AFM screw vs Idle Speed Screw - how do they interact at idle?

[LBEEMING]

Hi Team, 3.2 85 btw,

Hoping someone can chime in on this. I am told that once the engine gets to about 70 deg the O2 sensor kicks in and adjusts idle, but during cold it's all about these two mixtures.

Am I right in thinking that AFM screw is purely for CO value at idle and the idle screw is only for adjusting idle when ICV is disconnected.

Issue I am having is that at cold start rpm hunts between 900 to 1100 for 10 seconds, then drops to 650. Once warm it idles fine at 850.

Now if I turn down the idle screw half a turn, it will start and sit at 1100 steady then drop even lower to 600 rpm till warm.

I'm wondering if my base idle AFM setting is too rich and that's forcing the idle to 600 until the O2 sensor kicks in.

Just a theory, have an afr gauge coming so will verify idle afr soon!

FYI - made a home made smoke tester, only leak found was from the air box intake to the airbox itself, nothing after the AFM module. Car drives well, no issues to speak of. Just want to get the warm up cycle 100% sorted. Maybe an exhaust pop every now and then on over run, but I did delete the CAT and have a monty pipe 😀

Cheers!

[ant7]

The Screw in the AFM [CO] affects the mixture accross the full range [its a seperate air channel that bypasses the barn door flap] However; the ECU has the final say, and will compensate and alter both timing and mixture to be within a pre-determined parameter, especialy if you have a cat and lamba sensor fitted.
Also check the tightness [torque] on the inlet manifold studs.
Hope this helps.

Originally Posted by LBEEMING

Hi Team, 3.2 85 btw,

Hoping someone can chime in on this. I am told that once the engine gets to about 70 deg the O2 sensor kicks in and adjusts idle, but during cold it's all about these two mixtures.

Am I right in thinking that AFM screw is purely for CO value at idle and the idle screw is only for adjusting idle when ICV is disconnected.

Issue I am having is that at cold start rpm hunts between 900 to 1100 for 10 seconds, then drops to 650. Once warm it idles fine at 850.

Now if I turn down the idle screw half a turn, it will start and sit at 1100 steady then drop even lower to 600 rpm till warm.

I'm wondering if my base idle AFM setting is too rich and that's forcing the idle to 600 until the O2 sensor kicks in.

Just a theory, have an afr gauge coming so will verify idle afr soon!

FYI - made a home made smoke tester, only leak found was from the air box intake to the airbox itself, nothing after the AFM module. Car drives well, no issues to speak of. Just want to get the warm up cycle 100% sorted. Maybe an exhaust pop every now and then on over run, but I did delete the CAT and have a monty pipe ��

Cheers!

[911obgyn]

Once you have the AFR guage its a matter of the dance at idle adjusting both the afr and idle with the car hot and ICV jumpered so its centered. I tied a string to the hex key so i could retrieve it when i dropped it while adjusting.

[LBEEMING]

Thanks, still scratching my head though. O2 sensor when connected drops my idle by 200 rpm. Once disconnected it raises back to 800 steady. After a long drive this behavior at some point stops and it sits nicely at 800 at all times.

Im wondering if perhaps its just time for a new 02 sensor. If the O2 sensor is dropping RPM the O2 must be reading Lean I assume, and adding more fuel slowing combustion.

Another reason for thinking its the O2 sensor, is that I adjusted the AFM mixture from 12 turns out (set by PO) to only 4 turns out. This would have richened IDLE, so doesnt make sense that the O2 at idle is throwing more fuel in via DME just as before.

[Spetrow]

Glue the 3 or 4mm hex into this and mix adjustment is super easy. You wont lose the hex key under the intake. Flex shaft allows you to count the turns. https://www.harborfreight.com/33-piece-precision-screwdriver-set-93916.html

[LBEEMING]

Hi Team,

Well solved it I guess. Kind of. I turned the cog in the afm clockwise 8 turns ( leaning it out). The afm mixture screw to 4 turns out. Finally got the the o2 sensor to fluctuate between 0.2 and 0.7 volts. Previously I couldn't get it to move off 0.9vs. 0.9 v being rich! I think it was so rich it was out of the range of the o2, hence taking such an adjustment to get afr near ideal. So either PO changed the afm cogs or I don't know. What else could make it so rich?

Anyway, now my idle is perfect. I can remove o2 connector and idle stays as is. I rev it, and it settle at 800 rpm and doesn't move! So happy, was pulling hair out to try get it to idle well.

[ant7]

Good to hear that it now seems to have been sorted, however; its still concerning why everything was so badly adjusted in the first place, although from my own experience ie; from what had been done to my car's fuel system before I got it, nothing surprises me!

Originally Posted by LBEEMING

Hi Team,

Well solved it I guess. Kind of. I turned the cog in the afm clockwise 8 turns ( leaning it out). The afm mixture screw to 4 turns out. Finally got the the o2 sensor to fluctuate between 0.2 and 0.7 volts. Previously I couldn't get it to move off 0.9vs. 0.9 v being rich! I think it was so rich it was out of the range of the o2, hence taking such an adjustment to get afr near ideal. So either PO changed the afm cogs or I don't know. What else could make it so rich?

Anyway, now my idle is perfect. I can remove o2 connector and idle stays as is. I rev it, and it settle at 800 rpm and doesn't move! So happy, was pulling hair out to try get it to idle well.

[kevingross]

If you haven't already, check the engine temp sensor. Early ones had grounding issues.