Uneven at idle and low revs

[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.

[DrRobin]

[QUOTE=blue62;643432]

Originally Posted by DrRobin

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.

I am running on a stock air filter, it was just changed, but didn't make any real difference.

I measured my MAF voltage today, in fact I measured it several times as the engine warmed up at idle.

Ignition on, engine off MAF = 1.008V

Engine on, fully warm MAF = 1.400V, RPM = 793, MAF 4.2g/s (15Kg/Hr)
The short term fuel trims on both banks were around -4%, slightly more negative on Bank 1.

I took readings as it warmed up, once the fast idle had stopped, as follows:

MAF=1.490V, RPM=857, MAF=5.2g/s
MAF=1.475V, RPM=836, MAF=5.1g/s
MAF=1.450V, RPM=806, MAF=5.0g/s

These are lower air flow than I saw a couple of days ago (6.5g/s at idle), not sure how much it changes with temperature, but it is a lot colder here today.

I appear to be just in spec (1.2 - 1.5V at idle) and 13 - 15Kg/Hr, but at the high end and given that my fuel trim still shows the engine as running rich should I still suspect the MAF?

Unfortunately my 5 point security Torx didn't turn up today, so I haven't had chance to remove the MAF for checking/cleaning, perhaps tomorrow it will be here.

I fully charged the battery yesterday between the two tests, with it sitting on the drive, opening and closing the hood and not driving the battery was a little low, not enough to cause any starting problems. Not sure if the battery charge level from 15% to 95% would make any difference?

Robin

[blue62]

[QUOTE=DrRobin;643444]

Originally Posted by blue62

I am running on a stock air filter, it was just changed, but didn't make any real difference.

I measured my MAF voltage today, in fact I measured it several times as the engine warmed up at idle.

Ignition on, engine off MAF = 1.008V

Engine on, fully warm MAF = 1.400V, RPM = 793, MAF 4.2g/s (15Kg/Hr)
The short term fuel trims on both banks were around -4%, slightly more negative on Bank 1.

I took readings as it warmed up, once the fast idle had stopped, as follows:

MAF=1.490V, RPM=857, MAF=5.2g/s
MAF=1.475V, RPM=836, MAF=5.1g/s
MAF=1.450V, RPM=806, MAF=5.0g/s

These are lower air flow than I saw a couple of days ago (6.5g/s at idle), not sure how much it changes with temperature, but it is a lot colder here today.

I appear to be just in spec (1.2 - 1.5V at idle) and 13 - 15Kg/Hr, but at the high end and given that my fuel trim still shows the engine as running rich should I still suspect the MAF?

Unfortunately my 5 point security Torx didn't turn up today, so I haven't had chance to remove the MAF for checking/cleaning, perhaps tomorrow it will be here.

I fully charged the battery yesterday between the two tests, with it sitting on the drive, opening and closing the hood and not driving the battery was a little low, not enough to cause any starting problems. Not sure if the battery charge level from 15% to 95% would make any difference?

Robin

Your voltage numbers look ok.
But you stated in post #6 that the engine runs much better with the MAF unplugged.
That makes me suspect the MAF as the issue. Cleaning it may help. At the least you can clean it and see if anything changes.
You could do a vacuum test as vacuum issues can cause your problem. But improved running with the MAF unplugged points to a MAF issue.

The MAF is really two sensors in one unit. "Intake Air Temp". is measured by the MAF as part of it's function. Look at intake air temp when at idle and see if the reading looks reasonable.

Low battery voltage has a huge affect on modern day cars.
When ever your doing any tests or having any issues always make sure basic's like battery voltage is at spec.
Also that battery connections are clean and tight.
That is a first step item in any diagnostic procedure.

[DrRobin]

I have a Ctek battery sense fitted, it was that which said 15% capacity, voltage was 12.3V but it was quite cold. The car started ok and when running voltage was measured at 14.1v, pretty normal for my car.

The car did run better with the MAF disconnected and with it circuit my fuel trim and plugs are showing it is running rich, so it definitely points to the MAF.

I am going to take it as soon as my 5 point T20 arrives, give it a clean if it needs it and unless it is as good connected as disconnected then I will order a new one. I should know pretty quickly now what I know what to look for.

Thanks for all the help.