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Vehicle is a 2002 Subaru Legacy Wagon 2.5 SOHC 5 speed.  220,000 miles.

 

Problem would only occur after 30+ minutes of driving on a hot day, as of last week.

 

Now it occurs as soon as engine is warmed up.

 

Plugs, wires, fuel filter, and a million other things have all been replaced in the past couple thousand miles.

 

Once the car warms up, I'm getting a terribly low idle and hesitation/jolting so bad it isn't even road worthy. It's so bad that I can barely get it into 1st gear sometimes.  The car won't cut out, but seems pretty close to it most of the time. At idle, it revs up just fine. 

 

Could this be a TPS issue? Injectors? Fuel pressure? I don't see why it would only do this once fully warmed, it runs fine (seems a little slower than usual) until it warms up. There is no check engine light.  In the past, it threw a code for IAC valve, but it went away once I cleaned it.

 

Thanks guys.

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Update, problem now persists constantly, regardless of engine temp.  Code reader tells me P1507 code for idle control.  I've checked all vacuum lines, cleaned IACV and replaced gasket, ran some seafoam (lol), and am really not about to spend $300 on a new IAC valve when that may not be the issue.  About to try the idle "relearn" procedure.

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Cool, I ordered that neutral safety switch and MAP sensor, really hope that solves it as I need my daily driver back.

 

After performing the "idle relearn" procedure, there are no trouble codes and the car idles properly, but still has a crazy hesitation and is not drivable.

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It could be a lot of things, but just to start, take out the IAC and the throttle body.  Clean it all with carburetor cleaner.  You will need to buy a $5 gasket to reattach the TB.  You don't have to disconnect the coolant lines to the TB.

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Haven't solved it yet, but the neutral safety switch came in today and I'm betting on that. Didn't think the part would be here for a few more days, so I have to finish installing differential bushing kit before I can do it :/  Hopefully I finish tonight.

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Not fixed, so it's not the NSS or MAP sensor. I'll look into the vacuum gauge thing, gonna put a new coil pack on it since the Autozone guys will let me return it if it doesn't work. Everyone says not to screw around too much with the throttle body, but that's where I'm headed.

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if it's stumbling at idle - one thing to try, spray some starter fluid into the intake. If it is starved for fuel - it should smooth out. If the problem gets worse, maybe it's being flooded. And if there's no change, could be a timing/compression/spark issue.

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Unfortunately the bad idle stopped, it actually idles okay now but hesitation persists.  I replaced the coilpack, that wasn't it.  I put a vacuum gauge on it and it acted completely normal, no problem indications.  I'm thinking fuel pressure? 

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Just throwing this out there, the throttle body could be dirty. Had a very similar issue with my 96' Impreza. Ended up cleaning the throttle body off the car with a tooth brush and then a second time while the car was running with that throttle body cleaner. Ran much better after that, and the issues went away completely during the next drive cycle. 

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try refreshing all the grounds you can find.

 

might be time for compression/leakdown testing.

 

maybe there's a dropped valve guide?

 

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Took the throttle body off, as I suspected it's still pretty clean. I'll give it a good spraying out anyways.  Ugh it's a bad day when I have to run a compression test... At least a dropped valve would give me an excuse to do a delta cam grind lol.  All grounds were sanded and cleaned when I put the engine back in.

Edited by 1986Brat

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kinda doubt a valve issue with smooth vacuum - but.....?

 

live data may be a good next step - Torque app with an elm327 BT adapter, or a Tactrix cable to a laptop......

 

maybe free ssm;http://www.subaruoutback.org/forums/65-parts-accessories-performance/39426-freessm-complete-access-your-ecm-tcu.html

Edited by 1 Lucky Texan

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Found fuel in the vacuum line to the fuel pressure regulator, replaced that, no cigar.

 

Half way through compression test, cyl 1 and 3 have good compression, may have to do leakdown test.

 

Oh jeez computer stuff sounds fun.

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Well compression test showed low compression on cylinder 4.  Cylinder 1, 2, 3 had 210 psi, and cylinder 4 had <100 psi...

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To provide closure, or lack there of:  three months later I'm still having the same issue.  Rebuilt and ported heads, delta 1500 cam grind, reseated valves (at The Head Shop).  Now I have good compression on all cylinders, leakdown test doesn't show anything out of the ordinary, and the car is much faster... for the 1 minute that it drives properly.

Code reader shows codes P1507 and P0507.  IAC Valve cannot be the issue since I took a functional one off of a friend's ej251, performed idle reset procedure, and car eventually behaved the same.  

I downloaded FreeSSM, got a VAG COM KKL 409.1 cable, installed drivers, hooked up to the car and got an "interface test successful" message followed by "No or invalid answer from ECU/TCU" whenever I tried to use it.  I've just spent a solid 6 hours trying to solve whatever issue I'm having running this software... guess I'm throwing in the towel on that. 

Not sure where to go from here. Maybe I should get it towed to the dealership for their diagnosis... 

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Have you checked fuel pressure with a guage?

02 has the funny cap thing on the fuel pump that will break and bleed fuel pressure back into the tank.

 

Does it have a MAF sensor? Or is it MAP?

Edited by Fairtax4me

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Yes fuel pressure was checked, MAP sensor replaced a while back.

 

PROBLEM SOLVED, I think! Turns out the computer was sending the wrong codes because the throttle cable and cruise control cable were slightly misadjusted, sending codes for idle control and not picking up on the real culprit... upstream oxygen sensor!  After adjusting the throttle cables and performing an idle relearn procedure (disconnecting battery, etc.), it threw codes p0130 and p0171 (I think) for o2 sensor and subsequent lean air:fuel mix.

 

I feel like a serious moron, but am very happy to have my daily driver back.  Now to get the AC leaks fixed...

 

Thanks for the help guys.

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