Hello,

Last time I drove I noted a slight misfire at idle, not under load. I did a visual inspection, and pulled one spark plug to take a look. That spark plug had cracked porcelain and the spark was obviously tracking up the plug. So I bought some new plugs.

Today, I pulled all of the plugs to replace them. I found one more plug that was missing the inner diode!! All of the other plugs were fine, with the exception of these two. I'm freaking out right now, because in all my years I've never seen a plug come out of an engine with a missing diode--where did it go?

Has anybody ever seen this? If so, what was the end result? Do you think this diode will get caught up in something and cause engine damage, or do you think it melted away and/or is sitting in my oil pan? Even though I've driven the car plenty without any driveability issues noted, I'm nervous to fire it up. HELP!!!

 

What brand of plugs?
Same cylinder?
Why are you changing plugs so often?

 

This is the first time I've replaced the plugs since the engine was rebuilt 3000 miles ago. I replaced them because I found one that had the porcelain cracked.

They are Bosch platinum plus plugs, which I used originally and had good luck with.

 

Very odd problem if I follow you...
Are you referring to the inner electrode (metal rod thingie surrounded by the white ceramic insulator)?
No diodes are in the plugs...
Can you post a clear closeup picture of the one with the missing piece?

Any liece that came off would either be in the cylender or hopfully expelled through the exhaust. If it is in the oil pan, then you have major trouble like a holed piston.

 

I don't see how in the world it could get past the piston into the oil pan. I would suspect that either:

1) It was ground down by the piston and the cyclinder - though you don't mention any noticable performance issue, discoloration of oil, or excess oil being burned out the tailpipe.

2) It somehow got stuck on the piston either by melting, or getting on stuck in carbon build up but the latter problem is unlikely if u just rebuilt

3) It dropped off the plug as u pulled it out and didn't notice it fell on the floor.

4) And finally what I think is highly unlikely and that is it blew past the valve into the exhaust.

5) Actually there is another unlikely scenario and that is the plug didn't have a diode when it was originally installed, but then I don't know how it would fire properly.

 

I'd be contacting Bosch - at least make them aware of it!
IS IT THE SAME CYLINDER??
A very lean situation in one cylinder could burn an electrode, but unless there's a massive vacuum leak, or plugged intake port, I can't think of any other reason for it!

 

Couldnt preignition or detonation cause this? My newly built 455 had detonation at part throttle until I adjusted my vac advance. I also had an AC Delco spak plug outer porcelain crack on me causing a miss. After getting rid of the ping it hasnt happened again.

I was also thinking about the piston hitting the plug. Are they the correct plugs? You did say its new rebuilt engine right?

 

Sorry, I meant the porcelain center electrode (spark travels between this and the ground electrode) and didn't mean oil pan, I meant cylinder. I had a lot racing through my mind when I typed that first post.

This engine was rebuilt just under 3000 miles ago, and I've never had any driveability issues. The misfire I noticed to even make me look at the plugs was really minimal, but it still bugged me. These plugs are correct for my engine, and have the proper reach. The ground electrode wasn't pushed down or anything, so a piston hitting it is unlikely.

This engine has always ran strong, even the day I was driving and noticed the slight misfire at idle. Once again, this was only at idle and not under load.

I hope the electrode either fell off on the ground when I pulled the plug out or else it melted into nothing inside my engine, causing no harm. Wishful thinking I guess.

 

How about some pictures so we can see exactly what happened and give more informed answers?

- Eric

 

Detonation or pinging will cause that part of the porcelain to break off. I would listen carefully for pinging while driving. Warm up the engine real good and go up a hill full throttle and listen closely for pinging. Could just be timing a little too advanced.

 

I have never had good experiences running Bosch plugs in an Oldsmobile engine. Never had one do what yours has, but I had plenty of cracked insulators and quickly burnt-away side electrodes with them before I decided they were unsuitable for Oldsmobiles.

Trouble is AC-Delco no longer really offers a "correct" plug for these engines, and NOS is drying up. I've had better luck with Autolites than modern ACD plugs. NGK are the best plug around IMHO, but often hard to find.

 

X2

I run NGK XR5 plugs gapped @ .035". on the street.

Same plugs when I race but that set of plugs I do some rounding off of the grounding bridge to shorten it up a little and to remove the sharp edges and I gap them @ .030" when spraying it with Nitrous.

 

I think your probably running to hot of a spark plug and it burnt away. You might consider a plug 1-2 heat ranges lower. If you inspect the rest of the plugs carefully you may find deterioration on them also!

