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1998 Lexus Ls400 Two Year Mystery Solved! Sluggish, Misfire Up H


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Hi Everyone,

Today I am super happy and wanted to share.

My 1998 Lexus LS400 has been feeling really tired. I bought it in 2009 with 194,000KMs, and had the spark plugs changed along with the timing belt. For the past two years, it would misfire up hills, acceleration was so-so, and the fuel consumption was terrible -- about 18L/100 km. Today it has 260,000 km. I live in Australia, so unfortunately my car does not have OBDII. Even my Lexus dealer was not able to get much info out of their machine...

So, I started replacing things:
1. New O2 sensors
2. Coolant temperature sensor
3. New fuel injectors
4. Decarbon treatment (professionally done)
5. New fuel filter
6. Seafoam treatment
7. Search for vacuum leak
8. New air filter
9. New mass-airflow sensor
10. I'm sure I'm forgetting some things...

Nothing really helped fuel efficiency, except for increasing the tire pressure to 42 psi. But still the car felt sluggish. Did I have clogged cats? Did I need to replace 8 coil packs? I was thinking that maybe the car was just getting old. I started test-driving some LS430's and started saving for an LS460 (Australia is expensive!). I bought my wife a Prius so that our average fuel consumption would be similar to having two normal cars.

Well, this weekend I finally got around to changing the spark plugs again with Denso iridiums and WOW! Night and day difference!!! The car feels more powerful than ever. No hesitation and tons of acceleration. It's so smooth and quiet -- quieter than some of the LS430's I've recently driven with half the km's. I've been driving it hard all day and I'm getting 10L/100km.

This is unexpected, because the plugs I just replaced are NGK iridiums. Aren't these plugs good for 120,000 miles? My mechanic would look at one plug every few months (#1 cylinder) and always told me that they were fine and did not need to be replaced. I'm wondering if these are counterfeit plugs (I went to an independent garage) -- because when the NGK plugs were new in 2009 my car did not drive as well as it does now.

I have attached photos of the 8 spark-plugs. These are the iridium spark plugs that after 40,000 miles increased my fuel consumption by a factor of 2 and halved my horsepower.

Maybe these plugs look terrible and I am showing the obvious -- I would not know because I don't have much experience in reading plugs. Any insight on what is going in my engine, and thoughts on the health of my engine would be most gratefully received.

Thank you for all the helpful posts on this forum. It has been a great resource.


Images of spark plugs:


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Those plugs don't look that good so I can see why they might have been giving you problems. There seems to be a lot of carbon build up on them and they also look wet from oil maybe. You might have other underlying issues that are fouling the plugs over time.

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Welcome, Ken and thanks for sharing that info. The #1 cylinder plug looks the best and your mechanic kept pulling that plug never seeing #7, which is probably the worst with carbon build-up. The electrodes do look a bit worn away so you just need to keep an eye on your new plugs and see how they last and perform. You should use anti-sieze on the threads when you replace them.

You have 161k miles on the engine so the rings and such may be getting a little leaky. You would have to do a leak-down or compression test to see what the health of each cylinder may be. BTW, what kind of petrol do they have in Australia? Is it high octane with detergents? Is it the so-called Tier one gasoline?

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Juding from your photos, those spark plugs are BKR6EIX-11 of NGK. The BKR6EIX-11 doesn't have the plutinum tip at the outer side electrode and it doesn't last long. I'd say less than 1/4 or less compared with the genuine IFR6A11, BKR6EIX-11P or VK20.

You have driven 66,000km in 5 years. I assume that it was 40,000km after the change when you begin to notice the problem. According to the NGK's paper, the BKR6EIX-11 is only good for 20,000km when used for the good direct ignition system.

Did you select IK20 or VK20 of Denso this time?

You should select VK20 of Denso or BKR6EIX-11P of NGK. These have the platinum tip at the outerside and last long. These are also easily available in the market too.

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Thank you all for the great info.

@Landar - I use regular unleaded 93 here in Australia. Occasionally I put in the E10. Having grown up in the US, it is painful to pay $7-8 a gallon as we do down here. I might do a compression test in the future when the car gets sluggish again. It is running so well now that I don't want to break anything :)

@Yamae - Thanks for this great insight on different kinds of plugs. I selected IK20's this time because I did not know what I was looking for. Great to know that I will need to replace the IK20's in 20K.

Thanks again! Ken

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