hi guys, ive got a digi gti with a thirst for fuel...not sure if its throughout the rev range or just at high rpm, it drank from just over quarter tank to deep in the red in 20mins (26miles) when driving between 4750-6300k RPM ! I assume this must be between 13L(2.86Gallons)-16L(3.52Gallons)! rather excessive something between 7-9 MPG!!!!!!!!! it was splitting flames out the back on changing up gear especially into third. anybody know what I can do to get it down? as the MAF screw is only good for idle adjustment as far as i know?? its got a 282 cam with vernier pulley and standard fuel pressure reg. is it most likely someone has played with the MAF 'wheel' in the past? How can i measure the Air/Fuel Ratio? Would an exhaust gas analyser help at all? never adjusted the vernier pulley as the adjustments are so small i cant measure them, any ideas how to make sure they are EXACTLY right? would all this fuel lower the performance of the car?? i dont want it economical i just need it fast, but 7-9MPG? surely that's rather excessive? oh the timing light is out by 3-5mm to the left of the 6 deg BTDC mark not too sure what that equates to is that too much advance?! didnt want to smash the dizzy freeing it up and thought it wouldnt matter much, the car goes well just the MPG but if i can make it faster all the better. any help really appreciated!! cheers dubbers.
i would set the timing up correctly, as the ignition will be wrong. However the fuelling above 4k is controlled by the ecu map irrelevant of the MAF position as its maxxed out by then. longer duration cams will affect mpg, your right tho less then 10 mpg is silly, A trip to stealth may help to get the cam setup spot on.
could be a faulty fpr not dumping excess pressure to the tank, it is also vacuum controlled to make sure all the vac lines are tip top. The ign timing really needs to be sorted you wont be getting the best out of the engine till it is! The AFM screw is for mixture adjustment, idle is adjusted at the throttle body. All the info you need to know is in my digifant tuning guide
You really need to get it on a rolling road to fully set it up, but it sounds like you've got a more fundamental problem than just fine tuning. I've got a Fuel Pressure gage and AFR Meter, but I'm a long way from you at 9 mpg! Besides, you really shouldn't drive it unless you have to, as the excess petrol will bore wash and wreck your engine. Are you confident in the temp sender, and is the air filter in decent condition? A chisel on the base of the dizzy will free it up. Check the throttle switches (closed and WOT). I'm wondering if the WOT enrichment might be stuck open.
The way I free off the dizzy is to spray penatrating oil round the base then dig out as much crus as possible with a thin screwdriver/pick/pin whatever. Then spray a bit more round it. Come back in 10 mins then clamp mole grips round it and tap it back n forth, the shock will free it off. Also, dont rely on the gauge to estimate how much fuel you have used and the volume in the tank. I put 17l in from 'empty' and it read just over half full, which is total rubbish since the tank holds 55litres Best way to measure the MPG is to brim the tank, drive it for a bit, then come back and brim it again, measuring how much it takes to fill compared to how many miles you have done.
ok guys as you might be able to tell this is my digi racecar, next race is next sunday so i need to sort it out a little by then, i thought about getting it to stealth, but they always say they are busy for the next 4-6wks in my experience, by then the season will be over. and according to fthaimike my engine may well be dead by then.[:^(] EDIT stealth on holiday next week!!! Vac lines and blanked ones all brand new. K&N filter cleaned and good condition, blue temp sender brand new vw part (150miles) I am sure I can get the timing spot on, dont know why I didnt just move the dizzy! i though a few mm's was nothing but i suppose it could be upto 4 degs out. my bad come to think of it I might have the throttle body front vac pipe/blanked pipe the wrong way round, but would the car still run? ill check against my other digi. can I just drill a hole/weld a nut onto the exhaust downpipe and connect a lambda to it and a permanent AFR guage, is this how they work? I've got a 'spare' digi, my daily driver, so i can pilfer/swap AFM, ECU, FPR at will, just dont know how to test for improvement apart from a blowtorch behind the exhaust the FPR is a second hand jobbie, its not been on long as it had an ajustable FPR on. maybe someone had to turn it down in the past rather that up... maybe someone put a 'chip' in the ecu if the ecu controls high rpm fueling straight off the map,i will open it up and take a look. although sometimes they look the same even with a different map on rubjonny i've looked at your guide many a time, but comments have been made saying none of those adjustments will affect high rpm fueling? so would turning the AFM screw/turning the wheel on the spring help at all? fuel consumption seems normal at low rpm. would I be right in saying a buggered FRP with too high fuel pressure would push more fuel into the cylinders than required each time an injector opens? its abit odd because a digi often leans out at top end eh? cant work on the car till next wednesday due to work so just trying to get a feel for where i'm going to start! any help appreciated, sorry for the long posts, cheers
the fpr should be connected to the vac nipple closest to the passenger side of the engine, i did look into these on a tbody i was cleaning up and the ports are different sizes so im pretty sure it will make a difference! as for fueling under wot you got me on that one, what i know is whats written in the guide has been said though when at full throttle the afm flap is pegged at max anyway so even if it was buggered it wouldnt effect anything, though if the base idle/co was miles out due to a faulty afm this may effect how the car behaves at WOT. As I say though, dont base your calculations of the ammount of fuel burnt on how much the gauge moves, they arn't that accurate. I'm not even sure if the gauge works in a linear way?
i have always connected the FPR to the Nearest vac point with the shortest possible piece of hose. Lambda sensors aren std sizes m18 x 1.5 i think, but if you could find a nut and weld it on, then yes it would work. if you need owt let me know, im only about half an hour away and have a digi engine sat on the floor.
Test the throttle body switches, or just swap the whole TB if you've got a spare. Does the WOT switch click when you open the throttle wide? Swap the ECU or MAF out if you've got a spare too. I'm not confident it's that, but it's easy to rule it out. You'd want a bit of extra fuel pressure on a racer. Std reg should work to 3 bar at tickover, and I've seen modded engines running at about 4 bar. The pump only works at about 4.5 bar, so you'd struggle to get to 9mpg just by fuel pressure.
