Alfa Romeo Forums banner

1 - 5 of 5 Posts

·
Registered
Joined
·
209 Posts
Discussion Starter #1
Hi All, I'm seeking some L-Jet knowledge on a V6...

I've been having fueling problems recently that ensued after changing from the stock downpipes to the Centerline aftermarket (CA Legal) drop in replacement.

The symptoms are rough running, misfires, and virtually no power until high rpm. I boiled the cause down to a really rich mixture reading at the O2 sensor. The reading is 0.94V from the single spade connector. The range I found in research is 0.2-0.8V where 0.8 is "rich". When I disconnect O2 and run in open loop the car runs fine.

Worth noting: I've tried two O2 sensors (the old one which was in there and a new Denso 2343048), the voltage at the exhaust is the same as the ECU (grounds are good), and the fuel pressure seems to be okay (runs okay in open loop).

Thoughts? Did I just get two bad O2 sensors?

Thanks ~M
 

·
Premium Member
Joined
·
922 Posts
Hi
When I disconnect O2 and run in open loop the car runs fine
Okay so actually the car runs fine, it only runs poorly when it is receiving bad information from the O2 sensor. The O2 sensor is putting out bad information because, either it is defective, or because the exhaust gases it is measuring are contaminated by outside air. In other words I think you have an exhaust leak before the O2 sensor.

If you have a leak or a crack in the exhaust system the fast moving exhaust gas in the system, will suck outside air through the leak/crack, and into the exhaust system. The O2 sensor will interpret this extra oxygen, as evidence that the engine is running lean, and instruct the ECU to richen the mixture.
However since the engine was not actually lean, its now running too rich, which gives you all the symptoms you were having
rough running, misfires, and virtually no power until high rpm
When you replaced the old down pipes, you had to loosen or even remove the exhaust manifolds right ? Go over those area and look for leaks.

Hope this helps
 

·
Registered
Joined
·
209 Posts
Discussion Starter #3
Hi Milanoguy, thanks for the response.

That makes sense, my one question is why does it read more towards the "rich" side of the scale when measuring voltage at the O2 sensor? Shouldnt it be leaner? Could it be ingress of oil (it doesnt smoke though...)?

This issue mostly occurs after the car has warmed up, so I guess that also supports your idea (any cracks or gaps will grow with heat).... Ill lift up the car tomorrow an and take a look...

Thanks again,
 

·
Premium Member
Joined
·
16,232 Posts
I don't think you have two bad O2 probes. The two exhaust systems (stock and aftermarket) are different in material, thicknesses, and heat retention, and maybe I suspect that these changes make a difference in the temperature and burned mixture in the exhaust gas, and thus the readings at the probe.

It is important to remember that the probe (usually quite reliable) is just sending info on what it reads, not that it is a bad one. Owners of 164s, with the diagnostic systems the 164s have, assume that when the error code comes up with a bad O2 reading, it is a bad probe, but actually it is just the probe saying something wrong somewhere else.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #5
Good to know probes are reliable, I guess my only questions are then:

1) Can someone confirm if the range for voltage is 0.2-0.8V with 0.8v rich?
2) If it were running 0.94V rich would it run poorly even in closed loop? It seems as though when I plug the O2 back in the ECU max "leans" the fuel trim and that's what's causing rough running, but the voltage output from the O2 sensor remains the same, AKA lambda doesn't change. If it would run the same maybe leaky CSI?
3) Are there documented cases of the different y-pipes on Centerline FWD cat sections causing poor/undrivable performance on 2.5L's? I haven't found them yet on the BB
 
1 - 5 of 5 Posts
Top