Michael Benno

Hi folks, I recently changed my spark plugs and I had a few newbie questions.

Here is what the old plugs looked like when I removed them. The car seems to run and drive fine before changing them. Does the condition of these plugs give any clue to how the engine is running?




They look okay or they look like they are running rich or cold according to this cross reference I found on the web. I could not really tell.


My second question is around the gap. The old plugs were the correct spec: Bosch WR7DC. I measured the gap and they all measured at around 0.36" (0.91mm). The factory specification is 0.7mm +/-1. So these old plugs were about .11mm out of tolerance from the specification. How would this larger gap impact the performance of the motor?



Lastly, replaced the plugs with Bosch WR7DC+ Copper 7900 from Roger at 928srus. The boxes claimed the plugs to be pre-gapped at 0.31" (0.79mm). I checked the gap and they varied from 0.66mm to 0.73mm. So I recapped them all to 0.76mm. But then I read that I should not re-gap certain types of plugs. Did I do something wrong?

I drove the car 5-hrs and it seems to be running fine with no noticeable difference. Thanks in advance

Speedtoys

Iridium tip and such you should not, but cheapo coppers, no issue

FredR

Michael,

With wear the plug gaps can only increase so comparing your gap to the spec makes little sense as that is the range they should be set within when new. Manufacturers may specify slightly different values. From my point of view I always expect to check and adjust new plug gaps as needs be. As to what constitutes a need for retirement of the plugs I am not sure there is a definitive answer. I have seen plugs still running with the electrode burnt down to the insulator- not that I recommend such. At the cheapo price they are I put a new set in every 5 years [roughly every 30k km] and they still look new.

Regarding your query about spark plug - you more or less answered this yourself- no perceived difference before and after suggests nothing really wrong with what you had. The bigger the gap the longer it takes for the spark to jump- not that you would notice the difference. Not sure at what point the gap becomes "too much" but I suspect anything up to 1.0mm the motor will run quite ok. That your plug gaps were all at the same value [0.91mm] is a good sign. When I last changed mine they were a bit more than the value in the WSM and I set the new plugs as per the WSM.

The photo guide Jeff submitted is an excellent guide to plug reading - saved it to my files already. That should tell you most of what you need to know.

Finally, please post of pic of your plugs with a "gap of 0.31" inches -

dr bob

It's even OK to re-gap the more exotic metal plugs if you bend rather than pry on the ground electrode. The multi-electrode plugs are where you generally can't check or adjust gap yourself.

The gap ultimately determines the firing voltage required. As gaps get bigger, the voltage required goes up. Since we depend on a collapsing magnetic field in the coils to generate that voltage, higher voltage required equals later spark. There are additional risks associated to insulation breakdown in tired plug wires, and bigger risk of arcing and tracking in the distributor caps. The worst is an insulation breakdown in the coil when the firing voltage is too high, the major reason why you don't want the coil to "fire" without a plug or other path to ground. Internal arcing is fatal to coils. All good reasons to keep good plugs in the car -- saves a lot on collateral damage to coils and other pieces. Plugs are a lot cheaper.

The color looks OK, but is a little darker than what I'm used to seeing in my very gently-driven car. Mine are generally very light tan, enough away from original white to know they've been used but not by a lot. Meanwhile, If I took out 8 plugs that all looked like yours, I would be very satisfied. Your choice of the copper Bosch plugs from Roger is excellent.

Michael Benno

Wisconsin Joe

I'll agree with Dr Bob that they are a bit darker than I'm used to seeing, but not 'bad' in any way. The gap is big because the electrode is worn down some.

You could easily bend the tip down a bit to get the gap correct and keep running those.
New ones are cheap enough that replacing them is a 'non-issue', though. No harm in getting new ones in.

Also no harm in 'perfecting' the gap. They are supposed to be gapped correctly out of the box, but as you found, that usually is not perfect.
I always check and adjust the gap on new plugs.