Sherl

I have 98 LS400 with 229k miles on it (I just did a trip from California to Carolina, last week) and its been with the problem for a quite a while. I often get P0420 code (and P0430 but not as often as P0420 when I clear them). Also sometime after short stop intervals (ideally less than 20-30 mins), the engine might crank but wont stop, I need to crank/try-starting them for 3 or 4 times to get fired up and once fired up it doesn't seem to have any trouble. This happens only in very short stop-ride situations (say ride for 4 miles, stop and go for purchase for 20 mins and try starting back on - the engine would simply keep cranking until I try three/four times. On/Off cycle with key). If I apply gas before cranking the engine, it seems to alleviate the problem - the engine starts almost instantaneously in the above situation.

I recently replaced fuel filter and cleaned MAF sensor as well - but should I suspect and replace any o2 sensor or check for Cat leaks (or may be a clogged cat, is that even possible)? The car has been in throughout in Southern California and I suspect had even seen a snow until last week.

Losiracer2

have you already replaced the coolant temperature sensor? By now, yours is probably cracked on the connector/corroded and it plays a very important role in how the engine operates, fuel economy, power delivery, etc.

Sherl

The coolant sensor or thermostat was replaced by previous owner around 170k. And I see my temp gauge is almost at the middle and I have driven 800 miles a day in past six days, 3100 miles and it comfortably cruised all day at 77mph and around 25mpg. Didn't stutter at 24°F either, while I was passing a range in New Mexico covered on Snow. Indeed my dashcam memory card stopped working low, I just realized that it wasn't rated for 0° C

Losiracer2

Ok, then that's probably good. If you want to get rid of the P0420/430 codes without having to spend thousands on catalytic converters, I added Spark Plug antifoulers and spaced the rear O2 sensors out away from the direct airstream of the exhaust. I stacked two on each outlet and then screwed the O2 sensor into those. You'll have to drill out the 2nd one to allow space for the O2 sensor to fit into but its been like that for over 30k miles and I still get 27mpg hwy on my nearly 300k mile car.

Sherl

Thanks for that. Can you send some pics and please post how this works? Sorry sometimes I feel like total noob.

jaaa

Have you checked for leaks on top of the flanges just past the cats while the car is running and up on jack stands? It usually leaks on the top side of these flanges. The Y pipe side of the flanges erode/rust over time and start to leak(hence the 420/430 codes). I managed to fix mine by taking out the Y pipe and grinding down the flanges flat again and using liberal amounts of Permatex Ultra Copper Sensor-Safe High Temp RTV on the both sides of the gaskets as I put it back on.

Sherl

So I was reading this thread and my car also exhibit the same symptoms. I did change Fuel Filter under the right rear passenger side. Not the filter/pump inside the tank. So my mechanic did suggest me a possible fuel pressure drop instead. So where should I start? Assuming I have not checked the Fuel Pressure in the line, should I check before and after in this case or start replacing components beginning with Fuel Pressure Regulator? I still do have P0420/0430 codes and probably would start working on this as well.

jaaa

I doubt the p0420/0430's are related to your no start. I think you need to concentrate on fuel pressure/delivery problems. Have you tried cycling the key on to off a few times before you try starting? If you never have a no start doing that, I would be suspicious of a low fuel pressure situation.

Sherl

So what I was aware about that LS400 doesn't prime its pump when its turned to ON either. Its starts priming only when the ignition cranks. I might be wrong but I read in an old thread on a 92LS400

Also here is my o2 sensor reading. So I was in the conclusion that I post-o2 in both cats should read as 0.7-.8 atleast at 2k RPM but mines 0.2-0.3 - what does that mean.


At Idle, 800 RPM

At 2200 RPM

dicer

The legend red colors look the same on here. So are the shorter amplitude graphs checked at the same volt settings as the larger ones? And are the smaller ones the down stream ones?

Sherl

Reference

python

iTrader: (4)

fuel or spark..have to figure which it is

Sherl

Fuel filter is changed what else could be? Spark plugs are like 60k miles old.

python

iTrader: (4)

many things..best to take it to a rep. shop

Sherl

Honestly I haven't found any in Charleston SC