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  #1  
Old 01-05-2021, 05:47 PM
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1985 300D Broken EGR Pipe Mystery: Bad Oxy Catalyst?

Hi all,

This is my first post, though I have read through Pelican Parts guides and threads for years. I am the owner of a 1985 300D (123.333) California model, and my question has to do with the dreaded Oxidation Catalyst.

Background:
I am the third owner and this car appears to have been well maintained and cared for, including having done the warranty replacement of the trap oxidizer with the oxidation catalyst in 2003. Flash forward to 2020 and my mechanic and I have a mystery on our hands: I had been noticing exhaust smell in the cabin and my mechanic traced it back to a broken EGR pipe (617-142-02-64) and replaces it. The smell goes away, only to come back a few months later. Once again, the EGR pipe is broken. Oh well, faulty part - my mechanic dutifully replaces it under warranty. A few weeks ago, the smell comes back and, sure enough, the EGR pipe is broken once again. My mechanic thinks that the oxidation catalyst (to be clear, it’s not an old trap oxidizer) is likely clogged to the point of building up too much pressure in the exhaust system such that the EGR pipe keeps failing. Interestingly, I have noticed no diminishment in power. He recommends replacing the entire catalytic oxidizer with the ‘straight pipe’ system that is very popular on these forums. Of course, he can also replace the oxidation catalyst and the jobs will be similar in price. However, my philosophy tends to be repair-before-replacement, if at all possible.

Questions:
1) Has anyone experienced backpressure in the exhaust system building up to the point of breaking pipes? Could excessive vibration due to worn motor mounts be the culprit instead, as another mechanic I spoke with hypothesized?
2) Beyond getting a new oxidizer or straight-pipe system, does anyone have any experience repairing/declogging a catalytic oxidizer? Can it be done?

Many thanks for the help!

Rob

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Old 01-05-2021, 10:32 PM
dkr dkr is offline
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Why don't you just delete the EGR? Your intake and car will thank you over time.

Dkr.
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Old 01-06-2021, 12:28 AM
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If you can post a picture of the broken EGR tube.

One would think that if there was enough back pressure to burst the EGR Tube it would happen while you are driving and there would be a loss of power then when the tube bursts you power would come back.
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Old 01-06-2021, 10:39 AM
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I'd expect if you had enough back-pressure in the exhaust to burst a stainless steel tube that you'd have much bigger problems (like burnt valves or extreme loss of power). I'd be looking for a vibration problem that's causing that pipe stress. Something is loose and allowing movement or that engine must be idling like a jackhammer.
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Old 01-06-2021, 07:55 PM
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Quote:
Originally Posted by Diseasel300 View Post
I'd expect if you had enough back-pressure in the exhaust to burst a stainless steel tube that you'd have much bigger problems (like burnt valves or extreme loss of power). I'd be looking for a vibration problem that's causing that pipe stress. Something is loose and allowing movement or that engine must be idling like a jackhammer.
Yes.
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Old 01-07-2021, 02:28 PM
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Thanks all! Yes, I highly suspect it's a vibration issue and not backpressure, for the reasons stated on the thread (i.e. car running normal otherwise). It's likely something loose since the idle is within normal behavior (no jackhammering) and the motor mounts were replaced in the last few years. I will post how I eventually resolve the issue.
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Old 01-07-2021, 06:13 PM
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Quote:
Originally Posted by BlauBenz85 View Post
Thanks all! Yes, I highly suspect it's a vibration issue and not backpressure, for the reasons stated on the thread (i.e. car running normal otherwise). It's likely something loose since the idle is within normal behavior (no jackhammering) and the motor mounts were replaced in the last few years. I will post how I eventually resolve the issue.
Take a look at the motor mounts. More then a few people have had the mounts collapse with in a year or two. Also the Transmission Mount also does part of the duty.

Also it does not take heavy vibration to break stuff. Just a resonance that the part does not like. Take a look at your crankshaft damper.

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