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Check engine Light

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First guess - vacuum leak so I checked Tekalon vacuum lines and other hoses, cleaned the MAF.

Oil Filler is new type and replaced in last 15k miles -given that the code relates to one side (4-6), I don't think it would be MAF or oil filler/dipstick related anyway.

My service manager suggested that I check the accordions on flexible line that attaches to the lower part of the Air/Oil Separator. It has some oil on it but not a great deal.


1 Where does the lower AOS line attach to the engine?

2 The AOS doesn't appear to be too difficult to replace (doug_d's posts are helpful)

3 I'm not opposed to replacing the AOS as a matter of course as failure is a nightmare .

4 What else should I check before ordering a new AOS kit?

5 What parts/processes does Oxygen Adaptation Area 2 relate to?

BTW - I have slightly increased fuel consumption - no smoke in the exhaust, no residue on the pipes. CEL returns 100-150 miles after reset. Idle is smooth performance doesn't seem impacted.

Edited by LemmyCaution
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I forget which code is which, but if your fuel consumption *increased* then it's probably running rich, not lean.

Anyway; the AOS is easy to replace. Just the two vertically aligned 10mm-head bolts into the block and the two plastic connectors. Under the bottom of the AOS is the suction line into separator for the pass-side cylinder back. It's a little accordion-pleat rubber hose about 3" long (if that). It's held on by a spring-band clamp but it attaches to a smooth metal tube so it can be pulled straight off too. You can reach it from below or from above if you have small hands and a high pain threshold. I pulled it off and then used a screw-type clamp to replace it. Much easier.

Anyhoo. The other thing to check is the PCV and the crossover line. The crossover line connects from the side of the AOS, runs across the motor, and snaps into an elbow piece on the cylinder bank... roughly diagonal from the AOS, just ahead of and below the fuel rail on the drivers side. That elbow has the PCV molded into it.

I am tracking down a vacuum leak myself and replaced both the crossover line and the PCV/elbow. The elbow is ~$20 and the crossover line is ~$75.

Interestingly, I had not planned to replace the crossover line at all. However when I bought the elbow piece from the dealer (they do NOT know that it is also a PCV) he was out of stock but he mentioned that they had several of the crossover line. That struck me as odd, so I asked, and he claimed that they went through the crossover lines frequently. As they age they become brittle and then mechanics crack them unintentionally when working on the motor (lean on them, unplug them with too much force, etc). He mentioned this as a 996 problem, not a 986 problem, but who knows.

The elbow is easy to replace, the crossover is not, since it runs across the block UNDER every other %%$!$@#! line there is.

I forget which term is which. There is both short-term fuel trim (produces no CE lights) which flips all over the map in response to of-the-moment conditions. Long-term fuel trim will trip the CE if it hits either the rich limit (car running too lean to compensate for) or the lean limit (car running too rich to compensate for). Those values are maintained for two RPM ranges IIRC. One for idle to some low range, and the other for higher RPM. Area 2 probably means that the high rpm LT fuel trim is off. Can't look it all up now (at work).

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  • 2 weeks later...
One for idle to some low range, and the other for higher RPM.  Area 2 probably means that the high rpm LT fuel trim is off.  Can't look it all up now (at work).

Thanks for your reply- - I've been using my ScanTool and saving files.

Still not sure what's going-on.

Here's the freeze frame from the last time the code was thrown:

Fuel System: Closed Loop

Calculated Load: 1%

Coolant Temp: 199F

Short Term FT: 0.78%

Long Term FT: 5.46%

Short Term FT: 0.00%

Long Term FT: 4.68%

Fuel Pressure: Not Reported

Intake Man. Press: Hot Reported

Engine RPM: 793 r/min

Vehicle Speed: 0MPH

Just about the same readings each time - seems to always happen stopped in traffic. Engine runs rough for 30 secs light comes-on - engine smooths out after light changes.

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The code indicates that cylinders 4-6 are running lean and engine management cannot restore balance to that bank.

Logic says to look for a vacuum leak which I did initially. Further tests with a scanner indicated that primary 02 sensors are functioning well secondaries somewhat less well on that side (because of the rich mixture).

Fuel trim is higher on that side as well by 15-20%.

The MAF is stable, levels are realistic.

There is evidence of oil seeping mostly on the right side of the engine.

Replies to my post a few weeks back suggested replacing the MAF, replacing 02 sensors etc. (the MAF may still need replacement - FYI)

Conclusion - upon inspection of the right side of the engine (cyls 4-6) I found that the valve cover and crankcase were seeping and that some bolts did not meet torque specs (10 ft/lbs). I re-torqued the valve cover per sequence posted elsewhere and the crankcase boltsb.

Upon cleaning the area I found that in addition to the aforementioned oil seepage there is a leak in the lower line of the Air/Oil Separator. This had been my suspicion all long but, testing with carb cleaner did not reveal a vacuum leak.

So - Monday, I will order a complete A/O Separator kit along with 6 plugs, oil filter etc. for my 6 year service.

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