Hello, we have a 1997 LS 400 that's in good condition. Recently the catalytic converters and oxygen sensor(s) were replaced, and it had a full inspection and routine servicing. However, we are still having an intermittent problem. At times, the engine will suddently run rough and have barely enough power to climb a shallow hill, as if it has gone into limp mode or only half of the cylinders are firing. However, the Check Engine light does not come on, and when we take it to our mechanic, they find no codes at all. Also, our mechanic is not able to get the car to exibit the problem. So most of the time the car runs fine, then all of a sudden it will run as if only half the cylinders are firing, and we pull over, stop the car, and have it towed to our mechanic. Then our mechanic starts the car and it runs fine, and they can find nothing wrong and no codes. The most frustrating part for me is that there are no codes.

Any advice on how to diagnose this? Has anyone experienced this and found a solution?

Thanks,

-Tom

 

Well you can imagine if your mechanic could not figure it out in person inspecting the vehicle, you can imagine how much of a fun time we are going to have analyzing it over the internet. Sounds electrical though maybe by judging by the recent 02 sensor & converters work that was done. Maybe something is heating up and causing some kind of short. If it didn't do that before, maybe some wires somewhere got messed up and its shorting out somewhere because that probably wouldn't code.

 

It sounds like this lack of power a crappy running engine was occurring before the converter and O2 sensor replacement, yes?

I suspect a fuel delivery problem (pump, filter, in-tank filter, pump pickup screen, cracked in-tank hose). It's not so much a fuel pressure issue as it is a volume issue.

Even though it's not listed a routine maintenance item, when was the last time the fuel filter was replaced. You may want to start here, then work your way back to the fuel pump.

My Ducati has an in-tank pump and filter. Often with this vintage motorbike, times the hoses that connect the pump to the filter will harden and develop cracks (the hoses are submerged in fuel). This provides a return path for the fuel back to the tank so the engine sees less fuel volume.

 

No code but the poor running often relates two issues. One is the poor fuel supply caused by the failing fuel pump or the clogged fuel filter. Another is the failing ECU mostly caused by the worn QAS capacitors.

 

I'd agree with Yamae, and paulo, more so with the fuel delivery. On my 93 I have a hiccup while under load. Just cruising it seems to be OK. I've tried 3 different ECUs and it happens with all of them. 2 were rebuilt, one by Denso. I have a wicked shake but that's from the motor mounts, which I still need to change. I do sometimes get a code for the sub O2 sensor but that has nothing to do with how it runs. When I do change the fuel pump I will be changing the filter also. "eventually"

 

It could be the torque converter go bad.
A faulty torque converter can lead to a variety of issues that might seem like engine performance problems.