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YOUR RESEARCHRev hang problem -00 C2
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Performed · outcome unconfirmed · Original poster
-I�ve cleaned the throttle body and according to diagnostic tool it operates linearly with the gas pedal
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Performed · outcome unconfirmed
However I have changed the tires and they are now all the same size.
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Suggestion
Of course your right foot isn't visible but a small bit of lag is normal as it's air mass that has to be sped up to rush into the plenum and fill the cilinders when the throttle opens and keeps being sucked into the cilinders from the plenum when the throttle is closed.
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Hi!
I have this annoying problem with my -00 C2 996 that when I�m accelerating and taking my foot off the gas and press the clutch to shift up my RPMs keeps climbing for a little while before it starts to drop. This feature makes quick shifting difficult as revs are just too high for next gear and it also sounds annoying because it�s like I�m still on the gas while engaging clutch even though I�m not. This has been bugging me for two summers already and I just can�t figure out what�s causing it. A year ago I did a complete engine overhaul and during the assembly I paid special attention to ensure there were no vacuum leaks but the problem stayed exactly the same.
Things I�ve checked/changed
-New fuel injectors
-both varicam solenoids tested
-Cam deviations ok
-Intake side completely stock
-Exhaust fister modded
-UAOS fitted
-I�ve cleaned the throttle body and according to diagnostic tool it operates linearly with the gas pedal
-No fault codes
-e-gas calibration done many times
-MAF cleaned with MAF cleaner
-Dismantled and cleaned gas pedal
-Tried to disable clutch switch but no effect on the problem
-According to diagnostic tool all of the temp sensors seem to be functioning properly
-I�m certain I�m not pressing the gas pedal, when engaging the clutch. I�ve had the car for 3 years and at least I think it wasn�t like that the first summer I drove it.
Also my RPMs increase randomly in cold starts. Sometimes idle RPMs won�t increase at all even if the car has been sitting in cold for several days. When I bought the car, RPMs increased for a little while every time in cold starts. I don�t know if these two issues are related.
Here�s a video of me first blipping the throttle in neutral and then doing the exact same thing in first gear while clutch engaged. Btw engine temp is normally not that high.
You Tube
What could be causing that?
Perhaps the short bowden cable between throttle pedal and throttle position sensor is dragging, making it harder for the spring to pull it back.
Don't know if that's a thing on 996 with e-gas, though.
You sure it's not just an electrical issue with the tach, like maybe the RPM packets are stacking and making the engine speed read higher than it actually is?
As it doesn't quite sound like it is racing at 4000rpm based on that vid.
Do you have Durametric data to confirm the rev hang?
Also, replacement status of your clutch/flywheel?
I have seen this several times before...
Does your car have T/C or PSM ?
This problem usually stems from wheel speed sensors, or mismatched tires/ air pressures..
What type test equipment do you have ?
If you have T/C or PSM turn them off and see if the problem persists....
Post any fault codes in the ABS/PSM system, and tire size/brand/air pressures............If there is any problems with wheel speed sensors, tire sizes,air pressures, the computer will think a wheel is locking up or the car is going out of control and will try to intervene by not letting the throttle suddenly close..
Thank you all for replies.
Great! this sounds like a reasonable explanation. My car has PSM and I practically never turn it off. I will try this the first thing I get my car back together and out of winter storage. I have OEM 18" turbo twists fitted with pilot sports and the tyre pressures should be fine.
I only have Foxwell NT530 diagnostic tool and I've never had any codes from ABS/PSM system. Maybe it's time for me to get Durametric...
Tyre sizes, we need the tyre sizes...
Bigger rear tyres rotate slower than fronts at any given speed and when coasting/engine braking the MSR function within PSM interprets the lower rear rotating speed at risk of too low, thus risking stability and adds gas.
https://www.tuningblog.eu/en/categor...ontrol-267465/
Sizes are correct. Are they same brand and model/type?
Pressures also as specified on the sticker on the inside of the frunk lid?
BTW I haven't noticed anything wrong in the video above. Of course your right foot isn't visible but a small bit of lag is normal as it's air mass that has to be sped up to rush into the plenum and fill the cilinders when the throttle opens and keeps being sucked into the cilinders from the plenum when the throttle is closed.
Yep. The tires are the same brand and model and they are the same age. And yes I use the recommended air pressures and usually check them regularly with a separate pressure gauge.
In the video 0:14-0:21 I�m blipping the throttle from about 4k RPM and revs increase to about 5k. From 0:40 �> when the car is moving and I�m in gear with clutch depressed I�m blipping in the same way but it sound like I�m pressing the gas pedal more/longer even though I�m not. I pressed the gas even less than at the beginning to prevent the RPM from reaching the limiter. I know it�s hard to tell from the video because you cannnot see what my foot is doing, but when driving it�s like night and day.
I have a similar issue with my 2001 986, there's a half second delay when lifting off the throttle before the revs come back down. That car has Egas, but no traction control or PSM. I have a 2000 C2 as well, and that car is fine, so it sounds like this behavior is not specific to a particular model / year and rather an issue with your 2000 C2 and my 2001 Boxster.
I would follow Skip's thoughts above.
But fwiw your video looks pretty normal when sitting and reving the car without any load. If you want, post a video under acceleration with and without the psm on to see if there is a difference.
Are you running any aftermarket software? Alot of folks complain that an e-gas pedal has a slight rev delay versus a cable pedal. Better e-gas throttle response is easy enough to fix with good software. But even with software, there is a slight delay in revs dropping off in the 996.
Shifting in the 996 is a rythym, especially on the track, which is why i never went to a short shift kit in my 996.
I would unplug the PSM pump and see if it goes away.
If it does not, I would suspect the throttle body. The feedback pot gets dirty and the ECU can't modulate the throttle correctly.
I have a very similar problem to mrop. The rev.s "hang" but only when the car is moving and the clutch is depressed. No issue when I am stopped and in neutral. The car is a 2001 C2. I dont have PSM or traction control. However I have changed the tires and they are now all the same size. The car has been converted to a Safari car. Based on the symptoms and the comments above it would appear I have a computer issue. Anyone know what the fix might be?
He wrote that his 996 doesn't have TC or PSM, the only two systems that would open the throttle.
No need to unplug the ABS, it does not play into this.
While I agree, the ABS does receive torque info from the engine and does transmit wheel speeds to the engine. Since the larger wheels have thrown off the calibration of the wheel speed to RPM, I think it's worth a quick test to ensure that the ABS is not interfering with the throttle by giving confusing signals to the engine. The test should take about 5 minutes. I'd unplug the connector on the ABS, although it's probably OK just to pull the fuses: looks like fuse B9 is the one. Also E9 and E10 if they exist, though those seem to only be on the PSM versions.
Well theprf you were right. Thank you. I unplugged the ABS and the problem went away. Its a pleasure to shift now instead of making me feel like somehow I am not getting my timing right. So now the question is how do I have ABS but not have the "rev hang" issue.
I didnt read all of the posts but do u have a tires size issue or abs sensor issue? If its the abs sensor, get a good scanner and see which corner is bad. If its tire size...get the correct size tires
.
How much do you want to spend? A motorsport ABS will fix this for upwards of $5k USD however it will disable your instrument cluster.
You could try diaconnecting the output wires from the ABS to the ECU. Grab a wiring diagram and an Amp Junior Power Timer pin removal kit.
I made custom front stub axles with a different number of teeth on the ABS sensor wheel. That is not practical for the rear I don't think.
So there is no way to get closer to an equal diameter between the front and rear tires? The overall diameter should not matter if u can get them close enough.
