AFM mixture screw.......!

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AFM mixture screw.......!

Postby spanishfly » 04 Jun 2019, 17:31

I know I'm not supposed to touch it ever but am getting frustrated at the rich running just want to start using it so after marking the position I screwed the AFM in 4 1/2 turns then out 11 turns and nothing happened! Is that normal? I was expecting dementors in the yard or plagues of locusts, atleast some thunder and lightening!

Before this I've done all the other checks in Bentley bar the fuel pressure test as I don't have a tester or a C02 analyser.

All passed according to the figures there with the exception of cylinder 3 injector showing only 11 ohms rather than 17 ohms.....?

Coil is grey and in spec.

Temp sender 2 new and in spec.

Temp sender 1 in spec.

TPS in spec.

ECU seems good checked against known good spare.

ISCU good according to Bentley.

ISV buzzing, cleaned and checked with a spare.

Plugs, leads, dizzy cap and rotor arm new.

Timing set at 3000 rpm against new mark 1.75" from TDC and checked at tick over with green wire unplugged, box leads joined and vacuum pipes off.

Air filter cleaned.

No vacuum leaks I could find.

Engine bay earths remade.

Dash earths checked.

So my question is.... shouldn't turning the screw on the AFM make noticeable differences to the running?

If it should and mine isn't, what is wrong and any advice about what I can do to it please. I can't quite see how that screw interacts with the flap (which moves freely) and is about 6mm open at rest.

Taking the top cover off the aircleaner revealed the quantity of fluff and dust you'd expect from a van that spent more than a decade in a dry barn....!
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Re: AFM mixture screw.......!

Postby silverbullet » 04 Jun 2019, 17:44

The adjustment screw is an air bypass, allowing you to trim the volume of air entering at idle and so set the idle mixture.
You need a CO meter.

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Re: AFM mixture screw.......!

Postby spanishfly » 04 Jun 2019, 17:56

The idle screw on the throttle body affects the sound of the engine, screwing it out slows the revs whilst screwing it in increases them but shouldn't the normally covered screw that I suspect the PO fiddled with and that I have now screwed all the way in and out with the engine running do something obvious to the engine note?
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Re: AFM mixture screw.......!

Postby silverbullet » 04 Jun 2019, 19:20

The screw on the throttle body (which isn't the AFM) is also an air bypass. You are going to have to go back to basics, probably delve into Bentley at length and/or swap all the parts that have been "tweaked" for known, good, unmolested ones.

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Re: AFM mixture screw.......!

Postby spanishfly » 04 Jun 2019, 20:10

Thanks Ian, I appreciate that sourcing unmolested parts may become necessary in the end. Sadly it often is!

What I'm seeking to understand is how the covered AFM mixture screw affects the moderation of air into the manifold.

Is it blocking or unblocking a hole and adjusting the volume of air that way?
Does it or should it move the butterfly through a mechanical intervention?

And if yes to either of these why is there no apparent change to how the engine is running? ?

I guess I'm hoping something maybe gummed up or stuck through lack of use that could be persuaded to work again!
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Re: AFM mixture screw.......!

Postby Aidan » 04 Jun 2019, 21:22

as Ian said in his first answer the AFM CO adjusting screw on the AFM is an air bypass adjustment, ie it allows adjustment of the volume of unmetered air into the air intake at idle when the AFM flap is basically shut; by adjusting the volume of air it adjusts the air/fuel mix ratio and thus the burn of the fuel and thus the CO content of the combusted fuel. It won't noticeably change the engine note you need a co meter to measure the variation it achieves - it's a simple aperture limiter no mechanical connection to the flap

The idle screw on the throttle body may change the revs at idle as it is screwed out as it allows air to bypass the throttle body flap (butterfly valve) which is shut, there are holes in the flap that allow some air to enter the system at idle and a flat cut aperture on the flap which is limited by the idle screw (again no mechanical connection just an aperture limiter) but remember the idle is controlled by the idle control unit so the effect may not be noticeable as the control unit may cut the air passing the air bypass valve to compensate so keeping idle constant

The idle bypass valve allows air into the plenum chamber in addition to that passing through the holes in the throttle body which the idle control unit adjusts it to ensure the revs stay at or around the idle speed depending on load and temp so if you have power steering or air con they also inform the idle control unit of their demand

If you completely compress the hose to the idle bypass valve then you should have a reduction in revs even to the point it stalls which proves that air is being allowed past the bypass valve, if that makes no difference then the air bypass valve is closed, either by the idle control unit electrically or gummed up by deposits in the valve, if you limit the hose you should see a decrease in revs initially and then the revs raise back to a happy idle as the idle control unit increases the valve opening to compensate for the lack of air as it works to keep the engine at idle speed
Last edited by Aidan on 04 Jun 2019, 21:29, edited 1 time in total.
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Re: AFM mixture screw.......!

