Uneven at idle and low revs

[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.

[blue62]

Originally Posted by 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.

Ok your battery voltage looks ok. The 15% capacity was throwing me off.
I do think it is most likely a MAF issue. I also thought it could be a vacuum leak.
But with negative fuel trims and and plug color showing a rich condition a vacuum leak is unlikely.
Keep us posted I like to know final outcomes.

[DrRobin]

Originally Posted by blue62

Ok your battery voltage looks ok. The 15% capacity was throwing me off.
I do think it is most likely a MAF issue. I also thought it could be a vacuum leak.
But with negative fuel trims and and plug color showing a rich condition a vacuum leak is unlikely.
Keep us posted I like to know final outcomes.

5 point Torx arrived, but won’t get enough grip on the bolts, both are rusty, so I had to drill them out. Fortunately I found a couple of screws that are almost the correct size, so they will do until I get some new bolts.

The MAF looked reasonably clean, just a small amount of dust on the sensor, cleaned off and left to stand.

I put it back in and after the engine was warm MAF reads 4.2g/s at idle, same as before. I took it for a drive, 100% better, probably about the same as it was with the MAF disconnected.

My short term fuel trims settled down to around 0%, long term on bank 1 is -0.78, but bank 2 is -3.91. I suspect this might pull back towards zero after a few more miles, but given it felt smoother I am not going to worry. I will try to get a few miles on over the next days.

Intake air temperature was 20C, actual air temperature is 9C, I guess the intake gets a bit f heat from the engine when warm.

I think I will buy a new MAF, now that I know it’s part number and hopefully that is it.

Robin

[blue62]

Originally Posted by DrRobin

5 point Torx arrived, but won’t get enough grip on the bolts, both are rusty, so I had to drill them out. Fortunately I found a couple of screws that are almost the correct size, so they will do until I get some new bolts.

The MAF looked reasonably clean, just a small amount of dust on the sensor, cleaned off and left to stand.

I put it back in and after the engine was warm MAF reads 4.2g/s at idle, same as before. I took it for a drive, 100% better, probably about the same as it was with the MAF disconnected.

My short term fuel trims settled down to around 0%, long term on bank 1 is -0.78, but bank 2 is -3.91. I suspect this might pull back towards zero after a few more miles, but given it felt smoother I am not going to worry. I will try to get a few miles on over the next days.

Intake air temperature was 20C, actual air temperature is 9C, I guess the intake gets a bit f heat from the engine when warm.

I think I will buy a new MAF, now that I know it’s part number and hopefully that is it.

Robin

Sounds like the MAF was the issue.
Dirt even so small and fine that you can not see it can have an effect on the MAF.
They are very sensitive. Another thing that happens with them is that very fine dirt particles erode the coating on the hot wire. You can't see it with the naked eye. I have seen magnified pictures of the issue. Looks just like it had been sand blasted.

Yes Intake Air Temp. is affected by engine temp.
If your short term fuel trims stay at or near 0% they will eventually pull long term fuel trims down.

I think you stated you have 93k miles on the car.
MAF signals tend to slow down with age.
A new MAF "may" give you better throttle response and fuel mileage.

[DrRobin]

Originally Posted by blue62

Sounds like the MAF was the issue.
Dirt even so small and fine that you can not see it can have an effect on the MAF.
They are very sensitive. Another thing that happens with them is that very fine dirt particles erode the coating on the hot wire. You can't see it with the naked eye. I have seen magnified pictures of the issue. Looks just like it had been sand blasted.

Yes Intake Air Temp. is affected by engine temp.
If your short term fuel trims stay at or near 0% they will eventually pull long term fuel trims down.

I think you stated you have 93k miles on the car.
MAF signals tend to slow down with age.
A new MAF "may" give you better throttle response and fuel mileage.

Both long term fuel trims are about -4% now, but I suspect I know why, I am running on E10 fuel, it changed from E5 a few months ago. The air/fuel ratio for petrol is 14.7/1, but E10 is A/F 14.04/1, the difference is -4.4%. The ECU in built map is almost certainly set for 14.7/1, so would indicate a negative fuel trim.

I have a new MAF sensor arriving tomorrow, so will get this installed on Saturday. My Boxster ran great on the 10 miles each way to work today, so I am not going to worry about LTFT unless it works it’s way up to -10%.

Robin