Whilst the MR2 Roadster has very few flaws as either a fun cheap sports car or as a precision tool to hit the track with, there is one thing that we as a Club feel every owner should be aware of, and that is the pre-catalytic converters (or pre-cats for short). There have been a huge amount of questions on here since the forum began regarding these, and this thread is here to hopefully answer any and all questions that have cropped up about the pre-cats, as well as dispel some myths about them.
*Please note: The Club neither encourages nor advocates the interference with emissions equipment on any motor vehicle, and we take no responsibility for any action taken by any person as a result of reading this article. All text and pictures here are for information purposes only.*
What is a pre-cat?
To put it quite simply, the pre-cats sit before the main catalytic converter in the exhaust system and help to keep the harmful emissions as low as possible for a short period after you start the car up. Of course, there is slightly more to it than that…
The main catalytic converter in the Mk3 works best at converting the harmful compounds contained within the exhaust gas at high temperatures: However, since the engine takes a while to heat up to it's optimum running temperature, there is a time when a great deal of harmful emissions are allowed to simply pass straight through the cat and are dispersed into the air. Toyota obviously wanted to keep these emissions to an absolute minimum to enable the car to be classed as a ULEV (Ultra Low Emissions Vehicle) to allow the Mk3 to be sold in California (they have practically the most stringent rules on car emissions anywhere in the world there!), so between the engine and main cat they placed two pre-cats contained within the main manifold itself. The manifold itself looks like this:
The four headers run into the two chambers containing the pre-cats, and then they're passed onto the main cat to let it do its job. The pre-cats are made from a ceramic material, which whilst excellent at absorbing the noxious gasses at low temperatures, is also highly brittle…
Why are we worried about them?
As stated above, the pre-cats themselves are not the strongest material known to man, and they have been known to break down and enter the engine, causing serious damage to the internals. When this sort of damage has occurred, you are almost certainly looking at needing a new engine.
Woah, wait a minute! How can the pre-cat get back into the engine: Surely the exhaust flow pushes it all out?
True to a certain extent, but here's the clever bit…
The 1ZZ-FE engine (Toyota's designation for the engine inside the MR2 Roadster) is a very clever piece of kit, and arguably its main party piece is the VVTi, or Variable Valve Timing Intelligent. This increases engine response all over the rev range by altering the timing of the cams, allowing for differing amounts of valve overlap in order to give great low-down torque as well as good top-end power. The 1ZZ also uses it's VVT to perform EGR (Exhaust Gas Recirculation) functions without the aid of a specific valve like other cars. Under certain operating conditions (usually steady cruise) the cams are timed to scavenge some exhaust gas back into the cylinders, as a way of reducing the high hydrocarbon emissions that modern petrol engines generate at certain times.
Unfortunately, when you combine this with some very sharp ceramic pre-cat particles, you can imagine what happens: The pre-cats start breaking down, and get dropped into the main cat which then causes excessive pressure, leading to oil blow-by in the engine. When the VVTi kicks in, the pre-cats are sucked back in and scratch and score the cylinder walls, leading to more oil passing by the piston rings and being burnt off without you even realising it. No oil in an engine leads to massive failure as every moving part grinds against metal, and in short you end up with a practically useless engine. When this happens the situation is compounded by the fact that hot oil is now allowed to drip directly onto the pre-cats and break them down even quicker, which in turn allows large chunks to block the main cat even more, which then stops any smaller pre-cat material escaping at all and sucks even more back into the engine to cause even more damage… A vicious circle of the very worst kind.
Some common symptoms of pre-cat failure are extreme oil loss, very noticeable lack of power all the way through the rev range, and horrible noises coming from your engine bay. Essentially, if you've got any of these problems and they are directly related to pre-cat loss, then it's too late. Even the oil warning light won't save you here, as by the time it comes on there's almost zero oil left in the engine anyway.
But I've read elsewhere that the pre-cats themselves are fine, it the piston rings which are the weakness…
This is where we come across a real conundrum, and a question to which no-one has a definitive answer. It's true that on very early MK3s there was a known problem with the piston rings themselves on a 1ZZ, and Toyota issued a technical document to the dealers around the world stating as such. They also changed the design of the piston rings for the facelift version of the Roadster, which became available in 2003.
Now whether it's a case of the piston rings failing, oil dripping onto the pre-cats and breaking them up, or the pre-cats self destructing and taking the piston rings with them, we just don't know. All we do know for certain is that whilst you can't take the piston rings out of the engine, you can remove the pre-cats from the manifold. No pre-cats = Nothing to get sucked back into the engine.
Okay, so the pre-cats are obviously a bad thing, but what can I do about it? Is there any way to tell if they're okay on my car?
There is only one sure way of telling, and that it to remove the entire manifold and check both the top and bottoms of the pre-cats for any signs of damage. This is the only 100% way.
I'm not very mechanically minded, so is there another way? Even if it's not 100%?
Yup, and this is the way 99% of people do it (myself included). It's very simple, and requires nothing more than a 22mm O2 sensor removal socket (Available from here for one, but you can get them at many other places as well, this is just an example), a can of PlusGas or similar penetrating oil (WD40 will do at a push, but it's a lot easier with the PlusGas), and a torch.
The picture above shows the heatshield which covers the manifold itself, and is how your car looks when you open the engine bay. Coming out of either side of the heatshield are the O2 sensors, which need to be removed to see the pre-cats from the top only.
1. Get the engine nice and warm first, it'll make this job a lot easier!
2. Spray the PlusGas liberally onto the joint where the O2 sensor meets the manifold. Leave for 10 minutes, then spray it again. You cannot use enough of this stuff, trust me! Don't worry about the steam coming off; it's not doing anything any harm.
3. Being very careful not to burn yourself on the heat shield, use the O2 socket to remove the sensors, Unplug them first from the plastic clip (it's a simple push-tab-and-release connection), and make sure you turn them anti-clockwise. If you have an older vehicle, you may find that these are very stubborn, but do persevere and don't be afraid to give it a little elbow-grease!
4. Pull the sensor out of the socket and place carefully on the floor, away from your feet. You don't really want to tread on it now you've done the hard part, do you?!
5. Take the torch and shine it into the holes. You're looking for a completely solid honeycomb matrix with no cracks or large holes in it, like this:
6. When you've finished checking (and hopefully found that they're still intact), simply screw the O2 sensor back in and nip it up with the socket. Oh, and you may want to plug it back in too. 
My pre-cats look fine! I'm safe! *dances*
Not quite: They're still very fragile, and remember you can't see the bottom of the matrix from that angle either. All this means is that your engine is still fine and you're not in any immediate danger of the pre-cats failing.
Oh, okay. So what's the next step then?
The only 100% sure way to protect your engine is total removal of the pre-cats from the manifold. This isn't a particularly hard job, but it is more involved than simply removing the sensors.
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