Welcome! Log In Create A New Profile

RPM Varies at speed/load


Expect the best, and accept no substitute.

Products for your Boxster, Cayman and Carrera.
Just got back from a road trip. I noticed that the RPMs on the highway varied by 25 to 50 RPMs at around 2000 to 2400 RPM levels. It got worse maintaining speed up hills. If I down shifted to 3000 RPMs, I didn't notice any variation. Idle RPM is also steady. Originally, I could hear the "blipping" and looked at the needle to confirm (can see the needle bounce up and down by about 25 to 50 RPMs). At lower speeds, it is harder to tell, but it still seemed to be there. I hope it's not the MAF, or worse yet, something with 7th gear, but any thoughts on what this can be before I contact the dealer? Thought it odd that at higher RPMs I could not hear or see the variation, and at idle it is pretty steady, but maybe the variation at that level is less noticeable (audible and/or visual via needle and idle isn't at load). Thanks.
If this is a manual, the RPMs at a given speed should not vary at all. Ever.

That would imply a slipping clutch.

I am confused when you say "If I down shifted to 3000 RPMs, I didn't notice any variation."

Do you mean you down-shifted? if so, you reduced your torque multiplication, putting less strain on the clutch, and therefore it held. If true, this further confirms that your clutch is slipping.

Its actually the classic test.

Grant

Grant gee-lenahan-at-vee-eff-email-dot-net
It is a PDK (2009). Yes, down shift from 7th to 6th upped the RPM to 3000K from about 2200 in 7th. No CEL is on the car.
it appears to be slipping. That might be a simple adjustment.

A PDK, while it operates automatically, is in architecture a manual transmission, with two physical plate clutches, and solenoids that operate them.

Grant

Grant gee-lenahan-at-vee-eff-email-dot-net
Thanks, Grant and Mark. Oh how I yearn for the simpler times again. I'll try the 5th gear test on the way home tonight. I have a longish, gradual incline so can get a bit of a load at 2200ish RPMs.
be the first report of this I've come across if that proves to be the case.

But consider this: At steady cruising speed only about 40 or 50hp is needed so the engine is not making gobs of torque. For the clutch to slip then engage -- if this indeed accounts for the varying RPM -- and do this repeatedly is not what I would expect. However, thinking out loud, if there was some hydraulic/electronic circuit malfunction that resulted in the clutch being slighly disengaged then engaged ever so slightly this might explain the behavior. I would think in this case though there would be an error code and a CEL even though the error wasn't related to the engine. Those PDKs are as much computer as tranmission.

One "test" you can do is with the engine/drivetrain all warmed up is take the car out someplace and put it in 7th gear. I'm not sure how you can avoid the transmission downshifting but under the same conditions -- speed/RPM -- give the engine some throttle as much as you can while avoiding triggering the downshift. If the clutch pack was slipping under a steady load it should slip when you increase the load and the slipping should be rather obvious.

You can also under the same conditions put the transmission in 5th gear. This gear uses the same clutch pack at 7th. Then at about the same RPMs give the engine as much throttle as you can without triggering the auto downshift feature.

You are looking for the engine RPMs going up at a rate or to a level that does not jive with the vehicle's speed.

My WAG is the behavior is not coming from the PDK but from a fueling problem.

Oh, you can try this. Do an e-Gas calibration. The steps should be in the car owner's manual. From memory at least for my cars: With the key off and your foot off the gas pedal turn the key to the first position the on position and leave it there for 60 seconds. After at least 60 seconds turn the key off. Leave it off for at least 10 seconds. The next engine start the calibration will be complete.
I had trouble believing a PDK clutch was slipping too. wont repeat what he already walked through.

Grant

Grant gee-lenahan-at-vee-eff-email-dot-net
I think the reset may have worked, but will need to get it out on the open road this weekend. Coming home from work just isn't a long enough, fast enough test, but I must say, ever since doing the recalibration, my throttle response seems to be much better. Will do this maybe twice a year, and highly recommend others do it. Didn't see the process in my owner's manual, so I did some research on line and found another person explaining the same procedure, but they had leave it off for 5 seconds, not ten, then turning it back to position 1 for 15 more seconds before turning it off. So, it was insert key (position 0), turn to position 1 (one position to the right) for 60 secs; turn it off for 5 secs (position 2???), then back to position 1 for 15 secs; then off. I think it is important to turn it to off and hold the key so it doesn't pop back to position 0 while you are doint the 5-10 second bit where the key can be removed/inserted. Thanks again for the advice. If it doesn't cure the RMP variation, at least my throttle response is better. I may also get a container of the fuel additive to clean the injectors, and run it steady on the highway at higher RPMs for a good 20 or so miles.
Can you please update this, did you find a solution ?
Hello

Have you ever resolved this issue. My vehicle has the identical symptoms and I'd like to know your experience with this case. Thanks a lot!
These cars all operate using fixed gears so if there is ever a time when it's in gear and the wheels aren't spinning at a fixed proportion (based on the gear) of the engine speed, then the clutch is slipping.



Edited 1 time(s). Last edit at 02/13/2020 02:26PM by Boxsterra. (view changes)
Sorry, only registered users may post in this forum.

Click here to login