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Old 07-12-2017, 05:57 AM   #1
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RPM fluctuation

Anyone have a situation where the rpm fluctuates while running along at a steady speed? My 2004 boxster S has about 50 to 100 rpm fluctuation in the rpm when running along at highway speeds. More like 50 when level with little strain on it and closer to 100 when going up a slight grade. It idles perfectly at 850 and has not lost any power and performes just fine in every other way. A couple of thousand miles ago I had a check engine light which the durametric said was a fuel tank vent leak. I cleared it and it has not returned. Has 101,000 miles on it. Recently serviced with plugs, air cleaner, etc. It is a tiptronic. Maybe dirty MAF or low fuel pressure? You would think it would give a check engine light for either of those. Any ideas?

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Old 07-12-2017, 07:01 AM   #2
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This kinda sounds like a vacuum leak to me. You should begin looking at the rubber in the engine bay for cracks. On our 2000 we have been chasing vacuum leaks for 2 years and the brake booster diaphragm finally failed. it wasn't even on our radar as the source of the problem.

A smoke test would help locate any leaks.
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Old 07-12-2017, 02:01 PM   #3
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May need to clean throttle body, cleaning my steadied everything out nicely.
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Old 07-20-2017, 11:54 AM   #4
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rpm fluctuatiom

Cleaned the MAF and throttle body and checked around for vacuum leaks, which I didn't find. Still have fluctuation. Any other ideas?
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Old 07-20-2017, 12:43 PM   #5
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I found a gas station with pure gas and eliminated the fluctuation idle
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Old 07-20-2017, 02:25 PM   #6
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Quote:
Originally Posted by jcslocum View Post
This kinda sounds like a vacuum leak to me. You should begin looking at the rubber in the engine bay for cracks. On our 2000 we have been chasing vacuum leaks for 2 years and the brake booster diaphragm finally failed. it wasn't even on our radar as the source of the problem.

A smoke test would help locate any leaks.
Very useful.Thanks for sharing.
This is why vacuum leaks can be so difficult to find yet easy to fix
.But first you need to find them. And the suggestion to just pinch off Vacuum Hoses at strategic points to diagnose does not work well for the Brake Booster !Nor does carb cleaner or propane.Maybe a smoke machine:
"Brake booster leaks: Disconnect the booster’s vacuum supply hose and introduce smoke directly into the booster (eliminate the check valve for this test). Check for smoke exiting the booster canister (crimped areas, etc.), and check underdash for smoke exiting through the rear of the booster (don’t depress the brake pedal during this test)."

Last edited by Gelbster; 07-20-2017 at 02:28 PM.
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Old 07-21-2017, 06:47 AM   #7
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Cleaned the MAF and throttle body and checked around for vacuum leaks, which I didn't find. Still have fluctuation. Any other ideas?
There's a LOT of hose on a Box, so you will need a smoke machine (easy to make) to get a full test. Duramertic would be helpful if you had one.

