car's bunnyhopping/kangarooing at low revs... igntion and mechanical timing is spot on

but... what is the ideal air/fuel ration on a g40 then.... mine goes right down to about 10.5 AFR which is, well, wwayyy tooooo rich, and wont help the idling problems...

i've done some research on air/fuel mixtures etc which seem to say stick with 11.5 ish or so max on full throttle.... i guess its a case of adjusing the CO pot screw (fuel trim i think its called?) until i get a slightly leaner mixture (and better fuel economy)

someone on here must have some input - i've seen yoof's video on youtube (karlcl coupe posted it i think) but that didnt show much :p

Firstly, what are you using to measure the airfuel ratio?? If it's not a wideband or exhaust gas analyser- I wouldn't trust it much.

On a stock G40 chip I'd say you're looking at around high 13:1 a/f ratio- max power on a forced induction engine is usually made at an a/f ratio of 12.5:1 but this will vary from engine to engine.

10.5:1 isn't just rich, it's bloody rich, that was what i set my nitrous up for with an 80 shot!!! Whip your plugs out and have a smell/check- if you run very rich a/f levels for a while (sometimes even 20miles or so) despite cleaning a plug, it may not recover, and will be a dud.

There's a guide for setting the CO pot up, forget where it is, seems like this and a new set of plugs would be a good start

Hope that helps a little bit..

Yoof

its a gauge off the Lambda sensor (running through a shielded cable so no interference from anything electrical to upset the ECU - common mistake by people adding a gauge is not using shielded cabling)

they are new plugs... or they were with the engine strip and rebuild 1000miles ago...

thanks for that yoof, appreciated.

the only thing i could find about the CO pot - says it should be roughly 550ohms..... but... my question is... what exactly does it adjust?! mixture at idle is it?

I had a play with the CO pot (changed it to 250ohms and also 750ohms to see what effect it had, and it didnt do much!!!!

i forgot to mention the car has a superchip fitted... and yes, it is using a lot of fuel and you can smell that its running rich too...

its a gauge off the Lambda sensor (running through a shielded cable so no interference from anything electrical to upset the ECU - common mistake by people adding a gauge is not using shielded cabling)

Agree with the shielded cable comment but unless the meter itself is a properly calibrated meter (i.e. not an off-the-shelf £30 gauge or a DIY kit) then it's only any good for a rough idea. Best bet is to take it to a friendly garage and hook it up to their gas analyser, that's the only way to properly set the CO pot too.

they are new plugs... or they were with the engine strip and rebuild 1000miles ago...

can kill a set of plugs in a couple of miles if it's far too rich, ask Dan Knight

rae - some afrm do interfere with the signal and can cause your car to run SHIT -a duostyling one did the same for dan and sam

Anyone know where that CO Pot guide is?

rae - some afrm do interfere with the signal and can cause your car to run SHIT -a duostyling one did the same for dan and sam

i can agree with that, hooking laptop up shows that the ecu still recieves the same information from the lambda with the gauge hooked up and also when unplugged on mine, so at least i dont have to worry about it interfering.

little bit more co pot stuff : http://forums.vwvortex.com/zerothread?id=862194

i cant find the original post about the CO Pot , doesnt help that i cant remember who posted it!

so, basically from what i've read elsewhere.. i need a different chip!! even though this one was made for this particular engine at some point in a past life

and i forgot to give you the chip i had in my pocket last night!

doh!

argh! oh well, i'll get it soon! (next week probably)

thanks for the zorst... and you didnt say bye to us last night :p

i know sorry mate - everyone was off so we scooted home for a cup of tea!

As mentioned previously by Yoof, you need to have 100% faith in the accuracy of the equipment you are using before adjusting the fuelling as you may be starting a mission of a futile wild goose chase if the data you working from is not accurate.

You really need a five wire wideband probe to take an accurate and real time reading of AFR. This information if fed back to a controller and accurate AFR data can be logged on a laptop or displayed on a gauge. I replace my five wire sensor on regular basis as they soon become slow to respond after being in and out of several cars being mapped subject to being exposed to lean/rich mixtures.

If you are using a narrow band sensor (OE G40 sensor) the readings will be inaccurate as this type of sensor is only really designed to be accurate at 14.7:1 AFR as the digifant ECU try’s to maintain that value at idle/light cruise. Values at the other end of the spectrum will be inaccurate as the sensor is only really designed to be homing in on 14.7:1.

