1998 v70xc - CEL, EFI-436, driving fine . . .

[eslupin]

Hi there,
I've got a v70xc (1998), and in the middle of a multi-day drive the check engine light came on. Stopped, stared at it, made some phone calls, and drove it another few hundred miles to Seattle. Nothing apparent is wrong with it -- still getting the same good gas mileage, starting and running well, no blue smoke, etc. Took it in to the dealership, against better judgment, and they said it's giving a 436 code for the rear o2 sensor (thus setting off the CEL) which they traced to a "BIG INTAKE VACUUM LEAK". Their proposed course of action is to replace the breather box and associated hoses, recheck, then replace both O2 sensors. Code has been reset, so CEL is off now, and I'm wondering what to do. If there's a big vacuum leak, why not look for that and see if it's a hose – rather than replacing the breather box? What am I missing, and how are they making the leap from vacuum leak to the breather box needing replacement? What can I do to hone in on what's causing this code, or to determine whether it's actually a bad O2 sensor, or something else . . ?

(Also, this is the first time the car has been up to high altitude (~6000 feet then back down); CEL came on an hour or so after descending back near sea level. I don't imagine that would have much effect, but some of the threads on here seemed to suggest that altitude can be a factor in some of these fuel mixture problems. . . )

Lastly: Presumably the CEL is going to come back on / the rear o2 sensor will continue to have this problem. But if it's not blowing through fuel or otherwise running strangely, what motivation do I have to fix this, beyond the obvious wanting to not have the CEL on all the time?

thanks so much - - -

[RickHaleParker]

Check for Air leaks and uneven compression before even thinking about replacing the O2 Sensors.
Do a Smoke test on the intake & exhaust system. Do a Compression test.

EFI-436 Rear heated oxygen sensor (HO2S). Compensation

Condition
The rear heated oxygen sensor (HO2S) constantly monitors the front heated oxygen sensor (HO2S), ensuring that it adjusts the short term fuel trim integrator so that it corresponds to the exhaust gas oxygen content. If the short term fuel trim integrator deviates, the rear heated oxygen sensor (HO2S) will make fine adjustments to it. If this adjustment is too great the engine control module (ECM) interprets this as a damaged front heated oxygen sensor (HO2S) and diagnostic trouble code (DTC) EFI-436 is stored.

Substitute value
twin heated oxygen sensor (HO2S) control disabled.

Possible source

• uneven compression
  air leakage in the intake system
  air leakage in the exhaust system
  defective rear heated oxygen sensor (HO2S)
  defective front heated oxygen sensor (HO2S).

Fault symptom
may result in poor performance.

[eslupin]

Thanks - will try smoke test first. Is it worth doing The Glove Test ? (Still trying to understand why they jumped to replacing the breather box )

[Klausc]

The 'air leak' has to be between the intake manifold and the air mass meter. It has nothing to do with air pressure in the engine (PCV) or the breather box. It is easy enough to check for air leaks. Check all of the small vacuum lines that go to the intake manifold, especially the small one next to cylinder #1 which tends to crack.

[volvolugnut]

I have seen major air leaks in the plastic duct between MAF and the intake manifold. These have been at the folds to allow the duct to flex. Best to remove and inspect all surfaces.
Hoses generally will be split at the ends where pressed on the hose barbs.

volvolugnut

[eslupin]

Hi, thanks to both of you. I have been peering and poking at hoses and don't see anything super obvious - but can anyone link me to a good diagram / walk through which would show exactly which hose(s) you're referring to, between the MAF and intake manifold? would be much appreciated !

[RickHaleParker]

volvolugnut is talking about the Accordion Hose. Take it off and stretch it, sometimes air leaks hide in the folds.

Smoke test both the intake and exhaust.
Never hurts to do the glove test, however a failed PCV system would not be the cause of EFI-436.

[eslupin]

Thanks. Will check that.

[eslupin]

So . . .
Smoke test (per shop) shows no leaks anywhere – but there's a bypassed (capped off I think) line in the PCV system. Shop wants to replace the O2 sensors, and also to replace the missing/bypassed PCV hose (and go ahead and do a full PCV service). Is it remotely possible that the missing/bypassed hose is causing the EFI-436 code?

Wondering if there's any chance doing the full PCV service (I'm more or less due for it), which would include replacing the missing hose, would take care of the code, saving me the expense of replacing the O2 sensors?

edited to add: the shop seems confident that it really, truly is the O2 sensors – not just a leak. But I'm trying to exercise caution for the sake of my pocketbook, and because my sense from this forum is that EFI-436 is significantly more likely to be from a leak than the O2 sensors themselves.

Thanks in advance for any/all advice.

[eslupin]

Belated updated: shop replaced the more accessible O2 sensor, and the CEL has not returned, 6k miles later.