Vacuum leak at air flow sensor plate?

[grantsmells]

Hi everybody!
I tried doing a vacuum leak test on my RoW 1982 SC with a vacuum cleaner on reverse, stuck into my pop off valve, and i can feel a considerable amount of air coming out near/past my AFS plate. I'm no expert but i don't think air should be coming out here, am i right? and what could my problem be?

The history:
Since rebuilding my leaking fuel distributor, the car now starts for a few seconds but will refuse to continue running.
It started and ran fine before i removed the FD (except for the 2 cylinders fuel was leaking into). I have tested the FD and injectors and they are spraying fuel fine when the AFS plate is lifted, the fuel pump and relay both seem to be working as they should, and when i start the car with the AFS plate lifted a little, it will start and run for a little while, which made me suspect a vacuum leak.

Thanks in advance for any help that anyone can give me!
Grant.

[T77911S]

no you are no expert, sorry, could not resist. air does "leak" past the plate. there is nothing wrong.
the other thing that cuaght my eye was, "it started and ran fine before i removed the FD".
who rebuilt the FD?
fuel coming out injectors when it is not suppose to is a sign that the injectors are bad. then look at the FD if it still does it.
when you lifted the sensor plate and started it, does it keep running?
2 things, since the FD has been "rebuilt" and you are lifting the sensor plate to keep it running it sounds like you are too lean. yes could be an air leak too. also could just be that the fuel flow on the FD has changed with the rebuild.

you need to:
check air leaks. make sure all the intake bolts are tight, vac lines are good and the rubber boots on the runners are not leaking. (dont know what all you took off). injector O rings and sleeves are another place to check.

check fuel pressures cold and warm
replace plugs, cap and rotor. wires if bad
set timing
set mixture.

yes in that order.

[boyt911sc]

Originally Posted by grantsmells

Hi everybody!
I tried doing a vacuum leak test on my RoW 1982 SC with a vacuum cleaner on reverse, stuck into my pop off valve, and i can feel a considerable amount of air coming out near/past my AFS plate. I'm no expert but i don't think air should be coming out here, am i right? and what could my problem be?

The history:
Since rebuilding my leaking fuel distributor, the car now starts for a few seconds but will refuse to continue running.
It started and ran fine before i removed the FD (except for the 2 cylinders fuel was leaking into). I have tested the FD and injectors and they are spraying fuel fine when the AFS plate is lifted, the fuel pump and relay both seem to be working as they should, and when i start the car with the AFS plate lifted a little, it will start and run for a little while, which made me suspect a vacuum leak.

Thanks in advance for any help that anyone can give me!
Grant.

Grant,

You are not doing it right in the first place. The metering unit should be isolated for this type of test. As you have experienced, the metering unit specifically the air flow sensor plate has no capability to resist the air pressure from your vacuum pump (reverse).

A simple test to check the condition of your FD (fuel distributor) is to test run the FP and all six (6) fuel injectors removed. Place the ends of the fuel lines into suitable receivers or containers or bottles. The fuel lines less fuel injectors should not deliver any fuel at all unless the FD plunger is lifted up. After you released the FD plunger, it should slide down by its own weight.

You need to test your fuel pressure too. What problems were you having for rebuilding the FD? Keep us posted.

Tony

[grantsmells]

Originally Posted by T77911S

no you are no expert, sorry, could not resist. air does "leak" past the plate. there is nothing wrong.

Just to be clear, I am saying that with positive pressure in my air box, air is getting out backwards past the closed AFS plate, not simply lifting the plate to travel into the engine.
Could you explain in lamens terms why this happens? I thought it would need a decent seal in order to have enough vacuum to lift the plate when the engine is turning over. In all the reading up on vacuum leak tests people have done, nobody has mentioned the air coming out here. I imagine if I did a smoke test, so much smoke would come out here that I wouldn't be able to spot small areas of smoke elsewhere if I have any other leaks. Or am I wrong about that too?