 

I pulled the new plug out and looked inside with a flashlight. I could get a good view of the top of the piston, no marks noted. I cranked the engine a few times and found some of the piece came out, although it looks like it busted the electrode up pretty good in there. I tried a magnet, but it doesn't appear the electrode is magnetic like the ground electrode is. I ran a vacuum over the plug hole and hopefully got the rest...I'm assuming and hoping the rest blew of the small pieces blew out the exhaust and didn't get caught in a valve or damage my cylinder walls too much. I'm going to borrow a friend's boroscope and see if I can see anything notable.

On a side note, I typed "electrode broke off plug into engine" into google and found that 5 out of 6 posts were Bosch platinum plus plugs. Apparently people are having the same issue with these plugs. Some of these occurences were bone stock vehicles, and a few were performance vehicles. Note to self: use a different type of plug to prevent possible engine damage!!

 

Wow - very interesting... I have not heard many good things about Bosch anything, but this one could could ruin your engine!
A boroscope? Like a tiny camera? That would work very well.
Get rid of the rest of those bosches - take em back (if you can) and get a refund as they were defective.
Hopefully you will be able to get the rest of the broken pieces out. Maybe make an adapter of some sort (with a cap to something) to connect a piece of thin vacuum hose to the big hose of your shop vac to hopefully get in there and suck out any remaining debris.

 

A Borescope is a VERY useful tool.

http://en.wikipedia.org/wiki/Borescope

Expensive, but often much cheaper than a full teardown if you suspect an internal problem.

I've run Bosch plugs in my VW's for decades and never had a problem with any of them. Normal single pole style all the way up to dual, tripple, and quad platinums. Of course those engines came with them from the factory and were designed for them. I wouldn't run them in an old-school V8 at all. Champion, NGK, Autolite, etc. are all that I would consider using for that application.

I hope the borescope shows that you got all the rest of the electrode out. If not, try fishing the small pieces out with a coat hangar with some gaffers(black) or duct-tape(silver) wrapped around the end in reverse so the sticky side is out. Wrap it up several inches so that you don't put it all down the hole and chance losing that as well. You can leave a little ball of it at the end, but make sure it's a continuous piece, and the cloth-style tape, and not the cheap plastic stuff.

I had to do that once to get out a piece of debris that fell down a plug hole while cleaning an engine, and it worked well.

-Jeff

 

I stay away from champion plugs as they don't have a solid electrode. I like the motorcraft plugs and NGK's in performance motors, and I've had good luck with a/c delco in stockers.

I would still check to see if they were burnt up by having to high of a heat range in there!

 

God to know. I haven't messed with enough of the older engines lately to be in tune with this tidbit. A buddy's shop uses "Mighty" plugs for most of the older applications like this as well, and has been successfully for a few decades now.

I also know there's only a few actual spark plug manufacturers/factories on the planet, and a lot of the plugs are all made on the same lines with the name changed on them to suit the company that's marketing/selling them, so you can buy a ton of differently packaged ones, but they are all the same plug.

I still have a set of Champion "Gold" plugs sitting in the toolbox from my 66 425 that are waiting on a new engine to go in. They probably have less than 200 miles on them. Just waiting on a new hole to call home. Any clue if these would have the same issue with the electrode you mentioned above? They were purchased new in 1990 or thereabouts...

-Jeff

 

I had issues with Bosch plugs as well, not in American iron but everyday European cars, Fiats, Fords, & Peugeots in particular.
I replaced the old plugs with Bosch platinums and after a few hundred miles the customers were bringing their cars back because of starting issues. Once I figured out the cause I would only fit NGK or Champion plugs to a customers car.
It cost me a lot of goodwill from the customers despite my efforts to trace the customers with those damn Bosch insulating terminals for a free plug change.
I won't ever use them again.
Roger.

 

I've had similar issues in the OTHER direction with VW's. Especially the VR6 engine. Those were designed from the factory to run on the dual-plats, and putting anything in them besides those or the triples or quads will cause issues. For the standard 4-banger, I usually stick to the green "supers" and never had any problems.

Bottom line, try and stick to the closest thing you can find to the ones that it came from the factory with for any engine.

Where I see more problems than anything is when they first started using factory platinum 100K mile plugs in aluminum cylinder heads. The plugs may last that long, but good luck getting them out of an aluminum cylinder head after being in there that long. I don't know how many times I've seen one pull the threads right out of the hole. That's a miserable mess to get into.

-Jeff

 

Thanks for that info tidbit. The Caddy will not get a plug change by me...

 

Autolite AR 72 or AR73 are unbeatable in a BBO ( in my opinion). They cost a little more but they perform perfectly. Pre-gapped from factory too --- just screw em in and go! Always run the coolest heat range plug that your air/fuel mix will allow!! Your problem sounds to me like you are(or have) burned the electrodes off your plugs from detonation or too lean situation. With the lousey fuel of today, if you are running 87 gas, you are more than likely detonating under hard throttling and that will burn/blow those plug tips away in a hurry!