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Discussion Starter · #1 ·
My car: 2010 Boxster S manual, 44k miles (40k service done a year ago)

I've been fighting an intermittent power reduction for some time. On some starts from full stop, if I engage the clutch at a low rpm but get into the thottle quickly the car is very slow to accelerate. I am beaten by F-150s. I have the accelerator pedal fully on the floor but the car is very slow even in the 4k-7k power band in first/second (partial) gear. It corrects itself usually somewhere into second gear when I get an instant kick of the full engine power. No CEL is shown, and I don't read any fault codes with my cheapo OBD scanner.

It felt to me like the ECU was restricting fuel. My first thought was PSM was tricked, but I've reproduced this with PSM off. It doesn't at all feel like a slipping clutch, the engine speed matches the road speed, the engine rpm rise slowly, and there is no clutch smell. I used a bluetooth OBD scanner and was able to log one of these slow runs with the Torque Pro app. What I noticed between a good (fast) and bad (slow) run was while the ECU logged the accelerator was fully pressed, the throttle (at manifold) was limited to about 40% on the slow run vs 80% on the fast run (at same point in power curve). I see the ECU is very slowly increasing the throttle percentage during this run, but just to highlight how slow it is the app logged my 0-60 at 9.8 seconds when I was fully on the gas and shifting at 7k. Bogging the engine on a start wouldn't explain why it never recovers into the power band.

For some reason the ECU is limiting throttle on some starts, which seem to be somehow related to how the car was launched. I have an appointment with my independent Porsche mechanic, but from talking over the phone he didn't have an immediate suspect. I thought maybe the collective wisdom of this board might help me with the diagnosis.
 

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I wonder if one of your wheel speed sensors is going bad. The car thinks it's in a skid or losing traction and so it cuts the throttle.

Might be worth your time to pull the wheels and check that each sensor is solidly mounted and the connectors are making good contact.
 

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Discussion Starter · #3 ·
Greg, would your suspected problem happen if I reproduced it with PSM Off? I've seen this with PSM Off (I turned it off specifically to trouble shoot this).

I had my mechanic run diagnostics (no codes) and test drive. He thinks the ECU is responding to a bogged engine. I didn't think the restricted throttle would last until 70mph, but he says if you stay on the gas it might. I know the best way to reproduce this is to lightly bog the engine, but I swear I've seen this without bogging. I'm going to try to pay better attention and drive more often with my OBD logger active.
 

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Greg, would your suspected problem happen if I reproduced it with PSM Off? I've seen this with PSM Off (I turned it off specifically to trouble shoot this).
Hard to say. I know that PSM will turn itself back under certain conditions, but it may just be a braking situation (ABS kicking in). I know if I start my car (2007 CS) out at a low RPM, it bogs for a second but responds very quickly, so I don't think you are having normal behavior. It's clearly some kind of sensor or measuring glitch somewhere that the ECU is reacting to.

Please keep us informed as to what you find!
 

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Discussion Starter · #5 ·
I had this happen yesterday when I definitely did not bog the engine. I slightly prolonged a clutch slip @ 1800 rpm which would normally be a pretty quick start. When I think of other times I've seen this (when I didn't purposefully bog to see it), a slightly prolonged clutch slip does seem to be a common factor.
 

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Discussion Starter · #8 ·
Also looks like it could be the clutch switch. I had one of the clutch switches (to detect full press on engine start) replaced, but I wasn't aware of the other until recently. The ECU may be restricting throttle because it thinks I'm starting off. I agree swapping both would be a cheap experiment, although I'm not sure I'm limber enough to get down in there to do it :eek:
 

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If the clutch switch was going bad, you would have intermittent start failures (i.e, turn the key and nothing).

You're right, access to those switches is tough. When I did my clutch switch, I set a small step stool beside the open driver's door, and laid on my back on the stool. Then I could scoot my shoulders into the footwell and reach up to do the switch.

Good luck!
 

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Discussion Starter · #10 ·
What I learned is there are two clutch switches, one to lock ignition (detects if the pedal is fully pressed) and one to assist with shifts (detects if the clutch is fully released). The ECU looks at this second switch and changes engine timing and throttle response parameters. See this post:

https://rennlist.com/forums/996-for...od-their-clutch-delay-switch.html#post9620709

and this full thread:

http://www.planet-9.com/987-cayman-...5405-clutch-delay-switch-defeated-joy-16.html

If the clutch delay switch is giving the ECU a faulty reading when I release the clutch, i.e. if the ECU thinks the clutch is depressed, it could easily explain the throttle values I'm seeing in my logs.
 

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Discussion Starter · #12 ·
After driving around with the taped plunger on the switch for a few weeks, I replaced the old switch with the new switch. I was able to reproduce the problem again on the new switch but never while the plunger was taped down. My thoughts are that while it is related to the clutch position input to the ECU, the main culprit could be pre-ignition prevention. I can't find the link now, but I read that a quick transition from near idle to full throttle can cause conditions with the fuel/air mixture ratio where the ECU retards timings.

For now I'll pay more attention to increasing the rpms higher prior to clutch engagement and keeping a more steady and progressive throttle when I want to launch quicker without being a full-on drag start.
 

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Discussion Starter · #14 ·
After three months of driving, I'm going to call this fixed after replacing the clutch switch ( Clutch Pedal Cruise Control Cancel Switch,Part #: 996-613-114-01-M100)
Unfortunately it's come back again--it seems more prevalent in the summer when the a/c is on. I'm at a loss, my indy mechanic couldn't come up with any ideas. Is it time for the dealer?
 

