Bill Ball

I have a vexxing little problem I would like to throw out to the group.

Presenting Symptoms:
Regular miss only on idle after the engine warms up. No miss on acceleration. The miss is a minor irritant, as the car runs perfectly otherwise. It just completed a 1500 mile trip over 4 days last week that included 100 miles at and average of 144.6 MPH with long runs at 165- 170. With that kind of performance maybe I should ignore the miss. :-)

History:
Idle previously smooth with no miss.
Miss appeared immediately after SC installation.

Physical Findings:
- Regular miss traced to cylinder #7 once the engine warms up.
- When you come off idle the miss is not there for a few seconds, then appears and gets progressively more apparent and regular over 5-10 seconds.
- Pull #7 plug lead - no change in idle (same miss), but when rev car the miss is apparent. Put the plug wire back on results in same miss at idle, but the miss on acceleration disappears.
- Pull #7 injector lead - same result as pulling the plug wire lead.
- Compression good: Cyl #6=225, #7=225, #8=210
- Plug condition good. No difference between 6, 7 and 8. No wetness or fowling. Plugs and plug wires are new, installed after the miss appeared. Caps and rotors are recent but not new.
- Put a loose spark plug on the end of the plug wire to cylinder #7 and it fires fine on idle.
- Unplugged idle stabilizer and sprayed WD-40 everywhere around cylinder #7 and all around the manifold and any intake junctions. No evidence of a leak (doesn't mean there isn't one, but we could not find one).

Differential Diagnosis - all not highly likely
- Bad injector
- Bad plug
- Bad plug wire
- Bad coil
- Bad distributor cap
- Vacuum leaks near cylinder #7

Plan:
- Swap injector leads and see if the miss follows the leads.
- Swap coils, then caps and rotors and see if the miss follows the swapped items.
- Swap spark plugs.
- Carefully examine the intake area around cylinder #7 for leaks.

Any other ideas?

The first person who provides a suggestion that leads directly to the discovery of the cause of the problem or who successfully guesses the cause receives a very nice set of Porsche crest aluminum valve stem caps in thanks for their contribution to my sanity.

bd0nalds0n

Did you have to change to 30# injectors for the SC? Were they new? Even if you used the old injectors, they had to be swapped from the old manifold. Maybe hole 7 leaks or the injector was damaged. Why not swap the injector for another before you go tearing into the coils/caps/rotors.

Seth W

I suggest an exorcism.

Sorry, humor (bad as it is) is all I have to offer.

: )

bd0nalds0n

Although now that I think about it, to take off the fuel rails at the rear, you'd probably have to take off the blower, which is a bit time consuming. It does get much easier the more you do it.

justin

My guess is the injector harness.

Bill Ball

The SC can stay in place. You just have to remove the rear intake adapter to get to the fuel line fittings.

Bill Ball

A miss at idle only is peculiar. I've shown it to the heads of our religion and so far they are scratching their heads. Maybe the new Pope can help.

Bill Ball

Yeah, Brian, that is the order I had planned, although the fact is it is easier to switch the coils and caps than the injectors.

The 30 lb injectors were new. I find it odd that one would not work at idle but work well otherwise, but that is on my list. I tested the #7 injector for leaks extensively by spraying WD-40 all over and around it.

Bill Ball

Noted. If it is the harness, you get the valve stem caps!

Bill51sdr

My bet is a vacuum leak. If it were an injector, you would have the miss at all engine speeds. I'd start looking for a cracked or loose vacuum line.

ColinB

WD40 might not be the optimum leak detector. Try carb cleaner of quick start (ether)

CB

Bill Ball

Yeah, WD is like kerosene. I have propane too. I do plan further leak tests.

SharkSkin

Bill, it could be a sticky injector that is slow to open. Just pulling some numbers out of the air to help you visualize what I'm saying.... suppose the injector is activated for 5ms per cycle at idle, and 50ms per cycle at WOT. That's just when the coil is activated. Now suppose that the #7 injector is a bit sticky and takes 4ms to open. So, at idle it's actually open for 1ms/C, at WOT it's open for 46ms/C. You're not likely to notice a <10% difference at WOT, but it would sure make a difference at idle.

Garth S

Another vote for a 'misguided' injector .....

Thaddeus

I say sticky injector also.