What do you guys think...this is the story...my intake gasket failed, the water jacket by #5 was leaking. I replaced the gasket and All leaks are gone. But I have a very ruff idle, then smooth after 1200 rpms. All the plugs are perfect except #5. It's black and dry, so I don't think it's a injector problem. I think the water (steam) from the leak damaged the valve...I'm Not doing the heads again (I built the motor 5 years ago). If I'm correct, will a hotter plug help, maybe a different gap...

 

Change plug for any option IT'S CHEAP ENOUGH!
AND use another different plug wire.
Cheap testing equipment.

 

Definitely check the spark going to blackened plug. A new wire JUST for that cylinder, as well as a brand new plug - exactly like the rest of the plugs and gapped the same size - for starters until you diagnose what the problem is. if it still gets black, check the dizzy and cap next. Either that cylinder is not getting the spark correctly, or there is something wrong in the air/fuel delivery or compression. I had a similar problem once and it was a collapsed lifter (valves not closing all the way, so lack of compression was not firing the mixture properly). Hopefully your solution will be easier than mine was.....

 

Let me add to this...I checked the plugs a month ago and noticed the same problem (only #5 was black). I cleaned them and put them back in. This weekend I did a tune up, wires, plugs, cap, & rotor. The #5 was still black. I also did a compression test and all is good there. The car still has a bad idle, so I'm sure that #5 will be black next time I check...Everything points to the valve...

 

Just for grins, you might swap the injector connector between #5 and #7 just to eliminate the injectir harness.

Since you've changed all the wires and cap, that's probably not the problem. However, I'd still put a NEW plug in #5 since all you did was clean them. You could have a bad plug - I've seen ones that were bad out of the box. Could have a fine insulator crack, or an internal problem.

While it's probably excessive, I usually check the resistance on every plug while I'm gapping them before putting them in. I typically see one plug out of every 15 or 20 plugs that has zero or much lower risistance than the rest of the plugs.

I'd replace that the plug just to play it safe if it were me.

 

i've had the problem, and it was fixed with a new set of plug wires. that is a good i dea. i didn't think to check the plugs before install. thanks

 

With the plug being 'It's black and dry', I'd think is was a leaky injector. If the new plug wire doesn't correct the problem, it may be time to look at the injectors.

What year/engine is the car?

RBob.

 

i agree with rbob. there is a problem with there eithor not being a hot enough spark due to too much fuel(leaky injector) or the spark isn't getting hot enough to burn the fuel properly. eithor way it is making a rich condition in only that one cylinder.

 

Not to side track any one, but what would cause the exact opposit problem? My #5 plug all ways runs to lean. It's been doing it for years, and haven't figured it out yet.

 

bad injector, also if the car was sittin for a while with the gas in there, you may have some lacquer build up in the fuelrail around #5 injector.

 

OK, maybe your right and it is a leaking injector. How can I tell? Do I pull them out (leave them in the fuel block) and see if one is leaking?

 

do a resistance check on them? send them off to get cleaned. ask around the board, maybe someone has a couple for a couple of bucks that they can send to you. if not, go to parts store and pick one up. i would bet that if one is bad, that it is the one from that cylinder.