I have a 2001 subaru forester s, 192k miles, automatic transmission and no troubles codes to be found.
straight to the issue, when coming to a stop from either a 40mph road, or if I just move the car forward 20 feet using idle to push me, when coming to a stop the car drops in RPM most times. Sometimes this leads to the car stalling over 3-6 seconds. Other times it'll sputter and struggle for a bit before catching up and shooting up into the 1000-1,200 rpm range. I've noticed if I come to a gentle stop it's more likely to keep a steady idle. This isn't always the case though. This has been an issue for almost a year but now it's becoming more frequent this summer. What seemed to lead to this was when one day while driving the car suddenly stalled coming to a light. Started up just fine but seemed to be in limp mode for a bit. The car idles about 50-100RPM lower since then. It used to rest in the 700-800 range but now sits closer to 600-670. That day was when we replaced the battery, upgraded the alternator and replaced the map sensor according to the codes we got. After that it drove a bit better but never the same. Car will also randomly hesitate when coming from a stop.
Example: car is sitting at a stop sign or light, I release the brake to move but when I hit the pedal nothing seems to happen for about a second, after a moment the car catches up and fly's away as usual. After it gets that short cough out, the car will drive completely fine until the next time I come to a stop. It's a mixed bag as to what is going to happen during those moments though. You never know what it's going to choose to do. The only semi-consistent thing about this whole situation is when coming to a stop very gently will it be more likely to stop with no issues. Car starts up just fine from ever stall and has never had an issue. The only other weird issue is in the mornings after it has sat all night it can take quite a few seconds to get going. It sounds like it's sitting on the verge of turning over but just can't quite do it right away. This is a new issue that has only started a couple months ago. That's all I know on that matter.
I've tried new wires, spark plugs, coil pack, cleaning the intake, new battery, alternator, new map sensor, new fuel filter, oil and oil filter is always maintained, I couldn't find any vacuum leaks, TPS seems to read fine, new upstream O2 sensor, car drives fine when accelerating and at high speeds so I don't believe it to be a fuel pump issue either.
I guess my question is what should I be looking for that I haven't already checked or tried to fix. I'm lost on where to go and don't have money to throw at a mechanic to get a "Maybe it's this thing or that thing" I'm not dis-crediting anyone but It's also not cheap for me.
I may have done more to the car but can't think of anymore at the moment. I will update if need be.
Thanks for reading!
Have you checked your brake booster for leaks? It seems you only have issues when you're slowing down, or stopping. Idling with your brakes off and it's fine, and your car struggling when they're on is a classic sign your brake booster is the culprit. Checking your brake booster is easy. Get underneath the hood on the driver's side and there will be a large, cylindrical object, that's your brake booster. It uses engine vacuum to boost your brakes. When your brakes are on, and there's a vacuum leak, your car will struggle or stop because there's too much air, all of a sudden. They tend to not trip codes because it's too sudden, and when you're not using your brakes, everything is fine.
Great catch @justin-shepherd , kudos.
Thanks
Pressure test the fuel pump, see if it's delivering enough fuel.
Would that cause it to only stall at idle? I was under the assumption that was a wide issue throughout all running conditions of the car. It never struggles once it gets moving past idle. I'll see if I can have it looked at though.
@imperator if you don't mind, what's your thoughts?
I couldn't find any vacuum leaks
How did you search? Sometimes you can't see or hear them. Only verification with something like smoke test can give you a definite answer. Only then can you cross it off the list. (just an example. Vacuum leak usually causes high idle)
I'm lost on where to go and don't have money to throw at a mechanic to get a "Maybe it's this thing or that thing"
Well that's not a mechanic. A real mechanic returns a working car to you.
I appreciate all of the comments on it. I'm going to be taking the car in this Monday to see if someone else is able to pin-point the issue. I'll bring up a pressure test on the fuel and see if they offer a smoke test as well. Hopefully we can get something figured out there. I'll be sure to keep the Topic updated with anything new going on or any fixes. Only new info I have as of now is I noticed when my car hesitates and really struggles to move form a stop, The fuel trims like to fly way up, I'm talking like positive 15-30%. sometimes a little higher! It's a cheap tool so I'm not sure of the accuracy but it seems like it's reading decent. Once I get moving though it'll drop back down into a normal range (about -5% to 10%). Sometimes it'll go a little higher than that when accelerating past the hesitation but not often at all.
