'99 2.5L 4eat, 150k - Hesitation/Misfire/Stumbling

g_man80 · 11-03-2009, 01:56 PM

I've been searching here, subaruforester.org, and scoobymods the past few days, but haven't gotten far. The car is a '99 Forester auto with 150k miles.

Going from closed throttle to 1/4 or more open the engine will hesitate and stumble for 5 seconds or so and then will accelerate normally. It does this hot or cold and from a stop or coasting (at speed I let off the gas for a few moments and when I get back on it the jerkyness occurs). Constant cruising is fine and it will run up to redline ok. Gas mileage is pretty bad and it doesn't have a ton of power (it hasen't been great at either the past few years).

Pulling out into traffic is a little sketchy as you can't give it too much gas or it hesitates. If you accelerate VERY gently it's smooth.

Idles fine (cold or hot). Ran a few tanks of gas (different stations and grades), so it's not bad gas. Misted the coil pack in the dark last night to check for sparks and nothing was seen. No cel. Replaced knock sensor last year.

I was thinking front O2 sensor, although it happens hot or cold. Maybe fuel filter? I the fuel pump failing? MAF sensor (no cel though)? Bad wires/plugs (can I check these before replacing)?

Any suggestions? TIA

Panops145 · 11-03-2009, 02:46 PM

do u think it could be a tranny problem? or possibly a misfire in possibly two of the cylinders because it lags that bad...is ur check engine light on?
my engine was misfiring with 2 cyl

Checkers · 11-03-2009, 03:40 PM

Replace the plugs first, check for fouling or gaps that are out of spec. Then get new plug wires. Then coil pack, go up the ignition system.

I would bet on one of the above being the problem. Get new plugs, go from there.

It might be something more complicated, but I've had similar issues with my RS that were nothing more than either plugs or wires. Costs $40 and an hour to change both.

mlangejan · 11-03-2009, 03:55 PM

if its not your ignition system it very well could be a fuel filter that is restricting flow or your fuel pump.
i had the same problem in my rs and it was finally cured after i put a 255lph fuel pump in the stock pump was no longer keeping up with the volume required to supply the system I've seen it happen in a couple of other subies as well

g_man80 · 11-04-2009, 12:20 PM

Thanks for the input everyone. I was hoping to avoid replacing random parts, but it sounds like that's the only way to narrow it down. I'm going to start with plugs and fuel filter.

Also, it started about a week ago and seems to be getting worse each day or two.

01LGT · 11-04-2009, 12:47 PM

I had a similar problem although it did it the worst when it was hot... Upstream O2. But when was the filter changed last? same for the plugs? upstream o2 is about $63 at napa. my experience it would lurch and sputter, it would hold at low rpm then take off. compare the costs with the problem if it doesnt fix it then u have to pay more, if it does fix it u will be paying less.

g_man80 · 11-04-2009, 09:30 PM

Maybe fixed. Changed the fuel filter and plugs this afternoon. Changed the fuel filter first and it did nothing. Next were the plugs and I think they did the trick. Acceleration seems smooth and consistant, although it's only been about 6 miles. Time will tell, but for now I'm going to say the new plugs fixed the problem. I'll update the thread if the hesitation returns.

g_man80 · 11-19-2009, 08:42 PM

The problem still exists. Plugs and fuel filter did nothing. No fault codes. I think I heard a few backfires if I let off the gas for a few seconds and got back on it hard. Gave up and took it to a garage. I don't want to keep replacing components and chasing the problem. They checked the easy stuff this afternoon (scan, O2 sensors, etc) and found nothing. Tomorrow they are going to do more testing. I'll keep the thread updated.

