A recent development in 1999 LS400...165,000 miles.

In park or neutral if I set engine RPM at or over 2,000 RPM and then release accelerator, at around 1,500-1,600 RPM , I feel.a slight bucking in engine...as if something is attempting to catch up...

Occurs on deceleration only.

Original; Oil Control Valves, Throttle Position Sensor, Pedal Position Sensor, Fuel Pump, Fuel regulator (gas tank).Fuel injectors, igniters, cam position sensors, knock sensors..

Most seals hoses and gaskets replaced with OEM.

Am wondering if a stretched timing belt would have this effect...mine is relatively new but from an Aisin Timing Belt Kit and there were comments about stretching.

Motor mounts were replaced 3 years ago....but am not ruling them out due to dyno testing on projects.and (infrequent) hard acceletation.

Anyone encounter and resolve this?

 

did you have your transmission mount replaced as well?
when was the last time your replaced your transmission fluid?
this could be driveline slop as well, so check your guibo (flex) bushing on the driveshaft.

 

Thanks..but not transmission or driveline related.

 

sounds like its ignition related. I would check spark plugs, distributor caps and rotors and coil packs for any obvious signs of wear or damage.

 

I wonder if the EGR is acting a little wonky. EGR typically opens on decel.

 

If its a fly by wire throttle, I'd suspect that system.

 

1999 LS400, so hybrid fly by wire. (Has throttle cable that will open throttle body if fly by wire fails)

Will investigate Throttle Position Sensor and Pedal Position Sensor...

 

1999 LS400 VVT-i does not have EGR...

 

Spark plug change about 6 months ago, no caps/rotors...

coil packs possible, but not running poorly at idle or at driving RPM's.

I have spares and will try replacing one at a time

Will inspect all spark plugs for clues...if nothing unusual, then will address fly by wire components in throttle body.

 

Consider the effects of electronic idle speed management. If there is any sort of delay in reaction time, you will be able to perceive it. For example, turning the A/C compressor on will load the engine. The idle stabilization valve is kicked in and the idle recovers. If the stepper motor (?) were to be slow, you would perceive a dip in rpm, recovery, then, when the A/C cycles off, you would have a mild surge. I am only considering GEN1 technology, I have no idea on the later Lexii.
Colin

 

Hah! Didn't know that. I have literally zero minutes wrenching on 98-00 LS400's, and it shows.

 

Found this in 1999 LS400 shop manual which seems to coincide with RPM point in question...and appears to address some event upon deceleration...

Anyone know what exactly the engine management sysyem is doing, or is supposed to be doing at 1,400 RPM?

Referring to last Line on the page.

 

Could it be that below 1400 RPM, the ECU triggers the fuel pump relay to switch the fuel pump resistor into the circuit?

I know the ECU does switch the fuel pump resistor in and out of the circuit to reduce unnecessary wear on the fuel pump when fuel demand is low. And making it a simple switch at 1400 RPM would not be my first guess as to how they do it, but they might.

I'm sure your manual will show the exact circuit for your '99, but here's a post I put up with a scan from my '91's manual when I was debugging fuel issues. It shows how the resistor works in the circuit, etc. I put in a lot of notes there regarding debugging, taking resistance measurements, putting in jumpers, etc.

https://www.clublexus.com/forums/ls-...ml#post9777959

Probably easy to measure voltage at the fuel pump relay to see whether the resistor is in the circuit or not.

Also, measuring fuel pressure at the rail would be nice info if you've got the adapter you'll need to do that.

 

The fuel pump/pressure regulator assembly are originals, incliding sealing o-rings..

The 1998 LS400 does not use a fuel return tube...apparently earlier models did, but was dispensed with due to increased emissions (fuel churning in tank? ) so pressure from the tank must be more precisely regulated.

I perform proactive maintenance, and have held off replacing the fuel pump, (the only thing that has consistently stranded me on every car I have owned is fuel pump related) but am contemplating a change.

 

Some fuel injection systems shut the injection off on throttle overrun and you can feel them come back on line at specified rpm.