To better answer your question about how I attempted to look for the vacuum leak all I did was run around with some carb cleaner around hoses. It was bad wording on my part in the original post talking about how I looked but didn't find anything. I'm aware I need to have it tested more professionally, just figured I'd add that it wasn't a loose hose or anything obvious.
Thanks for the tips though guys! Let me know if there's any more info I could give! I'll be sure to add more as more happens
Couldn't that also mean there may be a large amount of air getting in somewhere as well
maybe, but like I mentioned, vacuum leaks typically cause high idle, not low.
Bogging on acceleration is more often fuel starvation.
Anyway, more investigation is needed not guessing.
I forgot to include that I have already cleaned the IAC valve to the car. that was cleaned with the intake
Would that cause it to only stall at idle? I was under the assumption that was a wide issue throughout all running conditions of the car.
I would normally agree with you. You've changed a lot of parts but still have the same issue. No spark or ignition issues, no codes, nothing I'm aware of that's indicative of a possible electrical issue of some kind.. to me, then fuel delivery is the most logical place to look. That system starts at that pump, so that's where I'd start diagnosis at this point. (Yes, I noticed you had replaced the gas pump)
You could have some type of restriction/issue getting gas into the engine. Pressure test rules it out. @imperator if you don't mind, what's your thoughts?
I noticed when my car hesitates and really struggles to move form a stop, The fuel trims like to fly way up, I'm talking like positive 15-30%
This means your engine is running too lean. You ECU is compensating by adding more gas to the fuel mixture. This is why I suggested checking the fuel pump's pressure. You now have live data supporting a lack of fuel. Second step, if it that passes, would be a leak down test. (continue a logical process of elimination)
Alright alright. Just a quick question because I'm uneducated and curious. Couldn't that also mean there may be a large amount of air getting in somewhere as well, Sending more fuel to compensate for the abundance of air that may be going un-metered? Or would that have something to do with the leak down test that I'm unaware of. I just wanna cover all of my bases in case it ends up not being either of those options you have posted.
Thanks!
Couldn't that also mean there may be a large amount of air getting in somewhere as well, Sending more fuel to compensate for the abundance of air that may be going un-metered? Or would that have something to do with the leak down test that I'm unaware of.
Np. "Running lean" means too much air/not enough fuel. Conversely, "running rich" means too much gas/not enough air. Knowing 'where to start looking' is usually more dependent on one's experience, and the engine/vehicle's 'pedigree'. Btw, a leak down test is something that can/should be done right along with checking fuel pump pressure. You have your gauges out, why not?
Anyway, more investigation is needed not guessing.
Correct correct. I was just asking in the chance that maybe that's something I could look into later down the line if it wasn't something to do with fuel delivery. I'm getting it in monday to have it looked at by someone more equipped than I am with what you guys have told me so far. Thanks for the insight though!
It's all good.. we're offering you our advice and hope you might find it helpful.
Update!...kinda
So I decided just to put in a new fuel pump myself because the shop I was supposed to take it to had a missing member (specifically the one that was supposed to look at my car) and had to schedule me out until thursday. The new pump is a bit louder but has seemed to help remedy the hesitation issue that was becoming an issue so that's some good news. Bad news is my car is still idling a little lower than usual when coming to a stop and it tried to die on me as I parked in the driveway, right when I thought all was well. I did however notice that when I am coming to a stop that my fuel trims tend to go in the negative quite a bit. Anywhere between the -9 to -25 at peak. accelerating seems to be perfect now ranging from -6% to +6%! I'll keep watching and let you know if anything changes with that.
I will check tomorrow morning to see how the car starts when cold. Hoping that it fixes the long start time with the new pump. we'll see. I have an appointment tomorrow morning now to have them look over the car.
Thanks for keeping up with the posts and helping!
One more thing I wanna include is I have noticed a very slight but noticeable rough idle? When parked you can intermittently feel a slight almost rumble in the car. Like a light shake as if a piston isn't firing correctly or something else? It's not consistent but you can feel the car bump a few times randomly.