Postby silverbullet » 04 Jun 2019, 21:23

From the Book of Lies:Image

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Re: AFM mixture screw.......!

Postby spanishfly » 05 Jun 2019, 07:15

Thanks both of you, the fog is clearing!

I'm confident I can return both the screws to pretty much where they came from which still leaves me with a CO2 of 4.4%...!
Clearly progress without a CO2 meter will be difficult but in theory should I not be able to lean out the AFM by turning the screw anti clockwise. (24.15) At the moment it's 3/4 of the way in.

I have read tencentlife's and vanistans essays on the topic, they use a multimeter and the lambda to achieve Nirvana, my short term goal is to correct any changes that might have been made by the PO as a result of the rat attack on the loom in order to leave the van nearer the zone. I have no desire to start messing with wipers, cogs or springs and accept getting spot on won't be possible without access to a CO meter!

With reference to the bypass hose that goes to the idle valve this does seem to have the integrity of an old sock! Is that how it's supposed to be or a result of 30 years of oiling?

Last question, is the lower resistance on cylinder 3 injector a concern, significant? 11 ohms when the others are 17 ohms...... I'm unsure how to decipher low resistance in a cable or is it the injector.
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Re: AFM mixture screw.......!

Postby Aidan » 05 Jun 2019, 10:02

quite possibly an issue, measure the injector unplugged to take the wiring out of the equation and clean the contacts and recheck

the injectors are batched fired, ie they all squirt at the same time

low impedance may be a sign the injector is stuck open, if you pull it out of the manifold as per Bentley you can turn the engine over with the king lead removed from the disi and earthed so there's no spark and observe if it is spraying continuously or in bursts and measure the throughput; the injectors can be ultrasonically cleaned, there are companies that offer the service, I've used the place in Hull with good results
a stuck open injector would increase the fuelling and thus give a poor co reading and a high HHC reading which would be picked up on the MOT, iirc we're allowed 1200 HHC

The plastic body of the injector may have split due to corrosion so maybe water has ingressed
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Re: AFM mixture screw.......!

Postby spanishfly » 07 Jun 2019, 06:47

The low impedence was it seems a sign of secondary windings breaking down in the injector (according to injector reconditioner) replaced with rebuilt injector now reads as 17 ohms same as the others, sounds smoother but when driven buzzer of doom goes off.
Heads are clearly new so suspect electrical gremlin still.

One step forward.....
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Re: AFM mixture screw.......!

Postby ajsimmo » 07 Jun 2019, 09:38

Is buzzer of doom a new thing? Have you knocked the wire whilst changing the injectors?

I just hope you didn't have a hosing injector which could cause bore washing, putting petrol into the oil, thinning the oil to the point pressure drops dramatically... That'd explain raised hydrocarbons whilst also high CO. Let's hope it's not that!

Anyway, if it's been running poorly for some time, probably worth changing the oil again anyway.

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Re: AFM mixture screw.......!

Postby spanishfly » 07 Jun 2019, 13:16

The whole van is a new thing not actually driven anywhere yet though it has spent some hours ticking over! The buzzer did go off a couple of times then I hard wired the dash using one of Chris's fantastic kits and it still went off.

Just tested higher pressure switch with a power probe and it is off until 2000 rpm ish then on above that. Buzzer seems to fire a bit randomnly so will check the wiring.

Blue is off with engine running. (as it should be I think) so looks like a high pressure wiring issue...... hopefully!
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Re: AFM mixture screw.......!

Postby ajsimmo » 07 Jun 2019, 13:52

spanishfly wrote:Just tested higher pressure switch with a power probe and it is off until 2000 rpm ish then on above that. Buzzer seems to fire a bit randomnly so will check the wiring.


That's a bit too close to the limit for comfort. Is that hot/cold/80°? If you tested at anything cooler than hot, and you have anything other than the 1.8bar switch, the pressure is too low. Could really do with a wet gauge on it to be sure.

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Re: AFM mixture screw.......!

Postby spanishfly » 08 Jun 2019, 19:14

So replaced oil pressure senders still going off immediately, after idling for 10 minutes and just in to 2nd gear, probably nearer 2500 rpm when warm than 2000. Will look at computer board next and track down a wet guage? Tester to screw in between the tubes....

The buzzer comes on immediately, but not the flashing light that you see at start up. Could that be significant?
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Re: AFM mixture screw.......!

Postby ajsimmo » 10 Jun 2019, 08:13

spanishfly wrote:The buzzer comes on immediately, but not the flashing light that you see at start up. Could that be significant?


Weird?! That should never happen. Summats very odd going on there... Could possibly be a faulty BOD board, but what's triggering it? Might be two faults there.

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