I very studiously and carefully rebuilt the FD myself, it was clean inside, no blockages, just had to replace the 6 o-rings around the centre peice as they had hardened somewhat.
Being that the FD wasn't dirty or blocked up when I opened it, and I didn't touch the metering screws on the FD at all, could my mixture still have changed?
I already have new plugs and cap, not rotor yet but will as soon as I can find one. Tested wires and they seem fine.

Originally Posted by boyt911sc

You are not doing it right in the first place. The metering unit should be isolated for this type of test. As you have experienced, the metering unit specifically the air flow sensor plate has no capability to resist the air pressure from your vacuum pump (reverse).

I'm not sure if I understand this, sorry! Should I put the airbox lid and filter back on and pump the air/smoke into that instead of into the pop off valve?

Before rebuilding, the FD was leaking fuel into cylinders #4&5 causing them not to fire.

I did this flow test, and swapped the injectors around which ruled out the injectors, and isolated the leak to fuel lines #4 and #5. Since rebuilding, I did the flow test again, both without and with injectors attatched, and had no drips with the plate down, and equal metering of fuel in all 6 cylinders.
I am trying to get hold of a CIS pressure tester for a reasonable price locally but am not having much luck.
Also, fuel pump is definitely coming on when plate is lifted (ignition on), so that's not my problem.

[ossiblue]

Originally Posted by grantsmells

Just to be clear, I am saying that with positive pressure in my air box, air is getting out backwards past the closed AFS plate, not simply lifting the plate to travel into the engine.
Could you explain in lamens terms why this happens? I thought it would need a decent seal in order to have enough vacuum to lift the plate when the engine is turning over. In all the reading up on vacuum leak tests people have done, nobody has mentioned the air coming out here. I imagine if I did a smoke test, so much smoke would come out here that I wouldn't be able to spot small areas of smoke elsewhere if I have any other leaks. Or am I wrong about that too?

I very studiously and carefully rebuilt the FD myself, it was clean inside, no blockages, just had to replace the 6 o-rings around the centre peice as they had hardened somewhat.
Being that the FD wasn't dirty or blocked up when I opened it, and I didn't touch the metering screws on the FD at all, could my mixture still have changed?
I already have new plugs and cap, not rotor yet but will as soon as I can find one. Tested wires and they seem fine.


I'm not sure if I understand this, sorry! Should I put the airbox lid and filter back on and pump the air/smoke into that instead of into the pop off valve?

Before rebuilding, the FD was leaking fuel into cylinders #4&5 causing them not to fire.

I did this flow test, and swapped the injectors around which ruled out the injectors, and isolated the leak to fuel lines #4 and #5. Since rebuilding, I did the flow test again, both without and with injectors attatched, and had no drips with the plate down, and equal metering of fuel in all 6 cylinders.
I am trying to get hold of a CIS pressure tester for a reasonable price locally but am not having much luck.
Also, fuel pump is definitely coming on when plate is lifted (ignition on), so that's not my problem.

The area under the pop-off valve is an open chamber that accesses the intake runners and the throttle body. By pressurizing through the pop-off valve, air is forced through the runners and back up through the partially open butterfly valve in the throttle body. From there, the air passes through the rubber boot to the chamber housing the plunger and, since the plunger disc does not form an air-tight seal, out through the opening where the air filter would be.

If you are testing all areas of possible air leaks, including the air box, you need to close up the air box and connect your pressure hose to the air horn. If you want to test the pop-off valve, intake runners and all hoses downstream, you need to remove the rubber boot and put your hose directly over the throttle body.

[grantsmells]

Aha! I see, thank you. I thought the air plate would make a seal, as my theory was that a vacuum leak would mean there wasn't enough suck to open the air plate upon starting it up. But now I'm doubting that theory.

[grantsmells]

turns out it was nothing to do with vacuum.
i went against contrary opinion and adjusted my air/fuel ratio and that did the trick. turns out when i put the fuel distributor back together, the centre bit that houses the piston was slightly further up into the FD than when i took it out, and with the piston remaining in the same place, it was lower in comparison, therefore not delivering any fuel at idle, a few turns of the air/fuel screw and it starts as good as gold! i don't have any fancy oxygen measuring tools, so will have to take it to a shop to get my air/fuel ratio to the optimal level.