Uneven at idle and low revs

[DrRobin]

Sorry for the long post, I hope all of the detail points towards the real problem as I feel I have been skirting round the issue. 2001 986 2.7L with 93k miles

This has been on-going for a couple of months now and I have been through a few things as follows:

- Serviced at my local indy as normal in May. The water pump went when the garage reved the engine to get it through emissions. No other problems.

- Over-heated in a traffic jam in middle of a hot June day. Turned out to be an old expansion tank filler cap, replaced and no further problems.

- Late June, noticed there was something blowing from the exhaust, tracked it down to a hole in one of the CATs.

- Early August, New CATs fitted at local indy, ran okay away from the garage, but a couple of days later I got O2 age codes on all four O2 sensors and then later a misfire. To be fair some of the O2 codes might have already been there as my code reader didn't identify the post CAT codes.

- New O2 sensors obtained and fitted, late August by me. They took ages to arrive and I could only get generic Bosch sensors (correct sensor), so spliced them in to the original connectors using the crimp connectors which came with them. O2 codes gone, but I still getting a random misfire.

O2 outputs look fine, pre-CAT move up and down, post CAT fairly stable.

- Sept, random misfire continued until I realised the battery was very low, charged up and no misfire. My CTEK battery sense said the battery went down to 8V whilst the garage were changing the CATs, I guess they left the key in the ignition over-night and drained the battery.

- Also cleared the ECU.

- Sept/Oct, no misfire recorded but uneven at idle and low throttle openings. Cleaned throttle body, small improvement.
Removed oil cap, ran very rough.
Checked coils, all six had a small crack, so replaced all 6 with Beru.
Plugs looked okay, may be slightly rich/unburnt fuel, but replaced.

- Oct, Long term fuel trims have settle down to about -4% on bank 1 and -5% on bank 2. Short term fuel trims look fine.

- Nov, uneven running at idle and low revs continues. I have just bought a new air filter as the old is okay, but a bit old. I will also have a look at and clean the MAF sensor.

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This all started when the CATs were changed, I didn't notice any problem before. The O2 sensors were probably on the way out before the CATs were changed and in hindsight probably should have been changed with the CAT as swapping them over finished them off.

If it turns out not to be the air filter or MAF then I suspect the O2 sensors, even though the graphs from them look fine.

Could O2 sensors cause uneven running?
Is there anything else I should check?

Ta

[Stl-986]

1st....get the correct o2 sensors. You can get them on Amazon. Using the generic ones that crimp are known to cause issues.

Also, it helps to post the actual codes that are in the computer.

how many miles on the plugs?

What are you using to get the fuel trims? Standard obd readers are going to most likely give you the wrong pid for those and not be correct. Seen it myself using Torque. Correct ones can be seen in durametic/pwis. Fuel trims are going to be a symptom of your issue so I wouldn't bother looking at them for now, deep rabbit hole there.

With what you describe it sounds like an O2 issue and/or a vacuum leak.

[DrRobin]

Originally Posted by Stl-986

1st....get the correct o2 sensors. You can get them on Amazon. Using the generic ones that crimp are known to cause issues.

Also, it helps to post the actual codes that are in the computer.

how many miles on the plugs?

What are you using to get the fuel trims? Standard obd readers are going to most likely give you the wrong pid for those and not be correct. Seen it myself using Torque. Correct ones can be seen in durametic/pwis. Fuel trims are going to be a symptom of your issue so I wouldn't bother looking at them for now, deep rabbit hole there.

With what you describe it sounds like an O2 issue and/or a vacuum leak.

Thanks for the reply, some further information:

The generic sensors were all I could get at the time and that wasn't easy. I have now found a UK source for the correct ones and since all these problems started when the CATs were changed O2 sensors are the most likely culprit.

There are no fault codes stored. Prior to changing the O2 sensors I had 5 codes, two for pre-CAT O2 ageing, two unknown or vendor specific (turned out to be post-CAT O2 ageing) and a final unknown which was engine bay fan low voltage. Apparently this comes up if the battery voltage was low, which it was.

A couple of months ago when I had a random misfire and CEL, these would show up, but nothing since.

Plugs were changed with the coils, 2 weeks ago, so only 200 miles.

I have Torque Pro installed in an Android Head Unit and an Autel, both show the same for fuel trims, O2 sensors and pretty much everything else I can monitor. After a battery reset all fuel trims reset to 0% and the long term fuel trims then go negative from there. The old plugs confirmed it was running rich.

Is there an easy way to check for vacuum leaks? I have had a look round the vac lines and listened around the intake, but can't hear anything. The vac gauge on my scanner looks normal, but may be a small leak wouldn't show up?

[Stl-986]

your obd scanners wont show a vac. there is no vacuum pid for porsche. what you are seeing is some math that is being done based on a couple things but it wont be true vacuum.

Watch some youtube videos there are few on checking for vacuum but I think your issues will go away with real o2 sensors.

[DrRobin]

Originally Posted by Stl-986

your obd scanners wont show a vac. there is no vacuum pid for porsche. what you are seeing is some math that is being done based on a couple things but it wont be true vacuum.

Watch some youtube videos there are few on checking for vacuum but I think your issues will go away with real o2 sensors.

Thanks.

4 x Bosch O2 Sensors with correct plug fitted ordered for delivery tomorrow. I will get them fitted on Saturday, together with the new air filter and if it needs it MAF clean.

Hopefully we will then see what happens.

Intake manifold pressure is PID 0B (hex), I thought all vehicles had this, but that must be wrong.

[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.

[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???

[Stl-986]

the maf screws are a security torx bit which has a hole in the middle

[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.

[Stl-986]

did it have a k&n filter previously? If not, doubt the maf is the issue. If it isn't throwing any maf codes I wouldn't bother with the maf.

[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.

[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