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Discussion Starter · #15 ·
It hasn't happened since my July post. I switched gas stations after the July occurrence after I pieced together that all of my occurrences had been with this particular station. I had been using what I thought was a high quality Exxon, but they did have lower prices so maybe something was off? It could also be that I've changed my launch method as well, being smoother with the throttle on quicker starts. I'll report back to this thread if it happens again, but I'm hoping I can call this case closed.
 

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Unfortunately it's come back again--it seems more prevalent in the summer when the a/c is on. I'm at a loss, my indy mechanic couldn't come up with any ideas. Is it time for the dealer?
Based on the symptoms, I dont have the same problem but I feel your frustration! Occasionally, my car feels lethargic and loses its torque, like the exhaust gets plugged. It's consistent over a single drive but intermittent in that It could run well for a week, then have one or 2 bad days in a row, then back to good.

I've been looking at my ODBII Vitals for a long time and dont spot anything. Most recently, my AOS replacement seems to have reduced the occurrences a lot and now so have assumed there could have been vaccum issues. Currently, my issue resembles more of a aging O2 sensor sensitivity problem or intermittent backpressure issue so it's quite frustrating not having a car run consitently!

Could you share other aspects that you have covered over the course of this investigation?
- do you have any mods like headers/exhaust
- MAF reading/cleaning, sparks, coils check etc
- What are your fuel trims telling you? (RKAT, FRAU, FRAO) etc .. - fuel rail/regulator checks etc?
- physical engine/transmission mount checks etc
 

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Discussion Starter · #17 ·
My car was stock and the spark plugs were fairly new when I started experiencing this. From the OBD2 logs, I noticed the throttle not opening up fully on the slow runs, like the ECU made some decision to hold back power. I'm not sure what to make of the timing in the logs, I'll admit it's beyond my knowledge how to interpret. My mechanic was insistent it wasn't MAF sensor as he said it always throws codes when there's an issue (?).

I can share the spreadsheet of the OBD2 logs if you are interested, PM me.
 

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I am no expert nor mechanically inclined so don't listen to me either! Just sharing what I've learnt from the internet and other discussions.

In short, The ODBII runs in a closed loop, which means any one of the sensors could be sending a bad signal which makes the DME pull back (I've read about torque limiting routines but haven't identified the specific criterial DME will use to pull back torque :)), making it difficult to troubleshoot or conclude from the ODBII Data alone.

Your situation reminds me of this
https://www.youtube.com/watch?v=FJobCD6y8fk&t=149s

Also
This vid mentions fuel pressure under load as a possible cause so a test of fuel rails and fuel pressure regulator could eliminate this as a possibility
https://www.youtube.com/watch?v=VoiLJd2E0EI&t=40s

I like this guy's approach. Very methodical. Essentially he uses ODBII Data to find clues, then logically list causes, think through which ones are most probably given the symptoms, then run test as a process of elimination. Could always try to msg him for advice!

Troubleshooting an issue like this is tricky and takes a lot of time = $$$$ with an independent from my own experience.

The positive thing is it seems you can reproduce it at anytime?

I was thinking through any anomalies in the air/fuel delivery or spark. Might take a look at the spreadsheet anyway.
 

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Discussion Starter · #19 ·
Here's my spreadsheet I logged with my phone. I have a few runs in there. Start with the two rows I highlighted. Pay attention to the accelerator pedal position (column R), and then the throttle position (column Q). Both of these are a few rows before I hit 60mph. On the good row, you see the pedal and throttle are pretty similar. In the bad row, you see the thottle is about half what you'd expect based on the pedal position.

I see the fuel pressure looks fine, but not sure what else to look for.

Good acceleration run: row 143
Slow/bog acceleration: row 2871

https://www.dropbox.com/s/dvm9xvc8srabudn/Log of acceleration bog.xlsx?dl=0
 

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Here's my spreadsheet I logged with my phone. I have a few runs in there. Start with the two rows I highlighted. Pay attention to the accelerator pedal position (column R), and then the throttle position (column Q). Both of these are a few rows before I hit 60mph. On the good row, you see the pedal and throttle are pretty similar. In the bad row, you see the thottle is about half what you'd expect based on the pedal position.

I see the fuel pressure looks fine, but not sure what else to look for.

Good acceleration run: row 143
Slow/bog acceleration: row 2871

https://www.dropbox.com/s/dvm9xvc8srabudn/Log of acceleration bog.xlsx?dl=0
To mirror up how you're driving and the data, could you confirm these runs are captured when you floor the accelerator?

Good run seems to start from row 83 when you see the engine load spike up, then row 93 you see RPM rev up from IDLE. In these rows, the accelerator pedal position looks like you press half way then 3/4 down. So if you are flooring it, I think there's data missing.

If I compare to what I see in my durametric data to draw a baseline what to expect, it doesn't match up. As soon as my RPM rises, load follows. There seems to be a delay with your data capture and I think it's caused by sampling rate which is limited by the device. Another example is the RPM going from 6xx -> 2xxx. This makes it difficult to analyse if something is happening. I uploaded mine data which has a higher sampling rate for you to look at. Filter in the same way with "pedal value". Flooring accelerator = 100

https://www.dropbox.com/sh/iv6c9ikkg1kpi6y/AAAN8Y7OHL2QNxDZLHT9MiPEa?dl=0
 
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