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Discussion Starter #1 (Edited)
I purchased my first Subie a few months ago. Everything runs great, but I noticed a weird "thud" noise while idling a few days ago. It occurs at 8 seconds, and 19 seconds int the video. It will never do this while I'm moving. The noise is coming from the back end of my exhaust. When it happens, there is a slight jolt for the entire car. I run 91 Octane, and I have ONLY had this problem since my last fill up for gas... It's 2004 2.5 RS

In the video, you're looking at the back end of my exhaust. My muffler is detached and is on hangars (For CA laws) and the heat shield is above. Any help would be awesome! Thanks.

http://www.youtube.com/watch?v=tYMcIv7dYC0

The above was a copy/paste of mine from another forum (NASIOC). I did an update on my car since then:

Did a tune-up and changed spark plugs, as well as my valve cover gaskets. Also installed a cat-back exhaust (The exhaust system if fully connected now). The hesitation went away, as well as the thudding while idle.

However, while driving out today, I was at a stop sign and the thud noise came back. What the heck could this be!?

Today I also changed my header gaskets... but the thud noise is still there. It is more frequent now, maybe once every few seconds.
"I can only describe the noise as someone sitting down quickly onto a pillow full of air."
 

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Says video does not exist. Without hearing it, it sounds like a vacuum line detached, going bad or leaking.
 

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The sound itself unburnt gas reaching a combustible mixture and popping in the exhaust. Now, WHY it's doing that could be a number of reasons... Is the car catless? Does it throw codes for the front or rear O2 sensor?

Our cars are known to pop on decel by nature (they run rich), but not usually at idle, unless it's either running VERY rich or the catalytic converter is gone/bad.

The one time I had that issue it was because one of my spark plugs had completely blown out and one cylinder wasn't working. Clearly that's not the issue here... However, you said you changed plugs, but not wires? The wires might be the issue (though again, it would be odd for them to be misbehaving at idle and NOT at load).
 

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Discussion Starter #5
The sound itself unburnt gas reaching a combustible mixture and popping in the exhaust. Now, WHY it's doing that could be a number of reasons... Is the car catless? Does it throw codes for the front or rear O2 sensor?

Our cars are known to pop on decel by nature (they run rich), but not usually at idle, unless it's either running VERY rich or the catalytic converter is gone/bad.

The one time I had that issue it was because one of my spark plugs had completely blown out and one cylinder wasn't working. Clearly that's not the issue here... However, you said you changed plugs, but not wires? The wires might be the issue (though again, it would be odd for them to be misbehaving at idle and NOT at load).
I am running with stock cats. I have 158k miles. Later today P0301 came on. That is misfire for cylinder 1. Why would it misfire while idle?? never when im driving. Going to check wires/coils. but do you think it could be a clogged cat? im not getting any other codes besides p0301. I also cleaned my map sensor, and the problem seemed ALOT less, until i started driving it. After being driven for 5 minutes, it would go back to "thudding" alot while idle.
 

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2000 Ver 6 STI - 1996 Gravel Express
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Things to try.

1. check exhaust for leaks
2. check spark plugs and their gaps, maybe swap plugs around
3. check plug wires
4. check injectors, maybe swap cylinder 1 with another cylinder

I'm just going for the no cost checks.
 

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Discussion Starter #7
I replaced all gaskets except where the cat connects to the headers. that one is next (autozone gave me the wrong gasket -.-) But 1 person said this was his solution.

After just doing my sparks, I dont see what the problem could be :/ But I guess I'll check them all anyways... I got .44 gap, which was recommended on forums here.

Going to check plug wires next, as well as move around injectors. Just in troubleshoot mode. Thanks for the help!!

Also, idk if this would matter or anything, but when I do a hard reset (unplug positive and negative from battery and step on brakes/horn), my engine light does not reset. It is there immediately as i turn on the car. Same codes. Would't the sensors need time to calculate data? Also, after unplugging, cleaning, and replacing my MAP sensor, p0107 showed up. Not sure if this code goes away a few mins after driving, I'll follow up with that.
 

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So you just have the donut gasket left to replace. I guess you can look at it and see if there's a leak coming out of it. Not sure if there's any good way to check if it's leaking. Maybe feel for a leak or put some thick liquid on it, like slathering toothpaste on the donut gasket area, fire up the car, see if anything bubbles out.

As for the light coming on right after resetting the ECU, it might not be resetting. Disconnect the negative terminal connector, pump the brakes a few times, turn on the lights, leave, and then come back in like 30 mins. Just to make sure it fully drains for a reset.
 

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Discussion Starter #9
So you just have the donut gasket left to replace. I guess you can look at it and see if there's a leak coming out of it. Not sure if there's any good way to check if it's leaking. Maybe feel for a leak or put some thick liquid on it, like slathering toothpaste on the donut gasket area, fire up the car, see if anything bubbles out.

As for the light coming on right after resetting the ECU, it might not be resetting. Disconnect the negative terminal connector, pump the brakes a few times, turn on the lights, leave, and then come back in like 30 mins. Just to make sure it fully drains for a reset.

Awesome, thanks! Will do. Also, do you think a bad fuel pump could cause a misfire in only 1 cylinder? Fuel pump is only 20$, not sure when it was changed last, as I bought the car less than 10k ago.
 

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It's possible, but I'd expect to see a lot more symptoms than just, loud thud while idling. You can replace it if you want, just for peace of mind. It's not like it'll hurt anything, except your wallet.
 

