Catalytic Converter - Help With Diagnosis Please

[Mike Floutier]

I have a 2002 LS430 with 300,000 miles on the clock. I have had a mis-firing problem that I've been driving around for 15,000 miles.

4 or 5 days ago the car stopped responding to any more than 50% throttle; the engine still sounded smooth and the transmission would kick down BUT no more power to the wheels.

The next morning the problem went away BUT when I restarted the car it came back; slightly worse.

This pattern repeated each day with it struggling to get up hills and finally refusing to start at all - stopping immediately after starting (when hot) had become common anyway in the last week or so.

As well as the P0301 code which I've had for 15,000 miles, I was now getting P0420 & 430 codes for the Catalytic Converters - although only when I put my foot down, not when running gently.

Anyway, to cut to the chase, I suspect that I've got one or more blocked CATs. I've googled how to test for blockages and that seems fairly straightforward .

Now, bearing in mind there is one CAT after each bank's manifold and then the pipes join together to feed into the third CAT, what I want advice on is this-----

For the blockage to cause such a severe reduction in power and finally the engine not to start at all, COULD this be just one of the upstream CATs OR must it be BOTH.

Following on from that, what is the likelihood of both upstream CATs failing at the same time - in the space of just a few days. Would not a more likely diagnosis been the third (rear) CAT.

Any suggestions as to how I should approach this would be welcomed:)

[dave1]

Driving 15k miles with cylinder 1 misfiring? Very difficult to diagnose your cat problems. The codes you give are that the cat system is below the efficiency threshold. Unburnt fuel from your misfire entering your cat when it is warmed up will damage your cat as the cat will burn the fuel and overheat. I would suspect that this is what has happened.....possibly melted core.

[Mike Floutier]

Thanks Dave, yes I agree that the CAT on bank 1 is most likely blocked.

What I'm wondering is whether a blockage on just one bank alone could cause such a severe loss of power - including causing the engine to fail to keep running immediately after ignition.

I guess the answer may be yes. If you consider the effect of the force of "engine braking" available under normal circumstances and then consider how this force would be magnified with the back pressure of a blocked CAT....does that make sense???

My reason for trying to understand this is because, as part of diagnosing the problem, I intend to remove the upstream o2 sensors in turn and watch for improvements in running. I need to know whether to expect an improvement.

[dave1]

I think that it is quite possible that it your main cat that has gone. Have you had a compression check on your engine? It would seem that the first thing you need to do is establish why you are getting a misfire. Fuel leak, air leak, clogging of injector, plugs, coil pack, head gasket or head? So many things could be causing it. Any damage to your cat will have been caused by the misfire so that misfire problem needs sorting.

[Mike Floutier]

Thanks Dave, I totally agree that the underlying cause of the CAT deterioration needs dealing with if the same thing is not to repeat.

I've actually checked all the things you mention regarding the mis-fire - the car operates on a dual-fuel system (propane/petrol) so this helps a lot.

I've been reading around on the internet and it seems that exhaust valve recession (a common problem with propane) - causing mis-firing - is often not picked up by simple compression tests (even cylinder leak-down tests) - see second half of para. 3 in http://atsnm.com/pressuretransducerscasestudies.htm#TheMisfiringRangerMystery

Further reading threw up - http://atsnm.com/pressuretransducersarticle.htm#hidden - which I found very helpful because the main problem with my ideas of diagnosis was exactly as it says - ie. removing o2 sensors and unbolting exhaust components (after 10 years and 300,000 miles in my case) could be difficult.

I guess what I really need, both for diagnosing the CAT restriction AND the underlying mis-fire is a tech who is equipped and competent to carry out this kind of work.

On the basis that it would not be cost-effective for me to invest in that sort of gear I will concentrate my efforts now on finding that Tech.

Will let you know how it goes.

Thanks again!

[steve2006]

If you can find a suitable pressure gauge and adaptor remove the pre cat O2 one at a time, connect the pressure gauge to where the O2 sensor normally fits,start the engine and then take a reading.Reasemble this side then repeat for the other side.

I think the normal reading for a clear exhaust is around 3 psi so if you have one bank in excess of this or a large imbalance between the two this would suggest a collapsed cat on that side.

Another test which tends to be more accurate than a compression test is a cylinder leakage test where compressed air is fed into individual cylinders on the compression stroke at a predetermned pressure which is then monitored over a period of time to see if pressure is lost indicating poor valve seating, piston ring blow by or other causes.

[Mike Floutier]

Thanks Steve,

I agree with the idea of removing each o2 sensor in turn BUT as my current Lexus tech points out, this could lead to cross-threading. So, tonight he is going to try removing both CATs and seeing if this allows the engine the breathe sufficiently to run better and get power to the wheels. One thing's for sure, we will notice the difference (even if it's only the noise - lol!

If this does the trick then it remains to determine which CAT is the culprit. This seems a bit tricky to my mind.

In scenario 1. where either one of the front CATs is blocked BUT the rear CAT is ok, THEN replacing the CATs one at a time will show which is to blame (since only one blocked bank is clearly sufficient to cause the symptoms).

In scenario 2. where the rear CAT is blocked, (has anyone ever heard of this???) then, reinstalling both front CATs (having tested OK in scenario 1.) and then disconnecting the "Front Pipe" (section of pipework containing the rear CAT) from the downstream end of both front CATs this should 1. prove that both front CATs are OK and 2. show that the rear CAT is blocked.

I've got a whole set of new Lexus CATs on order along with all the nuts and bolts in anticipation.

Assuming this all goes to plan it will then just be necessary verify what caused the CAT blockage and fix it so preventing a recurrence .

I like your leak-down test idea for this Steve as it will be a lot cheaper than getting a $2000 atsnm pressure transducer/scope system.

[Mike Floutier]

Ok well this will make you laugh, he removed both front CATs and it didn't make the slightest difference!!!

Then I got this inspiration that I'd replaced the MAF only a couple of weeks or so before this horrendous problem arose.

We had already considered this and for some reason ruled it out. Fortunately I had kept the old (300,000 miles old) MAF in the garage. We put it back on and the problem disappeared.

The moral of the story (for me anyway - I'm sure no one else has done this) is not to put too much faith in cheap after-market parts.

Fortunately LexusPartsDirect were kind enough to give me a full refund on the £1300 worth of CAT parts that I'd pre-ordered.

Thanks for helping me with this!