CEL and Short Term Fuel Trims

[Steve 986]

1998 Boxster, 81,xxx miles. Have been fighting a persistent CEL with various codes, most common of which is P0150 and P0130. (P1319, 1318, 1316, 1317 were present at the start of all this and have since gone away during troubleshooting) Short term fuel trim runs 25% positive on both sides at idle but at any rpm above idle (currently 800 rpm) fuel trims go to zero then fluctuate within 5%. Long term fuel trims around 3%. Suspect vacuum leak at idle that is so small it goes away with any throttle because of course idle is the maximum suck in the intake. Have checked for unmetered air leaks with smoke multiple times. Found a few minor leaks around clamps on intake manifold crossover and found a crack in the idle air control valve and replaced it. Replaced fuel cap when an evap code popped up in the mix. Have a new AOS and rechecked all the connections. Replaced front O2 sensors with new Bosch and appear to be functioning properly on the real time O2 sensor graph. New MAF sensor, Intake manifold has new seals. Fuel pressure checks out ok. Running out of places to look. Having said all this, the engine is strong and idles perfectly, no hint of a problem other than the CEL. So are the short term fuel trends the clue to this problem, which to me indicates a vacuum leak at idle, or is that just a red herring throwing me off the scent? Thanks in advance for any ideas!

[blue62]

Originally Posted by Steve 986

1998 Boxster, 81,xxx miles. Have been fighting a persistent CEL with various codes, most common of which is P0150 and P0130. (P1319, 1318, 1316, 1317 were present at the start of all this and have since gone away during troubleshooting) Short term fuel trim runs 25% positive on both sides at idle but at any rpm above idle (currently 800 rpm) fuel trims go to zero then fluctuate within 5%. Long term fuel trims around 3%. Suspect vacuum leak at idle that is so small it goes away with any throttle because of course idle is the maximum suck in the intake. Have checked for unmetered air leaks with smoke multiple times. Found a few minor leaks around clamps on intake manifold crossover and found a crack in the idle air control valve and replaced it. Replaced fuel cap when an evap code popped up in the mix. Have a new AOS and rechecked all the connections. Replaced front O2 sensors with new Bosch and appear to be functioning properly on the real time O2 sensor graph. New MAF sensor, Intake manifold has new seals. Fuel pressure checks out ok. Running out of places to look. Having said all this, the engine is strong and idles perfectly, no hint of a problem other than the CEL. So are the short term fuel trends the clue to this problem, which to me indicates a vacuum leak at idle, or is that just a red herring throwing me off the scent? Thanks in advance for any ideas!

Interesting that your long term fuel trims stay around 3% when your short term fuel trims are maxed out at 25%. It is more common see long term fuel trims follow short term trims. Then as long term trims go positive that brings the short term trims back down.
perhaps that is not happening in your case due to short idle times.

Another thing I see in your post is:
You say it idles perfectly:
But a vacuum leak has the most effect at idle and commonly causes lumpy or high idle.

I do agree that your short term fuel trims do look like a vacuum leak issue.
But your P0150 and P0130 codes are O2 sensor malfunction codes not lean limit codes. At least that is how they read in the Bentley book.
But you replaced the sensors so that is confusing.
What are your O2 sensors doing when come come down from say 2500 RPM to an Idle?

Have you checked for exhaust leaks ahead of the Pre-Cat O2 sensors or leaks at the O2 sensor bung?
An exhaust leak ahead of or at the O2 sensor has the same effect as a vacuum leak on fuel trims.

[Steve 986]

BLUE 62 Thank you for the response. I will check for leaks in the exhaust ahead of the front O2 sensors, we have not done that as far as I know.

I agree, Long terms seem low but as I said as soon as it comes of idle the Short terms go to nearly nothing. Had one Porsche friend/Inde suggest the spark plug tubes.We did replace the seals on the tubes when we had the engine out for the Cam chain rails but did not replace the tubes themselves. A cracked one could be the culprit? Smoke didn't show anything there. They are easy enough to change and inexpensive but that seems like a long shot. Ever seen that?

Thanks again for the exhaust suggestion I will check it out and report back.

[blue62]

Originally Posted by Steve 986

BLUE 62 Thank you for the response. I will check for leaks in the exhaust ahead of the front O2 sensors, we have not done that as far as I know.

I agree, Long terms seem low but as I said as soon as it comes of idle the Short terms go to nearly nothing. Had one Porsche friend/Inde suggest the spark plug tubes.We did replace the seals on the tubes when we had the engine out for the Cam chain rails but did not replace the tubes themselves. A cracked one could be the culprit? Smoke didn't show anything there. They are easy enough to change and inexpensive but that seems like a long shot. Ever seen that?

Thanks again for the exhaust suggestion I will check it out and report back.

So now that I have the info that you removed and replaced the engine.
Together with your P0150 and P0130 codes which the Bentley says are sensor malfunction codes. Look carefully at your wiring to the O2 Sensors make sure all is correct. Make sure you don't have the pre Cat sensor plugged into the Post Cat wiring harness. I know it sounds crazy but things happen. Your problem could be related to the engine work and something is not quite correct in the replacement process.
So double and triple check your work.

As to the spark plug tubes they are just oil seals. They have nothing to do with vacuum. If they have problems and leak you get oil leaks and then the oil can short the plugs or rather the coils and you get possible misfires.

[Steve 986]

To follow up on my CEL problem. We removed the idle air control valve and replaced with new, the one we put on was not cracked but was a used one we had. Now at 250 miles with no CEL! Kind of interesting, the new valve raised the idle a hair and the ST Fuel trims are still somewhat high at idle but no CEL and Long Term Fuel trims are still low. Hoping this lasts more than one tank of gas!

Thank you for all the input!

Steve

[blue62]

Originally Posted by Steve 986

To follow up on my CEL problem. We removed the idle air control valve and replaced with new, the one we put on was not cracked but was a used one we had. Now at 250 miles with no CEL! Kind of interesting, the new valve raised the idle a hair and the ST Fuel trims are still somewhat high at idle but no CEL and Long Term Fuel trims are still low. Hoping this lasts more than one tank of gas!

Thank you for all the input!

Steve

Well that makes sense.
I have a 2000S so no idle control valve, so I never think of it in relation to vacuum leaks.
Sounds like you have it sorted have fun with it.