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Guest Message by DevFuse

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hesitation below 2500 rpm -maf?

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7 replies to this topic

#1 freewheel


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Posted 20 October 2003 - 08:49 PM

Ive been trying to solve this problem for a while, but haven't made any progress. My 98 Legacy hesitates while accelerating until it reaches 2500 rpm, then the power surges and it accelerates fine. Always 2500 rpm, and in any gear. Ive replaced the plugs, wires and added a K+N filter. I had a mechanic pull the codes and it indicated a bad O2 sensor, so at 72,000 miles I replaced both. But the problem persists and now is getting worse. Ive read here that the MAF sensor could be suspect, but no CEL light or codes indicating such. Today I disconnected it and started the engine as per a previous post, and it idled fine but revved very poorly. Can anybody shed some light on this problem or at least point me in the right direction? Thanks

#2 ScoobySchmitty


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Posted 20 October 2003 - 09:46 PM

I'm assuming you have an automatic, or is it a manual? It could be you are having a problem with your ignition timing (maybe the knock sensor). I would check your timing with a timing light, if you have one available.

#3 DerFahrer


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Posted 20 October 2003 - 11:10 PM

Welcome to the club, man. It's a Subaru thing. I have been chasing it in my Legacy for years! I have replaced: the knock sensor, the motor mounts, the alternator, the coilpack, etc. It has been hashed over time and time again on the BC-BF Legacy BBS, and those parts have solved the problem for some people, but not me. My engine is in a PERFECT state of tune even with 124k miles, but I can't rid myself of the hesitation. By the feel of it, I'm almost sure it's timing related to the knock sensor, but I think it's the ECU's end that's at fault, perhaps the stock ECU reads the sensor wrong or something...

And... yes. My XT does the same freeking thing!! Now that is a bit different, since my XT has a disty. The XT will ping on 89 octane, and only wants 91 octane. The disty is as far retarded as possible, leading me to believe it's a tooth off on the cam. That will be attended too as soon as my friend will lend me his timing light:-\

#4 97subaruGT


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Posted 21 October 2003 - 07:45 AM

I got exactly the same thing happening to my 97Legacy. When going from a stop, the engine coughs for a sec and as RPMs go up, it's back in business.
My knock sensor died some months ago so that was replaced. I replaced spark plugs, air/fuel filters and 02 sensor but nothing helped. When i disconnected the battery a few months ago and cleared the ECU, i think that was went away for almost a week but then the hesitation and rough shift from 1st to 2nd came back too.
Oh well, will have to live with it.

Quick question: :confused:
How does one use a timing light? do i have to have the timing cover off to get the reading?


#5 ScoobySchmitty


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Posted 21 October 2003 - 08:09 AM

I believe the timing light has a bypass to your #1 sparkplug wire, allowing it to flash when you get spark on your #1 cylinder. you shine this flashing light at a special gauge that is near the top of your underdrive pulley (the crankshaft pulley). The light will flash when a mark on the pulley is near the gauge, and will give you a reading on how many degrees + or - your timing is altered. I don't know engine specifics for the EJ25, you will have to look them up, or get a Haynes manual for your Legacy. That's about all I know about the timing light, and I don't know whether advancing the spark or retarding the spark will improve performance. :rolleyes: ANYWAY, that's all I know about the timing light and how to use it. Hope this helps out


#6 Commuter


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Posted 21 October 2003 - 12:05 PM

It's been discussed before. I've suffered from it. And have some of the same issue again right now.

These engines are known for running rich and building up carbon. I recall Emily of CCR Inc posting one time that when they tear down one of these engines, you'd think you were dealing with a carburated engine from the '70's.

What I experienced was hesitation while taking off from a stop, or during very very slow motion (eg, parking lot). Idle was fine, but it would bog between 600 and 1000 rpm. As it got worse, it crept up into higher revs. If I was loafing thru town with the engine turning over at about 1400 or 1500 rpm, I could feel little jerks, almost like a misfire. Easing into my garage to park (tight space) became quite the challenge with the ragged throttle response. However, if you put your foot into it a bit, it would be fine. Trying to "feather" the throttle was where it really showed itself.

My dealer did the MotorVac procedure on my car. This runs cleaner thru the fuel rail and injectors for several minutes, then the cleaner is run thru the engine. (As I understand it.) I don't know if the fuel rail / injector portion matters or not to be honest. Anyway, the problem disappeared. I was around 100k miles when this first happened. The diagnosis of carbon left me a little speechless. These cars have ECU's, MAF, fuel maps, etc., and I do 90% highway driving. Whodathunkit.

Over time, I would sense it coming back. A bottle of cleaner or a tank of premium would usually fix it up. Until recently, around 200k miles now. I've been trying to avoid doing another MotorVac (for various reasons).

I changed out the O2 sensor. That helped considerably. Although no CEL, it was probably going and causing the mixture to be even more rich. My gas milage had slipped a little (maybe 5%), and that has improved again with the new O2 sensor.

I've been meaning to try the "water" treatment to "steam clean" the engine. I just haven't gotten to it. (There was a post recently about this.) For right now, I'm living with it. I have to wonder if the MAF might be going. I know it can cause issues like this. Personally, I think it is still residual carbon.

Of course, there can be many other reasons as discussed in the prior posts. But if you've ruled them out, then it might very well be carbon build up.

For what it's worth...


#7 frag


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Posted 21 October 2003 - 12:46 PM

Thanks for making things even clearer Commuter.
Engine stopped missing intermitently at idle last year when I put a bottle of the strong kind (good for 5,000 kilometers they say) of fuel system cleaner through the system (gas tank variety)
I'm preparing to do the same shortly but also bought a complete system (80$ Can) that I will administer next spring: box comes with throttle cleaner, gas tank injector cleaners, oil anti acid (?) and a bottle of metered cleaner you connect to the intake manifold thru the PCV.
I'll report on the result then.
I suspected the engine was running rich and that carbon was building up (most of the time the exhaust pipe is black with at least a thin layer of soot).
But the only symptom I've had is an intermittent miss (one in a while) at idle (that dissapears when i do the treatment). Never an hesitation accelerating or cruising.

#8 PAezb


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Posted 21 October 2003 - 06:07 PM

Might what to check the EGR system also. If the EGR valve is opening early (rpm below 2000) this will cause hesitation and engine stumbling people are discribing.

Instead of using water in the vacuum of the throttle body, I've let the engine suck up a can of Seaform Motor TuneUp - good results.


96 OB w/125K

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