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A question for SDi owners about the exhaust


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Earlier today my car was in a garage. She was having a new alternator fitted, which was an expense I didn't need, but ho-hum.

Anyway, the mechanic told me that whilst the car was on the ramp and he was underneath, he spotted a hole near the cat and told me that soot was coming out.

I remember reading somewhere once that a cat isn't needed on a diesel, but is this right? Surely VW wouldn't have fitted one if it wasn't needed? Have any SDi owners taken the cat off and put a straight through pipe in its place? If so, where did you get the replacement pipe from? And has your car passed the MoT without a cat?

Any help, advice and tips would all be gratefully received.

Thank-you :)

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It's not a cat it's a DPF, but people call it a cat.

It should fail an mot with out one, as the rules are if it had one from factory it has to be present at MOT time, how ever Diesels don't have an emissions test as such it's more of a smog test, and it will pass the test fine with out the DPF providing everything is ok.

Just depends how fussy the tester is on the day really.

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It's not a cat it's a DPF, but people call it a cat.

It should fail an mot with out one, as the rules are if it had one from factory it has to be present at MOT time, how ever Diesels don't have an emissions test as such it's more of a smog test, and it will pass the test fine with out the DPF providing everything is ok.

Just depends how fussy the tester is on the day really.

What? No, it's not a DPF, it's a Catalytic converter. No Lupo was fitted with a DPF, they are all too old for one of those.

Diesel cars only have a smoke test at MOT time. This checks for soot coming out the exhaust. The Cat makes no change to this so you'll still pass an MOT without it, however the rules might change in the future so you might be best off keeping it fitted.

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Thanks everyone for your advice :) I'm still in two minds about what to do though :unsure:

I'd do it. If you do, let us know if you notice any MPG gains. I think Geoff said he de-catted an SDI and saw a slight performance increase and MPG increase. There's no reason why you shouldn't in fact. It's a case of just welding a straight pipe in too. Nothing too intensive I believe.

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It's not a cat it's a DPF, but people call it a cat.

It should fail an mot with out one, as the rules are if it had one from factory it has to be present at MOT time, how ever Diesels don't have an emissions test as such it's more of a smog test, and it will pass the test fine with out the DPF providing everything is ok.

Just depends how fussy the tester is on the day really.

As jon has said. This is all wrong. DPF's work differently.

Get a decat. It will free up a tiny bit of power and give very slight mpg gain IMO. Plus when it's a straight pipe there is nothing to fail. MOT wise old diesels get away with no cat so it's no problem. I doubt the rules will change aswell just yet. Plus some people switch them over every mot time on petrols and the decat is worth money if sold.

Edited by Dblock
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Decat on a diesel is great, better exhaust note, better performance, better mpg and a cat isn't required for an MOT, did it on my Peugeot HDi and it was ace

It does a little more on an engine with a turbo but I agree. Did you turbo seem to spool a little faster and pull harder up high too?

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It does a little more on an engine with a turbo but I agree. Did you turbo seem to spool a little faster and pull harder up high too?

This. I know someone who de-catted a Bora and said it made a hell of a difference...

At the end of the day, like I said earlier, you can go ahead an de-cat. As far as I'm aware, it's just a case of welding in a pipe. I'm sure that Geoff has done this, I'm waiting patiently for his input :D He'll be around sometime soon I'm sure. Logically speaking, you'll get a slight performance increase, but potentially a significant MPG increase and that's what attracts me to a de-cat, even when my cat is perfectly fine. I've been tempted to rip mine off before. I'm sure one day I will...

Edited by Skezza
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Why is it a mot fail.

It shouldn't be. I think it's a fail on petrol's though so it makes sense for some testers to forget they're working on a diesel and just go by petrol rules and fail it.

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It shouldn't be. I think it's a fail on petrol's though so it makes sense for some testers to forget they're working on a diesel and just go by petrol rules and fail it.

That's my point. On a petrol if a cat was fitted originally a cat has to be fitted during mot. Not so on diesels.

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Well I have recently been informed that the rules about this have now changed. My MOT tester friend also confirmed that a decat on a diesel on now an MOT fail. But I can't find any official info anywhere - only stuff about missing DPF's now being an MOT fail.

Now for the record my TDi has a decat so I'm not against it, but if it's true then I think it'll be going back in...

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believe me it's not an mot fail I've de catted, blanked my egr and removed the intake trumpets. no engine about to die lights or nowt. diesels of our era do not require cats for mot- simple. Geoff. by the way I haven't been able to see properly, that's why I haven't been on, still can't.

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believe me it's not an mot fail I've de catted, blanked my egr and removed the intake trumpets. no engine about to die lights or nowt. diesels of our era do not require cats for mot- simple. Geoff. by the way I haven't been able to see properly, that's why I haven't been on, still can't.

What's going on Geoff? Your eyes have been knackered for a while?

I've heard mixed reviews about the EGR blanking. Logic says it's a good idea, but some reviews have been quite negative suggesting it can wreck fuel economy etc.

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How can or wreck fuel economy?

No idea. I just remember reading on another VW forum that someone who blanked their EGR noticed about a 5mpg drop which is quite significant in my opinion. There was mixed replies. Some people said it was merely placebo after changing it, others said that the warm air can help the engine running warmer or something, I don't remember the exact words used. It's not something I've looked into yet if I'm honest.

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No idea. I just remember reading on another VW forum that someone who blanked their EGR noticed about a 5mpg drop which is quite significant in my opinion. There was mixed replies. Some people said it was merely placebo after changing it, others said that the warm air can help the engine running warmer or something, I don't remember the exact words used. It's not something I've looked into yet if I'm honest.

believe that 1 thing out of the 100's telling you

facepalm-card.jpg

But seriously choice is yours. I've only seen increases in economy and power. Plus your making the engine more reliable. EGR's only really work for the first 30k miles after that they are a silly way to reduce NOx.

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believe that 1 thing out of the 100's telling you

facepalm-card.jpg

But seriously choice is yours. I've only seen increases in economy and power. Plus your making the engine more reliable. EGR's only really work for the first 30k miles after that they are a silly way to reduce NOx.

Now now Dblock no need for a facepalm :D I guess his problems may have due to something else. Perhaps a dirty manifold? I'm going to blank mine off eventually, but when I do, I really need to clean the intake manifold too. 120k worth of crap there. That's a job for another day however, probably nearer the summer.

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The dirty manifold is also thanks to thr egr. If yours is vacuum controlled you can test blank it off. If so why not compare what you prefer?

I need to clean my inlet manifold but it's a horrible job on a diesel with a working egr. It's like caked on carbon then it becomes gummy and horrible. You might be surprised how much there is. That's going on turbo diesels but I don't see why an sdi would be much different.

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The dirty manifold is also thanks to thr egr. If yours is vacuum controlled you can test blank it off. If so why not compare what you prefer?

I need to clean my inlet manifold but it's a horrible job on a diesel with a working egr. It's like caked on carbon then it becomes gummy and horrible. You might be surprised how much there is. That's going on turbo diesels but I don't see why an sdi would be much different.

I've been told one of the best ways is to take it off, then completely cover it in oven cleaner with a frothing agent and leave it for about an hour. Then when you return rinse and dry.

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I've been told one of the best ways is to take it off, then completely cover it in oven cleaner with a frothing agent and leave it for about an hour. Then when you return rinse and dry.

Yes that can work but some people worry if the manifold is made out of aluminium. The oven cleaner has lye in it and that reacts or eats aluminium. Pressure washer or parts cleaner is always handy too.

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