AIR FUEL MIXTURE

[chris7ian]

HI ALL, MY MULtivan has always run very smoothly... until recently. It started at the MOT whe I was told the emissions way WAY out so i had my mechanic look at it. He adjusted the air/fuel mix (just the screw under the blue cap on the MAF) and now my bus has no guts. Worse still, I've since found out the MOT station was basing emmissions on a far more current vehicle so of course mine appeared high

Under instruction from my mechanic on the phone (he's quite far) I've tried readjusting the screw (about 2 1/2 turns from the end stop) ad although it runs far better, it's still sluggish so I think the mixture is still lean. I realise too rich is preferrable to too lean so can I just keep going until it feels better? Nothing else was adjusted and the mech did it by feel so couldn;t give me any figures (I know... )

Engine is a '89 2.1 MV

I wish I'd realised the MOT garage's mistake straight away and saved myself a whole lot of effort

cheers
c7

[slowcoach]

HI ALL, MY MULtivan has always run very smoothly... until recently. It started at the MOT whe I was told the emissions way WAY out so i had my mechanic look at it. He adjusted the air/fuel mix (just the screw under the blue cap on the MAF) and now my bus has no guts. Worse still, I've since found out the MOT station was basing emmissions on a far more current vehicle so of course mine appeared high

Under instruction from my mechanic on the phone (he's quite far) I've tried readjusting the screw (about 2 1/2 turns from the end stop) ad although it runs far better, it's still sluggish so I think the mixture is still lean. I realise too rich is preferrable to too lean so can I just keep going until it feels better? Nothing else was adjusted and the mech did it by feel so couldn;t give me any figures (I know... )

Engine is a '89 2.1 MV

I wish I'd realised the MOT garage's mistake straight away and saved myself a whole lot of effort

cheers
c7

That screw (Allen head isn't it?) Is indeed the mixture screw, but it's adjusted while using the exhaust sniffer thing. So you're likely on the right track, if that's what's been meddled with. But without the readings, you're fumbling in the dark somewhat!

Sent from my MotoG3 using Tapatalk

[chris7ian]

Tell me more about this 'sniffer - is that the probe they insert at MOTs or another device? If so then I reckon the MOT station owe me a 15minute favour - As far as I've been told, the allen headed mixture screw is the only thing that was touched

[kevtherev]

Yes you need a gas analyser to set it correctly
Otherwise it's just guesswork

[chris7ian]

...but what figures should I be aiming for? I can't remeber what the original reading was when everything ran supersmooth

[kevtherev]

CO content @ idle .5 % volume
ignition timing 5°±1° BTDC @ 2250RPM with vacuum

some more MV data for data's sake

https://www.brickwerks.co.uk/mv-engine-data/" onclick="window.open(this.href);return false;

[chris7ian]

Thank you!

[Theclash]

I would set CO to 1.8 they run better

[California Dreamin]

WHS ^^^^ as far as the MOT is concerned you are allowed up to 3.5% for your engine. VW may have stipulated 0.5% but given the engines age/mileage it will run better a little higher 1.2 - 1.5% (engine up to full running temperature, best checked within a couple of minutes return from a run).

Martin

[chris7ian]

Cheers - my brother, who was with me at the MOT, seems to remember it was running at 1.5%ish when the tester raised his 'concerns'. 3.5% is what I found on google since.

It really ran well before the fiddling - smooth and pulling hard. Immediately after the adjustment it just hesitated getting the revs up from idle. After I'd tweaked it myself it ran better but still comparitively sluggish (and stalled a few times when cold). Just goes to show the old adage is true: If it ain't broke, don't try to fix it!!

[chris7ian]

OK....soooo - I popped into the garage that did the MOT and asked if I could pop the bus on their sniffer machine to which they happily agreed and left me to it
The initial reading came up as 0.8% bearing in mind I'd already adjusted the mix (richer) a bit after my mechanic (different garage to the MOT) adjsut it down, so who knows what it was before!
After adjustment I finely settled on 1.3% and the engine seemed to run alot smoother

I think I may have a little wiggle room (up to 1.5%) as it still feels a little 'restricted'.
I'm hoping that's now the end of it as a couple of nights before I did the last adjustment, on the short drive home in the chilly night air, the bus had no drive and could only be limped along. With the pedal at half throttle, the revs would barely rise above idle for about 4 seconds then drop off for another 4, then repeat. After stopping twice and rev'ing in Neutral (Auto gearbox) everything went back to (pre-adjustment) 'normal'

Thanks all for the input and figures provided above