Rough idle when secondary distributor disconnected - SOLVED
 

SOLVED - see thread post #12

When I disconnect one of the two distributors, I am assuming that the engine should still idle and run smoothly. Please correct me if there's more to it than that.

When both distributors are connected, all is good with the exception of an occasional miss. Usually this happens when I don't take 1st gear up to 3K rpm before shifting to 2nd. Not a critical issue, the engine doesn't die, it's just a miss. But I wanted to see if I can diagnose what is going on. This has been infrequently occuring since I bought the car in December, 2011. Both distributor caps and rotors were replaced within the last 1K miles.

The engine idles smoothly when the primary distributor is disconnected and the engine is running on the secondary distributor only. I found this to be encouraging as the distributor belt must be working.

The engine idles rough and misses when the secondary distributor is disconnected and the engine is running on the primary distributor only.

I believe that I have ruled out the coil (and perhaps everything else upstream) as a suspect by observing the same behavior when I swap which coil I source the spark from.

What should I look at next?

Have you swapped the caps and rotors? What about the coil to cap wire? I would have suspected the coil first, but you seem to have eliminated that already....

I have found when disconnecting the one on the left -the primary things don'run as smoothly.. never had s reason to disconect the secondary on the right. Don't assume you have a problem.

Primary vs. Secondary
 

I haven't swapped caps/rotors, I am thinking about trying this. I did swap the coil to cap wire, so I believe I have eliminated that.

I may be mis-interpreting primary versus seconday. I thought that the primary distributor was below the cap on the right hand side as you are looking at the engine from the rear of the car. A different way to describe this is that I thought the primary was toward the 'passenger' side and the secondary was toward the 'driver' side. Have I got this wrong?

Have you checked your plug wires for electrical leaks in a dark garage? Check those behind the mufflers that go to the lower spark plugs.

Check the inside of your distributor caps for arc tracks between contacts. Clean them.

Ohm out your spark plug leads with the caps.

All wire caps on the distributor correctly inserted and in the proper location?

Alex - The 'I' and 'II' in the linked image, Do these refer to the top ('I') and bottom ('II') sets of spark plugs? And which distributor then is primary? I thought it was the distributor closest to the fan shroud, now I'm not so sure. I think I need to get this cleared up so that I am describing my diagnostic attempts correctly.

Yes.

It is.

The distributor with the shaft driven by the gear is the so-called "primary"; the one where the end goes into the engine. The other one is the "secondary" which is the one that is capped off on the underside.

..... there remains the possibility that there is one defective plug amongst the six off the primary distributor ....

One can usually pin it down by either pulling the wires from the cap sequentially on a running engine, or inserting a noid light. Also, if you have a timing light, connecting it to each wire will often reveal a misfire as you observe the Xenon bulb flash pattern.

Cap, rotor or plug leads. Most likely plug leads, we're having to replace lots in our workshop recently.

Plug wire(s) in wrong position on primary cap?

Misses / stumbles / pops are from left exhaust outlet
 

In order to minimize the amount of time laying on the garage floor...

All of the misses (pops, stumbles, backfires) are coming from the left tailpipe. You can feel a substantial pop of air along with the noise. I am pretty sure that the exhaust system is OEM stock.

Does this help narrow it down to the left bank of cylinders (1, 2, 3) as the pops are coming out of the left tailpipe?

Solved!
 

Thank you all for your tips and hints.

When I followed wires II-1, II-2, and II-3 from the primary distributor cap to where this set goes through the body to the bottom side of the engine, I noticed that the group of 3 wires was twisted 180 degrees. At that point I started to suspect that the number 1 and number 3 wires might be swapped. Thankfully, these two wires could be swapped at the primary distributor to test the theory.

Swapped II-1 and II-3 at the primary distributor and smooth running now from the primary distributor with the secondary distributor disconnedted!

The next time I have the car jacked up and the engine tray removed I will get everything returned to specification.

Interesting how the dual plug design masked the issue except for an occasional miss.

Thanks for a very interesting thread and glad to hear it was solved, but what's an engine tray?

The question now is, who installed them. ;)

I'm referring to the black plastic thing under the engine. It prevents oil from dripping on the garage floor and prevents me from seeing what the heck is really going on :). I suppose you have removed yours.

Indeed.

Since I bought the car in December, 2011, I haven't done anything in this area except put in new distributor caps and rotors. And I faithfully replaced the wires as they were previously put on the caps. And they were correct before I performed the temporary swap to address the routing issue.

There is a service note at 61K miles in November, 2009 that mentions a "slight stumble at idle". They performed a 60K service that included replacing the spark plugs. Funny that a 60K service was performed again at 68K miles in September, 2011 that also included replacing the spark plugs.

Maybe the fault occurred back in 2002 when the car had 44K miles on it. That's when the valve guides were replaced and a new clutch, pressure plate, and flywheel were installed.

I'll probably never know for sure.