Thread: Backfire Protector Valves (and a couple of other questions)

RR-LPG 009.jpg

1. Impco model L gas converter
2. MAF or mass air flow meter. Controls petrol mixtures and is original car part.
3. Impco model 200 mixer (or generic copy) also known as cork-in-the-air-intake, AKA boston strangler
Rubber bulb at back of engine: cruise control actuator

One tag appears to be on the engine fumes breather. Can't read all the tags.

When you click on the pics you get bigger versions. Next time do the tags with black texta. You can do up to 5 attached pics per post. Use the "manage attachment" button below your post window.

Easily seen in the 3rd photo:

#6 is the big casting (with "3.9" embossed) on top of the motor bridging the heads so it has to be the intake manifold for air?

#5 is an electrical device just before the inlet throat of the casting: oxygen sensor? air temp sensor?

So! The converter is a heat exchanger to convert fuel from liquid to gas.

MAF is self explanatory.

The mixer is thus the de-facto LPG carburettor?

GEEZ! With such a big volume of air/LPG always in the system, no wonder they pop-off with extreme prejudice! No surprise injected LPG systems are getting popular?

I thought there was some sort of an interface between the two systems ie separated by some sort of diaphragm?

So, where is the damage done during backfires?

OK I can get that a backfire will inactivate the LPG supply but why does it conk out on petrol as well?

No.6 is then the plenum chamber
No.5 is probably the throttle position sensor (TPS) and has a round 3 wire plug.
Interface? Diaphragm? To run on gas you disconnect the injectors using a relay or two. They will be cut into the main wiring loom somewhere between the engine and the ECU inside the vehicle.

If you remove the mixer, unscrew the 5 screws holding the round cover on, you will see a diaphragm and spring. They are what vary the gas flow relative to the air flow. If the mixer is jammed shut no air will enter the engine. The cover can be warped and not seal the diaphragm, preventing it from rising under vacuum generated by throttle opening and allowing air to pass through on either fuel. If the mixer has shattered/jumped off no air will be metered by the MAF and the engine will not run well.

Do get yourself a workshop manual, click the "shop" tag at the top of the page and order a CD manual called "RAVE" suitable for your vehicle.

Originally Posted by geodon

Easily seen in the 3rd photo:

#6 is the big casting (with "3.9" embossed) on top of the motor bridging the heads so it has to be the intake manifold for air?

#5 is an electrical device just before the inlet throat of the casting: oxygen sensor? air temp sensor?

So! The converter is a heat exchanger to convert fuel from liquid to gas.

MAF is self explanatory.

The mixer is thus the de-facto LPG carburettor?

GEEZ! With such a big volume of air/LPG always in the system, no wonder they pop-off with extreme prejudice! No surprise injected LPG systems are getting popular?

I thought there was some sort of an interface between the two systems ie separated by some sort of diaphragm?

So, where is the damage done during backfires?

1st exam showed that the hose bet the MAF & the mixer has been blown off at the mixer end. Easy.

The MAF was wobbly- the mount was a typical crude LPG installer thingy so I tightened it up a bit after detaching the 5 pin plug.

It ran well then I realised I forgot the plug & when I connected it, it was just as crappy as before.

What does denying the MAF power mean? Does it have a failsafe setting that over rides the problem?

Are they fixable? I notice there are MAF cleaners available. Is it possible the backfire dislodged some debris & affected the sensors?

MAF sensors fail for all sorts of reasons, and there's nothing to lose by cleaning one. Use care, MAF or contact cleaner, no harsh solvents, no brute force air jets.

Also a MAF disconnected enables the ECU's "limp home" mode and it will run badly when you reconnect while running. Switch off the engine and only restart with the MAF plugged in.

Dismantled the MAF unit to gain access to the bypass with the sensor wires.

Gave them a good blast with CRC MAF Sensor cleaner ( 25 BUCKS for what I'm certain is nothing but Iso-Propyl Alcohol!?).

Fired up & goes a treat on petrol but "not a sausage" on LPG.

Time to get intimate with an Impco 200 series mixer, I guess. I got a good download from Impco with an exploded diagram & a parts list.

Am I safe in assuming there are all sorts of gizmos that stop the stuff coming out when I undo fuel lines under the bonnet? I use golf tees to plug petrol lines when they start to flow but I don't think that's gonna cut it for LPG, they're too damn big.