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Clogged cat issue (hopefully!!!) can anyone help to verfiy? WARNING - LONG!!! HELP!!!

3K views 7 replies 6 participants last post by  300se126 
#1 ·
I searched forever and think I am right about this, but would appreciate some feedback. Thanks in advance for reading all of this.

Our 1990 300SE has 196,000 miles on it now and recently developed some issues. Originally it was just a longish time cranking to start when cold, accompanied by crappy idle. I went through the whole big mother list to fix, including:

-New fuel pump and filter
-Cleaned injectors and replaced all seals including cold start and holder seals Injectors were replaced at 110K, so should be good.
-Replaced plugs, cap, rotor, coil, air filter
-Checked all tubing for vacuum leaks, replaced most tubing
-Resealed fuel distributor and EHA, tested idle control valve, tested fuel pressure regulator. All good.
-Tested new MAS, not the issue
-Replaced or tested any temp sensors
-Anything else you can think of, probably done once or twice by now.

No luck. To make matters worse, after all of that, the idle roughness and long cold start remained, but now the car has begun running great when cold after the long start, then stalling when shifted into Drive when the temp gauge starts to move. If I hit Drive when cold and start moving, an occasional sputter happens but otherwise I am good until I let off the gas. The only way I have been able to overcome this is to start the car cold and then get on the highway fast, then drive at least 20 minutes.

After the car reaches full normal operating temp, it runs perfectly, like a brand new car. No missing at idle, tons of power at all speeds, etc. Just great. It will start perfectly every time and will run all day like this. When in the evil sweet spot between cold and normal operating temp, the car drives fine with very light and progressive throttle input, but if the throttle is pushed aggressively, the car sputters as if about to stall. Backing off to zero and then starting throttle application again but very gingerly produces smooth operation.

The clue I got that made me suspect cat cloggage is that today when it died while only "warm" not "hot," a long time of cranking, waiting, and repeating (35+ minutes) eventually led to it making a popping noise form the cat area and then starting immediately and running in Park very well. Shift to drive... dies instantly. I opened the hood, and saw smoke from the back of the engine. Crouched at the side and looked in at the pipes - the smoke was coming from the very hot cats. Car had been running from cold only about five minutes. Rear pipes were cool as a proverbial cucumber.

Cranked some more and it backfired again, followed by instant starting. I held it at 2000 rpms until it got up to just over 80 degrees, then hit drive and sputtered to a rough start. Kept on the gas for ten minutes of driving and the problems were gone - car runs perfectly. Will restart just fine, shift great, no stalling, hesitation, etc.

So, tomorrow morning the cat is going to die a painful death. I don't need it for emissions, so I am going to use Jono's suggested method of beating the crap out of the guts with a long rod and a monster hammer to "straight pipe" the cat without needing to weld the O2 sensor to any custom pipes.

I am killing the cat regardless, since it is not needed and will provide some very cathartic brutality in the name of chasing the problem. That said... does this sound plausible as the culprit? I am so tempted to say yes given that I have literally tested or replaced EVERYTHING else... has anyone else here had identical symptoms they could trace to a clogged cat?

I would suspect a temperature sensor somewhere if it were not for the smoking cat and the backfire followed by starting - as if the engine is pressurizing the cat until the blockage blows clear of the outlet. Help!
 
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#2 ·
1- CAT problem- You know it is clogged. You want to confirm this, get emission test- if it fails -confirmed. Solution- change CAT.

2- Do Timing chain stretch test- if off - Do timing chain job.

No cut short from here- Sir.

Sahai
 
#3 ·
My clogged cat allowed the car to start and idle fine but with low power response that got worse as it warmed to the point that it would not yield >4000 rpm. The temps also kept climbing to the point that I would shut it off but it NEVER ran fine after getting up to temperature. Food for thought....
 
#4 ·
My car would not pass emissions no matter what I did. Also, it had reduced power (a key Point) and ran a lot hotter than normal. Took the cat down to replace it and stuff started to fall out. Replaced it and now fine. I'm sure the bad CAT also contributed to the higher temps along with other issues. Replaced the CAT got a valve job and now it runs fine, temps are always in the normal area and it doesn't burn oil.
 
#6 ·
Thanks for all of the replies, guys. As to all suggestions...

I have checked the chain and tensioner, which are fine. I checked the plugs for weepage from the VC gasket, which is getting leaky, but they are fine - and hard starting was never an issue when cold. Compression is perfect for all cylinders, and newer head gasket to boot.

Since I don't have to pass emissions, I cored out my cat yesterday, just hollowed the whole thing front to back. Now it is just a big fat can with an O2 sensor stuck in it. I let the repair sit overnight so the exhaust junction cement I used can fully cure - I will take the car out for a few hours today and report back. Based on all of the responses describing shot cats, I am VERY confident I did the right thing here. That and, you know, I replaced everything else!
 
#8 ·
Fixed! Not sure what clogged it, but it was very clogged...

I hollowed the cat out to keep the O2 sensor where it was, and the car now runs soooo much better! I needed to tweak the air mass meter adjustment under the tamper-proof ball to get it running like a new car again - I think one of the cat-related backfires may have messed up the metering plate. A little fiddling and it is like I have a brand-new engine! Plus, the hollow cat sounds kinda badass, so I got a free mod out of the deal to boot.
 
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