11-03-2009, 01:56 PM	#1
g_man80 Scooby Newbie Member#: 93466 Join Date: Aug 2005 Chapter/Region: NWIC Location: Logan, UT Vehicle: '99 Forester	'99 2.5L 4eat, 150k - Hesitation/Misfire/Stumbling I've been searching here, subaruforester.org, and scoobymods the past few days, but haven't gotten far. The car is a '99 Forester auto with 150k miles. Going from closed throttle to 1/4 or more open the engine will hesitate and stumble for 5 seconds or so and then will accelerate normally. It does this hot or cold and from a stop or coasting (at speed I let off the gas for a few moments and when I get back on it the jerkyness occurs). Constant cruising is fine and it will run up to redline ok. Gas mileage is pretty bad and it doesn't have a ton of power (it hasen't been great at either the past few years). Pulling out into traffic is a little sketchy as you can't give it too much gas or it hesitates. If you accelerate VERY gently it's smooth. Idles fine (cold or hot). Ran a few tanks of gas (different stations and grades), so it's not bad gas. Misted the coil pack in the dark last night to check for sparks and nothing was seen. No cel. Replaced knock sensor last year. I was thinking front O2 sensor, although it happens hot or cold. Maybe fuel filter? I the fuel pump failing? MAF sensor (no cel though)? Bad wires/plugs (can I check these before replacing)? Any suggestions? TIA

11-04-2009, 09:30 PM	#7
g_man80 Scooby Newbie Member#: 93466 Join Date: Aug 2005 Chapter/Region: NWIC Location: Logan, UT Vehicle: '99 Forester	Maybe fixed. Changed the fuel filter and plugs this afternoon. Changed the fuel filter first and it did nothing. Next were the plugs and I think they did the trick. Acceleration seems smooth and consistant, although it's only been about 6 miles. Time will tell, but for now I'm going to say the new plugs fixed the problem. I'll update the thread if the hesitation returns. Last edited by g_man80; 11-19-2009 at 08:36 PM.

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11-03-2009, 02:46 PM	#2
Panops145 Scooby Newbie Member#: 200310 Join Date: Jan 2009 Location: Massachusetts Vehicle: 1999 Impreza 2.2l Silver	do u think it could be a tranny problem? or possibly a misfire in possibly two of the cylinders because it lags that bad...is ur check engine light on? my engine was misfiring with 2 cyl

11-03-2009, 03:40 PM	#3
Checkers Scooby Newbie Member#: 188440 Join Date: Sep 2008 Chapter/Region: International Location: desert Vehicle: LOL must die	Replace the plugs first, check for fouling or gaps that are out of spec. Then get new plug wires. Then coil pack, go up the ignition system. I would bet on one of the above being the problem. Get new plugs, go from there. It might be something more complicated, but I've had similar issues with my RS that were nothing more than either plugs or wires. Costs $40 and an hour to change both.

11-03-2009, 03:55 PM	#4
mlangejan Scooby Newbie Member#: 205567 Join Date: Mar 2009 Chapter/Region: W. Canada Location: kelowna bc canada Vehicle: 2000 impreza rs 2.5 pearl blue	if its not your ignition system it very well could be a fuel filter that is restricting flow or your fuel pump. i had the same problem in my rs and it was finally cured after i put a 255lph fuel pump in the stock pump was no longer keeping up with the volume required to supply the system I've seen it happen in a couple of other subies as well

11-04-2009, 12:20 PM	#5
g_man80 Scooby Newbie Member#: 93466 Join Date: Aug 2005 Chapter/Region: NWIC Location: Logan, UT Vehicle: '99 Forester	Thanks for the input everyone. I was hoping to avoid replacing random parts, but it sounds like that's the only way to narrow it down. I'm going to start with plugs and fuel filter. Also, it started about a week ago and seems to be getting worse each day or two.

11-04-2009, 12:47 PM	#6
01LGT Scooby Newbie Member#: 224816 Join Date: Sep 2009 Location: Rapid City, SD Vehicle: 2001 Legacy GT	I had a similar problem although it did it the worst when it was hot... Upstream O2. But when was the filter changed last? same for the plugs? upstream o2 is about $63 at napa. my experience it would lurch and sputter, it would hold at low rpm then take off. compare the costs with the problem if it doesnt fix it then u have to pay more, if it does fix it u will be paying less.

11-19-2009, 08:42 PM	#8
g_man80 Scooby Newbie Member#: 93466 Join Date: Aug 2005 Chapter/Region: NWIC Location: Logan, UT Vehicle: '99 Forester	The problem still exists. Plugs and fuel filter did nothing. No fault codes. I think I heard a few backfires if I let off the gas for a few seconds and got back on it hard. Gave up and took it to a garage. I don't want to keep replacing components and chasing the problem. They checked the easy stuff this afternoon (scan, O2 sensors, etc) and found nothing. Tomorrow they are going to do more testing. I'll keep the thread updated.