A stock G40 cycles between 14.6:1 - 14.9:1 AFR at idle and cruising at motorway speeds. Light acceleration, but not on boost will drop the AFR to low 14 to high 13:1 AFR and full boost values drop into the 12's ending up at a constant 11.8:1 AFR at full throttle.

The CO pot will only trim fuelling at idle and up to 5psi of boost. Above these values the ECU switches to a pre determined map and does not use the CO pot value or o2 sensor to determine fuelling values. The only part of the CO pot sensor that is used is the Air temp sender.

10.5:1 is stupidly rich and you might as well be pumping fuel out of the back of the car though a hose. Most G40's I try and maintain 11.3-11.8:1AFR and they deliver good power at full throttle without blowing the internals to bits.

By the sounds of it you need to go back to basics. Most running problems on EFi cars are due to dead senders, broken wires, and bad earths, poor calibration of trigger switches such as throttle idle/full throttle or low voltage to the ECU or fuel pumps. Once you are happy the foundations of the system are spot on and you have the appropriate equipment for accurate meaningful data acquisition then you can act on your findings.

top notch reply, thanks.

on the gauge i got, and using the vwtool/odbii port, via the ecu the lambda read between 0.2 ish and 1.8v at idle (it "cycles") and on the gauge it reads between 13 ish and 17ish from memory... i know this isnt accurate.

from the sounds of it - i could do with taking it to a Rolling Road when i've got the exhaust sorted and sticking a mixture analyser up the exhaust pipe!

the bit about the CO pot @ idle is what i wanted to know about to be honest - the map in the ecu must be okay as its been running for xxxx number of years. it might not be perfect but it'l do for now. will try some other chips though to see what happens :) (stock/one petes lending me etc)

all the sensors work (tested thoroughly on vagcom lead) and the earths are all new / clean / bigger cables. trigger switches work spot on - have tested with multi meter etc. i cant see it being a power to the ecu problem, but i will check this out tonight.

any clues as to why it would take so long to start ? (crank it for up to 7-8 seconds before it fires up) every other G40 fires up straight away ! [edit: i assumed this was because it was sticking too much fuel in, plugs were new... will check the state of them now tonight!]

Edited 2 November, 200718 yr by rae

Poor starting might be due to a weak fuel pump taking time to make good pressure at the rail. The engine may be down on compression taking a number of turns to build up enough cylinder pressure to kick the engine into life.

i did a compression test after 200 miles or so and all was okay (was still taking ages to start) but will do one again tonight or tomorrow

fuel pump is the one that was on the car as a GT - haven't touched any of the fuelling stuff so cant see that being a problem, ... i cant understand the car running rich but having low fuel pressure?

more testing needed i got some other g40 injectors to try so will do that this weekend too!

thanks for all the info, i'm sure a few people will learn something from it

i did a compression test after 200 miles or so and all was okay (was still taking ages to start) but will do one again tonight or tomorrow

fuel pump is the one that was on the car as a GT - haven't touched any of the fuelling stuff so cant see that being a problem, ... i cant understand the car running rich but having low fuel pressure?

more testing needed i got some other g40 injectors to try so will do that this weekend too!

thanks for all the info, i'm sure a few people will learn something from it

Don't discount the fuel pump. I had a car in a few weeks back that was running 12.7:1 AFR at idle and this leaned off to 14.4:1 on boost. The car had a knackered ECU earth causing it to run rich and only 11.2V at the fuel pump as the clifford alarm wiring was very special.

Don't discount the fuel pump. I had a car in a few weeks back that was running 12.7:1 AFR at idle and this leaned off to 14.4:1 on boost. The car had a knackered ECU earth causing it to run rich and only 11.2V at the fuel pump as the clifford alarm wiring was very special.

hellooo again.

i've just tried a standard G40 chip - AFR reads approx 11.75, whereas on superchip reads more like 11 (not 10.5 as i said before - i couldnt watch the afr & drive very well!!)

on the hunt to tryout some other chips and maybe get one made for my spec.

thanks again for all that info!

found out why car doesnt start first time - i've run vac hoses off wrong place... need to get another brake servo pipe with the vacuum output for the ecu and the fuel regulator instead of coming from where the charcoal cannister hooks up @ the ISV!! starts straight away with this pipe blocked off