Ignition problem
 

Hi Guys
I recently purchased a 1978 911 Targa (first 911!!!) and i decided to change the oil and plugs etc. Its was running fine before the service until i changed the plugs and now it seems to be missing on one cylinder. i put the old plugs back in and i still have the same problem - can anyone help :confused:

First off, let me welcome you aboard. Second, where's the pic of your car?

You probably have either mixed up the firing order with the wires or one of the plug wires may not be fully seated on the plug.

Go over the firing order double checking that the correct wire is on the correct spark plug.

You could also have a bad spark plug cable.

Thanks for the welcome - i will try and get a photo posted soon - only if i can get it running!!!!

If wires are swapped you have at least 2 cylinder not firing. Check for loose plug wire, gap on plugs, loose plug.

Possibly have pulled a boot loose from the wire. Give it a close check on the wires and firing order. Did you change the rotor? Good luck!! Tony.

Pull the cables one by one and check the resistance.

I'll tell you how to do that after you ask 'how do I do that'....

When you push the Beru connector onto the plug, you want to feel and hear a click. That means it's on. A little Vaseline helps.
The SC distributor runs counter clock wise (CCW)
Check the firing order with each wire accordingly. 1-6-2-4-3-5
.
The Beru connectors can fail but are easily checked as follows with a Volt meter:
Long Beru: 3k OHMS
Short connector at the cap: 1k OHMS
Total resistance per complete wire end to end: 4k OHMS
Rotor: 1k OHMS for a standard, 5K OHMS for a rev limiter.

When you change the cap or rotor, make sure that the cap seats correctly in the notch.
The rotor also has a notch in the shaft to fit into.
Don't over-torque the plugs: About 20 ft-lbs. I use anti-seize on the threads.

Also, get a Bentley SC Repair Manual. Lots of good info!

Thanks for all your advice so far -

An update, we ordered new leads, dist cap and rotor arm and we fitted all these last night but are still having the same trouble, its not running right and missing.

If there is anything else you can think off i would really like to hear your ideas

Thanks Elaine a frustrated 911 Porche Owner!

At this point I am going to guess that you cracked a spark plug. I've done it before, and I spent a lot of time and money before my dealer finally solved it.

Solution: Buy one new plug and go through the engine changing the plugs one at a time. Put the new plug in cylinder one and run the engine, if it doesn't run any better put plug number 1 into cylinder number 2, and run the engine again, repeat until you have tried all six cylinders.

Make sure your getting a big fat blue spark at each plug boot. Pull the boot off the plug and ground it with a philips screwdriver and watch for the spark. If they're all nice...I'd bet you have the firing order incorrect.

A version of PBH's suggestion is to short out one cylinder at a time and observe any difference in the idle characteristics. This is sometimes referred to as a "cylinder drop" test or "dynamic compression" test.

With engine OFF, loosen the grip of the ignition wire boots on the distributor cap. You're going to physically remove each wire in turn with the engine running. Then, engine ON. Using insulating pliers or heavy rubber gloves and pliers, remove each wire in turn (engine runs alternately on 5 cylinders). If you feel lucky, do this with bare hands while standing in a puddle of water with no shoes. You get the idea.

On a normal-running engine, the rpms will drop as you remove the spark on the test cylinder. The cylinder that isn't affected is the weak cylinder and one to further diagnose.

It could be a bad/loose wire, bad or fouled spark plug, plugged injector, crack in dist. cap, and/or the other items listed in the previous posts up to and including no/little compression in that cylinder, but we won't go there now. But at least you've narrowed the cylinder search.

Hope this helps,
Sherwood