91 LS400 Stumble at Low Throttle

[jmellander]

Hi all:

 

I've just joined the forum and hope that someone can give some insight onto the problem I'm having.  I have a 91 LS400, 180K

When the throttle is just feathered, the car stumbles (speeds above 40 are most noticeable).  When I take it out of overdrive, the same symptoms occur at higher RPMs, but the same feathered throttle, so it doesn't seem to be RPM related, but more due to the throttle.  When the throttle is pressed harder, normal acceleration (disclaimer: not having driven another LS400, I can't be sure that the acceleration is as it should be - how to check?).  Items replaced or changed so far:

• Timing belt - partially corrected the problem (old belt had jumped a tooth or two), needed anyway since it appears that it had the original timing belt with 170K miles on it!!
• New plugs, plug wires, distributor caps & rotors, air filter
• New Throttle Position Sensor - no change
• Hacked fuel pump relay to deliver constant 12 V to fuel pump (partially corrected the issue)
• New fuel filter

Other:

• No Check Engine Light codes.
• 15-17 MPG mix of freeway and city.
• Just barely passed smog last inspection.

I've been considering replacing the fuel pump next, but am at somewhat of a loss as to what the symptoms indicate.  Any suggestions?

 

Jim

91 LS400

 

 

 

[TrevC]

HI Jim

First thoughts are that it needs a fuel cleaner running through the engine and a very good blast at high revs to clean out the carbon build-up in the engine and exhaust system.

This would cost very little to do and I'm sure you would see some benefits from it even if it does not cure the fault.

Secondly, I would check for air leaks in the inlet manifold area which could also cause the symptoms you describe.

Keep us updated on what you find

Cheers  Trevor

[Exhaustgases]

Also check tps.

[The Wrench]

Agreed, check the TPS, if you can, reset the ecu. because if you changed any inputs, it has to re-learn its parameters.

also see fuel pressure, pressure regulator, and Air flow meter/air intake for any suspicious leaks or blockages.

stumbling when you press the gas can sometimes be from either too much, or too little air.