MFI dropping a cylinder

[Strato 102]

I have a 73 S with 79,000 miles. It was commissioned after a six year storage with flush, filters, lines, etc. for the fuel system about 1500 miles ago. Lately, it has been dropping cylinder #6. Either on or off as if it were a spark problem. It is a fuel problem. If the car otherwise didn't start right up stone cold, warm up without a hiccup, rev smoothly to 5000 rpm and then pull like my son's M5 BMW and then hold a smooth 800 RPM idle, I would carefully wrap the pump in a blank check and send it out. Question is; is there any kind of witch's brew that I might try first to free up a plunger/check valve, etc. in what seems to be a healthy pump/set up? The injector is new. Thanks very much for any kind of input.

[RSTarga]

I'd try some techron or some Sta bil for a couple of tanks. Have you given a really good run? Leaving these things sit is really bad for the pump. They also really like to be wound out and driven hard. Use it or lose it as they say.

[Strato 102]

I hear your shout loud and clear. The PO's receipt shows it being done. It is a good idea to do it again to get a baseline/troubleshoot starting point. Thanks.

[304065]

Back in the day the factory recommended B-12 chemtool.

If, in addition to the shouted admonition above you have your fuel tank thoroughly cleaned, and I'm talking about sandblasting the inside and having it plastic coated and having the 8mm hose strainer replaced, and have the silver can filter on the MFI filter console changed, and verify both the flow and the pressure in the loop between the console and the pump per Check Measure Adjust, if you do all of those things, here's what I would do.

After letting the car sit for a day or so, use a disposable rag and hold it over the #6 injector line on the top of the pump. This is to catch the fuel that will inevitably spray out. Use a 17mm open end to remove the #6 line, SLOWLY. It would be a good idea to extinguish your omnipresent Monte Cristo #2 while performing this operation, because of the likelihood of causing a fireball. Seriously be careful.

Pour some B-12 down inside the #6 and let it sit for a couple days, reassemble and try it.

[Tom F.]

Why don't you swap the #6 injector with one from another cylinder, and see if the miss moves to that cylinder?

[edmayo]

In addition to Tom F's great advice, don't just rule out an ignition problem. That is more likely than a fuel problem. Check the whole #6 plug wire system from the contact in the cap to the end of the plug connector, then either replace #6 spark plug with a known good one, or swap it with #5 just like the injector advice. The liklihood of this being a pump problem, although possible, is very slim.

[Strato 102]

Why don't you swap the #6 injector with one from another cylinder, and see if the miss moves to that cylinder?

After being certain it's not a spark problem, I swapped it w/#4's injector. #6 is now running with a brand new one. I am not qualified to do a proper CMA. The most ominous thing lurking in my 67 Normale's fuel system is maybe having to line bore and re-sleeve for the shafts. I am really loving that car right now!

[304065]

What do you mean you aren't qualified for CMA?

Any competent Porsche driver tracks faster if brakes come smoothly.

Air filter, compression loss, plugs, dwell, timing, fuel pressure, injectors, belt, correlation, smog

http://forums.pelicanparts.com/porsc...es-thread.html

I hereby qualify you in the name of Robert Bosch. . . .

[Strato 102]

Thanks, guys, for the input. I've only had this car for a few months. Right now, my position is to be in awe of the MFI system and the people who know it. The car I sold to help buy this was a CIS T. Here is all you needed to know about that injection system; it was the same as the Volkswagon Rabbit's that were routinely driven 200,000 miles, junked, and then the junkyard guys would use them as a get around car in the yard. I guess it's time to start learning this system. I'll put some Berryman's in the pump outlet and let it sit. Then I run it with some Techron in the fuel. The tank will come out this winter. Again, most of the time this car runs superbly. Once in a while, it acts like someone is playing with the plug wire. It has new Bosch WR5 plugs and Beru wires (those are getting cheaply made!) and a very good ignition guy was checking it while the cylinder was down. I really appreciate everyone's knowledge and help.

[MountainRoads]

I sincerely hope this question doesn't display my ignorance, but how do you know it's a fuel *delivery* problem isolated to upstream of #6 injector, and only #6? An ignition problem limited to #6 or a bad injector I can understand, but how are you so sure it's only #6 injector that's not getting the right feed? Just curious, because these MFI systems are near and dear to my heart, also.

- MR