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Low Speed Flat Spot


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Under light throttle application from a standing start, the engine feels hesitant as the revs rise - similar to a blocked jet in earlier times.

1/2 and full throttle positions seem fine - plenty of power (Denver altitude not withstanding).

The flat spot is more noticeable when the engine is warmed up. When cold, it is quite sharp off the mark, and there is no hesitation or lag.

Any similar experiences? The vehicle has 98,000 miles.

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  • 2 weeks later...

My 92 LS400 does the same thing - has a small hesitation off idle when warm, but only when the throttle is opened quite gently. It is obviously going a bit lean, and the enrichment during the warm-up phase prevents it. Perhaps the O2 sensors are trimming the fuel too much?

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i would say the throttle body and plate need cleaning as buid up of carbon and varnish buildup

buy a 5 dollar can and spray it while the engine is running

reving it with the intake removed for direct acess

also if it is this the idle may be alittle unstable with this kind of blockage

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Thanks for the comments, but on a multiport EFI engine, the fuel is injected into the manifold runners well after the throttle butterfly. How does the throttle become blocked and varnished? I understand fuel flowing from a carburettor causing throttle varnish, but not EFI.

What spray were you referring to? Carb cleaner?

Hot idle without A/C will sometimes fall off to 400-450 RPM. With A/C, up to 600 RPM in neutral, and with drive engaged, it idles about 700 RPM.

Cold idle 700 to 800 RPM.

Any further thoughts? Do the O2 sensors have a finite life in this engine?


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where the varnish and carbon builds up is from the car being turned off and the unburnt fuel in the combustion chamber backing up the intake plenum and around the throttle plate varnishing


hope i helped to clear it up

carb cleaner will work

but there are specific ones for throttle bodies

not sure of the difference though

and no o2 sensor last forever but a good 250 km should be fine

but a good motorvac would also help evrey 50 000 miles

to save gas and restore power

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My experience on other engines is that the throttle body deposits are the result of the gases from the crankcase ventilation system and in some engines the EGR flow also contributes.

Also the additional air flow needed during warm-up is normally provided by a Constant Idle Speed valve, which allows the main throttle plate to return to a base position in order that the Throttle Position Sensor sees the throttle angle range similarly under all conditions.

I haven't owned this car long enough, nor have I worked on it enough, to know how these systems work specifically - all I am going on is experience on other engine systems.

I only use intake system cleaners after removing the throttle body from the manifold - even with "O2 sensor safe" chemicals, I feel there is too much risk with the O2's and the converters to introduce any liquid solvents to the engine. I don't think I want to remove the throttle body just yet.

I will inspect the throttle body for deposits though - thanks for the advice.

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no problem glad to help

but anything that is o2 sensor safe is usually for silicone gasket makers

the mould replent in silicone can damage o2 sensors otherwise almost anything else will work fine no adverse affects

and there really is no need to remove anything to use the product

i use the tb cleaner every 6 months on my cars

and it keeps them purring perfect

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  • 2 months later...

Being an old carb man, I'm not too familiar with EFI. When you use a cleaner spray where do you find the place to spray it? The top end of this engine looks very tidy and I can't see any openings in which to place the nozzle of the spray can.

Thanks for any help!!

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