Uneven at idle and low revs

[blue62]

Originally Posted by DrRobin

4 x Bosch O2 sensors fitted, the post cat on bank 2 wouldn’t thread in, so I had to take the cat out and fit it on the bench, still done now.

ECU reset and then started, ran okay, but not really much different to before, a slight uneven exhaust note. The short term fuel trims went straight down to -8% and I noticed the MAF read 6.5% at 800 rpm when warm. I suspect my MAF is over reading, as the engine was running rich.

The bolts holding the MAF in place won’t come out, my security Torx bit won’t go in correctly, I am not sure if it because the bolts are rusty or the Torx is the wrong size. I was using a security T20 with six stars. I think it might be a 5 stars?

I didn’t manage to get it out, so fitted the new air filter and disconnected the MAF, the fuel trim was much closer to 0% and the engine ran much better. A short journey proved it, running much better and fuel trims close to 0%.

I will try to get the MAF out tomorrow, then try a clean, but probably need a new MAF.

When you disconnect the MAF the system defaults to open loop. There is no fuel correction (trim) happening while in open loop operation. That is why your fuel trims show close to 0%. Fuel trims mean nothing in open loop operation.

What is your MAF voltage with key on engine off??
What is your MAF voltage with engine running at idle?????

Have you disconnected your battery or had a low or dead battery in the recent past????
You say you reset the ECU... what did you do to accomplish that??? What do you mean by you reset the ECU???

[DrRobin]

Originally Posted by blue62

When you disconnect the MAF the system defaults to open loop. There is no fuel correction (trim) happening while in open loop operation. That is why your fuel trims show close to 0%. Fuel trims mean nothing in open loop operation.

What is your MAF voltage with key on engine off??
What is your MAF voltage with engine running at idle?????

Have you disconnected your battery or had a low or dead battery in the recent past????
You say you reset the ECU... what did you do to accomplish that??? What do you mean by you reset the ECU???

With the MAF sensor disconnected the fuel status still said closed loop using O2 sensors and short term fuel trim still varied by a few percent, it was just a lot closer to zero than with the MAF connected.

As I understand it fuel trim is set by the pre-CAT O2 sensor and if the MAF sensor is disconnected the ECU reverts to the inbuilt map for air flow. The MAF sensor fine tunes the air flow to control emissions.

I didn’t measure MAF voltage, my reader gives g/s, it was zero with the engine off and 6.5g/s at idle.

I disconnected the battery for 10 minutes to clear the ECU as recommended when replacing O2 sensors.


Regarding ECU bolts I knew they were Torx T20 security bolts and had a driver, but the security Torx we use here are 6 point and Porsche used an unusual 5 point, I have the correct TS20 5 point coming tomorrow.

Robin

[blue62]

[QUOTE=DrRobin;643425]With the MAF sensor disconnected the fuel status still said closed loop using O2 sensors and short term fuel trim still varied by a few percent, it was just a lot closer to zero than with the MAF connected.

As I understand it fuel trim is set by the pre-CAT O2 sensor and if the MAF sensor is disconnected the ECU reverts to the inbuilt map for air flow. The MAF sensor fine tunes the air flow to control emissions.

I didn’t measure MAF voltage, my reader gives g/s, it was zero with the engine off and 6.5g/s at idle.

I disconnected the battery for 10 minutes to clear the ECU as recommended when replacing O2 sensors.


Regarding ECU bolts I knew they were Torx T20 security bolts and had a driver, but the security Torx we use here are 6 point and Porsche used an unusual 5 point, I have the correct TS20 5 point coming tomorrow.

Robin[/QUOT

Whenever you disconnect your battery you need to recalibrate your E-gas throttle.
The idle and low rpm issues your are having often happen after a battery disconnect.
You could also have a faulty MAF sensor, but I cannot tell if it is faulty without voltage readings.

[DrRobin]

[QUOTE=blue62;643426]

Originally Posted by DrRobin

With the MAF sensor disconnected the fuel status still said closed loop using O2 sensors and short term fuel trim still varied by a few percent, it was just a lot closer to zero than with the MAF connected.

As I understand it fuel trim is set by the pre-CAT O2 sensor and if the MAF sensor is disconnected the ECU reverts to the inbuilt map for air flow. The MAF sensor fine tunes the air flow to control emissions.

I didn’t measure MAF voltage, my reader gives g/s, it was zero with the engine off and 6.5g/s at idle.

I disconnected the battery for 10 minutes to clear the ECU as recommended when replacing O2 sensors.


Regarding ECU bolts I knew they were Torx T20 security bolts and had a driver, but the security Torx we use here are 6 point and Porsche used an unusual 5 point, I have the correct TS20 5 point coming tomorrow.

Robin[/QUOT

Whenever you disconnect your battery you need to recalibrate your E-gas throttle.
The idle and low rpm issues your are having often happen after a battery disconnect.
You could also have a faulty MAF sensor, but I cannot tell if it is faulty without voltage readings.

I did a throttle calibrate but it made no difference. I will try to get MAF voltage readings tomorrow. Hopefully my 5 point T20 Torx will turn up and I can get the MAF out for a look and possible clean.

By the look of the screws the MAF has never been changed and I haven’t seen it on a bill for the 14 years I have had the car so it stands a good chance of been faulty, especially at 93k miles. A new Bosch MAF is £90, so not too bad if it fixes the problem.

[blue62]

[QUOTE=DrRobin;643427]

Originally Posted by blue62

I did a throttle calibrate but it made no difference. I will try to get MAF voltage readings tomorrow. Hopefully my 5 point T20 Torx will turn up and I can get the MAF out for a look and possible clean.

By the look of the screws the MAF has never been changed and I haven’t seen it on a bill for the 14 years I have had the car so it stands a good chance of been faulty, especially at 93k miles. A new Bosch MAF is £90, so not too bad if it fixes the problem.

If you did a throttle calibrate then my guess is a MAF issue or a vacuum leak.
But you stated the car ran better with the MAF unplugged so I am leaning towards a bad MAF.

Voltage spec. for MAF key on engine off ...0.9-1.1 volts
Voltage spec. at idle 1.2-1.5 volts.

[DrRobin]

[QUOTE=blue62;643429]

Originally Posted by DrRobin

If you did a throttle calibrate then my guess is a MAF issue or a vacuum leak.
But you stated the car ran better with the MAF unplugged so I am leaning towards a bad MAF.

Voltage spec. for MAF key on engine off ...0.9-1.1 volts
Voltage spec. at idle 1.2-1.5 volts.

Thanks for the MAF voltages, I will check tomorrow. If the reader won’t give voltage I can always check with a multimeter.

[blue62]

[QUOTE=DrRobin;643431]

Originally Posted by blue62

Thanks for the MAF voltages, I will check tomorrow. If the reader won’t give voltage I can always check with a multimeter.

It does not happen often but they (MAF sensors) can meet those voltage specs. and still be faulty.

If you try cleaning the MAF only use a MAF cleaning spray. Don't touch any part except the plastic housing and air dry only. They are touchy little devils. Cleaning only works about 50% of the time.
Also if your running anything other then a stock air filter, non stock air filters are hard on MAF sensors.