# MK4 Jetta Idling Rough



## csaw18 (Mar 4, 2012)

I own a 2000 Jetta 2.0 and been having idling issues.

When I start the car it seems to be alright but when I rev the engine to increase rpms and let off the rpms drop to 800 then rise to about 1100 where it will idle which is high for my car. It continues to do this while driving. As long as I am traveling its fine but the second I push in the clutch it does the same thing. Living in the northeast and the varying temps we have had I though it could be moisture in the fuel so I tried dry gas and didnt help. 

It feels like the power wants to cut out but ive never had it stall out on me. I also dont have a check engine light on so scanning it may not show anything. Just throwing me in a few different directions with what it could be. It dont know if it could be a fuel delivery problem, air delivery issue, and I also have my alternator replaced 2-3 weeks ago and this started a week after that. Anyone ever seen this?? or have an idea of what it could be. Thanks


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## csaw18 (Mar 4, 2012)

*correction*

Correction to the previous post. My car drops to 600 rpms not 800


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## ps2375 (Aug 13, 2003)

Find someone with VAG-Com and have it scanned, there might be something there. How long ago was any regular maintenance done, like plugs, poss wires, air-filter done? HAs the TB ever been cleaned, but don't just pull it off and clean it w/o having VAG-Com avail to do a TB adaption.


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## RedVirus80 (Jan 17, 2008)

After you get it scanned let us know whats up...sounds like either maf or o2 sensor


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## Anony00GT (Mar 6, 2002)

Could be as simple as a vacuum leak or gummed-up throttle body.

Where are you located in the Northeast exactly?


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## csaw18 (Mar 4, 2012)

Just got it into the shop today and got it scanned. No codes, I recently also had my battery replaced along with the alternator so next step tomorrow is to reset the settings with the TB as I guess it can sometimes lose the settings when you disconnect the battery. Also going to look at cleaning it. hopefully its a quick fix

Annoy00GT
I am originally from Maine but currently live in Boston, so I just got back in Maine today so i brought it to my mechanic.


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## VW MAFIA (Dec 3, 2011)

Crank sensor........, vacuum leak.........


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## Anony00GT (Mar 6, 2002)

csaw18 said:


> reset the settings with the TB as I guess it can sometimes lose the settings when you disconnect the battery. Also going to look at cleaning it.


Exactly. More likely when they're dirty. If you've got VCDS, remove the TB from the car completely and thoroughly clean it. Then reinstall and do a TB alignment with VCDS.


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## Dubbington (Oct 26, 2005)

I don't have access to a vag-com or VCDS....located in the Norcal/bay area. 

Check engine light is on and buddy scanned it with a OBDII general scanner. P0420 I believe came out. 'Bank 1" it said. Possible 02 sensor? 

Would cleaning the MAF without access to VAG-com be a bad thing?


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## shill35 (Apr 1, 2012)

p0420 is a CAT Efficeny code.As far as the running issue, I would start with cleaning TB and running adaptaion most likely will fix erratic idle. If it does not it will be very hard to diag MAF without a scanner and data stream. One thing you could also try is with car off disconnect MAF and then start car. See how it runs. With MAF disconnected it will put MAF reading to a default spec. If the car then runs better with MAF disconnected suspect bad MAF sensor. As far as the P0420 the CAT will need replaced. VW monitors are very good and will not set codes if part is not bad. When replacing CAT i suggest replacing o2 sensors as well both upstream and downstream with a good OEM sensor not a cheapo. Same with CAT us a good OEM brand or your light will come back.


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## Dubbington (Oct 26, 2005)

shill35 said:


> p0420 is a CAT Efficeny code.As far as the running issue, I would start with cleaning TB and running adaptaion most likely will fix erratic idle. If it does not it will be very hard to diag MAF without a scanner and data stream. One thing you could also try is with car off disconnect MAF and then start car. See how it runs. With MAF disconnected it will put MAF reading to a default spec. If the car then runs better with MAF disconnected suspect bad MAF sensor. As far as the P0420 the CAT will need replaced. VW monitors are very good and will not set codes if part is not bad. When replacing CAT i suggest replacing o2 sensors as well both upstream and downstream with a good OEM sensor not a cheapo. Same with CAT us a good OEM brand or your light will come back.


Thanks for the info :thumbup:

VW told me I still qualify for a recall until 2013 for a bad cat. Think I'll tell the dealership the check engine light is because the cat and they need to change it so I can sell it.

Car has 114k on it. I was thinking about getting two Bosh universal O2's or two from Oxygensensors.com. They're around $100 each instead of $150+


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## Anony00GT (Mar 6, 2002)

Bosch Direct Fit O2 sensors only.

If the dealership will replace the cat for free under recall, have them do it. Otherwise, perform proper diagnosis first. It's not uncommon to have a P0420 when the cat is actually not the faulty component. Mixture imbalances can also (more commonly) cause this code, as the service information and VW TSBs indicate.

Driving with a mixture imbalance for too long will kill the cat though, making the faulty cat a symptom, not a cause.


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