The booster was hard to test because it takes a bit of vacuum to pull it down to check. A vacuum pump is helpful. We could hear ours leaking with my head under the dash when that car was idling.
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Old 07-26-2017, 01:18 PM   #8
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Took off the throttle body intake tube and had a little bit of oil in it and also in the hose running over to the AOS. Not enough to be smoking or anything, but enough to dirty up a towel. Wondering if an aos starting to go bad could cause the rpm to fluctuate at cruising speeds. Also thinking about taking off the resonance tube while I am in here and clean it. Anything to look for with it that could cause an rpm fluctuation or vacuum leak? Trying to cover all the bases while I have it apart.
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Old 07-26-2017, 01:22 PM   #9
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test the AOS with a digital manometer "Pyle" all in Search .Look for 5" or less iirc.
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Old 07-26-2017, 03:16 PM   #10
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Originally Posted by Gelbster View Post
test the AOS with a digital manometer "Pyle" all in Search .Look for 5" or less iirc.
afraid I don't have access to one. Probably just change the aos since I do have one. I also took the resonance tube off and there was some oil in there as well. The current aos has about 30K on it, so I have probably got the best out of it. Now I have to figure out how to get the lower flex hose off, I can't get my hand in at an angle where I can push the release tabs. Not the very bottom one, I have the special pliers for that one
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Old 07-26-2017, 03:44 PM   #11
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Try the hand on the oil filler tube test?
A digital manometer on Amazon/Fleabay is $30 .I got my Pyle for $20.Beats a needless AOS replacement job any day !
And a new AOS may be defective.
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Old 07-28-2017, 11:51 AM   #12
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Desided to go ahead and replace the aos since it already had upward of 30K on it and there was some oil in the intake. Finally figured out how to approach it for the best angle and it wasn't too bad. If you are right handed you can approach from the driver's rear fender and slide up over the engine and you can slide your right hand down the back of the engine and across to the aos and it is easy to push the release on the middle breather tube and slide it off. Then I just removed the 2 mount bolts and pulled the aos out of the bottom hose. Then you can access the clamp on that hose and remove it and mount it in one of the remote cable clamps for reassembly. Judging from the 2 hours it took me to change it, I would say the dealer is making about $500 an hour for their labor when you go to them. Haven't had a road test yet to see if it fixed the rpm fluctuation, but the aos needed to be changed and the intake and maf needed cleaning, so it was all necessary anyway.
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Old 07-28-2017, 12:00 PM   #13
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Carry the old (good) one as a spare because if your AOS fails in the boonies on a trip ,you could wait days for a spare part
Also be creful with the hose connections - they get brittle and difficult to seal correctly.That will give you vacuum faults.
A digital manometer could have confirmed if the AOS was marginal/good.Save a lot of work !
Search "PYLE Meters PDMM01 Digital Manometer"
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Old 09-05-2017, 09:28 AM   #14
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well, I have been working on my RPM fluctuation for a while and no change. It still does it at cruise speed ( anywhere between 1800 and 2200 rpm I get about a hundred rpf flux ). below or above everything is perfect. no other symptoms. I have cleaned the throttle body and changed the maf, oxygen sensors and aos. These were well overdue for a change anyway. I did a smoke test and found no vacuum leaks. I am wondering if the throttle position sensor or fuel injectors would give these symptoms. At 104K miles I am sure it could benefit from new injectors. Any thoughts? It is a 2004 boxster s with tiptronic. It does it in auto mode, or in any gear in manual mode and it does it with cruise control on as well..
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Old 09-05-2017, 10:09 AM   #15
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maybe take it to an Indie with Durametric and get the Freeze Frame data at the exact moment the fluctuation occurs.
Alternatively ,keep driving it until the problem deteriorates - may make the trouble shooting easier.
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Old 09-05-2017, 10:43 AM   #16
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Maybe try checking your fuel trims and MAF readings with an OBD2 scanner (you can get one from ebay for $10 and read the values on a free app on your phone)?

I had cleaned the MAF & TB, replaced the AOS etc, all of which helped a bit with my fluctuating idle issues but didn't fix them. Had no codes show up at all, but when I checked with an OBD2 scanner I found my Long Term Fuel Trims were pegged very high.

Finally replaced the MAF, turns out mine was reporting wrong values even after a good clean, and the LTFT readings dropped right down to acceptable levels on the first drive after. The car now runs and idles like a champ!
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Old 09-05-2017, 12:33 PM   #17
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Quote:
Originally Posted by Gelbster View Post
maybe take it to an Indie with Durametric and get the Freeze Frame data at the exact moment the fluctuation occurs.
Alternatively ,keep driving it until the problem deteriorates - may make the trouble shooting easier.
This is pretty much where I am now, waiting for it to get bad enough to make it easier to find. It runs great when I am above 2200 rpm which is where I want to be anyway.

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