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Part throttle hesitation


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12 replies to this topic

#1 Inane Cathode

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Posted 15 October 2010 - 11:55 PM

Hey it's me again!
Well i got the idling issue sorted out. Turns out i had the wire thats supposed to be an input from the starter solenoid circuit (tells the computer the starter is cranking) hooked up to battery voltage. This made it think the car was constantly trying to start, so it kept retarding the ignition thinking it was going to start any minute now until it almost died then rinse repeat.

Anyhow, it's been running pretty darn good lately but recently it's started up with a new problem.

After driving in a really heavy rain storm, at least thats when i think it started, it's started a part throttle hesitation/bucking. It's almost impossible to just cruise down the highway. If you're trying to crack the throttle it won't do anything, in fact it'll start to buck pretty good and act generally crappy about trying to accelerate until you get about a quarter into the throttle travel then WHAM it comes on like a light switch and you're off like gang busters. If you drive aggressively it'll run pretty good, but when you go to take off in first gear it'll do the same bucking/poor running thing until you get to half or maybe more throttle.

It's not always repeatable, and it seems to come on like a switch at different throttle openings, but it's always right off closed throttle or very near.

I've changed the air filter, oil, cleaned the MAF, dicked with the IAC valve (i can reset it pretty easy, index marks). I've unplugged the TPS and driven, and its had the same issue. I unplugged the MAF and it got ALOT worse, much more pronounced than with it plugged in. I unhooked the pressure regulator to simulate a vacuum leak (and higher fuel pressure) and it had the same issue, only idled worse heh.

Anyhow. I feel like i'm out on the furry edge of diagnosing here, so could anyone help me out with some ideas, i feel like i'm already stumped ><

#2 forester2002s

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Posted 16 October 2010 - 12:46 AM

When I get this hesitation problem, I make sure to NOT drive away immediately after starting the engine.

I start the engine and wait about 10-seconds (without touching the throttle, nor any electrics).
Then I drive off, and there's no engine hesitation.

Try it. It just might work for you.

#3 Inane Cathode

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Posted 16 October 2010 - 01:07 AM

It runs fine if you take off right away, or if you wait, it always seems to start up after its warmed up or a few blocks into the drive.

#4 grossgary

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Posted 16 October 2010 - 01:50 AM

year, engine, trans?

you said you had a "wire hooked up wrong", why was that? has the wiring been worked on/changed?

no check engine lights now or in the recent past?

good work, looks like a good list of things you'e tried.

what's the state of the plugs and wires? age/mileage? are they subaru wires and NGK plugs? sounds like possible ignition issues which could also possibly mean coil pack - but usually these all give a check engine light.

possible knock sensor - it's retarding the timing on you - but normally when they start to fail they give a check engine light, so that makes me want to rule it out. i've seen about 3 sensors cause issues without triggering the check engine light, the knock sensor hasn't been one of them but i bet it's possible. if it's cracked at the base that's not a good sign.

on older models you can unplug the O2 sensor and drive without it just fine, not sure about newer stuff but i keep meaning to try it on my legacy.

#5 1roughneck

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Posted 16 October 2010 - 07:28 AM

I think it is the TPS sensor. That or the Idle Air Solenoid. Many vehicles I've owned had TPS go bad that way. My YJ jeep is pretty bad about it. Water does not help.

#6 Inane Cathode

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Posted 16 October 2010 - 09:55 AM

year, engine, trans?

you said you had a "wire hooked up wrong", why was that? has the wiring been worked on/changed?

no check engine lights now or in the recent past?

good work, looks like a good list of things you'e tried.

what's the state of the plugs and wires? age/mileage? are they subaru wires and NGK plugs? sounds like possible ignition issues which could also possibly mean coil pack - but usually these all give a check engine light.

possible knock sensor - it's retarding the timing on you - but normally when they start to fail they give a check engine light, so that makes me want to rule it out. i've seen about 3 sensors cause issues without triggering the check engine light, the knock sensor hasn't been one of them but i bet it's possible. if it's cracked at the base that's not a good sign.

on older models you can unplug the O2 sensor and drive without it just fine, not sure about newer stuff but i keep meaning to try it on my legacy.


It's a 96 legacy engine and engine management swapped into a 72 beetle, but it's still a subaru heh. The wire hooked up wrong was the starter input wire to the computer, it tells the computer if the car is cranking or not, it's sorted now though.

Plugs and wires are brand spanky new, i checked for water intrusion and they're dry all over. How would i diagnose a bad coil pack? It seems a bit too 'regular' to be a coil pack though, although i've never had a bad coil, or pack before so i wouldnt necessarily know that.

As far as check engine lights, i've had TONS of them. Mostly from my own dickering with unplugging things but a few i've chased down and fixed per above.

The knock sensor is brand new too, the old one was all cracked around the base as you describe and whenever it got wet it'd throw the code until it finally just quit all together. New one fixed that.

I'm not sure if it's an oxygen sensor issue, it's too harsh i think to be a fuel trim issue, that and the upstream sensor is brand new as well. All the wiring has been gone through umpteen times so i'm pretty sure its not a wiring issue either.

