Check Engine Light!

[Kyrak]

Hey all,

I have been having a problem with my 97 Boxster for a while now where it stalls out occasionally when you are coming to a red light and you put the clutch in. It is throwing P1551 (Idle Air Control Valve Closing Coil - Open Circuit), P1514 (Idle Air Control Valve Opening Coil - Open Circuit) and P1115 (Oxygen Sensor Heating 1 After Catalytic Converter - Above Upper Limit). Based on all the research I have done, the symptoms, and the P codes, I had the IACV replaced, but the problem persists. It does seem like it doesn't happen until the car gets warmed up, but I am not 100 % sure. It also happens much more frequently when the A/C is on. My guess is now an O2 sensor, but wanted to get more expert advice than mine before buying more parts :)

Here is a chart of my runtime O2 sensor readings at idle. Let me know what you think.

Kyrak.

[Kyrak]

More sensor readings if needed.

[ar38070]

Well the O2 sensor readings should be oscillating back and forth between .1 ane .8 volts. The bank 1 readings looks pretty strange. The bank 2 readings look better but still look a bit odd. I have attached a spreadsheet that shows what my car (1998) looks like.

You could try pulling the sensors and having a look at them for any obvious problems. You could then swap them between banks to see if the readings follow the sensors.

[ar38070]

Here is the attachment. I could not upload the xls file so here it is compressed.

Porsche_07_Aug_05_18_27_13_Capture_Data.zip

[insite]

that's kind of wierd. since niether coil is getting voltage on the IAV, check that pin 2 to on the IAV wiring harness is getting ~ 12V at idle. your O2 sensor reading may be a symptom of the idle issues. when you reset the MIL, how long does it take to throw the codes again?

[Kyrak]

Once I reset the light, it typically comes back within 2-3 days or so. It seems if I am careful to not let the engine RPMs drop rapidly then it doesn't happen as much (the stalling). The wiring harness would certainly make sense since we already replaced the IACV. What is the easiest way to test the voltage to the IACV? Now that I have seen the above poster's O2 readings, mine seem to be somewhat normal alternating high and low.

Kyrak

Edited October 12, 2005 by Kyrak

[insite]

hmm. based on the fact that the light takes 2-3 days to come back on after a reset, i doubt if it's the wiring harness. the only other thing that would make both coils show 'open loop' on the IAV is the 'emergency mode' that the DME uses in the event that it can't 'trust' the IAV's readings. this could indeed be caused by O2 sensor or MAF issues.

you have your short term fuel trim graphs above. could you post graphs with your bank 1 / bank 2 short term and LONG term trims? that will help.

[Kyrak]

Here ya go, it seemed like this was a weird spike so I screenshotted this set of readings.

Kyrak

[ar38070]

Your maf readings (I assume they are 8% and 10%) are off. They should be around 0%. I do not understand your O2 readings. They should look like a sine wave. Yours do not unless you have some other graphs.

If you had the IACV replaced but it still throws the same codes then I would go back to the people that replaced and ask why you still have the codes.

[Kyrak]

The 8 and 10 % are the LT fuel trims that insite asked for. My MAF flow rate is at 5-6 which is within range (shown in the 2nd post). I am assuming that the IAVC was replaced correctly, but maybe the simple answer is either a bad replacement part or faulty installation. It was done by a shop I trust though. I asked them to replace it after researching and finding all my symptoms to be consistent with IAVC problems. Any other ideas? I can get some more O2 readings if that would help.

Kyrak

[ar38070]

MAF readings at idle do not mean much. You have to check it at higher rpms to see that the MAF functions correctly over its operating range. You can look at one of my prior posts to see that mine was okay at idle but underread at high rpms. Replaced the MAF and problems went away. I am not saying that the MAF is your problem but LT fuel trims should be about 0, if not then something is not right.

[Kyrak]

My foot is not as steady as yours, but hopefully this graph will do the trick. My readings certainly aren't anywhere near yours in your old post. Let me know what you think.

Kyrak

[insite]

i'm pretty sure it's your MAF. your readings are good at idle, but the long term fuel trims indicate that your car is running lean at speed (these should both be roughly zero). do you have any hesitations when driving the car (around 5K RPM or so)?

you can try unplugging the MAF and going for a drive to see if it runs better at speed. you may have to keep the engine alive at idle w/ the gas pedal, but once you get going it will run okay. the DME defaults to a base fuel map when no MAF signal is detected.

[Kyrak]

Well, won't I feel silly if I replaced the IAVC unnecessarily and it was the MAF. Yes, I have experienced the hesitation at 5K or so RPM under acceleration. The car also has had a K&N on it which I know can foul the MAF. If it ends up being the MAF, I will put a stock filter back in at the same time. I will be replacing a window regulator today, so will unplug the MAF at that time. Should I unhook the battery to reset the DME when I do this? If possible, I would like to drive it with the MAF unplugged for a few days to see if it prevents the stalling and the CEL.

If it ends up being the MAF, where is the cheapest spot the get one online these days? I assume I need the 123 00 one? I have the security bits so I know I can replace it pretty easily.

Thanks everyone for the help, much cheaper than the shop :)

Kyrak

[ar38070]

Cheapest place for a MAF that I know of is:

http://www.autohausaz.com/catalog/lookup_p...FToken=80622876

[Kyrak]

Well, yesterday I unplugged the connector to the MAF and drove around the block a few times. I was getting some pretty intense hesitation (almost like I took the throttle off) upon acceleration. The hesitation seemed to go away if I didn't give it much gas. Is this normal behavior without the MAF plugged in? If the MAF is only there to optimize fuel efficiency, I am not sure this behavior is consistent with MAF failure.

The good news is my window regulator swap went with no problems (note to self, make sure to not close or open the door using the window).

Kyrak

[insite]

Well, yesterday I unplugged the connector to the MAF and drove around the block a few times.  I was getting some pretty intense hesitation (almost like I took the throttle off) upon acceleration.  The hesitation seemed to go away if I didn't give it much gas.  Is this normal behavior without the MAF plugged in?  If the MAF is only there to optimize fuel efficiency, I am not sure this behavior is consistent with MAF failure.

<{POST_SNAPBACK}>

very interesting. the car should actually run very well with the MAF unplugged with the exception of idle. the default fuel map used by the DME w/o MAF would usually cure any drivability issues (above ~1500 RPM) if the MAF were the culprit. more on this later; i gotta run.

[Kyrak]

Well, I tried the historically ineffective "MAF cleaning" and so far so good, but have only driven for a few miles. I guess time will tell. I will update this post either way within a week or so.

Kyrak

[ar38070]

You should know sooner than that. Your OBDII software should be able to show you pending codes. They should show up in 20-50 miles.

[Kyrak]

Just when you thought it was safe to go back in the Boxster....IT'S BACK! P1514 and P1551 again. Not sure where to go from here. Any advice?

Kyrak

[Loren]

Just when you thought it was safe to go back in the Boxster....IT'S BACK!  P1514 and P1551 again.  Not sure where to go from here.  Any advice?

Kyrak

Both of those point to an open idle control valve. Have you checked the voltage to the coil of the valve?