# P0420 - Loud Engine Noise - Need Diagnosis Help



## chrisc351 (Feb 17, 2011)

First of all, thanks for taking a look at this thread.

My girlfriend has a 2005 Jetta 2.5 with just under 140,000 miles. She just had the P0420 light come on. Currently the car makes a very loud rumble at low rpm and is quite loud during acceleration but silent when off of the gas pedal. I've replaced the cat and both oxygen sensors as well as cleaned the MAF to no avail. 

A friend of mine thought it might be timing chains but it was changed just last year, so I doubt it. Does anyone have any idea what it may be? Could it be a bad fuel pressure regulator or possibly a vacuum leak? This thing is driving me up a wall :banghead:


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## Master_P (Feb 10, 2012)

Was the P0420 code resolved after replacing the cat?

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## chrisc351 (Feb 17, 2011)

No, the same loud running noise is apparent and the code came back after I erased it. My dad seems to think it could be the PCV system. I checked for general vacuum leaks on the engine cover but not on the crankcase cover. Could it also be the EGR Valve?


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## Master_P (Feb 10, 2012)

I don't think the 2.5L has an EGR.

P0420 is catalytic efficiency. Is this a BGP or BGQ engine?

If the cat and o2 sensors are good, you might want to look for an exhaust leak at the collector or head. Maybe the SAI valve is stuck open?

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## le0n (Jun 12, 2013)

honestly, unless we can hear what you're hearing, this could be anything.

youtube videos don't usually help isolate noises, but if it is an obnoxious thing, perhaps it will help in this case.

vacuum leaks are not fun on any engine. luckily on this engine, there are not too many places for them to occur.

like stated above, the SAI could be the culprit. check the hoses coming off of it and make sure they are properly connected and not cracked.

the other areas to check would be the N80 purge valve hoses and the PCV hoses.

those are the main places i would start with.

have you done a vagcom autoscan on it yet? it may keep you from chasing your tail.


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## chrisc351 (Feb 17, 2011)

Thanks guys, I'll check those out this evening. 

The only way I can describe the noise is that its a low grumble and when accelerating its almost like it has an exhaust leak. What makes me think it isn't a leak at the manifold or the head is that its dead silent outside the car. The noise is only audible from inside the car. It does make the low grumble when in park.


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## le0n (Jun 12, 2013)

cool. let us know.

my thoughts, even after acknowledging what you've already stated is that it could be a collapsed or damaged catalytic converter.

did you change it yourself, or did someone else change it for her?

also, was the cat replaced to remedy the rumble and p0420? i'm still trying to establish a timeline of repairs vs. symptoms.


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## chrisc351 (Feb 17, 2011)

She gets home later tonight so I'll get out there with a flashlight. 

I replaced the cat myself, the old cat was actually fine, so I'll probably sell it. The car has a good amount of vibration. 

The timeline was replaced the O2 Sensors and then the Cat. Do you think a fuel filter could possibly do it?


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## le0n (Jun 12, 2013)

chrisc351 said:


> The timeline was replaced the O2 Sensors and then the Cat.


ok, thanks.

Was the rumble there prior to replacing the stuff above?

Does the engine idle like it is supposed to, or does the idle jump around?

How's the gas mileage?

Does the exhaust smell like the engine is running rich? (your rear o2 sensor thinks it is)

Did you use the oem kits to replace the o2 sensors, or were they universal fit sensors?

Did you replace with an oem cat, or did you go with a hi-flow cat?

Again, did you or can you get a vagcom scan?

and no, the fuel filter won't do it.

what transmission does it have?

if it has that much vibration, i wonder if a worn transmission or motor mount is the culprit.

while you're there tonight, see if you can man-handle the engine (from a solid part of the engine (not the engine cover)) to get it to move around to possibly isolate a failed mount. this won't explain the emissions error code, but it may help solve the rumbling.


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## chrisc351 (Feb 17, 2011)

The rumble happened simultaneously with the P0420. This makes me wonder if it is the motor mounts. 

A few weeks ago she had to bring the car in to the dealer to have the whole wire harness replaced because it was misfiring, after new plugs and packs. I noted that the engine was shifting around a lot while it was misfiring. I wonder if that strained the harness enough to wreck it and possibly pulled a vacuum hose off somewhere. 

I'll let you guys know soon!


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## le0n (Jun 12, 2013)

^^ That's good to know. 

I would have expected a different error code if it were a broken harness. 

How long after the dealership work did this symptom occur?

Weak mounts could be allowing the opening an closing of an exhaust leak. Inspect all of those exhaust connections again while moving them around with your hands.


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## chrisc351 (Feb 17, 2011)

I couldn't find a vacuum leak anywhere last night, but I did take a closer look at the cat I pulled off. Obviously the flex pipe should move around but should it slide up and down about 6-8 inches?


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## le0n (Jun 12, 2013)

flex? yes.

slide? no.

are you stating that the old one you removed slides?

upload a photo of what you're talking about.


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## chrisc351 (Feb 17, 2011)




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## le0n (Jun 12, 2013)

yeah. that looks plenty broken.


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## chrisc351 (Feb 17, 2011)

Well, good and bad news. I bought the motor mount (expensive) and waited for the girlfriend to come home from work to get started. Got it out in the shop and started pulling it apart and bam, I noticed a bolt was missing from the bracket that attaches to the engine. 

The top bolt, I was able to turn by hand. After I got the bolt head out, it had been sheared off. Looking at the rear bolt to the bracket, it had also sheared off inside, just completely missing the bolt head, unlike the front. 

Going to get new bolts tomorrow, mount still looks fine. This would definitely cause vibrations!


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## le0n (Jun 12, 2013)

You're starting to shed some light here. 

Sheared bolts like that are not a good sign. Once one mounting point fails, it puts strain on the other mount points. Inspect all of the mounts. 

Check you new flex pipe for damage too.


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## chrisc351 (Feb 17, 2011)

The motor mount was fine just needed the bolts to hold it in. I put new bolts in and the car is silent again. Cat looks good and the code hasn't come back in over 200 miles! Hopefully just routine maintenance from here on out! 

Thanks Le0n and Master_P for the help! :thumbup:


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## le0n (Jun 12, 2013)

nice 

hopefully things are situated now.


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## Master_P (Feb 10, 2012)

Great find!!!!

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