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Volkswagen Jetta, Golf, GTI: 1985-1992 (A2) Great start then runs rough, burns super rich.


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Last Post: Apr 3, 2014 5:44 PM Last Post By: tcovenant2000
DragonHawk

Posts: 3
Registered: 03/31/14
Great start then runs rough, burns super rich.
Posted: Mar 31, 2014 12:40 AM
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89 Jetta with 1.8 single "RV" engine.

I was driving and it suddenly started running rough. Barely could accelerate as it is stumbling so bad. Pulled over shut it down and had a look (saw nothing unusual) Restarted and it purred for about a minute then back to rough. There is soot on the ground it is running so rich (none when first started).

Fuel pump, filter and transfer pump tested (was working at night and did not know about the soot bellowing from the exhaust)
New plugs, Cap and rotor,
New air cleaner.
Replaced Coolant Temp sensor (blue)
O2 sensor is producing approx 0.5V (between black and ground) so I assume its working.

A few days later,
It runs smooth. with full power until warm (first started) and then restarts smooth for about a minute after each restart.
My brother was falling me back home and said he could see when it started act up form the exhaust change. clear to black.

This video is not of my car but it appears to be the same problem, thought it might help in diagnosing,

https://www.youtube.com/watch?v=YPb-JT6tRcE

and ideas?

Desperately appreciative

James
tcovenant2000

Posts: 1,775
Registered: 12/09/03
Re: Great start then runs rough, burns super rich.
Posted: Apr 1, 2014 1:39 PM   in response to: DragonHawk in response to: DragonHawk
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Don't assume the O2 is working. It may fail when hot or be intermittent. Disconnect that sucker and see if the problem goes away. Remember that the most common O2 failure is to produce 0V which the ECU sees as "LEAN! ADD FUEL!".
Disconnecting it will force it into open loop mode and if the O2 sensor is the problem then it won't happen again.
It's a '98. Any codes?
Does it do this when cold? If so, that's an indicator of CTS problems. I know you've just replaced it, but I've seen bad ones out of the box.
Thomas
DragonHawk

Posts: 3
Registered: 03/31/14
Re: Great start then runs rough, burns super rich.
Posted: Apr 1, 2014 5:03 PM   in response to: tcovenant2000 in response to: tcovenant2000
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update, air flow sensor swapped out, and brand new Oxy sensor installed. Car runs good with o2 sensor unplugged and horrible with it plugged in (hence why I thought it was bad, Good call tcovenant2000 )

Its an 89 with the Digifant 2 system (No **** brains in the box). Any idea how it runs in closed loop but fails with the o2 (new) plugged in? Works great when cold (with or without the o2 plugged in) as soon as the temp comes up, engine goes for crap.

I have sunk $300+ into her and I still runs like sh*t.. I think a match may be in order now.

Edited by: DragonHawk on Apr 1, 2014 5:09 PM
tcovenant2000

Posts: 1,775
Registered: 12/09/03
Re: Great start then runs rough, burns super rich.
Posted: Apr 2, 2014 6:15 PM   in response to: DragonHawk in response to: DragonHawk
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If it runs well cold, O2 or not, that's good. It means your CTS at least works well enough to say the engine is cold. In cold running the ecu ignores the O2 sensor, which is why it runs well cold.
When it's warm, and O2 is plugged in, you are seeing tons of crud out the back, so it's running rich. Since it's running too rich, anything in the engine (bad injector leaking, etc) that would cause a rich symptom would be compensated for by the O2 (unless it was so bad the leaning out couldn't keep up.
But if it were a leaky injector or similar, so bad that the fuel control can't compensate, then it would continue to puke rich exhaust even with the O2 unplugged, so likely not.
You're possibly looking at a major leak in an injector, or some other reason for it to be so rich that the fuel control can't compensate. More likely you're looking at a source of ambient air hitting the sensor part of the O2 sensor. I'd start with looking for exhaust leaks at/near the O2 sensor.
Here's another thing to try. Disconnect the O2 sensor (it is a single-wire sensor?). The O2 sensor side is usually female. The ECU side is usually male. But make sure. Take the side that DOESN"T go to the O2 sensor and, engine running and warmed up, hold the wire on the engine or bare metal on the frame. This simulates an O2 sensor sending a SUPER LEAN signal, which should cause the engine to choke and spew nasty fumes. Next take a "D" cell battery (a fresh one) and hold the - side onto the frame or engine. Hold the wire onto the + side of the battery. This simulates a SUPER RICH signal and should cause your car to go really lean. If these tests pass then it's almost certainly a bad O2 sensor (you might have cracked it when you installed it?) or a leak that's allowing ambient air to touch the sensor part of the O2 sensor.

Thomas
DragonHawk

Posts: 3
Registered: 03/31/14
Re: Great start then runs rough, burns super rich.
Posted: Apr 2, 2014 8:26 PM   in response to: DragonHawk in response to: DragonHawk
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After confirming the o2 was working ok and retracing all wires from the ECU for resistance valves. I shorted the CTS to simulate extreme temp and it still didn't lean out the fuel mix.(O2 was reading 0.92V). Was about to replace the ECU when I saw it......

Fixed it.. Was a ground wire below the main group on the valve cover. There were two wires below on the side of the engine. One was cracked and the other was broken. Cut them back and regrounded them and all is good again.

Make sure you clean and check ALL ground points :)

Wow 1 lose wire cost me $400 in parts grrrr. but I have loads of spare parts now
tcovenant2000

Posts: 1,775
Registered: 12/09/03
Re: Great start then runs rough, burns super rich.
Posted: Apr 3, 2014 5:44 PM   in response to: DragonHawk in response to: DragonHawk
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Heh...did I mention to check all grounds? :)

Thomas

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