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 Old 12-26-2016, 09:26 AM   #1
 
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Default low RPM surging and stumbling

So I have read a few similar posts but nothing exactly detailed the way I've been experiencing my problem. Very noticeable stumble/buck/surge/hesitation whatever you'd like to call it, ONLY on light throttle <20% accelerator position, and ONLY below 2500 RPM. If i stretch out the legs for a bit (some sustained higher RPM driving), it goes away and does not return. Plugs and coils are brand new, gapped properly etc.. MAF is good, air straightener is in properly. I feel as though I've narrowed it down to the suggestions of either an Injector, or a bad low pressure FP. Any other ideas?


Also, it seems to be worse in the cold (below 40 degrees F), and I just noticed that the crankcase ventilation pipe is cracked in the middle, but I don't know that his would contribute the issue, that system is quite foreign to me.

Appreciate any helpful ideas or banter!
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 Old 12-26-2016, 06:31 PM   #2
 
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Do a boost leak test. If your crankcase vent hose is cracked, it will probably let un-metered air into the engine, messing up the idle and low air flow conditions. If you romp on it some, the ECU learns the "new" LTFT and possibly helps smooth it out some.

You can spray carb cleaner or something on the crack and watch your idle or STFTs and tell if that is the issue
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 Old 12-27-2016, 09:45 PM   #3
 
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Certainly worth doing before anything else. The thing with the hose is, as you may already know, its a hard tube sheathed in a soft rubber hose which runs nearly the full length of the tube. So my tube itself is broken in half but the overlapping hose is still connecting the two ends but im sure air is syphoning out under pressure. that being said, I wont really be able to spray at the breaking point because I will have to remove the sheathing hose and at that point the hose will be in half, thus skewing the results.

But the boost leak test is obviously a go-to. Thanks for your input
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 Old 01-02-2017, 11:27 AM   #4
 
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I've replaced the vent hose/pipe with an OEM part from Mazda. Didn't want to deal with dabbing something or using a breather filter. There were no findings during the boost leak test. Which is good, but doesn't get me any closer to a solution. I'm guessing im looking at injectors and fuel pump now
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 Old 01-02-2017, 11:53 AM   #5
 
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please let me know how this goes, i suspect my low pressure pump is acting up with similar concerns.
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 Old 01-02-2017, 12:02 PM   #6
 
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Originally Posted by BankShot View Post
please let me know how this goes, i suspect my low pressure pump is acting up with similar concerns.
yeah of course man. And if you find anything out on your end keep me posted. I have heard of this sort of thing being related to the HPFP, but only once.
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 Old 01-02-2017, 12:07 PM   #7
 
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i have been hunting a similar concern,i have already replaced my HPFP internals,cleaned intake valves,checked for boots leaks,i did my fuel injector orings to ensure injectors sprayed straight into the cylinders, its almost feels perfect but just kinda has a stutter, also i dunno if its relative but when i fill up my gas tank full it seems like the car jus runs stronger and better for like 50miles..........so low pressure pump just made me think..
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 Old 01-02-2017, 12:59 PM   #8
 
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Hmm, that's interesting. Mine seems to be more prone to occur and much more prominent after a cold start. And usually completely ceases after some spirited driving. Its hard to pin down. I've switch gas stations, changed tunes, swapped plugs and coils etcetera.
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 Old 01-02-2017, 08:03 PM   #9
 
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I don't think it sounds like any fuel pump or injector issues, maybe fuel quality? Are you using 93 octane?

I guess you're not getting any cels? Do you have any mods? AP or other logging device? If you do, I would suggest a closed loop MAF cal, or at least see how good it looks. Sounds like your ECU is having some issues with low cfm calibration, like the MAF cal is off or a small air leak. Have you checked all the other "hose" connections? The 2 small vacuum hoses on the EBCS, the BPV tube, MAF O-ring, etc
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 Old 01-02-2017, 08:19 PM   #10
 
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At this point I doubt its fuel related. I always use 93 and I even run a 91 tune in case I get some lesser quality gas. I only use Shell or Mobil gasoline and I've tried several different ones in different areas to no avail. I've changed my tunes, no changes.

No CEL's other than the "valve shutter stuck closed" on cold mornings, which was supposed to be solved by the recall, but my issue reoccured. But I don't see that code being linked, and again its only an issue for a split second on cold start. Other than that, no codes.

I do appreciate your input!! I will be sure to check those hoses! I have cleaned the maf, adjusted the air-straightener but I have not done a MAF cal in several months. Honestly the "How To" on MAF cal didn't really make too much sense to me, and im not exactly an idiot. I'll have to take another look.
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 Old 01-03-2017, 05:38 PM   #11
 
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You should look at your LTFTs and see how stable they are. What are they at low throttle input when you are having the issue and then do they change after you get on it like you described.

If it's not a leak, that's about the only (easy) thing I can think of now.
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 Old 01-03-2017, 06:13 PM   #12
 
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Originally Posted by sheston View Post
You should look at your LTFTs and see how stable they are. What are they at low throttle input when you are having the issue and then do they change after you get on it like you described.

If it's not a leak, that's about the only (easy) thing I can think of now.
They are at -0.2 at idle right after starting, thats at my home which is fairly high altitude. Stays around there on light throttle if I recall. I will take some logs tomorrow, good idea! I've had fuel trim issues depending on which tune I use, but its pretty steady at current. I'll get back to you!
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 Old 01-11-2017, 11:42 PM   #13
 
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Originally Posted by sheston View Post
You should look at your LTFTs and see how stable they are. What are they at low throttle input when you are having the issue and then do they change after you get on it like you described.

If it's not a leak, that's about the only (easy) thing I can think of now.
Okay so i took note of my LTFT's as you suggested. Honestly not sure what my readings indicate.

-0.2 at fuel cut, idle, above 65% throttle and WOT
6.9 at low throttle when the issue is presenting itself
4.5 at 20-40% throttle ) i'll still experience the issue at this reading, but only below 2300RPM and usually only after a cold start
8.4 40-65% throttle, no issue
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 Old 01-12-2017, 09:38 AM   #14
 
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Those LTFTs look good, would indicate that your MAF cal is good and no leaks.

Maybe it has something to do with the "valve shutter stuck closed" CEL you get occasionally. I believe that's the VTCS in the intake that is supposed to help cold and at low RPMs. Maybe search on the symptoms of that VTCS starting to die. I've removed mine and upped the rpms to 1000 so I don't have a lot of experience with that (because racecar)
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