Hi, technical query about low inlet manifold vacuum. I have a mk2 8v gti with several modifications as follows, pb head and standard ecu and injection , Kent gs2h cam, 2.0 agg block , mk3 agg downpipes to jetex 2.5 inch exhaust, mk3 fuel tank and pump, k and n panel filter in standard airbox with intake restrictor removed, aem wideband afr . The engine runs well with no vacuum leaks, timing correctly set and all components checked, the issue I have is with the fuel ratio as monitored with the afr gauge which I installed as I felt something was a bit off. At warm idle and light cruise it is about 12, under moderate to fairly hard acceleration it is about 15 to 16 and at wot is about 12.5.( which is the only time it seems right). CO is 1.5 at idle but the adjustment screw in the air flow meter is wound right out to achieve this. I would expect the afr to be about 15 at idle and light cruise and 14.7 or under on hard acceleration to achieve good fuel consumption and full power. I believe the issue may be due to the kent cam causing low manifold vacuum ( 0.2 bar at idle rising to 0.7 at 2000 rpm) due to the valve timing, which affects the fuel pressure regulator giving 3 bar of fuel pressure at idle , dropping to 2.5 bar at 2000 rpm no load, and then increasing back to 3 bar at higher rpms. I have adjusted the air flow meter flap tension to achieve the best balance I can but cannot find a setting where it is not too rich at idle or too lean on acceleration. Sorry this is a bit long winded, hoping someone may have some insights that would help get the best from this setup, thanks.
There's not a lot really you can do, it's down to the basic fuel control in the ecu. That said, mine, with almost the exact same spec, aside from ported head and extra compression, is fairly consistent across the rpm range, apart from leaning at the high end. I'd be temp to try a different metering flap unit if you could get one. I had to change mine, it was running rich and lean in different places up to that.
Hi Tristan, thanks for your reply. I know the ecu is hard if not impossible to re map so it is all about manipulating what it is seeing, or changing parts. I believe the root of the problem is the valve overlap from the cam, I had the same cam in it with the standard engine with a vacuum gauge instead of the afr and was always aware of the low vacuum at low rpm but with the bigger block and larger exhaust it has amplified the fueling issue so it stinks at idle and smells hot when giving is some beans. I was wondering if a vernier pulley may help, or a different cam, or is the exhaust too big? Had also considered an auxiliary air bypass for the air flow meter, whether drilling or machining the flap or adding an adjustable air bleed pipe between the airbox and throttle body would be feasible. The ultimate solution would be possibly a decreasing rate spring in the air flow meter but I have no idea how to make such a thing. I do also have the complete mk3 so if it came to it could use the injection system complete with O2 sensor but that is even more work. Happy to keep tinkering!
Running that poor under acceleration isn't really good. Values should be under 14,3 and for acceleration much lower. Curious how you get a CO of 1,5% with AFR 12. I had CO 7 with those values and even though the map is adjusted to run less rich when not needed, I don't think I'll get below the 3,5% which is the "MOT value" over here.
I agree that standalone management would be a solution and just map around the problems, but it would probably cost quite a lot which is what puts me off as this set up didn't cost me very much and will never make huge amounts of power. I'm aiming for a more basic solution like add a resistor or modify something to cheat the system. The engine does run well as it is and if the afr gauge wasn't there it would seem completely sorted. Perhaps I should just take it out and enjoy
The afr values do seem to be mostly the opposite of what would be ideal, they were much worse before adjusting the air flow meter preload. I had the car in the garage where I work to double check the ignition timing, so once I had set it I thought it would be rude not to pop it on the mot emissions machine and see what it was doing. Found it to be 1.5 CO and lambda of 1.05, whether that indicates the afr gauge or the mot machine are not quite accurate I don't know . It smells much better now than before I set it up as it was eye watering at idle and smelt very hot under load. In my mind if I can make the air flow meter 'see' less air at idle it should help by putting less fuel in. Still think the root of the problem is the fuel pressure not reducing due to the low vacuum being supplied to the regulator.
