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Let me get it out there...I am a Subaru Newb... please help.

So I am working on a 96 legacy outback 5 speed for a friend with a new machined 2.5l motor (600 miles on it) and after 2 min of test driving it, the throttle starts pulsing and the car bucks under power like the first time I drove a stick. It does this without me switching gears, on flat ground, in any gear. One minute-smooth, next I am a ragdoll. Any thoughts? It does this every time once it gets warm and it occasionally shuts off at a stop.

I thought it may have been a bad/wrong sensor but apparently it worked fine till 40 miles ago....I did see more metal flake in the oil then I expected after break in.
 

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2001 VDC/SC One of a Kind
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Fuel, air or fire and the sensors, many of them, that control the mix.

MAF
TPS
Fuel filter
Fuel pump
Front O2 sensor
Bad ground; either at the battery, block or the many lugs off the harness at the strut towers, firewall, fender and intake manifold.
Coil or crank sensor
Plugged exhaust, i.e. CAT damaged or clogged.
Vacuum leaks

If I were to guess at which one to check first, FUEL DELIVERY. Check the fuel filter, then the pump.

Next, fire from the coil.
 

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Discussion Starter #3
Fuel, air or fire and the sensors, many of them, that control the mix.

MAF
TPS
Fuel filter
Fuel pump
Front O2 sensor
Bad ground; either at the battery, block or the many lugs off the harness at the strut towers, firewall, fender and intake manifold.
Coil or crank sensor
Plugged exhaust, i.e. CAT damaged or clogged.
Vacuum leaks

If I were to guess at which one to check first, FUEL DELIVERY. Check the fuel filter, then the pump.

Next, fire from the coil.
So here is my logic and feel free to poke holes in it, but the problem only exists after 1-2 min of running, or at least is only visible after that time. To me, this makes the fuel filter or pump issue less likely since it wouldn't be changed by engine temp, only by run time which would make the problem go away after 5 minutes of shutoff, which it doesn't. The vacuum harness was changed but it was changed before 400 miles of smooth running so Ib don't think it is that, I checked the grounds, all are tight so no easy fix there :rolleyes:.

This leaves the exhaust, sensors and coil pack.

I know the exhaust is looking very black, from the engine seeming to run rich, but I think the possible clogging would be a symptom, not a cause, but I am pressure testing it this afternoon.
 

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2001 VDC/SC One of a Kind
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Are you saying there is black smoke out the tailpipe or there is a lot of carbon buildup in the exhaust pipe?

Are there any codes on the ECM?

A clogged CAT is a possibility and would cause issue after warmed up. You can also verify a clogged CAT with a scan tool by looking at the rear O2 data. If it is known to function properly and it sends abnormally low readings back to the ECM at 2500 rpm, less than .100 mv, then it is an indication of low or no flow out of the CAT.

Again, the list I provided previously needs to be rechecked. If you are certain of fuel delivery, then check the remainder.
 
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