Spark plugs looks terrible!

Submitted: Sunday, Mar 30, 2014 at 20:15
ThreadID: 106989 Views:1767 Replies:6 FollowUps:1
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Today I changed the spark plugs in our 100 series petrol/LPG. I was really surprised in the condition. The electrode at the tip was reduced by nearly 40% compared to the brand new ones! What is everyone's thoughts on this poor condition in regards to fuel consumption? Over the past few months the economy has gone out the window. Should I expect to get more kms out of a tank of gas? Currently I'm getting 285kms to 70 litres of LPG. I used to get 330 kms about 30000 ks before.
I'd be interested to hear you view.
Cheers Chriso.
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Reply By: Ross M - Sunday, Mar 30, 2014 at 20:23

Sunday, Mar 30, 2014 at 20:23
Is the fuel/air ratio correct ie rich enough for good power? if lean you may burn more relative quantity in order to maintain the power. That also may make the spark plugs run hotter and erode the electrodes faster than normal too.

Might pay to have the mixture checked and a more frequent check of the plugs.
Is it injected gas or has a converter? Sometimes the converters can um up a bit/get lazy and make the system run leaner. ie not so sensitive to demand.
AnswerID: 529457

Reply By: Member - Rob S (NSW) - Sunday, Mar 30, 2014 at 20:47

Sunday, Mar 30, 2014 at 20:47
Hi Chriso

It's recomended to change the plugs every 20,000 klm on lpg.
And get the valves checked adjusted every 40,000 klm, not a cheap exercise on a 4.5 petrol cruiser, unless you can do it your self.
the exhaust valves are made of cheese and don't last long at all on lpg.

330 klm out of 70 litres lpg works out to about 21 litres per 100klm.

That's the same as what i get out of my 80 series, same engine , manual, 285/ 75/ 16 bfg at tyres,lightly loaded on a steady high way run,no roof rack.
i usually get 420 klm out of 90 litres lpg, 21.4 litres per 100klm.

Depending on the set up of your 100 series, accesories etc, and vapor or injected gas
285klm to 70 litres of lpg is getting a little high( 24.5 litres per 100klm )if your not towing, unloaded etc etc.
time for a tune ?


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and that's when I thought I was wrong!

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Follow Up By: Member - Rob S (NSW) - Sunday, Mar 30, 2014 at 20:54

Sunday, Mar 30, 2014 at 20:54
Just to note that 21.4 litres per 100klm is the best i can get on a good day,and a tail wind lol.
goes down hill very quick towing ,or fully loaded with roof rack,
and a heavy right foot. around the 24.5 litre per 100klm mark

I only ever made one mistake
and that's when I thought I was wrong!

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Reply By: Athol W1 - Sunday, Mar 30, 2014 at 20:52

Sunday, Mar 30, 2014 at 20:52
As the plug gap increases so does your fuel usage, regardless of the fuel type used. This is the reason for regular tune ups with spark fired engines since they were first developed.

If you are predominately using LPG over petrol then you should be using spark plugs of a cooler (sometimes referred to as a softer) heat grade, that is plugs that have a shorter path from the electrode to the body so that heat gets away quicker. LPG burns at a higher temperature to petrol even when everything is correctly set and operating.

Even when using the correct grade of plugs for the LPG application it will be necessary to change the plugs at shorter intervals than you would normally change the plugs in the same engine running straight petrol.

Hope this helps
Retired Motor Mechanic
AnswerID: 529462

Reply By: 671 - Sunday, Mar 30, 2014 at 20:59

Sunday, Mar 30, 2014 at 20:59
Get a LPG specialists to have a look at the old plugs. A lot of information can be gained by looking at their condition and color. If it is running too lean then the combustion temperatures will be higher and that can cause a lot more damage than just plugs. If everything looks good then he may recommend a plug that is one stage colder than standard, particularly if you are doing a lot of heavy work like towing. A colder plug will dissipate heat faster but don't go any further than one stage. If you fit a plug that is too cold, they may foul up because the normal combustion deposits will not get burnt off them
AnswerID: 529463

Reply By: Member - Phil G (SA) - Monday, Mar 31, 2014 at 21:12

Monday, Mar 31, 2014 at 21:12
What brand/number were the plugs that you removed? Were the new plugs the same brand/number?
The reason I ask is because there are a variety of different plugs available these days and the electrodes on the platinum tyres can be considerably smaller.
AnswerID: 529531

Reply By: nootsa200873 - Monday, Mar 31, 2014 at 21:49

Monday, Mar 31, 2014 at 21:49
Well after all your replies and advice I feel I'm on the right track. The new plugs that replaced to old ones, were the same. NGK branded, with a '5' heat range. And from all the info gathered, I should be using a '6'. So with that all done, we are just about to leave on our first big trip. A whole month on the road from Victoria to NSW borders and back. This will put a few K's on the plugs and I may change then when I get home.
Thanks very much for all your advice and tips, but now....
Cheers Chriso.
AnswerID: 529536

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