Max Mixture On Afm But Still Not Enough.

[Grahamrally 0]

Right I've done lots of hunting around for a similar problem and there's all sorts but I want to post this just for a little feed back and a few ideas. Basically I've got a 309 gti, since I bought it a few days ago it's always stumbled on light throttle, doesn't seem to be a missfire, more a stumble.

 

I checked it on a gas analyser and it was 0.5% CO the way the AFM was set. So I adjusted the mixture screw until it read 2% CO, problem is, that's now at maximum on the AFM and there's no more adjustment. The car definately responded much better to more fuel, but yet I'm sure with the mixture set on max on the AFM it should be reading a much higher CO % than just 2?

 

This led me to think there was an air leak, so I did the old hand over AFM entry trick, low and behold there was. I found a couple of air leaks on the pipework. Then checked the inlet gasket and someone had put the wrong one on. The injectors were spraying against the gasket as it didn't have cutouts for them!! So I changed this, expecting it to run like a dream. It runs much better, but still there's a fundamental stumble on low % of throttle.

 

So I did a compression check, just to be sure, 175psi all round, no problem. Changed the plugs....no change.

 

Whats a little baffling right now are two things. The fact I have to run it on max mixture on the AFM says to me there's still too much air getting in from somewhere. However I'm not convinced there's air leaks anymore, so could the pressure reg be playing silly buggers? Could the SAD be feeding air constantly?

And it doesn't stumble anywhere else other than light throttle?!

 

The last thing, is there's a plug coming from the back of the engine, i think it goes all the way down to the sump and I guess is another oil temperature sensor. However the cable comes to the rocker cover, is clipped there but the plug isn't plugged into anything. Is that correct? I'll try and get a pic.

 

Any further thoughts would be much appreciated.

 

Graham

[Pugnut 9 1 Cars]

the SAD gets metered air anyway , but to check it just crimp the hose to or from it when the engine is up to temp and see if the revs drop. I've never heard of a fuel reg failing , but fuel filters do get blocked and fuel pumps can get old. Have you any means of checking fuel pressure? I would imagine a fuel starvation to be more of a problem at high rpm though.

 

the next thing i would try is to fit another afm

[Anthony 1,003]

Try another known-good AFM and go from there.

 

The last thing, is there's a plug coming from the back of the engine, i think it goes all the way down to the sump and I guess is another oil temperature sensor. However the cable comes to the rocker cover, is clipped there but the plug isn't plugged into anything. Is that correct? I'll try and get a pic.

That's normally if it's a three pin plug - it's used by the Peugeot diag machine when setting ignition timing, but it's useless to the rest of us.

[welshpug 1,657]

check the dipstick seals properly, that will also cause an airleak.

[Grahamrally 0]

Try another known-good AFM and go from there.

 

I've tried another AFM, not known if it was a good one. But it produced the same results. The mixture adjustment had to be all the way in. It was slightly better, but I put that down to the fact that not all AFMs are the same in calibration. I will try another at some point and re-track my current AFM.

 

Anything else that might cause a low throttle stumble from the management side of things?

 

The next thing I will investigate is if the ignition timing is correct. It goes without saying that if someone has fitted the wrong inlet gasket when they skimmed the head, they may have incorrectly fitted the dizzy back.

 

It's possible that the ignition is too retarded as it doesn't pink. However I would have thought it would run a lot richer if it was. So still probably not the cause.

[Pugnut 9 1 Cars]

I've tried another AFM, not known if it was a good one. But it produced the same results. The mixture adjustment had to be all the way in. It was slightly better, but I put that down to the fact that not all AFMs are the same in calibration. I will try another at some point and re-track my current AFM.

 

Anything else that might cause a low throttle stumble from the management side of things?

 

The next thing I will investigate is if the ignition timing is correct. It goes without saying that if someone has fitted the wrong inlet gasket when they skimmed the head, they may have incorrectly fitted the dizzy back.

 

It's possible that the ignition is too retarded as it doesn't pink. However I would have thought it would run a lot richer if it was. So still probably not the cause.

 

the only other sensor that affects fuelling is the coolant temp sensor ( 2 pin connector on side of thermostat housing)

 

but this normally gives rich fuelling when faulty. worth a try though. i'm sure someone will assist with expected imedances at certain temps (welshpug!)

 

Al

[welshpug 1,657]

If its running lean the Throttle stop screw may have been adjusted at some time rather than the idle screw, so even with the idle screw fully in its still pulling in too much air.

 

found these values for the Temperature sender.

 

-10c = 8200 to 11000 ohms

 

+20c = 2280 to 2720 oms

 

+80c = 290 to 370 ohms

[Grahamrally 0]

If its running lean the Throttle stop screw may have been adjusted at some time rather than the idle screw, so even with the idle screw fully in its still pulling in too much air.

 

found these values for the Temperature sender.

 

-10c = 8200 to 11000 ohms

 

+20c = 2280 to 2720 oms

 

+80c = 290 to 370 ohms

 

Excellent, I'll find these values out when I get back. Thanks for that Welshpug. Not done anymore yet on the car, but will tonight.

[welshpug 1,657]

may be worth setting the throttle body back to basic settings, well detailed in the Haynes manual if you have one.

 

check for wear in the spindle and the return spring tension too.

[luggy 13]

It goes without saying that if someone has fitted the wrong inlet gasket when they skimmed the head, they may have incorrectly fitted the dizzy back.

 

Might be worth checking that the exhaust gaskets have been fitted properly.

[Grahamrally 0]

Just reset the throttle stop, re-tracked the AFM and reset the mixture, still runs best on max richness. Anymore ideas other than change the AFM?

 

I haven't been able to check the resistance from the temp sensor yet. A little confused as to what is causing this really.

[Grahamrally 0]

How far out are your AFM mixture screws? Aimed at anyone who wouldn't mind taking a look and counting the number of turns?

[RINRIN 1]

11 turns from one end to another