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Can a cat throw misfire codes?

2502 Views 17 Replies 5 Participants Last post by  euro_flaw
97 E420 142000. It happened all of a sudden, stopped at 7-11, started the car, running very rough. Restarted, no help. Felt like misfiring 2 cylinders on the same bank. I had to fly cross country for a month to help my parents the next day, so I parked it. While I was gone my gurl drove it "sparingly" presumably. When I get back the problem is still the same, codes say cyl 1 misfire, cyl 3 misfire, and random cyl misfire. Plugs are about 2 years old so I start with that. Every plug looks like it's firing even, no carbon, but well used. The only issue I could see was #3 was loose enough I didn't need a ratchet to loosen it and there was approx 1/2" of oil in the plug well, but the plug itself was not showing any signs of fouling or oil in chamber. They were all Bosch super +, no signs of anything serious.
When I fire it up it runs much better, just a slight, random miss that smooths out with rpm's, more like a small vacuum leak. Leaving the top cover off so it's free breathing, by the time I get to the end of my block the "check engine electronics" reappears, and it stumbles at the stop sign. Thinking of the 1/2" of oil in #3 plug well, I decide to "clean it out". Huge difference in power. It never had this kind of power before, I'm thinking that might be that. But when I get back it still has that slight stumble at idle that goes away quickly with rpm, and when I look underneath the front passenger catalytic converter is about to go supernova it's so hot. The other 3 are hot, but not glowing so bright they look look like they might melt. The codes say cyl 1, 3, and random misfire only, no cat codes.
So here I am, new day. I'm next going to swap 1&3 coils to 5&7 to see if the misfire follows, and run it with the the top cover on, then off, to see if that has any effect on power. I'm thinking (praying actually) that the misfires are sending unburnt gasses to the cats, and the passenger cat by some twist of physics is receiving more gas than the other making it hotter, and that by taking off the top cover I get so much unrestricted air it irrefutably proves alll I need is a cold air intake to run laps as fast as nascar.
Any thoughts? Can it be a bad cat? Can a plugged cat throw misfire codes?
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From 60K miles of experience with the M119 E420 and experience with the 5 liter M119 in my SL500.

Unless you or a PO replaced ALL and I mean ALL the vacuum hoses and breather hoses within the last 6 years, there is a VERY high likelyhood you've got vacuum leaks. And until you get the fixed with new hoses you'll be chasing your tail all over the place replacing stuff that doesn't need to be replaced.

And what codes to vacuum leaks throw.

misfire, out of range mixture adjust, cat codes, air injection codes and a host of others. unless the car is idling very smooth, good chance you have a vacuum leak.

Also, on my E420, between 80K and 130K miles I had 6 of the 8 coil packs randomly go bad. gives P030x, P0300 codes. after replacing 6 I just went ahead and replace the last two and kept them for spares.

Interestinly, the SL, with 95K miles hasn't had a single coil pack go, and I have records since day 1 on the car. (Bought it 15 months ago). So it seems somewhat random on coil pack failures.

And again, vacuum leaks CAN and DO throw cat codes, been there, done that, and as soon as the vacuum leak is fixed cat code goes away.

get a good code reader and check for pending codes. and go from their. if it shows a pending misfire code, swap the coil pack with another cylinder and see if it follows.

And finally, the M119 like plain old plugs, the Bosch from the dealer. They do not like fancy platinum plugs, copper plugs, etc. etc. But the replacement interval on plugs on the M119 is 25K miles, NOT the 100K folks are now used to.

So, get the hoses replaced then see if anything comes back.
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I personally changed all vacuum lines I could find last year which fixed a slew of problems. How ever, I have yet to find any resource that shows ALL the vacuum lines and their respective locations. The gurl just had to go shopping so I haven't had a chance to try anything yet, ( poor decision, I know) , so I've been continuing the rebuild on my sons mustang until she gets back. Is there a resource that definitively shows ALL the vacuum lines? I'm kinda under the gun as the registration runs out on 3/04/15, and the state simply don't care I've been off for a month after shelling out the $ for a cross country trip and all the incidentals around my folks place. ( do you have any idea how much a rear tire costs for a Massey ferguson tractor?) I really appreciate the input, my friends. This is by far the most important resource available to all of us. Shared collective knowledge. Too bad the potential to exploit that is so high.
Did you also replace the breather hose from the PS rear cylinder head cover to the bottom of the intake manifold.?

And something that also can develop a leak or crack is the boot between the MAF housing and throttle body. It gets hard with age and if it cracks and/or the clamps come loose it can also give similar problems.

The vaccuum hoses are pretty easy to catch all of them. just look for all the rubber hoses on the intake manifold.

