CEL and Odd Vacuum Readings

[bspot]

I was driving on the highway and a CEL popped up. Read the code, it said it was a MAF error.

I cleaned the crap out of the MAF, and the code came back.

One thing I noticed though, is my vacuum readings are goofy. After accelerating hard then of the throttle at about 2000-2500rpm I'm seeing about 10in Hg.

After accelerating gently and letting off the throttle at the same RPM, I see about 15in Hg.

Usually I see at least 17 at idle, and more under engine braking.

Could this be the PCV?? They notoriously have problems in these cars and actually have a 10 year warranty on that part. One of the hoses cracks, or the diaphragm can malfunction.

I'm booked in with Volvo in case it's free, but I'm hoping to not get stuck paying for diagnosis if that's not it. Any good tips to nail down what is going on?

[TimH]

I had the PCV issue on my car. It threw the code: P2187 System Too Lean at Idle Bank 1 when the PCV failed for me. When the PCV fails the car makes a weird noise at idle. The noise goes away when you pull the dipstick or undo the oil cap. If you wait too long to repair the PCV it will blow out seals and cause oil leaks. The "recall" or whatever volvo calls it is something like 10 years or 160,000 kms, I can't remember exactly. I was the second owner of my car when it happened and I went to Valentine Volvo and they fixed it for me for free.

I also have had the MAF fail for me. Threw a P0101 code. Idle and driveability didn't change much when it failed. The only indicator of a failed MAF was the CEL and the MAF readings being stuck. I bought a new MAF and that fixed the problem.

Your vacuum readings don't look that bad. My off throttle vacuum readings are anywhere between -10 and -20 depending on what the car is doing.

[bspot]

Ugh.. now I don't know what to do. Do you think it's worth ordering a new MAF for $100 and giving that a try first?

Why would off throttle vacuum be 10? That seems really odd to me. Is the throttle plate behaving differently because of the CEL?

Also, how'd you check your MAF readings? Do you have DICE?

[bspot]

Could it be retarding spark because of the CEL and the vacuum readings are a total red herring that will be solved with a new MAF and no codes?

[TimH]

Originally posted by bspot
Ugh.. now I don't know what to do. Do you think it's worth ordering a new MAF for $100 and giving that a try first?

Why would off throttle vacuum be 10? That seems really odd to me. Is the throttle plate behaving differently because of the CEL?

Also, how'd you check your MAF readings? Do you have DICE?

You should be able to scan it with a generic OBDII reader and get a generic OBDII code. You shouldn't need DICE to figure out if the MAF is defective. You just need an OBDII reader that supports the CANBUS protocol.

With an OBDII reader you need to look at the "MAF g/s" PID. If the MAF is dead then the reading will be stuck at a value regardless of the amount of air flow. Idle should read around 4-5 g/s. Reving the car at idle should give a reading of somewhere between 5 g/s and 55 g/s depending on how much air flow there is.

Idle vacuum for me is either -10 or around -15 depending on what the car is doing. For instance, during warmup the car is usually running at -10. I wouldn't be too worried about the amount of vacuum unless the car is hunting for idle which would indicate a vacuum leak.

[bspot]

Excellent! Thanks for the advice man, I'll check that out with the ODBII, and try give it another clean. Luckily Autopartsway has the MAF for $100 so I don't get bent over at Valentine or wait 2 weeks for something from Tasca.

[bspot]

So I logged it, and I'd say dead MAF. I think the vac reading is because my boost gauge is dying. If I tap it, it reads correctly.

Monday,14,July,2014,18:13:22:32386,VCDS Version: Release 12.12.2 (x64),Data version: 20140212
OBD --- --- --,
Marker,TIME,PID A - 016,TIME,PID B - 000,TIME,PID C - 000,TIME,PID D - 000
,STAMP,Mass Air Flow ( g/s),STAMP,Not Running,STAMP,Not Running,STAMP,Not Running,
,0.17,0.63,,,,,,
,0.35,0.63,,,,,,
,0.52,0.61,,,,,,
,0.69,0.63,,,,,,
,0.86,0.63,,,,,,
,1.03,0.61,,,,,,
,1.21,0.66,,,,,,
,1.38,0.61,,,,,,
,1.54,0.63,,,,,,
,1.72,0.63,,,,,,
,1.89,0.63,,,,,,
,2.07,0.63,,,,,,
,2.25,0.63,,,,,,
,2.41,0.63,,,,,,
,2.58,0.63,,,,,,
,2.74,0.63,,,,,,
,2.93,0.63,,,,,,
,3.10,0.63,,,,,,
,3.27,0.63,,,,,,
,3.44,0.63,,,,,,
,3.61,0.63,,,,,,
,3.78,0.63,,,,,,
,3.95,0.63,,,,,,
,4.13,0.63,,,,,,
,4.31,0.66,,,,,,
,4.48,0.66,,,,,,
,4.65,0.66,,,,,,
,4.83,0.63,,,,,,
,5.00,0.63,,,,,,
,5.17,0.63,,,,,,
,5.35,0.63,,,,,,
,5.53,0.63,,,,,,
,5.70,0.63,,,,,,
,5.88,0.63,,,,,,
,6.04,0.63,,,,,,
,6.23,0.63,,,,,,
,6.42,0.66,,,,,,
,6.60,0.66,,,,,,
,6.77,0.63,,,,,,
,6.94,0.66,,,,,,
,7.11,0.66,,,,,,
,7.28,0.63,,,,,,
,7.45,0.66,,,,,,
,7.62,0.66,,,,,,
,7.80,0.66,,,,,,
,7.97,0.66,,,,,,
,8.14,0.66,,,,,,
1,8.31,0.66,,,,,,
,8.48,0.63,,,,,,
,8.66,0.66,,,,,,
,8.83,0.63,,,,,,
,9.00,0.66,,,,,,
,9.18,0.66,,,,,,
,9.35,0.66,,,,,,
,9.52,0.61,,,,,,
,9.69,0.61,,,,,,
,9.86,0.61,,,,,,
,10.04,0.61,,,,,,
,10.21,0.63,,,,,,
,10.38,0.61,,,,,,
,10.56,0.63,,,,,,
,10.75,0.63,,,,,,
,10.92,0.63,,,,,,
,11.10,0.69,,,,,,
,11.27,0.66,,,,,,
,11.44,0.63,,,,,,
,11.61,0.63,,,,,,
,11.78,0.61,,,,,,
,11.97,0.66,,,,,,
,12.14,0.61,,,,,,
,12.31,0.61,,,,,,
,12.48,0.61,,,,,,
,12.65,0.61,,,,,,
,12.83,0.63,,,,,,
,13.00,0.61,,,,,,
,13.17,0.61,,,,,,
,13.34,0.61,,,,,,
,13.52,0.63,,,,,,
,13.68,0.63,,,,,,
,13.85,0.61,,,,,,
,14.03,0.63,,,,,,
,14.22,0.63,,,,,,
,14.38,0.66,,,,,,
,14.56,0.66,,,,,,
,14.74,0.66,,,,,,
,14.91,0.63,,,,,,
,15.08,0.66,,,,,,
,15.24,0.66,,,,,,
,15.42,0.66,,,,,,
,15.59,0.66,,,,,,
,15.75,0.66,,,,,,
,15.92,0.66,,,,,,
,16.09,0.66,,,,,,
,16.26,0.66,,,,,,