I recently changed the distributor caps and rotors on my normally aspirated 993.

The engine idles quite well and drives very smoothly at slow speeds. Once accelerating, the engine misfires (misfires hard enough that it is easily felt in the car).

The distributor caps and rotors all look to be installed securely and are fitted correctly.

My question is: is it possible that I inadvertently switched a pair of spark plug wires?

If a pair of spark plug wires are switched, could the engine idle smoothly but misfire under acceleration at higher revs?

Thanks much.

 

Was the misfire there before the change?


Are all wire caps firmly pressed and snapped into the distributor caps?

A quick check will confirm proper location of wires.

 

Absolutely.

You likely have two wires swapped. Since we have twin ignition, a swapped wire can be masked because the 'other' plug handles the combustion for the cylinder.

To narrow it down, with the car off, remove one of the coil wires at one of the distributors. Place the end of the wire so it does not arc to something metal when the car is started. Then start the car and see how it idles.

The car should idle reasonably well on either distributor by itself. The lower left belt driven distributor cap sends spark to the top plugs. The upper right shaft driven distributor cap sends spark to the bottom plugs.

Good picture with labels - credit to IXLR8: http://edelweiss.smugmug.com/Cars/Po...4492&k=p4Q9Vrq

The picture and comparing with your installation should be sufficient to see what's amiss. There ignition wires should have labels on them, but they are often difficult to find.

EDIT: Yep, Alex has you covered. His response was not present when I started replying. Sorry for the redundancy.

 

Very nice diagram, Alex! You never disappoint

 

Those are great suggestions. Thanks for the help.

The misfire wasn't present prior to starting this project, so I am going to check the wires carefully. I think that is likely where I made a mistake.

 

I discovered after about a year of ownership that the plug wire harness that leads to the lower plugs on bank I was flipped over before it ran through the engine tin.

So although the wires were correct at the distributors, plug 1 and 3 wires on the bottom side of the engine were swapped.

https://rennlist.com/forums/993-foru...ed-solved.html

For me, finding and fixing this resolved an infrequent misfire when making a long left turn from a dead stop.

 

Just to follow up -- I had switched the plug wires for cylinders 3 and 5 on the bottom distributor.

Glad it was an easy fix.

Thanks for the help!