# Loss of power, sputter when warm



## slowride81 (Nov 21, 2011)

I'm trying to diagnose this issue. Here's whats going on... When cold (sitting overnight for example) my car starts and idles pretty well. However after a bit when it gets warm it starts having a loss of power and sputtering. If I take my foot off of the gas for a second and hit it again it will speed up but then 2-3 seconds later it will loose power. It only does this when it gets warm. Almost sounds like its missing really bad. When its first started up and I drive around the block everything is perfect. No power loss or anything. Any ideas? Its a 91 Jetta 1.8L 8v w/auto trans.

Edit: I was thinking possibly a fuel pump but I can hear it running when I turn the key. I have heard though that when the pumps are going bad when they get warm they don't pump properly. Seeing as I can hear the exterior pump maybe the transfer pump inside the tank?


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## randi dub (Jan 5, 2008)

*loss of power*

probably just the coolant temp sensor that controls the fuel injection , or your plug wires are bad its not the fuel pump


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## slowride81 (Nov 21, 2011)

I have a new coolant temp sensor(along with a heater core) on the way should be here tomorrow or tuesday will report back.


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## slowride81 (Nov 21, 2011)

Sensor did not fix the problem. I have 3 new ones now and they all do it. If I unplug the sensor while the car is running it runs pretty good. Plug it back in and :banghead:


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## slowride81 (Nov 21, 2011)

Made what I hope to be great progress today(this is my first vw so bare with me and i've only had it two weeks). The idiots who owned it before me had a ground wire spliced into the o2 sensor harness(their wiring was ruff i've been cleaning it up) from the valve cover ground. I don't know why but I pulled it when the car started to get warm and mess up and bam fixed. Drove it couple miles around town no issues but a slightly rough idle here and there but it never stalled or loss power and my temp gauge is working again. As a test I put it back on and the problems came back. Gonna finish the tune up (distributor cap/rotor, fuel/air filters ect.) and hopefully we are good to go. Yes, I did buy this car running like crap but for $700 with a pretty straight body and almost mint interior it was hard to pass up. I just need to to get through winter then its engine swap time.

Spirits are definitely higher now and it looks like I won't be taking a sledge hammer to the car


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## ziddey (Apr 16, 2006)

Get a new bosch o2 sensor. The one for the late 80s ford mustang is your cheapest bet (i4 and v8 use the same sensor). Will need to splice/solder onto your current harness.
Stock, I believe the o2 sensor harness for digi2 has a ground going to the intake manifold. O2 sensor itself is a traditional 3wire (2 heater + 1 signal) that grounds through the exhaust manifold.


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## Fat Rabbit (Nov 18, 2001)

There are two grounds from the O2 sensor. One is for the O2 sensor heating element which is a black and brown wire. The other is for the O2 sensor cable shielding -- that one is a black wire. Both of those wires connect to a point "on the engine block" along with a third ground wire which comes from the Digifant ECU. Although I have found them connected elsewhere. the original and most common connection point is the lower stud or bolt at the coolant hose flange at the transmission end of the cylinder head. If the ECU ground is not connected or cannot make ground the ECU will be ruined.

It is a common mistake to forget to connect those wires after a cylinder head rebuild or when replacing the engine thus smoking the ECU. FR


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## Killa_ganja (Apr 6, 2007)

Fat Rabbit said:


> There are two grounds from the O2 sensor. One is for the O2 sensor heating element which is a black and brown wire. The other is for the O2 sensor cable shielding -- that one is a black wire. Both of those wires connect to a point "on the engine block" along with a third ground wire which comes from the Digifant ECU. Although I have found them connected elsewhere. the original and most common connection point is the lower stud or bolt at the coolant hose flange at the transmission end of the cylinder head. If the ECU ground is not connected or cannot make ground the ECU will be ruined.
> 
> It is a common mistake to forget to connect those wires after a cylinder head rebuild or when replacing the engine thus smoking the ECU. FR




ok so ive searched all over .. just to clarify here.. I have 3 wires from the sensor(2 white wires for the sensor heater and a black signal wire) 
and 3 wires on the harness side(of which one -the black wire-is connected to a ground which is the"point on the engine block")
is this right?


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