Acceleration Hesitation at Low RPM

[delarryous]

At a stop, in neutral revving or starting up like normal in first gear, I get a slight hesitation/studder and then it begins to pull as normal. At higher RPM it pulls normal. I've changed the plugs and cleaned the MAF and nothing seems to work. Sometimes the condition happens in other gears as well.

Mods include:
VF-39
06 wrx 565cc injectors
STi TMIC
Custom open source tune

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[DZXN]

Does your idle hold steady or rise and fall? Check IACV threads and see if it sounds similar to your issue. The gasket is $8+-, cleaning takes 20 minutes, and getting the screws out is the toughest part.

[bushflyr]

I get the same thing. It's not the IACV, I've had that too, several times, and it's super obvious.

My situation, and it sounds like the OP's, feels like there is no fuel for just a split second after you hit the throttle from idle. It stumbled for a quarter second then revs build like it ought to. No idea what causes it. But replacing the IACV doesn't help.

[delarryous]

Originally Posted by bushflyr

I get the same thing. It's not the IACV, I've had that too, several times, and it's super obvious.

My situation, and it sounds like the OP's, feels like there is no fuel for just a split second after you hit the throttle from idle. It stumbled for a quarter second then revs build like it ought to. No idea what causes it. But replacing the IACV doesn't help.

This is exactly what happens. It feels like it doesn't make power at all. It was running great for about a year and half with this tune so I don't know what could have happened.

[The Phenx]

Originally Posted by delarryous

This is exactly what happens. It feels like it doesn't make power at all. It was running great for about a year and half with this tune so I don't know what could have happened.

Check your vaccuum and PCV systems for a leak, clean your MAF.

If you were good and changed nothing you always start with wear items.

[poopeebum]

I have this problem too, I have tried replacing the crank shaft position sensor as well but that does not help either. my hesitation at low RPM under load happens randomly not all the time. I have not been able to find out what the problem is as this has been happening for a while now. Same symptoms, hesitation under load at low rpms 500-2500 rpm, smooth acceleration after that

Im not sure if replacing the fuel pump, fuel pressure regulator, or the knock sensor will help or not. I've searched for years on forums but havent been able to find a definate solution

I should mention that i have no CEL's, currently the car is stage 2 custom tuned, but its been doing this even when it was stock so i doubt it is tune related.

[N/A_junkie]

Originally Posted by bushflyr

I get the same thing. It's not the IACV, I've had that too, several times, and it's super obvious.

My situation, and it sounds like the OP's, feels like there is no fuel for just a split second after you hit the throttle from idle. It stumbled for a quarter second then revs build like it ought to. No idea what causes it. But replacing the IACV doesn't help.

My situation to a T. I haven't bothered to ask about it, because it goes away after the car comes up to temp.

Mine also has a difficult time starting sometimes...not battery/electric related. It will crank...fire...hesitate...idle as normal. Almost easier to give it a bit of gas, like a carburated car, sometimes.

[sultanofsubguns]

The front 02 sensor was causing issues like this on my car and wasn't throwing any codes either.

[Douda]

2005 WRX, same problem, hesitates off idle when cold, seems to get better as car warms up.
Almost worse when the fuel is low so I'm going to try a new fuel pump.

currently has 45,000km.

[cosseywrx]

/\ same thing here! If my oil temp is under 180 mine hesitates in 2nd low rpm usually. 2005 wrx as well

[paulpat]

Same.... lol. I read a while back that with a stage 2 tune, sometimes, people have issues with cold starts. Something about being to hard to adjust the tune vs benefits and the tuners time. Lol.

[04wrbscooby]

Hey guys just registered but not new. I've been lurking hehe. Lots of useful info here and awesome people from what I can tell this is one of the best sites! Names Ben by the way.

Anyways I'm stage 2 ots map full cobb catless 3" turbo back and aem short ram intake. I use to have these issues but I cleaned my primary o2 sensor and all issues stopped. Mine was pretty black and especially since ots maps even stock run as rich as 10:1 af ratios. Try that if not replace! If not that try your coolant temp sensor! Check it with a voltmeter as if that's out of spec you closed and open loop fueling will be also.

Ben

[flash02]

I too am having this problem, this seems to be Not uncommon, anyone find a fix? Currently I do not have a CEL, this happens at lower RPMS as I'm going through the gears, WOT seems to be strong and this issue is not always constant, it will vary though seems to be better when the engine is good and warmed up.

I'm going to clean my Crankshaft position sensor as I've heard that can cause issue

I've read that any number of sensors could be causing the issues, including the neutral safety switch, it sure would be nice to find someone who has fixed this issue.

[meandean77]

I had same issues. I cleaned the brown relay under passenger kick panel and under hood for the front O2 sensor (clean connecters with elecrical contact cleaner let dry then apply a dab of dielectric grease). Now car runs very smooth.

[flash02]

Soooo strange how many "fixes" there have been to a problem that will more than likely happen to a Subaru. My plan of attack is first clean/inspect the cam and crankshaft position sensors, then go from there. Perhaps in have an intake leak, perhaps the front o2 sensor is malfunctioning, perhaps it's the neutral safety switch or perhaps 11 years of faithful service has caught up to the wiring and I need to get the meter out, the list goes on and on and on until I figure it out or I give up and move on. I guess I'm lucky as I'm the original owner if this 02 WRX, 168k miles org eng and trans, so I know what has been done.

