Rolocado

I picked up a LS 400 recently from a friend and it's had engine stumbling issues. Normally it happened around 45 MPH, then it started to stumble even when I was idling. I replaced the ECU caps this morning and it still persisted. Other things I've done recently are new air filter, cleaned the throttle body, new PCV valve/hoses, cleaned the MAF.

I unhooked the TPS when it was idling rough and it seemed to go up 100 RPM or so when I did and it sounded happier. I'm assuming that could be my issue? Am I able to adjust an old sensor or do you normally have to get a new one? Also, is there harm in driving without the TPS connected?

Thanks all

Scraape

what year is your vehicle? I am having this same issue on my 99, but mine is definitely misfiring bad

Superfast1

You'll need to check for code(s) so we can help you diagnose your problem.

Rolocado

Mine is a '92. I don't have any codes showing. I have a new TPS coming tomorrow. You might try unplugging yours as it's idling to see if it changes anything.

oldskewel

iTrader: (1)

I once had a soft failing fuel pump that gave really difficult symptoms, since it was a soft failure (i.e., the pump got weak / intermittent rather than just stopping). One symptom you mention - that problems can start once you get above 45 mph - could be due to the fuel flow being limited.

There are a bunch of electrical things in the fuel delivery circuit that can also fail. Here's a post where I laid out the circuit diagram and explained some troubleshooting to try.

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

In my case, the troubleshooting basically confirmed that everything else was working fine, so based on process of elimination, I tried replacing the fuel pump, and that solved the problem.

Measuring fuel pressure would be a great test if you can do that before replacing the pump, but for me, not having the special tool required, I just replaced the pump. They all do eventually wear out.

Rolocado

So I replaced the TPS and while it seems slightly better I still get a stumble at 600 RPM when my foot is on the brake. Now I'm annoyed. Going to unplug the MAF and drive around to see if that changes anything.

I really don't want to replace a $300 fuel pump if I don't know its not even bad. Any other suggestions

Yamae

So you have no code. The fuel pump is not directly checked by the ECU and there'd be no code even if it's failed. Other than the fuel pump. The ignition system is not fully checked by the ECU. The ECU detects the ignition problem getting the IGF (IGnition Feedback) signals from ignition coils. But the ECU can't get the tracking (leaking) problem at distributors , plug cables and spark plugs.

I'd rather check the tracking problem of the ignition system in a case now you are experiencing. I use an oscilloscope and a current probe to find out which part is causing the problem. Without those, it would be a time consuming job to find out the exact point. Sometimes you can find it very easily like these below but be careful for the electrical shock.

Rolocado

So driving tonight I realized it shudders when I am coasting and then when I’m stopped and am holding the brake in drive.

What’s more bizarre is when I’m coasting and apply the brakes, the shuddering goes away, but the shudder is coming from the engine....

oldskewel

iTrader: (1)

You're lucky that this problem is happening so much. It should help you find it.

On this last detail ... if you conclude that the problem is directly affected by braking ... braking should have no affect on ignition, but the brake booster uses manifold vacuum, and it suggests a vacuum leak could be the cause.