P0305 - Cylinder 5 Misfire

I have a '98 GTP with 190,000 km (120,000 miles). I changed the plugs at 160,000 km (100,000 miles) and wires shortly thereafter (both within the last 9 months). I'm now getting a P0305 error code and poor driveability. It just happened as I was driving home, prompting me to do the scan.

AC Delco wires were installed, as they seem to be the way to go.

Whatdya think? Maybe swap plug wires at both ends and see if the misfire moves to another cylinder?

Greg

What brand and heat range spark plugs are you using? Have you switched the #5 spark plug?

Plug wire swap sounds good since #5 is easy to get at. If that doesn't work swap out the coil packs with one another for troubleshooting. Good luck!

Welcome to AF.

I used OEM plugs from GM, so assume original equipment/heat. I haven't touched #5 since the whole set of 6 plugs went in. I'll start with the wires. Thanks!

Greg

For those who are interested, I pulled plug #5 and noticed plenty of carbon tracking down the sides of the porcelain insulator (maybe some oil and junk got in there to initiate the tracking?). I cleaned it up with some sandpaper and the problem seems to have gone away.

I can tell my wife now that my code reader has paid for itself :-)

Certain AC Delco Platinum plugs were defective. Which specific AC Delco spark plugs do you have? The ceramic will have what appears to be a hairline crack with carbon tracks. You may see the problem reoccur.

I have AC Delco plugs manufactured by NGK. Two sets of numbers are on the plug - 12568387 and ITR4A 15.

You're correct, the problem did occur again and the appearance on the plug is as you've described. I installed a new plug and after a quick trip to the grocery store, it seems o.k. again.

I spend a small fortune on those plugs and to have them defective sucks! If I gotta pull the ones from the rear bank, I'll be just a bit P.O.'d

Thats interesting. Did you get then from a GM Dealer, AC Delco authorized parts or regular auto parts store?

Do these NGK plugs have the AC Delco logo on them (at ceramic)? Any other numbers on the ceramic like 41-101? NGK did in fact make spark plugs for GM/AC Delco.

The OEM AC Delco plugs for a 98 GTP should be the AC Delco Platinum 41-921, Part # 25320671 but they are hard to come by. The AC Delco spark plug now recommended for use is the AC Delco 41-101 Iridium. The recommended Platinum is the #14 Rapidfires but I wouldn't use them because they are subject to the hairline cracks/carbon tracking. Out of 6, I had 4 bad ones in the past.

Other suitable alternative spark plugs for stock is:

Autolite Platinum AP606 or
Autolite Copper Core 606 or 605(one colder).

Also, make sure they are gapped to .060. And whatever you do do not use any type of Bosch spark plugs. You'll get similar misfire problems or they don't last long.

A lot of us with modded GTPs use the Autolites but with a colder heat range. They are a lot cheaper but effective and available at places like Walmart, if you happen to have any up North. Good luck!

I got the spark plugs from a GM dealer. The only numbers on the plug were the ones I mentioned earlier. I'm in the Great White North, so maybe there's some difference in sourcing.

However....I drove around for about an hour and the problem occured again on the same cylinder. I've noticed that there's been a bit of oil dropping from the car for about a month. I just figured that perhaps I didn't tighten the oil filter enough after the last oil change. Was also wondering why the car smelled so bad; I'm thinking it must be oil dropping from the valve cover onto the exhaust manifold. When I pulled the wire from the plug, I noticed that there was oil on it (the wire shroud). Perhaps this is wikking onto the plug and causing the problem?

The saga continues...

BTW, is the valve cover gasket easy to replace?

I got the spark plugs from a GM dealer. The only numbers on the plug were the ones I mentioned earlier. I'm in the Great White North, so maybe there's some difference in sourcing.

However....I drove around for about an hour and the problem occured again on the same cylinder. I've noticed that there's been a bit of oil dropping from the car for about a month. I just figured that perhaps I didn't tighten the oil filter enough after the last oil change. Was also wondering why the car smelled so bad; I'm thinking it must be oil dropping from the valve cover onto the exhaust manifold. When I pulled the wire from the plug, I noticed that there was oil on it (the wire shroud). Perhaps this is wikking onto the plug and causing the problem?

The saga continues...

BTW, is the valve cover gasket easy to replace?

I've had oil leaking from the valve covers gaskets and its never caused any misfire. The cables are made to prevent oil or water from getting in. The oil leaks are common on valve cover gaskets.

The valve cover gasket replacement is a Do It Yourself (DIY) job but not exactly easy. Note the right hand side (passenger) where the coil packs and wishbone engine mount is located. All of that has to be removed to get to the right side valve cover bolts. The rear valve cover is even more tedious.

I swapped the plug wires from #5 to #3 and the misfire moved to #3 cylinder, so it has to be the wire. I replaced the wire this morning and everything seems o.k. again. I'm thinking that all the arcing on the old plug damaged the wire.

In any case, Hot Rodder, thanks for the tips. Now on to that valve cover gasket...