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Discussion Starter · #1 ·
Hello there, I am new to this forum. I recently acquired a 2002 Beetle 2.0L manual trans with 180k on the clock. Got it cheap, and it runs fantastic aside from a few intermittent issues. Occasionally it will be running fine and suddenly it will lose half its power. Like a switch was flipped. I have run some tests with my Ancel gauge, and done a lot of reading here on the forums. I have two codes, a dreaded P0420, and a cylinder 3 misfire P0303. Rather than throw any parts at the problem (yet), I decided to check the thermostat temps. Only runs at 185 degrees F, and I have read thats just not hot enough for this engine. Maybe its running rich? I have very little service history, but I was told the injectors, ignition coils, and such were all replaced. No word on oxygen sensors or cat. I am new to the VW world, so I appreciate any input as to where to start narrowing the problem down.
 

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For misfires, I would pull the plug, check the spark plug wire, injector pulse and possibly compression testing. We have seeing coils fail and in rare cases, a bad ignition chip, inside the ecm.

Vacuum leaks are common, weak fuel pumps, failing engine speed sensors and dirty throttle bodies, can cause rough idle problems, dying issues. Checking your fuel trims; can help you diagnose, a vacuum or maf, fuel delivery issues (+ or - 10%; is considered, in the "normal" range").


Cleaning your throttle body and doing a alignment; would be a good maintenance procedure, especially @ 180k, as most owners and mechanics, do not clean them very often.


Checking and testing your cat; would be something to check and confirm, the cat it out of spec. This video is great and shows how to test things with a scan tool, a basic IR spot gun type thermometer, cheap these days at most stores or places like harbor freight.

P0420 How To Diagnose A Bad Catalytic Converter -EricTheCarGuy

If it doesn't pass, the testing shown in the video; some, have had luck with some "cata clean" type of treatments or a new cat from Bosal, tends to be not terribly expensive if needed.


Your thermostat, is definitely NOT working as it should; if your operating temps are 185 F, that is running cool and as you noted, probably running rich at that temp. Replace with a high quality oem thermostat; they are a common failure, on these VWs.

As to ALL the parts that have been possibly "thrown" at this car; getting and installing, high quality oem parts, is CRUCIAL on these German cars, in this case a Volkswagen. We have seen, many people install cheap parts; that are sold, at your typical auto parts store and cause, drivability issues, as these parts are just not up to oem spec and prematurely fail or make the car run bad.

OEM brands, tend to be suppliers to VW at the factory; a supplier, like FCPEURO, sells the oem parts but they are cheaper, as you skip the middleman, the VW dealer markup, etc.

www.fcpeuro.com (free shipping over $50 order and lifetime warranty on all the parts they sell)!

So, check the brand of your parts installed; we can confirm, what the correct oem parts would be and do some testing, checking the #3 cylinder, etc. Let us the results of your troubleshooting, testing and inspection, we can go from there.

For free online service manuals; check out these sites:


 

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Discussion Starter · #4 ·
Thank you for the very thorough replies and advice! I greatly appreciate it. When time allows, I will take a further investigation with the information provided, and update with any results.
 

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Discussion Starter · #5 ·
Alright, here is how what the current investigation has uncovered:

-The coil pack is new, OEM, and undamaged. Verified that it is functioning. (plug wires as well)
-I have replaced the fuel filter
-The throttle body has been cleaned, and reprogrammed
-I have used the VCDS to verify that the O2 sensors, and cat are functioning properly. (all the readings look good)
-I have cleaned the MAF
(also of note, I have replaced many suspect looking vacuum lines)

Now here's the fun part. I have narrowed down the misfire to cylinder 3. I determined in my live data readings that it is strictly cylinder 3 misfiring. However, it only misfires when engine speed is under 1,200 RPM. And even then its only sometimes.

I decided to remove the intake manifold to check spark plugs, and try and rule out fuel injectors as well by swapping the injector for cylinder 3 with a known good injector. It took me a good 4 hours because I stripped one bolt on the back of the manifold, and had to remove the wiper cowling to access it with a drill. Then, after pulling what appeared to be very old OEM injectors, I noticed (quite shockingly) that the entire o-ring and rim of injector #3 was completely gone. Clearly that is an issue. I have a new one on the way this week. I am also changing all the spark plugs as well.

Will update later
 

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Boom! Very detailed and solid troubleshooting; so, the fuel injector o-ring was bad? That would definitely cause a vacuum leak, a lean condition and a misfire! :)

PS: you may have already found this feature in VCDS; the misfire counter can be very helpful, to see specific cylinder misfire info.

Sent from my iPhone using Tapatalk
 
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