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Changed My Plugs In My 98 Ls400


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Last week i decided to pull my old plugs as they were 2 years old with about 80 000km on them. They looked fine and seemed lean all the way around by white tips.

I got a set locally of 8 for $50 . Which is the same NGK iridium's that were in there before.

The last time i changed them i noticed no difference at all in the car.

This time was drastic difference.

It starts up faster without any mis starts with the remote start when very cold -15.

The mileage has dropped from 9L/100km (26mpg) to 6L/100km (39mpg) in the same stretch of roads.

So what has changed !

I only did one thing differently , when removing them i noticed the threads were dirty . So i knew the head threads were dirty also. Also the ground screw on the coils on each of them were tight also.

So i sprayed pb blaster on each thread before installing them on the plugs threads as well as the screw holding in the coils.

Also the voltage on my add on display for my air suspension shows 14.1vdc at start with only a drop to 13.7 when the engine has reached temp .

It used to show 13.4vdc at cold start and down to 12.8 when hot. I have replaced all ground on the car a year ago as well as the battery so it should have all it needs to flow. My digital gauge is reading the voltage from my trunk which has a voltage drop of 0.3-0.7 volts from the battery up front . It has 0 gauge wire feeding the amp in the trunk so the drop is normal. The battery should be reading 14.4vdc at the battery the way it should be now taking the voltage drop into the equation .

Also had a tranny which shifted with the slightest effort added whether cruising or driving hard it would kick down easily like it was dragging bricks behind it .

So , conclusion , clean the threads properly and replace the plugs , use pb blaster and post the results. I am impressed by far.

Also my unusual mileage has to do with my car being lower with air suspension to reduce drag as well as the tires psi set to 40psi. The biggest factors are my cold air intake , exhaust being freed up and the electric fan which only turns on when i come to a full stop.

Oh yah , i mod it because i love it !

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  • 2 weeks later...

What exhaust and cold air intake are you using? What size tires/wheels do you have? This is awesome as I was considering doing some mods on my 91 but didn't want to hurt fuel economy. I currently get 24 mpg. I was getting 20mpg before stock air filter replacement. If I can improve perf and mpg that would be excellent! :cheers:

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What wires are you useing? I just put new plugs and wires in a month ago but im using all stock ones. I did notice a huge improvement in them from that though. since my old wires were dated 1991. The Plugs were factory plugs, and it had no service history in the dealer computer of ever having a tune up. sad.

I think the biggest improvement ive noticed was my Intake i built a few nights ago. Im only getting 26mpg though. and my tires are at 50psi. I will check out those NGK's and clean my threads as well.

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