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Not really sure why you cleaned the MAP sensor, they don't usually get dirty as they aren't in the direct flow of intake air, but hey. Can't hurt... Probably. Hopefully the P0107 (MAP Low Input) wasn't caused by that, though if it was I would suspect a failed sensor, not the cleaning. See if that one clears up from driving, if not, check that it's completely plugged in, etc.

The fuel pump seems unlikely. One cylinder misfire consistently doesn't fit with traditional FP issues, and if the pump was going I don't know how it could have idle issues and no running issues (stumbling, loss of power, etc).

Unfortunately, I'm seeing a similarity to my previous engine failure... you might want to check compression. If Cyl# 1 is too low, it can misfire at idle (thus dumping the unburnt gas and causing that pop), but still run just fine. I had that issue- one of my spark plugs fell apart and scored the cylinder wall to hell. Once replaced, that caused an intermittent lope at idle, but it ran "fine". It shook a teeny bit more than before (and consumed prodigious amounts of oil) but it ran that way for half a year of DD. This is what it sounded like with a spark plug that blew out (no compression):



IIRC, my compression numbers were something like 145, 145, 145, 120. Not low enough to trigger the code often, but enough drop that it didn't idle right.
 

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Discussion Starter #12
Not really sure why you cleaned the MAP sensor, they don't usually get dirty as they aren't in the direct flow of intake air, but hey. Can't hurt... Probably. Hopefully the P0107 (MAP Low Input) wasn't caused by that, though if it was I would suspect a failed sensor, not the cleaning. See if that one clears up from driving, if not, check that it's completely plugged in, etc.

The fuel pump seems unlikely. One cylinder misfire consistently doesn't fit with traditional FP issues, and if the pump was going I don't know how it could have idle issues and no running issues (stumbling, loss of power, etc).

Unfortunately, I'm seeing a similarity to my previous engine failure... you might want to check compression. If Cyl# 1 is too low, it can misfire at idle (thus dumping the unburnt gas and causing that pop), but still run just fine. I had that issue- one of my spark plugs fell apart and scored the cylinder wall to hell. Once replaced, that caused an intermittent lope at idle, but it ran "fine". It shook a teeny bit more than before (and consumed prodigious amounts of oil) but it ran that way for half a year of DD. This is what it sounded like with a spark plug that blew out (no compression):



IIRC, my compression numbers were something like 145, 145, 145, 120. Not low enough to trigger the code often, but enough drop that it didn't idle right.
Hey!! Thats EXACTLY what mine is doing. Heres an updated video.. http://www.youtube.com/watch?v=DCrMLCBuVYc&feature=youtu.be
It does it more frequent near the end...
 

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Discussion Starter #13
Just getting back to the forums, to let people know I solved my issue. Most issues are left unresolved on forums due to people not posting back.

I did a compression test, and all cylinders were 150. Thank God, I was praying for good numbers.

I retook my voltmeter and tried my spark plug wires again. Cylinder 1 was giving off hardly any reading. I then measured the rest of the wires, carefully this time. And the results of the test were all different from one another. MAJOR differences. The wires looked stock (I'm at 158k) so i went to my local Autozone and picked up new wires for about 25$. I hooked up everything and started my car.

An instant fix to my problem. My hesitation and "exhaust popping" was getting really extreme so I expected the worst. My car was hardly drivable and would almost die when I was idle. So here goes to simple fixes, and a rushed misread of wires for the 1st test i did. Learning as i go. thanks all!
 

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Because baller! I run the cheapest piece of crap gas in my car and sometimes 91. 91 actually gets a little bit better gas mileage, but I don't think the extra gas mileage makes up for the extra cost.
 

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Discussion Starter #16
Just part of my OCD. I know minuscule difference running 91 with N/A, but i like feeling like im "treating" my car everytime i get gas. The extra few dollars at the pump is no biggie, I spend my tips on gas money, and my paycheck on other things.
 

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It wont add any performance. Especially if it's tuned from the factory to run 87. Lower octane fuels are more combustable than higher octane so your engine will actually perform better on 87 than it would on 91. You only need to run higher octanes if you have higher compression engine which risks detonating prematurely before spark when running lower octane fuel. It's just a waste of money if you're not somehow built and tuned to run it. The backfiring is probably a result of using the wrong fuel :lol:
 

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It wont add any performance. Especially if it's tuned from the factory to run 87. Lower octane fuels are more combustable than higher octane so your engine will actually perform better on 87 than it would on 91. You only need to run higher octanes if you have higher compression engine which risks detonating prematurely before spark when running lower octane fuel. It's just a waste of money if you're not somehow built and tuned to run it. The backfiring is probably a result of using the wrong fuel :lol:
No more backfire :D Everyone gives mixed messages on which octane to run. Lol. Just like with oil filters, or different brands of oils. Sooooo many mixed messages and no conclusion after days of forums. haha. I'll look more into running 87. If there are no cons with running 91 other than $$, then no biggie ! Oil filter I use is the Subaru OEM factory one, from subarugenuineparts.com and Castrol 5w30 Full synthetic. Off topic, but yeah :lol:
 

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No more backfire :D Everyone gives mixed messages on which octane to run. Lol. Just like with oil filters, or different brands of oils. Sooooo many mixed messages and no conclusion after days of forums. haha. I'll look more into running 87. If there are no cons with running 91 other than $$, then no biggie ! Oil filter I use is the Subaru OEM factory one, from subarugenuineparts.com and Castrol 5w30 Full synthetic. Off topic, but yeah :lol:
You're doing it right ;)
 

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Lol, normally the first thing I suggest for hesitation is plug wires, but the way you described it I assumed you had no driving issues. Still, very glad to hear it was a simple fix!
 
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