[Dat8687]

I always thought this was normal its a boxer and the turbo is what 3 foot from the heads.. You cannot possible expect any response like a normal turbo car unless its Super high compression or a super small turbo

[Two Dogs]

I have something similar. 2013 STI hatch. At @2500 rpm, all gears, hot or cold it feels like it cuts out for a split second. I can replicate on demand, need to take it to the dealer for a check. Rather have a root canal.

[flash02]

Originally Posted by Two Dogs

I have something similar. 2013 STI hatch. At @2500 rpm, all gears, hot or cold it feels like it cuts out for a split second. I can replicate on demand, need to take it to the dealer for a check. Rather have a root canal.

From what I've read there are a number of things that can cause the issue, from MAF sensor to neutral safety switch and even poor grounds. What is frustrating to me is with all the Subaru owners who have reported this issue and even fixed it we don't have a write up in the FAQ or sticky section. I'm going to work on my car this week, as I replace and hopefully fix the problem I will update this thread. Yes I agree I'd rather have a root canal then work on this problem.

Cam and crank position sensors are easy to remove and clean, I'll start there, if that does not work I'll go to the IAC. Since I have an access port I should be able to look at what the various sensors are reporting. Currently I have no check engine lights so I really don't have a good starting point added to an 11 year old car it really could be any number of causes singular or combined.

[flash02]

Originally Posted by Dat8687

I always thought this was normal its a boxer and the turbo is what 3 foot from the heads.. You cannot possible expect any response like a normal turbo car unless its Super high compression or a super small turbo

Absolutely NOT true, turbo lag is one thing but a hesitation similar to a misfire is NOT normal. What is going on is almost like the ECU is creating a very small fuel cut, as some have mentioned dirty cam and crankshaft position sensors can cause this and even an intermittent neutral safety switch.

Possible sensors to check...

MAF sensor, I once had issues that made me think the tip in enrichment was off, turned out to be junk on the MAF sensor, once cleaned the problem went away

Dirty crankshaft position sensor will cause this too, as many have reported

Idle Air Control valve, the IAC system uses inputs from something like 4-5 different sensors to work correctly, any number of these have been known to cause the infamous Subaru acceleration hesitation

Neutral safety switch shorting out will also cause intermittent fuel cut, at least from what I've heard

Throttle position switch

The list can go on and on, oh and let's NOT forget about poor grounds

[flash02]

Removed and cleaned the crank and camshaft position sensors, the crank sensor was very dirty, although start up is much quicker and smother the hesitation on acceleration is still present, although not as bad. I still haven't checked the neutral safety switch and my IAC is not working very good, both could be causing issues. While I was getting dirty I removed the engine ground and clean it just for the hell of it. I am thinking about replacing the forward O2 sensor, but I'll check the free stuff first before I start spend money on sensors.

Any thoughts????

[Douda]

Originally Posted by Douda

2005 WRX, same problem, hesitates off idle when cold, seems to get better as car warms up.
Almost worse when the fuel is low so I'm going to try a new fuel pump.

currently has 45,000km.

After more reading I pulled and cleaned my MAF, made a big difference, still not 100% but much better.

[conklin_02]

Originally Posted by Douda

After more reading I pulled and cleaned my MAF, made a big difference, still not 100% but much better.

Did you clean it properly? A lot of people ruin their maf sensor when cleaning because they use to much cleaner or they just don't even clean it right. You need to clean that little eye that is hanging down in the sensor. Also, I had an issue with a cracked spark plug. Check all the wiring going to your coil packs, pull the plugs and make sure they are gapped properly (with plugs now they usually all come pre gapped to the cars specifications, except I have had a couple plugs that we're not gapped properly from the maker even though they should have been). Check the Pcv and the turbo inlet for any rips or tears because that's a common issue. Also, you could run sea foam through your vacuum line (in case you don't have access to you a smoke machine or do not want to pay a shop to perform a smoke test) and through your gas tank to see if you have any vaccum lines that are leaking. Or you could get a real smoke test.

[flash02]

Originally Posted by conklin_02

Did you clean it properly? A lot of people ruin their maf sensor when cleaning because they use to much cleaner or they just don't even clean it right. You need to clean that little eye that is hanging down in the sensor. Also, I had an issue with a cracked spark plug. Check all the wiring going to your coil packs, pull the plugs and make sure they are gapped properly (with plugs now they usually all come pre gapped to the cars specifications, except I have had a couple plugs that we're not gapped properly from the maker even though they should have been). Check the Pcv and the turbo inlet for any rips or tears because that's a common issue. Also, you could run sea foam through your vacuum line (in case you don't have access to you a smoke machine or do not want to pay a shop to perform a smoke test) and through your gas tank to see if you have any vaccum lines that are leaking. Or you could get a real smoke test.

Thanks for the input, yes I've cleaned MAF correctly.

[wrxpunk]

My 2006 Stg 2 WRX wagon started hesitating at lower RPMs a few days ago. Im gonna try some fuel system cleaner to see if that does the trick, but based on everything I have read, I'm guessing its the neutral safety switch (on the trans) that is causing the issue.

If anyone has figured out how to fix this, please let me know!

[ItsP]

I recently Replaced my fuel filter and it helped with low rpm hesitation. Easily had over 80k miles on the filter. I was pleasantly surprised with the outcome.