• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Spark Plug Query

Share this page

dklawson

dklawson

Yoda
Offline
I'm not sure which forum is best to post this in so I'll start here.

This is not thetypical “what spark plug is best for my car” question. This is a“have you ever measured” question. It is not specific for anymake or model of car. It is something that is bothering me.


I have a car that isgiving me some running problems so I cleaned the old spark plugs andthen decided to measure the resistance of all the high tensionignition parts. The resistor plug wires measured 500 to 1000 Ohmsdepending on length. The dizzy cap measured fine with no shortsbetween terminals. Then there were the spark plugs.


These are standardplugs (not resistor plugs) so I expected to measure Zero (0) Ohmsbetween the plug’s terminal and the center tip at the firing end. I only found Zero resistance on about 3 out of 10 plugs. (Once Istarted making measurements I brought out ALL my old spark plugs tomeasure). Apart from the handful of plugs that measured lowresistance, MOST of my used plugs measured several hundred thousandOhms or more.


All these plugs werepresumed working when removed. I have never had an engine startmisfiring due to a failed plug so apparently the resistance Imeasured does not affect the firing of these plugs… much.


So have you evermeasured the resistance of new and old spark plugs? Have you everseen a published value that says “if the resistance is above Xdon’t reuse the plug.”? Please share your thoughts andexperiences.


Thanks,
Doug L.
 
Probably not much help, but I have seen plugs that break internally.
I used to run Bosch Platinum plugs, until I had a rash of them where the center conductor would not only break internally (as shown by an ohmmeter), but slowly work its way out and close up the gap.
First time it happened, I assumed it was just one bad plug and replaced it with a new one. Next time, I found that all 4 were broken inside, and 3 of them showed the center electrode falling out (even though only one had stopped running). Replaced all 4 of those.

Next time it happened, I switched back to Champion and haven't had a problem since.

ISTR Champion had problems for awhile too, but I've forgotten what they were. Seem fine now, though.
 
I spent the evening in the garage after making my post. I figured out what's going on and it ties to your observations.

The electrode is fired in the ceramic, the ceramic is swaged into the threaded body, and finally the plug terminal is screwed and glued into the top of the ceramic. The top of the electrode is sized to just fit in a hole on the bottom of the plug terminal. The tolerance between the electrode and and the plug terminal has to be tight but lose enough to allow assembly.

I removed the plug terminal from four of my high-resistance plugs, did what I could to clean the bits, then placed a few strands of fine copper wire into the hole of the plug terminal, and screwed it back together. Measurements showed that the resistances had dropped from mega-ohms to nothing. I am not proposing fixing plugs... only suggesting that the high-resistance I was measuring was largely due to the assembly method and perhaps some minor internal arcing between the electrode and the plug terminal. The film thickness and clearance was obviously less than the spark plug gap so the plugs were functioning normally.

The problem you experienced with moving electrodes is a bit surprising. On the Champions I took apart the center electrode has a ring about where the hex on the plug body is. Once the ceramic is fired that ring would prevent the electrode from moving anywhere. It sounds like the ring must not have been used on the Bosch plugs.
 
Could be, I didn't think to open one of them up.
 
helpful information - especially if you are stuck after the stores close - could be a quick fix
 
Never thought to measure the plugs' resistance.

This may sound like a conspiracy theory, but: I've run Bosch Plats in Alfa engines with no issues but they'd foul nearly overnight in an English one. Heat ranges were always appropriate for the applications, too. And it was the same in "reverse," i.e. Champions would load up in an Alfa fairly quickly but were good for many hours/miles in the English cars. I just shrugged and used what had proven to work best.
 
When I was in business, I decided, based upon experience, the plugs would be as originally recommended or at least country (area of the world) specific on design.

Too many Nipondenso and NGK plugs on air cooled Folks Wagons with head threads still on the plug.

Heat ranges? Oh, yeah. Like, the ranges are RIGHT for what the range chart reads, but there is no correlation to real life.

Fords through 1967 or so, always Champions. Afterwards, Autolite.

General Misunderstandings, always his brother, AC.

Krau....ooops......GERman vehicles, Bosch (ever figure what what Bosch translates to in English?).

Had AC plugs blow out on friends cars on the freeway....entire porcelain came right out, stayed attached to the wire.....
Used to throw the best AC plug after a tune-up in customer's glove boxes just in case.

Seen plugs where electrode came loose and dropped down closing gap.

Where are you getting the plugs? Important. Had discussions long ago with factory sales reps, finally got them to admit there were at least two levels of QC.

Discount auto parts houses SOLD plugs sometimes substantially cheaper than the good parts stores could even BUY them.

The discounters made their arrangements for quantity and cost. The plugs were all assembled, but the ones for the discount houses came off the line BEFORE QC checks.

Who knows...maybe there were takeoffs on separate assembly lines.

My flatheads use Champion H-10. "hot" version for in-town is H-11. 30 or so years ago, H-10 became H-10C. 40 or more years ago, H-11's became obsolete.

I have not purchased a new plug for my three flatheads in 20 years at least.

I have 10-15 sets of H-10 (not "C") and at least 5 sets of H-11's......use one of two plug cleaners, blow 'em out, dry them off, check gap, put on the shelf.

Can't do that with modern plugs and engines.
 
Don't even know what they are but my '02 Accord V6 with over 160K miles is still on its original plugs. Never been off the car. (And may be impossible to remove by this time...).
 
More to Doc's comment on brand "swapability", I have a 20 hp Nissan Outboard. I put the book specified Champions in it in a tune up. It broke up at mid rpm. Changed back to NGK as installed by factory and it has run fine since.
Bob
 
Back
Top