Unhappy with Subaru acceleration from stop or slow

We have two 2004 Subarus - a Legacy sedan and an Outback wagon, both 2.5L, both low mileage. The sedan has always had an annoying problem - a moment of hesitation when I try to accelerate from a stop or slow speed. This makes it a heart-stopping experience to enter a rotary or try to grab an adjacent opening from a stalled lane.



A couple of years ago, I asked the service manager at the dealer about this - he explained that the ECU was working exactly as required by federal, state, and local regulations, and that I’m already living in the best of all possible worlds.



But, I asked, why doesn’t the Legacy accelerate as well as the Outback? He answered that the ECU was working exactly as required by… you get the idea.



We’ve been putting up with this for four years and I’ve got to believe there’s a better answer. I’ve started looking around, and wandered into the worlds of after-market mods, ECU modifications, and such… I’m not really that kind of guy, and don’t know enough to sort out the good stuff from the snake oil. I just want my 2004 Legacy to accelerate the way my 2004 Outback does.



Any recommendations on how I can solve this?

The best time to have pursued this was when the car was still under warranty, and to have gone to a dealership that was more cooperative than the one that you normally dealt with. At this point, your best bet is probably to go to a foreign car specialist who is very experienced with Subarus, so that he can figure out what type of glitch exists with your Legacy.

As you know, the two vehicles should have essentially the same performance, so no after-market modifications should be necessary for them to perform in the same way. A good mechanic who is willing to spend some time diagnosing the problem should be able to figure out the problem without doing any modifications.

As usual, I agree with VDC. It sounds like a temporary “lean out” condition which would make me suspect the throttle position sensor. However, a good Subie guy should be able to do a good diagnosis and get it working right.

Get the car scanned as a first step. AutoZone, Checkers, etc will do it for free and it only takes a few minutes.
If no codes are present then I would lean towards a vacuum leak, faulty MAF sensor, etc. as a stumble or surge often means a lean-running condition. The MAF could be unplugged and the car driven to see if the problem disappears. If it runs fine then the MAF is either dirty or faulty. Faulty should show up as a code.

NEVER put much faith into what a service manager or service writer tells you. The vast majority of them have never turned wrenches for a living and have little, if any, mechanical knowledge or aptitude.
They get through life by blabbing BS. There are a few good ones but they’re the exception, not the rule.

One question though. Does the engine idle perfectly smooth or does it have any stumble to it (even a subtle one)?

Thanks for all the insights. The car runs and idles very well - I’m not aware of any stumbles. It’s just a little too slow to respond at some very inconvenient times.

I agree with ok4450 that the MAF would be the prime suspect for these symptoms.

Thanks for all the insights to date. First experiment was to disconnect the MAF. Didn’t make a difference, except now I have an error code to reset - so the scan is next on the list. After that, it’s time to look for a Subaru place with better listening skills than the dealer I’ve been using.

Now that I"m paying closer attention to the problem, I’d revise my description a bit. When I hit the gas, I hear the engine start to rev, but the car is slow to gain speed - as if (to use a manual transmission analogy) I were engaging the clutch too slowly.

Your latest post is a bit fuzzy, but it appears you have an automatic transaxle vehicle and you’re suffering some slippage in the transaxle based on the latest info.

Before commenting further, is this an automatic transaxle vehicle? (transmission/transaxle; proper term is transaxle on a Subaru)

Yes, it’s an automatic. I suppose slippage is possible - certainly something worth mentioning when I talk to the mechanic - but its behavior is fine outside of the problem I’ve been describing. It runs and shifts smoothly and performs perfectly well at all speeds (except those first few MPH from a stop).

You might have figured out by now I’m not the primary driver of this vehicle, so this is the first time I’ve tried to pay such close attention to what it’s doing. If I didn’t know any better, I’d just say it’s a bit gutless… but I’ve also got that Outback - a virtually identical vehicle - proving to me that I can expect a little more acceleration from 2.5L than I’m getting from the Legacy.

This is a bit difficult to figure without car in hand. You should still get AutoZone, etc. to scan the car to see if any codes are present.

Based on the comment in the 6:41 post and a general gutless complaint, it almost sounds like a slipping transaxle problem; not real rare on Subarus.
I can tell you how to perform a converter stall test if you would like. Easy to do and only takes a few minutes.

Other than the possibility of a slipping trans you might consider a clogged converter. Depending on the severity of the clog, a clogged cat can cause a hesitation, gutless acceleration, poor fuel mileage, and in some extreme cases; can even roast an engine to oblivion.
A vacuum gauge can be used to check for a clogged cat. Unfortunately, many mechanics do not use this wonderful, easy to use tool.

Yet another possibility could be a problem with the ignition timing running retarded. The timing advance is computer controlled and retarded timing will cause hesitation and gutless acceleration. However, this should show up in a scan.

A few more possibilities (faintly) could be a clogged fuel filter or a sticking fuel pressure regulator.

Hope some of this helps.

Thanks - yes, I think we’ve gotten about as far as we can without car in hand, Hopefully I can take this information and make some progress with a mechanic or maybe even at the nearby AutoZone.

Back in simpler times, when dinosaurs roamed the earth and the workings of internal combustion engines were comprehensible, I learned some basic auto mechanics - could even use a timing light and dwell meter with some competence. Despite many other fine skills, I long ago stopped being handy around the car. Perhaps that will change - I’m a software developer and my Subaru is essentially a large, greasy computer… I will get this damn thing solved.

While transmission slippage is a possibility, since the original post states that the car has always had this hesitation problem, I tend to doubt that transmission slippage is the source of the problem.

At this point, I am leaning toward a TPS, which could have been defective from the beginning. It is more likely for this part to have been defective from the start, rather than for the transmission to have been slipping from the start, but of course, we are all grasping at straws, from afar. It is just a shame that the OP didn’t fully pursue this issue while the car was under warranty.

Well gentlemen this is a common problem with 97-98 Subaru 2.5L engines. It is commonly caused by faulty programing of the ECM. The knock sensor picks up the typical Subaru vibration at low RPM conditions and temporarily retards the ignition timing. The only way to correct this is to replace the ECM with an aftermarket unit of your choosing. You can confirm this by using a error code reader to look at the signal from the knock sensor. If the reading is close to -10 deg. then you will need to replace the faulty sensor to correct the problem. Hope this helps you. I wouldn’t worry about it being a lean running condition, also you O2 sensor can confirm the correct mixture. Use a fuel flow gauge if you’re really concerned. All my money is with your ECM, triggered by a sensitive knock sensor.