onto the AFM connections was with the boot pulled back and the plug connected. I straightened out some piggyback crimps and cut them down thinner so they'd fit in, and crimped them onto a couple of test leads. When the DME temp sender is removed water will piss out, but an M12 bolt seals the hole up good enough. A vanity mirror from a sun visor placed under the throttle switch lets you see the terminal numbers. The AFM seems to be able to open so far without the output voltage increasing. I think this must be to facilitate a predetermined fuelling map at idle with a bit of wiggle room built in. If other air sources, such as the ISV, become clogged then too much flow will come through the AFM at idle and push the wiper up onto the track thereby richening the mixture.

Outstanding tests are fuel pressure and the dreaded ISV, but it's going in for another retest first

Out of interest, what do the plugs look like? After a decent run, not after idling, are they black? Any other signs of running rich?

Plugs are light brown.

It's gone through. However CO is now a miniscule 0.2!

Good news. Did you work or what it was?