Catalytic Conveter Failure?

[A26B]

I had a great day yesterday with a ZR-1 owner from Switzerland who came to visit. He is in Tulsa, on work, for a few days, happened to find me on Google & emailed to see if he could come to my "shop" to get some LT5 parts.

Background: 1990 ZR-1, with about 40,000 miles. he has owned the Z for about 6 years & does his own work.

Background: He has installed an EMS, stand-alone engine management system with A/F monitoring on each bank. This was done as a last resort to solve a chronic, inoperative secondary system.

Background: The engine began to smoke & use oil. On disassembly, he found the oil control ring lands , to be worn completely down to the same level as the center groove, on the RH bank only. The engine was repaired using NOS liners & Total Seal rings on the RH bank. Oil consumption was improved and compression was even on all 8 cylinders.

Tuning required about 10% less fuel on the RH bank. Endiscopic examination showed cylinder scoring on the RH bank & some flaking of the nikasil coating. The RH bank was again disassembled. There is no visible damage to the piston top or cylinder heads.

Endiscopic inspection of the catalytic converters showed the RH cat to have the catalyst "all pushed towards the rear", "blackened" and "somewhat melted but not seriously."

There is no miss, engine makes good power and virtually all sensors have been replaced before #1, including the ECM.

The questions are:
1. What could have caused the incredible wear to the original oil rings on the RH bank only?
2. What could have next caused nikasil flaking to the NOS sleeve/liners on only the RH bank cylinders?
3. Why would the RH bank need about 10% LESS fuel than the LH bank to achieve balanced A/F ratios?

I can think of no engine internal that could caused #1 & #2, with the possible exception of secondary throttle valves not opening on the RH bank causing #3.

I have heard of 90 model problems with cat failure and catalyst reversion into the engine, but there is no visible damage indicating that. The cat would seem to be bad but not to the extent of complete blockage. Could exhaust restriction/backpressure cause these problems?

What say you experts out there? Who has experienced Catalytic Conveter failure & what were the symptoms?

PS: I have invited him to the forum and to become a Registry member to take advantage of the fellowship & great help available here.

[Blue Flame Restorations]

First, let me state that I am in no way an engine expert (ask Pete..LOL)

Is it possible that the CAT on the right hand side restricted the heat to escape and this caused the RH bank to run slightly hotter -vs- the LH bank?

Hey, I'm a body man............:handshak:

[QB93Z]

I also am not an expert.

My first guess would be a leaking injector (some time in the past) that caused continuous fuel flow whenever the fuel rail is pressurized. The excess fuel in the cylinder would effectively "clean" the lubrication film from the cylinder wall, causing increased ring and liner wear.

In addition, the excess fuel in the exhaust stream would over heat the catalytic converter.

In the extreme case that I saw, which was an injector passing full flow, the catalyst media lasted about 15 minutes before it was totally destroyed and partially ejected from the tail pipe like a roman candle. Later examination found some of the substrate lodged in the X-Pipe causing exhaust restriction.

The change in tuning may be confusing the diagnosis because we don't know the engine condition when that "baseline" tune was done.

Good luck

Jim

[zr1don]

Jerry,

I've had two cat failures since 2006. The first failure was with factory cats in 2006, replaced w/aftermarket. In Aug 2010 I could not pass CA emissions testing and found the rt side cat was sans its internals. (thefirst failure was also rt side cat sans internals). I have data scans from both periods that show the rt bank was using slightly less fuel than the left. After replacing the cats both times the imballance went away. I don't know this for a fact, but I believe the failures were caused by oil in the intake coming throiugh the pcv system. I have installed an oil catch can. Befor installation, I found oil in the plenum mostly collected in the rear rt side. The vacuum line from the pcv had oil in it befor the catch can installation and after installation it is dry.

Hope this helps, I don't have an explanation for the ring problem.

[A26B]

Jim,
Thanks for the input. I considered the possibility of a bad injector washing the cylinder, but couldn't conceive of the same thing happening to 4 cylinders, all on the same bank.

Don,
Very interesting that both of your cat failures were also on the right side and even more interesting about the fuel.

You may be onto something regarding the cat failure, but I still don't understand the severely worn oil rings and the comprimised nikasil (which may not even be related). In any regard, a good PCV catch can system would be in order when the cats are replaced. I did recommend that he ditch the stock manifolds in favor of long tube headers & aftermarket cats.

[gbrtng]

I have heard of 90 model problems with cat failure and catalyst reversion into the engine, but there is no visible damage indicating that. The cat would seem to be bad but not to the extent of complete blockage. Could exhaust restriction/backpressure cause these problems?

Wouldn't back pressure cause piston and top ring erosion instead?
He should rule out the back pressure theory by instrumenting both RH and LH exhaust pipes in front of the cats for pressure. I think some of the professional tool shops would have suitable gauges.

Were all the RH liners sloughing nikasil ??

This sounds like a question for Graham -

[HAWAIIZR-1]

Wow, Jerry I find this very interesting. As you remember my 51K mile motor with the entire left hand (driver's) side bank had lower compression with burning oil and I was puzzled as to why only that side as a whole. My cylinders were highly glazed and wondered what caused that and why only on one side bank? I remember in our conversations about the chrome plated Total Seal rings and nikasil compatibility too. I wish I did not have my original exhaust manifolds and cats in long term storage with no access for 5 years or I would be curious to look inside to see that went on too.

[Marc Haibeck]

The description of the ring wear associated with catalyst disintegration matches stories that have been told by Graham Behan at the BG technical sessions.

As I recall, it was said that GM saw this kind of failure at least twice. The changes that were made to the ?93 ? ?95 exhaust manifolds were to improve catalyst anti-reversion. They even took a loss of about two hp accomplish it.

[A26B]

Craig,
Thanks for reminding me about your LH bank problem. That fact had slipped my mind, but does possibly point to the cat also.

Marc,
Thanks for chiming in with Graham's commentary. Excellent information. My recollection of that discussion somehow visualized significant chunks of broken catalyst getting back into the cylinders. Now however, I can more readily visualize loose, broken catalyst being pulverized and then, even more easily, being transported to the cylinder during reversion and distributed to all cylinders on that bank over an extended period of time, therefore affecting all cylinders similarly.

Just goes to show how valuable these fireside chats with the experts really are.