MOT 2020 - Part 4, Diagnosis and Preparations

MOT 2020 Aug 3, 2020

Like previously stated in “Part 3, MOT test”, the car failed to pass on both the head lights light image and the 4 gas emissions test.
The head lights are easy to check, just see the pictures. Not shiny and clear at all.

Pretty faded head light lenses
Pretty faded head light lenses

As for the emissions test, things are a tad different. I’m no mechanic, I have to do my own research. This usually means a lot searching for official repair guides, installation guides from 3rd party parts suppliers, and hopefully both subjective and objective forum posts.  
Both subjective and objective comments are very helpful to spot any trouble or annoyances, that are good to know before you work on something with no or low levels of experience..  
Because service writer said it’s a suspected catalytic converter failure, I did not want to guess my way out this time. Especially because I couldn’t see for myself what’s the problem, except for some reading from a highly specialized measuring device.

So on I went, to my trusty (computer)tech forum, to ask for some car troubleshooting.  

I shared some information, and these pictures:

Results 4 gas measurement
Results 4 gas measurement

The catalytic converter should have around 50 deg C difference in temperature before and after the converter. It was hard to capture with an idle engine, it kept cooling off rather quick. It was doing at least something.

I also looked for some obvious faults in the engine bay, and found none. The throttle body also looked clean from above.

Clean-ish looking throttle body
Clean-ish looking throttle body

I also shared some pictures from “Torque for Android”, a smart phone app that can extract live data from the car’s Electronic control unit (ECU). It can also read faults, log data, but most importantly: show (live) ECU readings from both oxygen sensors. The first oxygen sensor tells a lot about the way a engine runs (problems with fuel, air, or ignition etc., or if it’s a problem after the combustion cycle). The reason for two oxygen sensors is one that shares the "actual" information. The second sensor only verifies the correct working of the catalytic converter.
I decided against sharing screenshots from Torque. I made the UI very cluttered to fit as many things as possible in one screen, and it got hard to read.

Now for the complicated part, I’ve made a “TL;DR” at the bottom for the folks that are not dedicated to read all this. For those who want to try to understand my rambling, the text between the dividers is for you :D


What the oxygen sensors try to achieve is a tad complicated, and I do not understand it completely myself. But I’ll try to give an idea of how it works. There’s also other good sources on the internet explaining this way better.
The oxygen sensors itself are not necessary for a basic engine to operate, however, for good emissions there’s need for a good combustion. Because oxygen sensors also provide useful metrics for the ECU, both got integrated pretty well because of that. This also means that the ECU will fail to manage the engine properly, if the oxygen sensors go haywire. Faulty oxygen sensors can actually cause a way higher fuel consumption and combustion problems.
Both actually happened last year due a faulty oxygen sensor, and I’ll make a post of that soon.
Anyways, Lambda 1.0 is a “perfect” mixture of air to fuel. A lower value means too much fuel, a higher value means too much air.
The MOT test accepts values between 0,97 and 1,03. The readings show a Lambda value of 0.984 at stationary and 0.976 at a increased engine RMP (2500-3000 RPM). The increased RPM measurement is close to a fail, but a MOT pass nonetheless. This indicates it’s very likely not a combustion problem.  
CO2 being very close to 15% also confirms this idea, a value exceeding 16% could indicate combustion problems.
Both oxygen sensors did move a lot around while graphing, and that’s apparently a good indicator for a lazy catalytic converter.
The first oxygen sensor’s data should move around a lot. The second sensor should be fairly stable, only responding wildly to full throttle or coasting. This wasn’t the case.

Then there’s the high carbon monoxide (CO) value from the 4 gas test. A good working catalytic converter will convert most of the generated CO (with some other elements) in to H²O and CO², water and carbon dioxide.
In other words, it converts very toxic stuff in to not very bad stuff.  

Additionally, I know that the previous owner drove with a check engine light for over half a year, probably adding somewhere around 15.000km while doing so. This was due a faulty oxygen sensor, the resulting not optimal combustion probably damaged the catalytic converter in the process, reducing the effective lifespan.
From different sources on the internet I’ve also read that the average lifespan of a catalytic is somewhere around 10 years and 160.000km.
With 17 years and ~18300km, half a year of driving with a check engine light and a failure on CO in the MOT, it probably was time to replace it soon anyways.


TL;DR: A lot of things point in the way of a worn out catalytic converter, and not something else.

So with that in mind (while crossing my fingers for the correct diagnosis), I started searching for replacement parts.  
I did not like the idea of buying used car parts, especially because there’s no warranty at all that it will pass the MOT test. Used OEM catalytic converters were expensive as well, and new 3rd party replacement catalytic converters weren’t that much more price wise.  
For around a tenth of the total quotation I had a new catalytic converter with warranty, and “everything necessary for the replacement”. Which of course, wasn’t everything necessary for the swap. The single use nuts for cylinder head studs, most important of all, weren't included.  

New catalytic converter, with two new gaskets and new studs bolts for the exhaust pipe.
New catalytic converter, with two new gaskets and new studs bolts for the exhaust pipe.

So I ordered some new single use nuts, and a polishing set for the head lamps, and prepared the car in the mean time.

The single use oval exhaust locking nuts
The single use oval exhaust locking nuts

The location of the catalytic converter in question is rather toasty, to warm it up quickly and keep it hot. It’s integrated into the exhaust manifold, a place where all nuts and studs tend to get rather stuck after a while. Everything gets hot and cool in rapid successions, warping metal in the process. Oxidation can also play a big role, in breaking stuff or getting it off easily.

Engine bay, brown exhaust manifold located at the bottom middle.
Engine bay, brown exhaust manifold located at the bottom middle.
Exhaust manifold with oxygen sensor between the far right and middle-right exhaust pipe.
Exhaust manifold with oxygen sensor between the far right and middle-right exhaust pipe.

To remove as much oxydation as possible, I had organized me some creep oil and rust remover in spray form. This had to be applied before the actual repairs, and the more time it had to work it's way in, the better. So I applied it a few times in the week before I met in Bremerhaven to swap the catalytic converter.
In the previous pictures the exhaust manifold already got treated with both sprays.  

Creeping oil applied, had to work it's way in first
Creeping oil applied, had to work it's way in first
Rust remover applied to the creeping oil, it changes it's color to purple after a few seconds
Rust remover applied to the creeping oil, it changes it's color to purple after a few seconds

For now the preparations. This weekend will be actual work done on the car, I really hope everything comes off easily.

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