Any ideas?

Edit: I don't think it's the tps sensor, i unplugged it and it was still doing the same thing. Of course it was much worse after unplugging the maf and leaving the tps plugged in but i believe that just exacerbated whatever was wrong.

Edited by Inane Cathode, 16 October 2010 - 09:59 AM.


#7 Inane Cathode

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Posted 19 October 2010 - 01:31 PM

Alright wtf

New plugs: No result

Checked the coil resistance cold: Good

Checked the coil resistance hot: Good

Checked the fuel pressure: 28psi with the regulator hooked up to vacuum, 36
with it unhooked, fuel pressure climbs way up as it should when i pinch the return line

Unplugged the tps: better for a while, then its really bad

Unplugged the mafs: bad all the time

Re-did all the grounds and various power feeds to and from the computer, no change.

I'm quickly running out of ideas here, i don't know wtf is going on. It's not repeatable all the time, but when it starts doing it you can get it to do it over and over. Even when you're taking off in first gear.

Ok, so a repeat of what its doing: If you take off hard on the throttle its fine, accelerating quickly is no problem, doesnt skip a beat. Once you're moving, however, and you're holding your speed with light throttle it'll start to crap out, when you increase the throttle to compensate it just gets worse, bucking/sputtering, not having it at all. After you push the throttle about a quarter, sometimes more sometimes less, it'll correct itself and go off like a rocket like normal. If you take off with light throttle it'll do it until you push in the throttle by about half.

It doesnt do it when it's cold up to when its almost all the way warmed up, then it starts up with it.

#8 johnceggleston

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Posted 19 October 2010 - 01:52 PM

any one else besides me think it might be the coolant temp sensor??

try unplugging it, the one with 2 wires. (there are 2, the one with one wire is for the guage only.) it's on the coolant crossover pipe under the intake manifold on the passenger side. follow the top radiator hose, look on the back side, i think.

#9 Inane Cathode

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Posted 19 October 2010 - 03:04 PM

Been there done that with an idle surge fixed with the starter wire fix, coolant sensor is brand new.

I guess to narrow things down, it hesitates and stumbles under constand light throttle application, i get long winded sometimes.

#10 Inane Cathode

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Posted 21 October 2010 - 07:43 PM

I ran a can of gm top engine clean through the IAC in case it was somehow gunked up, alas, no improvement. I also put some gm injector cleaner in the tank, filled it up, i suppose i'll have to wait but theres no improvement with that yet.

Damn, this is frustrating. Does anyone have any idea what i'm talking about? Am i asking the right questions? How about how do i know if this is the ignition system or the fuel system?

#11 jparsons

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Posted 19 December 2010 - 12:16 PM

I ran a can of gm top engine clean through the IAC in case it was somehow gunked up, alas, no improvement. I also put some gm injector cleaner in the tank, filled it up, i suppose i'll have to wait but theres no improvement with that yet.

Damn, this is frustrating. Does anyone have any idea what i'm talking about? Am i asking the right questions? How about how do i know if this is the ignition system or the fuel system?


I have the same problem on my 01 Outback - I've been watching your post and similar posts at subaruoutback.org. There are a few posts that have said they replaced the TPS sensor and everything was magically fixed. I've replaced it twice with no change. I've run the stuff through the fuel tank, checked for vacuum leaks, and that's about it. I haven't checked really anything else under the assumption that if I watch the posts long enough someone will come up with an answer.

I posted at the other forum my experience driving to the mountains - I don't know if this will give you any other ideas - I live in Colorado Springs and go snowboarding in the mountains. I have to drive down about 1k feet to Denver (from about 6300 ft to 5280 ft) then I have to climb to 12-13k feet once I leave Denver headed west. The car did it's same behavior - bucking, hesitating, etc on the way to Denver but it wasn't any worse than "normal".

The minute I started the climb into the mountains (you turn a corner on the interstate and then it's just a straight climb up), the car started bucking, hesitating, losing power, etc like I turned a switch. I endured it for the next hour and a half as it repeated the behavior every few seconds. I was ready to throw it off a cliff.

I finally made it there.

When I turned for home in the evening, it drove flawlessly all the way down the mountains and back to my house. If there were any hesitations it wasn't any worse than "normal".

Like I said on the other forum, the problem increased greatly when climbing straight up into the mountains. That's why I don't think its something like spark plugs or fuel quality or fuel filters, or plug wires, etc - the problem would have repeated itself at the same rpm either up into the mountains or headed down.

I don't think we can blame it on air pressure because it went bananas within a few moments of climbing straight up. The air pressure wouldn't have changed that much in that short of time. I am not the mechanic you are - but maybe this is helpful info?

#12 zedhead

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Posted 20 December 2010 - 03:56 AM

Egr

#13 jparsons

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Posted 20 December 2010 - 09:53 AM

I decided to use justanswer.com and the Subaru tech suggested I replace the front o2 sensor that sits above the point where the two exhaust manifolds come down. It was $60 at Napa and the problems are gone. Hopefully they won return but just for fun if the egr is a cheap part I can just replace that, too.




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