A few weeks ago my car began misfiring. It came on suddenly, but not accompanied by any unusual sounds like bangs, clangs, clunks, thunks, thuds, whines, pings, or pops. And no blue smoke or extra oil leaks. I had been out on a spirited drive, stopped at the top of a local canyon and talked with some fellow enthusiasts for about a half hour, started it up (oil temp still warm) and proceeded downhill for home. I was using a lot more engine braking than normal- I normally coast down in 3rd but this day was giving it throttle/engine braking in 2nd. Everything seemed normal until the bottom of the hill, coming out of the last tight turn I got on the power and the car just sounded "off song". There was more vibration, but nothing violent, and the tone of the engine was more bass. I thought maybe an exhaust stud had cracked but everything looks fine on the engine. On the way home it sounded like an Audi 5 cylinder, though.
So I thought then it was ignition related. I put my hand over the velocity stacks one by one and #3 and #6 didn't seem to cause any change in idle speed. I pulled #3 plug and it looked pretty good. But the plugs and wires were replaced anyway, during a valve adjustment and oil change service. The leakdown numbers were all good (3% to 5%) so I don't think it is a broken piston ring and no broken rockers or springs were reported during the valve adjustment (to my relief). The #3 plug connector was reported to be bad, however.
On my first drive after the service the car seemed to run smoother, but still the engine note was a little more bass than it used to be. The next drive my dad took it out and it started shaking more and running rough like it did just before the service. I thought maybe it was fouled plugs (he doesn't use as many revs as I do...) but after an attempted Italian tune up nothing changed, and it never fouled plugs before.
So since I feel (equal) vacuum from all the velocity stacks, leakdown is good, plugs and wires are new, distributor rotor and cap look good, I am thinking it may be fuel related. The only thing fuel related I would consider a possibility is a clogged injector. The car runs 91 octane with 10% Ethanol and sits for weeks at a time. So maybe some gunk/rust got through the tight tolerances of the MFI pump and is partially clogging the #3 and #6 injectors? The fuel console filter was changed during the service, not the tank screen though.
So, what say ye? Engine is an RS spec on a '74 7R case, plastic stacks, 019 MFI pump, cold start squirters working, overrun fuel cutoff switch working, thermostat blocked off (due to headers) and set lean for summer running.
I see Mark Jung charges $20 per injector for cleaning and testing, which is a bargain compared to the >$150 per injector from Pelican.
Edit: I strayed and did not take it to TLG for the above mentioned servicing. I went closer to home to another pretty highly recommended mechanic so I wouldn't need to drive so far on the misfire.
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Originally Posted by gofstr
