MFI - found a weird problem.

[jkeyzer]

Still playing with MFI (it's a weekend routine now).

I was having a bad miss (really noticable at idle, thump thump thump thump) and traced it down to cylinder #5 (no rpm change when plug wire removed). I checked the plug and it was not fouled. Then I started playing with the idle screw on the throttle body. It turns out that if I open the screw so that #5 draws considerably MORE air than the rest of the stacks (using my unisyn), the miss all but goes away and the car idles a LOT better (around 1000 rpm).

What gives?

Why is the correlation wrong for this cylinder? Fuel injectors were tested ok, plug wires ok, plugs are all good, injection timing spot on as of this weekend... Is it time for a leakdown test? Compression numbers were all ok. Valves were adjusted a couple thousand miles ago.

I'm confused now.

[HawgRyder]

Sounds like an air leak in that cyl.
Check the base gasket for the MFI stack.
Also...make sure there is not a crack in that particular stack.
Bob

[jkeyzer]

I was thinking about that, but if air were leaking in below the throttle plate, wouldn't this mean I would have adjusted the air bleed screw for LESS airflow on my unisyn, not more as I found?

[john walker's workshop]

you want about 1000 RPM anyway. open all the screws the same amount to get there. maybe that alone will fix it. a bad plug connector or wire will cause a miss. you could swap injectors and see if the miss follows the injector, or stays put on #5. valve clearance ok?