Is that fairly consistent over the load and rpm range? Mine swings wildly but consistently between the 12 ish at idle and the 15 to 16 at light throttle/load. Before adjusting the air flow meter preload it was 10 at idle and 17 at light load. It was at this setting when it passed its first mot for 11 years at about 2.8Co. Just to give a bit more context, I have owned this car for 21 years, the cam was fitted in 2004 to the standard engine. I took it off the road in 2011 as it was very tired. It is now back following a full body restoration where I bought a mk3 8v gti for the power steering and found many mk3 parts were useful upgrades including the agg block. The fueling was probably not great for years and the increased capacity has amplified the symptoms.
Yes, fairly consistent, using an Innovate stand alone lambda reader. It wasn't as consistent on the old air flow meter.
Interesting, maybe the spring in the air flow meter is weak on mine. Did you swap meters like for like or for one off something else? I do have another one but when I compared the resistance readings when moving the flap they read the same so never bothered swapping them. Inside the air flow meter on top of the arm there is a factory set screw which must not be touched, wonder what happens if you do? Anyone tried?
Fitted my other air flow meter and found it to have similar running characteristics as the original, so decided to go for it and have a play with the phillips screw in the top of the arm. This allows the contact on the arm to be moved quite a lot changing the behavior of the unit significantly. After a bit of on the fly adjustment it has enabled me to wind off the preload quite a bit to get the range of movement necessary to get more fuel at higher load, and not have too much at idle, so a big step in the right direction. Did nearly 100 miles in it yesterday and saw much more consistent readings throughout with afr readings mostly in the 13 to low 14 range. Had a couple of spikes on acceleration of about 17 which may indicate an issue but with some fine tuning will be in a better place. Ultimately I would like to cure the vacuum deficiency which I still think is caused by the cam, but it drives nicely now and I have always wanted a abf conversion which would obviously change things entirely.
how does lower manifold vacuum affect ignition advance ? less advance in low to mid load ? are you using mk2 distributor with centrifugal and vacuum advance ?
I still don't understand how the number gets up during acceleration. That is exactly opposite to what it should be. But it looks like turning the screw is getting you in the right direction.
Hi 2swe, no vacuum advance as it is running the standard pb digifant injection system so the ignition timing is controlled by the ecu. I have followed the exact procedure for setting this so should be OK. I used the agg engine distributor body with the internals from the pb unit for the electronics, to get the right sized body and the 4 window hall sensor target. There is a definite swell in power at about 3.5k rpm which keeps on until about 5.5k which I think is the cam doing its thing.
Hi daNpy, when testing with a vacuum gauge and fuel pressure gauge connected simultaneously it was reading 0.3 bar vac and 2.9 bar fuel pressure at idle, this continued up to around 1900 rpm ( no load, vehicle staionary) where the vac increased to 0.7 bar causing the fuel pressure regulator to kick in dropping the pressure to 2.5 bar. As the rpm was increased above this the vac dropped as would be expected as the throttle flap was more open causing the manifold vac to drop as would be expected. So I think the afr going higher is caused by the fuel pressure dropping at a certain point. The pb digifant management has no way of knowing manifold vacuum or lambda values so just opens te injectors for a certain time based on what it thinks should be happening which is why I think I'm getting the odd readings. Messing around with the air flow meter is a crude but free way of trying to get around the problem. I suppose another possibility would be to try with the fuel pressure regulator disconnected as at least it would be constant rather than being up down up?
Yes, try it with the pressure regulator disconnected. If I'm not mistaken, the fuel regulator will return fuel to the tank above 3 (?) bar anyway. That will then be constant. And logically thinking, at acceleration you need more fuel, so when the vacuum rises to zero (throttle open/ no vac) the pressure regulator should not be dropping the fuel pressure.
I have disconnected the vac line to the fuel pressure regulator and blanked off the port on the inlet manifold, and will take the car out for some testing later in the week. Any opinions on what target air fuel ratios to aim for? The car is used on the road only, for fun trips out and some back road blasting so I would like to keep it civilised but make the best of it. I believe that slightly over 14.7 is OK for warm idle and light throttle for fuel economy, 13 ish for acceleration and 12 ish is good for WOT for maximum power, but happy to try anything under sound advice.
I always thought 14,7 was ideal but that's the theory. I was told when tuning the Golf 14,3 is more realistic/ practical otherwise it runs too lean. During acceleration it should go down and to know if it's too lean or rich you should read the AFR in the exhaust, not the input afaik.
