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Old 01-14-2017, 07:43 AM   #1
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5000ish rpm flatspot

Gents,

Driven my 99 boxster for about 800 miles and the car runs great except for a slight flatspot at around 5000rpm. The car doesn't break up and revs cleanly through this rpm, but it feels like a bit of a power drop before revving further. My research on this shows that variocam deactivates at around this rpm which can cause a bit of a flatspot at 5,120rpm or a failing maf can cause similar results. My fuel trims look great and no codes, so I doubt the maf is bad, but the flatspot doesn't feel right either.

Any input would be great, thanks

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Old 01-14-2017, 10:34 AM   #2
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I chased an intermittent power loss at high rpm's and finally found a vacuum leak in the top smog hose. (thanks to "itsnotanova" for the suggestion) Once leak was fixed, the engine ran perfectly.

I found the leak by spraying starting fluid (it's really flammable, be careful) at each vacuum hose on the top of the engine until the idle noticeably increased, which indicated the leaking hose.
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Old 01-14-2017, 10:47 AM   #3
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Quote:
Originally Posted by jvfante View Post
I chased an intermittent power loss at high rpm's and finally found a vacuum leak in the top smog hose. (thanks to "itsnotanova" for the suggestion) Once leak was fixed, the engine ran perfectly.

I found the leak by spraying starting fluid (it's really flammable, be careful) at each vacuum hose on the top of the engine until the idle noticeably increased, which indicated the leaking hose.
Good advice. In my case my fuel trims are basically bang on zero so no indication of a vac leak, but thanks for the input
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Old 01-14-2017, 09:02 PM   #4
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Disconnecting the battery seemed to do the trick. Weird, maybe a maf on the way out
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Old 04-04-2017, 05:48 AM   #5
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replaced the maf, problem continues, but its intermittent. I have to spend some time looking for vacuum leaks I think.
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Old 04-04-2017, 06:01 AM   #6
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Quote:
Originally Posted by Quadcammer View Post
replaced the maf, problem continues, but its intermittent. I have to spend some time looking for vacuum leaks I think.
Years ago we used to throw an Oscilloscope on the engine and check the loads during revs to see if it was wires, coil packs, spark plugs, distro, etc. This was preferable rather than guessing.

Do they still practice this process with today's cars?
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Old 04-04-2017, 10:03 AM   #7
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Where is your wear at with vario cams paddles? I would assume a higher deviation will cause a visible curve dip.
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Old 04-04-2017, 10:28 AM   #8
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will have to check. I would think though that paddle wear would result in a consistent condition, not an intermittent one.
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Old 04-05-2017, 03:36 AM   #9
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I chased a similar flat spot problem for a few years. It only happened at wide open throttle and not every time. It was driving me nuts. After a while I ended up getting a pending code for the EVAP system. Come to find out my charcoal canister was plugged. Got a new used one from Woody. OMG no more flat spot!! So I would say EVAP system is something to take a closer look at.
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Old 04-05-2017, 05:49 AM   #10
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Interesting, will have to keep that in mind.

Spent some more time looking at everything last night. I replaced the oil cap and also noticed a broken oil dipstick tube (I swear the whole car is just a big piece of plastic). I did a temp repair on that, so I'll see how it runs over the next few days.

thanks guys

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