Spark Plug Resistance, Range Spec?

[ttocs]

MY2004 Boxster Base 2.7 Tiptronic

 

I'm wondering if there should be any concern regarding differences in the resistance of a set of spark plugs?

 

I just bought a set of Beru 14FGR-6KQU spark plugs. Measuring the resistance between the terminal nut and the center electrode, four of them are in a tight range of 5.6k to 5.8k ohms, and the other two are close to 6.8k ohms.

 

Firstly, does the resistance matter?

Is there a range of acceptable resistance for a single spark plug?

Also, should a set of plugs be fairly well "matched" in terms of resistance?

 

And lastly, I'm curious to know if the coils are expecting to see a certain resistance to the plug? and can the plugs have a resistance that is out of range for the coils to function properly or suffer a forced failure?

 

[Loren]

Resistance does not matter unless one or more has a huge difference.

 

What does matter:

Spark Plug electrode gap: 0.7 mm + 0.1 mm

Check spark plug connectors. Resistance: approx. 2 k ohms

Check ignition coil(s). The resistance between terminal 1 and terminal 15 at 20°C: 0.3 - 0.7 ohms

[ttocs]

Perfect! I just wanted to make sure that a 1k variance was ok.

 

Thanks Loren!

[ttocs]

Loren, I want to make sure that the spark plug gap you gave is not a typo because it's almost 1mm different from Beru's web site and the Bentley Manual. They both list 1.6mm for the gap on this plug - 14FGR-6KQU. Please confirm. Thanks in advance.

[Loren]

10 minutes ago, ttocs said:

Loren, I want to make sure that the spark plug gap you gave is not a typo because it's almost 1mm different from Beru's web site and the Bentley Manual. They both list 1.6mm for the gap on this plug - 14FGR-6KQU. Please confirm. Thanks in advance.

 

Sorry, you are correct for the 2.7-liter engine: 1.6 + 0.05 mm. Most are new spark plugs are pre-gapped so you likely are just checking.

[ttocs]

Even perfecter! Thanks!

[ttocs]

I had a bunch of work done when I bought my Boxster three years ago, new plugs and check the coils were a couple of the things that were done. They said the coils were fine. I requested that all of the old parts be returned to me, but only the IMS was handed to me, so I don’t know what plugs were in the car when I got it with 30k miles on it. The indy replaced the plugs with NGK BKR6EK. No biggy I thought, I didn’t know any difference, especially since I was unaware of what was taken out.

 

Those plugs now have 13,000 miles on them and don’t look good to me. They are about 1.7mm longer reach into the cylinder than the Beru plugs, have two prongs vs four, and are gapped at .9mm to 1mm instead of 1.6mm. Maybe that’s how the NGKs are supposed to be gapped??? but I doubt it. The resistance of the NGK plugs is pretty consistent at 1.3k ohms plus or minus a tiny bit. Since this is way different from the 6k ohms I measured on the Berus, i would guess this is not preferred. Also, there is grease on the plug terminal nuts. Is this what I should do on the new plugs? or only on the boots that touch the porcelain? And lastly, it took so little effort (one plug only needed a little more than the weight of the ratchet to loosen) to remove the #4, #5, and #6 plugs that I used a torque wrench for the others to kinda get an idea of a measurement. On the other side of the engine it was 17 ft lbs for #1 and #2, and 19 ft lbs for #3.

 

Any thoughts on the condition of these plugs is appreciated.

edit: all the plugs were wet and smelled of gasoline.

 

As a side note, the coils are all cracked except one. Cylinders 2-4 have major cracks while 5 and 6 have hairline cracks, cylinder 1 looks fine. I’ll get new ones on Monday from Exchange Porsche, they treat me very well - as they would with any PCA member (yes, you gotta show them your current membership card for the discounts). All of the coils measure at .7 ohms at room temperature. The spark plug connectors all measured 1.8k ohms.

Edited April 15, 2018 by ttocs

[JFP in PA]

OK,let’s start with the most obvious:  Cracked coils are a problem, regardless of what plugs are used.  They need to be replaced.

 

As for the plugs, there are a wide variety of plugs that will work fine in these engines, ranging from the factory Beru to Denso Iridiums, but they need to be properly gapped.  Most shops do not have the tools to reset the gaps on multiple side electrodes, so they just slap them in the way they come out of the box.  Usually, the Beru’s are pretty close right out of the box and need no changes, which is one reason they are widely used.

[ttocs]

Update:

As you said JFP, the Beru plugs are within a pretty tight tolerance for the gap. The new coils also measured very close to each other.

 

The car runs much stronger, and consistently so.

 

Two years ago I started using Durametric to diagnose a loss of power since some major service work performed a few months prior - which included replacing spark plugs (this is when the wrongly gapped NGK's were installed). I logged many drives so quite a few things could be checked, including acceleration. I logged 0-60 times of 6.8 to 7.3 seconds and found the times to be erratic. Now, things are very repeatable at 6.3 to 6.4 seconds. Pretty good for a Tiptronic! And passing on the highway is more thrilling! It's kinda like having 30HP more all of a sudden. I used to feel a "bog" at the 1st to 2nd shift point, now that's gone. I also notice a pretty big jump in acceleration above 5,000 rpm where it used to be kinda flat or a little fall-off in power. And the engine runs smoother, not that it was bad before but it wasn't near this smooth.

 

I waited a week to post this to make sure I wasn't dreaming.

[JFP in PA]

14 minutes ago, ttocs said:

Update:

As you said JFP, the Beru plugs are within a pretty tight tolerance for the gap. The new coils also measured very close to each other.

 

The car runs much stronger, and consistently so.

 

Two years ago I started using Durametric to diagnose a loss of power since some major service work performed a few months prior - which included replacing spark plugs (this is when the wrongly gapped NGK's were installed). I logged many drives so quite a few things could be checked, including acceleration. I logged 0-60 times of 6.8 to 7.3 seconds and found the times to be erratic. Now, things are very repeatable at 6.3 to 6.4 seconds. Pretty good for a Tiptronic! And passing on the highway is more thrilling! It's kinda like having 30HP more all of a sudden. I used to feel a "bog" at the 1st to 2nd shift point, now that's gone. I also notice a pretty big jump in acceleration above 5,000 rpm where it used to be kinda flat or a little fall-off in power. And the engine runs smoother, not that it was bad before but it wasn't near this smooth.

 

I waited a week to post this to make sure I wasn't dreaming.

 

Sometimes the easiest answer is also the correct one.  Glad you got it sort, now go drive it like you stole it!