MFI assistance - one cylinder popping
 

I posted the other day that my early 911 smelled funny and I knew something was up. on the way home from work, I "lost" one of my cylinders. The car had a serious skip with continuous backfires out of the exhaust. This happened after it warmed up although it came and went. I drive it to my mechanics place (not an MFI guru but he is a skilled generalist) where we poked around. At idle, the offending cylinder pops into the intake with a loud metallic pop. We set the dwell and timing and cleaned off the blackened spark plugs - to no avail. Adjusting the various screws on the offending cylinder also do not make it better (only worse). I have new wires, plugs, rotor and cap and the points are adjusted properly. The compression was great a few weeks ago. The only thing that has happened recently is the carolina drive, where the car saw lots of 5500 rpm plus work. I developed a high speed miss on that trip but it was occasional. I have a rev-limiting rotor (6200 I think) and I have not overrevved the car since I bought it. The butterflies on all cylinders are making a terrible racket. I thought I had loose valves until I held the linkages steady and the noises went away so I'm thinking that a throttle body rebuild is in my future. How much noise from these is normal? In any case, the next step is to follow the 10 steps - a valve adjustment is next and then pulling and testing the injector. Since the problem is relating to heat, I'm leaning towards a tight valve. Any thoughts?

1.) swap plug wires on two cylinders and see if it moves. are all the plugs black?
2.) trya compression /leak down test to check the valves
3.) if your mechanic can test the injector and the spray is good, your pump may need some attention.



edit.....you could spray some cleaner/wd around the stacks to check for vac leaks, but i don't think you would get exhaust backfire with an intake leak unless the plug is skipping a few beats.
i replaced my worn linkages with old mb diesel links. (mucho cheaper)

Was the spark plug wet in the offending cylinder? If it was, then it is not firing and I would change the plug and check the wire leading to that plug. Also, since it is one cylinder, you might want to check the cap even though it is new and make sure it is not scored in that area. This will create arcing and create a miss.

If your stacks/throttle bodies are making a racket, maybe you have worn throttle bores that need rebuilding. You could have also gotten one stack out of sync. Have any adjustments been made to the rods between the stacks? If anyone has messed with these, you could have a problem with syncronization on that particular bank or between the two.

Hopefully, these questions will help you look more into the problem.

Thanks,

Marc

If the previously mentioned checks do not point to anything. Remove the side cover of the MFI pump associated with the bad cylinder, move the rack back and forth by pushing on it from the front of the pump and verify that the pump metering device for the cylinder in question is moving in unison with the others.

See if the throttle shaft of the offending cylinder can be rocked back and forth with finger pressure while the motor is running. See if it makes a difference. Air can be leaking around the throttle shaft, leaning out that cylinder.

Depending upon the year of the system, the throttle return spring placement help leads to the loose throttle shaft problem.

Good luck,
David Duffield

Hello,

I had a similar situation happen and here is what I found. One of the pistons in my MFI pump (cyl #2), actually broke and stopped moving up and down in the rack. The end result was a dry hole. I would suggest pulling the hard line off and putting it in a jar(slowly and safely and around no flame) to see if you are truly getting fuel.


JA

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