Voodoo Sparkplugs, Pls Help!
 

I think I need a witch-doctor.

My 1978 911 SC with a de-smogged 3.0 ltr was running perfectly, roaring like a lion and sprinting like a cheetah. The minute I installed a fresh set of NGK spark plugs ( BPR5EY 1233) the car started misfiring and stuttering.

The car starts, idles, drives, and runs through the rev range, but feels like there's half a cylinder missing. The stuttering misfire is most pronounced under a load, such as normal acceleration in 3rd gear.

I pulled the NGK plugs and checked the gap -- .028 Fiddled with the gaps to feel like I did something and re-installed the plugs. No change, same stuttering misfire.

Reinstalled the Bosch plugs (W5CS RO171) that had been running like champions, and... misfire, exactly like the NGKs! What changed?

What did I screw up? The spark plugs are the ONLY thing I've touched. Snag a vacuum line? I can't find it.

I think it's voodoo. The inscrutable NGK plugs infected the Bosch plugs with rebellion.

Advice and help with the exorcism is very much appreciated.

Tx, piscator

Possibly got the ignition wires mixed up, in wrong order or loose connectors on ignition cables? Often ignition cables will work fine for many years and then will fracture internally during a spark plug change. Check very carefully that everything is connected correctly, including the cable ends on the wires. If everything is connected properly you may need new ignition wires.

You will need a exorcist or a long legged blonde your choice.

LOL

Check the cap and wires...

Luck

Got this exact results after I change the spark plug wires a couple years ago.

Turn out that while I though I had all the wire properly "seating" on the spark plugs, I had actually jam a few between the spack plug and the engine...

P.S. as per Targalid who is a much faster typer

Targalid & JJ, I'll check the connections again to see that they're seating correctly. I'm quite sure the order is right, but I'll double-check that too. If it's the wires themselves, it's very weird. I had planned to replace them anyway, but would never have expected a sudden failure from simply moving them aside to change the plugs.

Warner53, Yes, I'll need a long-legged, blond, exorcist to generate spark. ;-)

Thanks guys!

piscator

The wires snap on and off when inserted correctly onto the top of the plug. If you don't feel the snap then you likley don't have a metal to metal connection with your plug. It's easy to get it wrong. Make sure the plugs are torqued nice and tight as well.

good time to replace the cap and rotor as well (after your first issue is resolved).

Chris

Sometimes disturbing brittle old Beru ignition wires with a simple plug change can wreak such havoc with the ignition to where you wished you never touched them!

Do a continuity check of the old wires. Sometimes just moving them around can break/or cause a crack in the insulation. Look for the light show with engine running in the dark. Spray water mist on the wires while idling.

It is possible to get a bad plug.
I have had bad ones show up in boxes or sets before...it's a pain but it does happen.
Do your best to isolate the bad one...and in a pinch...put an old one back in to verify it.
Bob

did you get the correct heat range?
5 for bosch isnt the same as a 5 for ngk

Thanks for the help guys! It was a frustrating detour, but the problem is solved.

A fresh set of wires cured my SC's ignition stutter. I think it was a wire that pulled out of its connector at the distributor. It also seemed that some of the old connectors may have been loose at the plug end. And/or, I didn't seat one or more of the old connectors properly. As Targalid and JJ noted, those connectors are kind of a bear to snug down onto the plugs.

In truth, I'm not 100% sure what the problem was, but thankfully, it's gone!

In addition, I changed the plugs again, just to make sure I had the exact part that Bosch recommends -- "Super Plus" PN#7905. And for good measure, I installed a new distributor cap and rotor.

The result? My classic SC Targa has recovered from it's demonic possession and roaring like the naturally aspirated lion she is! Now if I could just track down that long legged exorcist...

Thanks much!

piscator

What type of plug wires did you use? I'm probably due soon as well.

Timmy2,

I used the Beru brand wires available from our host as Part #: 10-8533-613-M47 Nicely made, well priced and appear to be working just fine.

piscator