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Old 06-30-2016, 08:21 AM   #1
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What are "correct" MAF and O2 readings at idle?

Been doing a lot of reading and investigating for the old 1128 and 1130 codes!
Seems it is very common.
I know MAF and vacuum leaks.

What are the ideal specs at idle?

Do my readings make any sense?

A/C off - MAF = 2.75 g/s (10 kg/hr) @ 730 rpm, Post Cat O2's = steady 0.73 v to 0.74 v

A/C on - MAF = 4.0 g/s (14.4 kg/hr) @ 700 rpm

The 2.75 g/s tells me there is not enough air flowing, but the 0.73 v O2 says exhaust is rich? Is this correct?

I understand the A/C is adding load.
Is the throttle opening up more with the A/C to compensate giving a higher air flow?

MAF was cleaned, even swapped with a new one - no difference.
AOS and Dip Stick tube (found slight crack) also replaced - no difference.
Check for obvious leaks / disconnected hoses...etc.

Before I go pay for a smoke test, just wanting to understand what everything should be.

Thanks in advance!

2002 Porsche Boxster Carrara White - 103,000 miles
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Old 06-30-2016, 08:44 AM   #2
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From memory it should be 13-15 kg/hr. Your 10 seems low probably reflects a lvacuum leak also but its pretty small. You need to look at the fuel trims, they will tell you if the brain is compensating for a vacuum leak.

Vacuum leaks can be a pain to find.. Theres all kinds of places in the Auxiliary air system and in the evap system that can develop a vacuum leak and they are very hard to track down.
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2000 Boxster 2.7l red/black

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1973 Opel Manta
1969(?) Fiat 850 Convertible
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1981 Alfa Romeo GTV 6
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Old 07-04-2016, 01:20 PM   #3
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I chased vacuum leaks for several weeks on my 99. Culprit was brake booster line connector as well as J-pipe going to throttle body. Spec for my 99 MAF is 15 kg/h +/- 1.25, but forget that: like san rensho said look at the fuel trim in real-time. Plus the pre-cat O2 sensors should be fluctuating between .07 and .79, according to the manual. Post-cat are static as you noted.

Once I fixed the vacuum leaks it was still not right, so I replaced the generic O2 sensors and the MAF sensor, one at a time, with Bosch and that took care of the rest of the problem.

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