Also, once you fix problems, you often need to clear the pending codes and see what comes back. otherwise takes drive cycles before they go away.
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I suspect water may also be an issues. you mentioned oil in one plug well. The M119 is EXTREMELY sensitive to water in any cylinder well. I'd remove all the coil packs, and use compressed air and/or a blow dryer to make sure the wells are completely dry.
misfires won't send fuel to the exhaust. Or at least very very litle. The reason is that when the engine detects a misfire, it monitors it, and if it misses enough to throw a code, it will then shut off all fuel to that cylinder till the code is cleared. The M119 has port injection, right at the intake valve. once fuel is shut off, the only fuel going to the misfiring cylinder would be any that goes back to the manifold during valve overlap. And the M119 has variable valve timing so at idle it has very very little overlap since intake valve timing is changed.

What would help us all figure out what may be going on is to use a code reader to clear ALL codes, set or pending with the engine off, start the engine and let it idle a few minutes and read any pending codes, note them, then see if it throws any codes while idling till it is warmed up. Again check pending codes, then drive it till it lights the check engine light and read all set and pending codes.
e-430 - that is based on system design being able to shut off an injector (s).
If I am reading the post correctly suggesting running the engine for any extended period with a glowing cat? I think is a bad idea.
M119 will shut of injector if a misfire is detected for very long. Like the minute or so of heavy misfire needed to set the CEL. It's outlined as such in M119 documentation for the M119's starting in 1996. Since it's not a direct injection engine, some fuel may still get into a cylinder from airflow resonance and valve overlap from other cylinders, but should be very little.

The fact that the OP's cat is hot indicates something else may be causing excess fuel or a blockage.
back pressure on both cats = .5 lb at idle- 1 lb at 2000 rpm steady and quick snap to 4500. I honestly thought it was the cats and nearly shelled out big $ for new. I highly recommend this test before replacing cats for any reason, either done yourself or a disinterested mechanic. (not trying to $$ for cats)

vacuum test = 13 at idle, dropped to nothing then back to 14 at 2000 rpm. removed a vacuum line and noticable difference vacuum and performance.

while engine was running unplugged the coils from cylinders 1 & 3 = no change at all. unplugging any other coil resulted in engine running worse. BINGO! at the very least a large part of the problem.

Heres where I got extreemely lucky. Since i had to go to the salvage yard anyway for a wiper motor for my truck, I purveyed the selection of benz's in the yard and spotted a relativly fresh 420. Along with a few other little do-dads I found that 6 of the 8 coils had been replaced recently. They looked brand new. I took 'em all. $5.50 each for Gods sake, even the 2 that looked older were in better shape than mine. when i got home i replaced all 8 with the ones i just got and 'Voila, smooth as silk. i could barely feel the engine running with my hand on the valve cover. An exhilerating test drive and i check the cats. Neither are even slighty red. I check the vacuum and now its steady 18 at idle, drops off at 2000 and comes back up to 18. It runs like it was a new car. Passed smog with no prob, now im good to go.

I really appreciate all the input from every one. It really makes for a thorough trouble shoot with so many suggestions and things I never would have considered otherwise. The back pressure test for the cats needs to be an absolute must suggestion for anyone thinking about their cats. That certainly saved me some coin. Thanks again.
sounds like your back and running fine. You scored big time on the coil packs at the junkyard. New ones run about $90 each at the dealer, and even ebay who knows where they came from ones, run about $25 each.

And as I mentioned before, the M119 can do lots of strange things and throw lots of codes that folks say "needs new cats". however as you have found, not necessarily so. I wonder how many replaced cats, MAF's, Throttle bodies on the M119 when the real problem was either a vacuum leak and/or bad coil pack(s).

And yes, when running as they should, the M119 is a very smooth engine. Even mine with bad motor mounts was smooth. (That is until one completely let go and would pull the purge valve hose loose!)
Thanks again. Be looking for more questions as im looking into the suspension. It just seems to me like it should ride better.
With the mileage you have, unless you or the PO did some work, I'd suspect at least new shocks. I got mine at 100K from my brother. New bilsteins definitely helped.

Also I'd suggest a good look at motor mounts and flex disks.

And ball joints/control arm bushings should be inspected.

On mine, last week at 155K miles, DS motor mount broke. completly seperated, luckily it pulled the hose to the purge valve off, ran rough, and when it did that a second time, it was hum..... whats wrong. Quick check and I knew motor mounts. I also suspected the control arm bushings were going as were ball joints. Not in need of immediate replacement, but not up to new par.

When the Indy did the motor mounts had him do a check and he confirmed ball joints and control arms while not in immediate need of replacement have seen their better days.

BTW, he also said the M119's seem to tear up motor mounts more than many other MB engines. Why, he doesn't really know.
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