Club 80-90 Forums

Hi All,
I'd be interested to know if anyone else has experimented with using Bosch air flow meters off other VW, or non VW vehicles, as mine is running quite happily on one from a 1.8 golf gti [mk 2 i think]
this was bought for a fiver on e-bay after my original one suffered a big backfire while running on L.P.G. [the ign. timing was out] which twisted the alloy flap inside, jamming it. i also got one from a scrapyard off a BMW320i which seems to work fine, but has a noticeably lighter spring tension on the flap to the golf one. neither have the same part no. as the damaged original on the black plastic top, but both seem to work just fine.
looking inside the top with the cap removed, its obvious that the spring resistance of the flap, and therefore the fuel/air ratio can be altered, but i decided to leave it alone incase i bugger it up! there's allways plenty of old BMWs, Audis etc, in the scrappers with these on, so it might be worth trying if anyone else has blown theirs up. i keep a couple spare now ready for the next big bang, but since getting the timing and air leaks etc. sorted i haven't had another one cheers all, wasserleaker

Cheers for sharing this, I also have experienced the "Big BAng" and I carry a spare airflow meter. Top tip cheers.

Someone i know with an EFI range rover v8 which has a bosch airflow meter on it, and L.P.G. has a solenoid operated gizmo fitted which opens the airflow meter's flap totally open when its switched on to gas, which would do away with damaging the A.F.M. in the event of a bang, i suppose this makes sense as the flap isn't actually needed when using gas cos the gas demand is sorted out in the vaporiser according to how much vacuum the engine is producing, the airflow meter only sorts out the fuel to air mix while running on petrol, then again i spose in the event of a big backfire, the airflow meter would be safe, but the airfilter housing would cop the blast and be blown open! guess it pays to find out the cause of the backfiring in the 1st place and cure it. handy to know that other A.F meters will work tho, as T25 breakers prob want daft money for one.

I suspect the Citroen BX, Peugeot 205, 405 GTi/Mi16/STi/SRi/16v range will have the same Jetronic airflow meter.

Wasserleaker, where abouts do you live in Derby?

I've had a bit of involvement in a Passat LPG conversion that has a non-return valve in the air intake ahead of the AFM to stop this backfiring wrecking the AFM.

hi Ian, its ilkeston where i live mate,
the passat L.P.G conv. sounds interesting. one thing i've noticed with T25 injection engines coverted to L.P.G. is that by 'sandwiching' the gas mixer plate, if thats the right term for it, between the air filter box and the air flow meter. you end up slightly squashing the rubber 's-bend' connecting hose, if you know the thing i mean, between the A.F.M. and the throttle butterfly housing, although in reality, i dont think this affects performance. adding yet another gadget, like an anti-blowback valve in there as well tho would probably start squashing the 's' bend rubber even further, enuff to make a difference i reckon. could be wrong tho
i've experimented with running on petrol, with and without the gas mixer plate in place, to see if performance was different as i was convinced that the reduced hole size in the alloy mixer block would be restricting the engine when running on petrol, but couldn't feel any real difference , so although it looks as tho a lot less air could flow thru, it doesn't seem t'be the case.

I don't know for sure but i have the feeling that fitting a non standard AFM wouldn't be the best idea, especially if it's got a different spring tension. The idea behind the flap is that the more airflow going into the engine the more the flap moves. As the flap moves the resistance on the electrical connection changes. The ECU is programmed to be able to calculate the amount of air going into the engine by looking at the resistance across the connection. It then adjusts the amount of fuel that's injected into the cylinder.

By fitting an AFM with a different spring tension or a different range of resistances to the engine the ECU could well be injecting too much or too little fuel which is not so good for the engine.

It could be that all BOSCH systems use the same map of resistances or that all AFM's have the same air flow to resistance relationship but without knowing for sure i'd be nervous about running a non standard one....

Dave

Appreciate the reply Dave,
I'm wondering if its possible to adjust the spring tension of a non std. airflow meter so that it matches the fueling of a dedicated 2.1 T25 one, as i mentioned above, the tension is adjustable, with the square black plastic top removed.
however as you say too much or too little fuel to air is not good for the engine! as soon as mine is warm enuff to switch onto L.P.G. i switch over, but maybe C.O. emissions tests at various spring tension adjustments would indicate if the fueling [on petrol] was about right, or perhaps 'reading' the sparkplugs colour.
i should have noted what position was standard on my knackered one before pulling it to bits, and adjusting the golf or bmw one to the same tension, the numbers on the toothed adjuster are all marked '500' with 6 poss gaps inbetween. any one able to shed anylight on any of this? ? ?
testing, comparing the spring pressure by poking it with ya finger is probably not a very accurate method! cheers all, wasserleaker

You can anti blow back vavles that you put onto or into your air box. I had one on my old Audi that was gased up. You just cut a hole in the airbox about 2 inches in diameter and screw it in. It has a rubber seal round the edge and is spring loaded so that it won't suck any extra air in, but if there is a back fire it opens out to release the pressure. Can't find a picture though.

Cheers, Dubstar,
Sounds a worthwhile addition that type of gizmo does.

sounds like it might be the solution to the only real problem with gas conversions

It could be that all BOSCH systems use the same map of resistances or that all AFM's have the same air flow to resistance relationship but without knowing for sure i'd be nervous about running a non standard one....

I felt the same as Irish.David, that whilst an alternative would get you running on petrol, it's highlly unlikely they are all mapped the same, irrespective of the flap sping pressure.
In fact, the T25, MVs at least, have several different part nos. for the AFM, and pretty sure they tie up stictly with the ECU.
DJs may be more forgiving, but I'd still keep an eye out for the right part

Also, a CO or O2 check at idle is a very rudimentary check of mixture, telling you little about the part & full throttle mixture under load or acceleration richening the engine is getting.

You don't want it running lean at high load for instance, so take care when the LPG runs out.

I found a picture on this website. Near the bottom, £20.
http://www.lpg-kits.com/mixers.htm