When replace plugs?

[Arminius]

I don't want to do it! Worst case is a miss anyway! Could be worse as this 200BR allows access from either side of engine after removal of partitions. 5.0 L MPI W/ #600000+. Pulled one and it's a platinum AC 42-932 as designated with .060 gap as specified. Replacement under service bulletin 2006-03 is platinum AC 42-933, .060. These things last forever as we all say. I had a set in my MB for about 140,000 miles and a miss developed which I cured by personally replacing all 12 and nothing else. I figure that is at least 10 times the 340 hours on this 2003 boat which mainly ran to keep the battery charged to play the loud radio. So tell me it is OK to be lazy or maybe just do the accessible cap and rotor.

 

[bobeast]

I inspect mine every two years. Then replace based on how they look. Obviously, if you are running into any issues, that timeframe may get circumvented.

 

[Arminius]

Glad to hear that you feel inspection is adequate. Routine replacement was my habit but they last a lot longer since they took the lead out and introduced pricey electrodes. I think I will change my thinking rather than my plugs cause they weren't located where I (optimistically, like usual) expected them to be. Last plug is always a challenge; here it might be impossible. "There will be blood!"

 

[scoflaw]

Iridiums should last a very long time, 5 or 6 years.

Not sure about that photo, but if it's as black as it looks, you have another issue causing that.

 

[Arminius]

Iridiums should last a very long time, 5 or 6 years.

Not sure about that photo, but if it's as black as it looks, you have another issue causing that.

The close-up flash darkened the photo but brought out the rounded shoulders of the insulator and of the electrode. I'll run it by our marina mechanic when I pick-up a new rotor and cap.

 

[scoflaw]

Looks to be burning oil.

 

[BillK2632]

Was the engine fogged over the winter? Plug does look a little oily. As far as changing, Mercruiser is vague on the frequency. On my 4.3 I fog every winter and change the plugs every 3yrs/300hrs, my engine is carbureted. On your MPI I think the plugs should easily last 500hrs. In your case since the boat is new to you and the plug you pulled out doesn't look perfect I would probably change them now and check in 20hrs of use to see how they look, then figure on replacing in 300-500hrs based on a periodic inspection. I also do a compression test everytime I change the plugs - it's easy while the plugs are out and gives you a good baseline picture of the engine's health.

 

[sbw1]

I ended up waiting until there was a miss in my 8.1s. Those engines are very forgiving and the plugs can be pretty far gone by the time it is noticeable in the form of misses or mileage loss. When the boat was brand new, I changed them per the manual, but honestly they looked too good to change but I went ahead. There was no noticeable change in performance. So, I adopted the waiting practice.

 

[Henry Boyd]

I used to replace the plugs every two years mainly not because they were worn out, but to prevent them from rusting in place. Basically, I did plugs one spring and the raw water impeller the next.

 

[juggernaut1]

I recently replaced the NGK iridiums in my 8.1's. After 5 years a miss developed on one of the motors and exhaust smelt of unburnt fuel. Whilst I too was a little concerned they may have rusted in place, there were no such problems.

Not my preferred way to maintain the boat and I did have the plugs onboard to do for the last year but never got to it.

 

[scoflaw]

Plenty of products available to put on the plug threads so this doesn't become an issue.

The whole idea behind the 8 dollar iridium is not having to change them for a really long time.

 

[atrick]

I thought marine plugs were supposed to have stainless bases so they wouldn't rust. The plugs in my two boats have M in beginning meaning marine and are stainless bases-MR43T

 

[Henry Boyd]

Plenty of products available to put on the plug threads so this doesn't become an issue.

The whole idea behind the 8 dollar iridium is not having to change them for a really long time.

Eight iridium spark plugs, sixty-four dollars
Not having to heli-coil a stripped hole, priceless

 

[scoflaw]

Eight iridium spark plugs, sixty-four dollars
Not having to heli-coil a stripped hole, priceless

If you did your own wrenching, which I know you don't, and you owned a twin engine boat, you would understand that changing these plugs are no joke. Don't understand why you would think your plugs would seize in the heads at 2 years.

 

[Arminius]

Plug seizure was a concern with aluminum heads on motorcycles and furrin cars. Eventually I learned of moly anti-seize that could be messy but worked fine. Seizure was never a problem with old fashioned cast iron heads and steel plugs. Stainless on stainless can produce seizure in the form of "galling." Is there some concern that a stainless plug in a cast iron head may seize?

 

[Henry Boyd]

If you did your own wrenching, which I know you don't, and you owned a twin engine boat, you would understand that changing these plugs are no joke. Don't understand why you would think your plugs would seize in the heads at 2 years.

A) How do you “know”? You don’t, like you also don’t know I paid for my education by fixing V-12s and flat 6s. And yes I fixed a lot expensive cars that had been owner repaired. That made me realize that knowing how to do something is not the same as doing it every day. Fixing it right is a always better than fixing it cheaply.

To answer your other question, because a boat engine’s operating environment is not the same as a car. Nor is the environment, that is also conducive to corrosion. Sure you can get five years out of a set of plugs (that’s also about five short of what GM expects when the plugs are in a Silverado). But is the additional expense that big a deal?

 

[Arminius]

There is a psychological element here. Obsessive people like myself know better than to start a difficult project whose last detail may be a problem best anticipated rather than mentally obscured at the outset. Telling "war stories", I considered risk-reward as I soloed over Renton for the first and last time and realized I'd be better off boating. The worst case for neglecting the plugs is that you will have to call the Coast Guard.

But Mercruiser is putting aluminum heads on its 4.5 and 6.2 (small block derived). These would need lots of molybdenum anti-seize on the threads to prevent seizure. Warm the engine up before plug removal because aluminum expands more than steel.

 

[Scott06]

Plugs as stated replace as needed one thing on the mpi engines the crab style distributor cap can be a known failure point. Don’t have one myself but seems folks changes these frequently

 

[Arminius]

My Mercruiser original cap had aluminum post contacts that were heavily corroded. My friend (with credentials and experience) says the caps are a known weakness with tracing inside. So long since I've seen a cap but recall that a "tracing" is a spark etched path between posts causing misfires. I went for the $30.00 BB replacement from Boise with brass posts and it is identical except the furnished screws holding it to the distributor had a different pitch and were not used. All 4 threads were missing the factory Loc-Tite blue gum. No "J-1XXXX" # on cap either which would have been evidence with compliance with SAE explosive atmosphere standard. Functionally, its an upgrade as brass is better. Yea, I moved the old SAE label to it and used my own blue liquid. Research indicated dielectric grease on plug boots only and coil lead into cap needs special Mercruiser sealant so don't take it apart.
Sure was happy to find the only plug I pulled was platinum and they were standard in 2003 !

 

[Lazy Daze]

So... you used a non-marine dist cap and moved the label over to it to "make it compliant"? A label doesn't prevent it from going boom. You don't have to buy the Merc brand, but for safety, get a proper marine version.

The current line of Merc/Quicksilver brand do have brass contacts, by the way (and for quite while).