1997 850 NA M56 Closed Loop Fault

Steve in Beacon · Post by **Steve in Beacon** » 23 Sep 2019, 07:26

1997 n/a 850 with manual. No vacuum leaks except in evap system but I have removed and plugged the evap hose off the intake manifold. Tested everything with starter fluid, no sign of any leaks in engine compartment

Front H02 sensor is new Bosch, rear is older Bosch.

Signals from front sensor range from .2v to .8v, tend to the higher end.

Coolant temperature is accurate and goes up rapidly and appropriately when started

LTFT is +.8

Questions are:

What factors go into setting a CL fault?

Can the post-cat sensor influence the system's not entering CL mode?

What should I be looking for here? I am completely stumped!

Post by **abscate** » 23 Sep 2019, 10:38

What code are you getting ?

Steve in Beacon · Post by **Steve in Beacon** » 23 Sep 2019, 11:04

Hi Abscate--it's an honor to have you thinking about it. I have no codes at all. The cat is new--a reasonably high-end aftermarket, NY and Cal. approved. The engine runs for a minute or less showing OL, then switches directly to CL_Fault. Revving the engine will bring up a OL-Drive reading for a second or so, then back to CL-Fault. I have driven it around for 50-60 miles under all kinds of conditions and no change.

Intake manifold is tight and gaskets good as shown by starter-fluid test, many vacuum leaks found and repaired as part of a pretty thorough going-over.

Is the downstream 02 sensor a part of this equation? I read different things, with a consensus--which may be wrong--saying that a faulty downstream won't prevent closed loop operation, although it will throw codes, etc. I have a downstream sensor that is potentially good in now, and have switched in and out a new aftermarket sensor, which I understand Volvo's don't like.

I just repaired the fuel tank overflow leak, and will check the under-the-fender charcoal filter and j-tube and nearby areas next for vacuum leaks.

This thing is just perplexing the hell out of me! I'd hate to have to cut the car up for parts. I do have a 97 850 with an auto trans, and when my daughter brings it back--maybe next week?--I can switch out the engine CPU to see if it is something real freaky like that.

With a CL fault most of the readiness monitors won't even run their tests, so the car is unable to be inspected in New York State.

I appreciate your reading and asking!

Steve

Post by **abscate** » 23 Sep 2019, 12:43

Im driving by Beacon on Thursday and can bring AUTEL - I bet we can read some pending codes and fuel trims and see what is going on.

sent PM

Steve in Beacon · Post by **Steve in Beacon** » 23 Sep 2019, 13:53

That's mighty kind of you! Alas I am around after about 4:30; I work for State Parks and have a training that day I cannot miss. If that happens to work for you that'd be great, and I appreciate the offer no matter what!

I do have an Autel scanner and an iCarsoft i906 specifically for Volvos. The iCarsoft doesn't seem to have a section for pending codes but the Autel does, and it shows none. I don't fully comprehend fuel trims, or at least how to interpret them, so knowing that would be a big help. The 02 sensor (front) seems to fluctuate just fine and the engine temperature is entirely appropriate (about 97C when mostly warmed up) so I would guess the problem is something else, but I just don't know what else goes into determining whether the system goes into closed loop or sets a fault. I have read that way abnormal fuel trims can cause that, but the LTFT is -.8, which doesn't seem too very far out. The engine does 'burble' as it slows down after revving, not really a backfire but more than my auto trans 850.

Again, I don't believe that the downstream 02 sensor factors into it at this stage, but there seems to be a lot of misinformation out there!

Thanks again!

Post by **abscate** » 23 Sep 2019, 14:12

I am back through on Thursday late - finishing in Piscataway at 2000 hours, so probably 2200 hours for Beacon, unless I bail and spend the night in NJ for fatigue.

What engine management system are you running (or can you photo the top of your ECU under the cover box??

Steve in Beacon · Post by **Steve in Beacon** » 23 Sep 2019, 18:31

I did get a photo of the CPU and if I have a minute in the morning I will attach it. My daughter may be visiting tomorrow evening, bringing my '96 850 along, so I will do some switching-out of components and let you know. I'm the country mouse that gets up early (0430) and goes to bed similarly early (earlier than I did when I was a toddler!). If you are passing through on Friday morning I'll take the day off--but I am hoping I will get some insight tomorrow and will let you know. If any brainstorms hit, let me know!

Thanks again!

Post by **abscate** » 23 Sep 2019, 20:39

What is the history on the upstream sensor?

Did the open loop start with the new cat install?

Steve in Beacon · Post by **Steve in Beacon** » 24 Sep 2019, 05:11

Upstream sensor is brand new Bosch exact fit.

As for if this started with the new Cat, I can't say. When I got the car it had a non-functioning, fully rusted-out cat and muffler you know NY's use of salt!). It has something like 9 codes for mostly things like misfires and so on; I knew I'd be replacing wires, rotor, cap, cat, plugs and so on so I didn't bother writing them down. But inasmuch as it was showing MILs, it must not have had a closed loop fault.

Steve in Beacon · Post by **Steve in Beacon** » 25 Sep 2019, 17:27

I switched out the fuel rail and pressure devise from my known-good '96, also the MAF sensor. No change. Also switched out the CPU. Nothing. That pretty much leaves a massive vacuum leak, a way over or under pressured fuel pump, or a bad brand-new Bosch oxygen sensor. I'll try switching that out tomorrow evening.