Hesitation When Accelerating

[cleverdick]

Hi all,

I have recently noticed that my 1990 LS400 suffers from a 'flat spot' at around 45 m.p.h. when accelerating, sometimes accompanied by a smell of sulphur. This seems to happen just at the end of the warm-up phase. I have replaced the spark plugs and HT leads (for Denso iridium and Solid Ace respectively) but the symptom persists. I have read about this on the forum - seems to be a common problem - although each posting seems to be a bit inconclusive. The throttle body on my car definitely needs cleaning, but would that cause a flat spot at 45 m.p.h.? Throttle position sensor, maybe?

But what about the rotten egg smell?

Any advice gratefully received!

[ColinBarber]

ignition timing may be incorrect or you have a vacuum leak somewhere.

Is the engine running hot?

[cleverdick]

ignition timing may be incorrect or you have a vacuum leak somewhere.

Is the engine running hot?

←

Thanks Colin...

The engine isn't running hot - temp gauge (when warmed up) is bang in the middle. Idle is smooth too, by the way.

Cheers,

R.

[davepruce]

Mine used to do the same (hesitate) and if I think about it at the right time it probably still does, but its so slight that I now ignore it. Dunno about the smell though!

I did the plugs, leads and throttle body (easy enough job - see http://www.lexls.com tutorials), and each contributed a bit to the result.

Lexls do say to drain the coolant/remove the whole thing, but I didnt!

I held the butterfly open with a mole wrench externally and stuffed half an old tshirt (securely fastened to a cord) past it and into the far end of the body to catch bits and liquids, then delicately scrubbed the inside of the body with a toothbrush (the wifes of course) and the aforementioned cleaner. When I finished, I pulled the cloth out and all the crap came with it. I actually did it twice as on futher inspection I'd missed some!

I think the TB clean probably had the most remedial effect!

[cleverdick]

Mine used to do the same (hesitate) and if I think about it at the right time it probably still does, but its so slight that I now ignore it. Dunno about the smell though!

I did the plugs, leads and throttle body (easy enough job - see http://www.lexls.com tutorials), and each contributed a bit to the result.

Lexls do say to drain the coolant/remove the whole thing, but I didnt!

I held the butterfly open with a mole wrench externally and stuffed half an old tshirt (securely fastened to a cord) past it and into the far end of the body to catch bits and liquids, then delicately scrubbed the inside of the body with a toothbrush (the wifes of course) and the aforementioned cleaner. When I finished, I pulled the cloth out and all the crap came with it. I actually did it twice as on futher inspection I'd missed some!

I think the TB clean probably had the most remedial effect!

←

Well,I cleaned the throttle body, and at the moment it does seem to have made an improvement. But time will tell!

I did remove the whole thing. You don't have to drain the coolant, as the TB is the highest point on the engine. Water loss is minimal and can be soaked up with a towel. The large 1" dia. hose underneath is an air pipe BTW. Hardest part is disconnecting the two small water hoses, one of which I found was easier to cut through and replace afterwards. A replacement gasket can easily be made from gasket paper.

Just allow plenty of time!

Rich