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940 Rough idle, fuel pressure issue?

I unplugged the 2-pin connector for the O2 sensor and noticed absolutely no difference in the way it ran. Is that normal or am I just being hopeful that this just might be the problem?
 
does it consistently run better at higher rpm, like you're flicking a switch, on or about 3000 rpm, based on throttle angle?

Also, did you check for codes?
 
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It’s not quite as abrupt as feeling like flicking a switch, it smoothly transitions. Over 3k RPM, it feels good. No issues from what I can feel. It pulls healthy. Throttle angle doesn’t seem to affect it. However in the low RPM, even with low throttle input, it feels and sounds like WOT. It just bogs down.

Checked for codes, ran the diagnostic tests for the IACV, injectors, etc. and everything checks out. No codes.
 
Depending on throttle input, you could be crossing over from closed loop to open loop around 3000 rpm. In open loop it would refer to fueling tables rather than o2 sensor, so yes, it would run correctly.

However, without a code for O2, I'm skeptical. I don't know. I'm signing off this one.
 
Just drove it again, and when I first left and went over 3000, it felt like a switch this time. Interesting.
Good point though, it makes sense. I wonder if the O2 sensor actually has anything to do with it.

It really feels like cam timing, but the vacuum is still a little on the low side. And of course it's jumping around like crazy. The really odd part is that it ran pretty much exactly the same prior to swapping the head and cam.

Basically, this is where I am...

-get an adjustable cam gear and advance the timing a little
-replace the trashed engine mounts
-adjust the valve that was a little tighter than the rest (just ordered the correct size shim)

I've pretty much ruled out anything and everything else. Timing, vacuum leak, fuel pressure, injectors, plugs, cap, rotor, wires, power stage, TPS, MAF (I suppose it could be failing, but typically that'll throw a code, no?), etc. I haven't swapped the fuel system relay yet, but if the pumps are working and I have good fuel pressure, I don't think it's a problem.
The only other thing I can think Same goes for the radio suppression relay.
 
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I can tell you when I had a bad O2 sensor, I had both fuel mixture error codes and a rough idle. And the exhaust smelled so bad that my hair and clothes would stink for the rest of the day after a 5 minute drive. I'm pretty sure that one was a pebble inside of an oxygen sensor housing though. There might be other failure modes.
The fuel and radio suppression relay failing will stop the car dead. If you suspect an intermittent failure, it's easy enough to pop them open, clean them and reflow the solder.
 
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Right, I figured those relays either work or they don't. I don't think I've heard of them softly failing.

That sounds like a pretty extreme O2 sensor failure! I guess mine could be bad, but it's probably not the main problem.
 
Ok, just voltage tested the O2 sensor. On a cold start, it bounced around quite a bit, but after running for about a minute, it settled around .85V. Little high, no? It should be around .5V. But maybe it’s just a little rich?

I had the ECU side of the connector grounded, per the manual. It ran exactly the same as it always does and the CEL didn’t pop on.
 
Google it noob

Pegged at .9v is rich or bad sensor.

I'm used to just looking at an OBD2 feed of the O2 sensors... the newer cars are too easy to diagnose. They've made me weak!
At this point, I have reason to believe it's a bad sensor. Fuel pressure is good, plugs are good, injectors are good, no obstructions to the intake, no vacuum leaks.
 
I'm used to just looking at an OBD2 feed of the O2 sensors... the newer cars are too easy to diagnose. They've made me weak!
At this point, I have reason to believe it's a bad sensor. Fuel pressure is good, plugs are good, injectors are good, no obstructions to the intake, no vacuum leaks.

In theory it should oscillate somewhere in the range of .1v to .9v while at idle

;-)
 
In theory it should oscillate somewhere in the range of .1v to .9v while at idle

;-)

Right. It fluctuated for a few seconds on the cold start, then settled at right around 0.8V. I?m going to check it again tomorrow, but I think this O2 sensor may actually be the issue. I hope so, I?m sick of this thing running so terrible. It?s terrifying to turn out into traffic because it takes longer to get to 20 than my V70R takes to get to 60.
 
O2 sensors don't oscillate on a cold start. Once the sensor and coolant temp is warm enough, the ECU goes into closed loop and you see oscillating voltage as the ECU oscillates the mixture.

O2 sensors create their own voltage.

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Maybe that’s the problem then. On a cold start it oscillates, then settles around 0.8-0.9. When I revved it, it stayed closer to 0.95 with some occasional drops.
I went ahead and ordered a new one earlier.
 
I unplugged the 2-pin connector for the O2 sensor and noticed absolutely no difference in the way it ran. Is that normal or am I just being hopeful that this just might be the problem?

That is because this two wire connector is just for the heater. Power and ground. Once the O2 sensor is warm, it doesn't need the heater. The heater is just there to warm up the sensor on a cold start and get



Maybe that?s the problem then. On a cold start it oscillates, then settles around 0.8-0.9. When I revved it, it stayed closer to 0.95 with some occasional drops.
I went ahead and ordered a new one earlier.

That's not right.
 
O2 sensors don't oscillate on a cold start. Once the sensor and coolant temp is warm enough, the ECU goes into closed loop and you see oscillating voltage as the ECU oscillates the mixture.

An accurate voltage reading occurs when the engine is idling at normal operating temperature, thanks for pointing that out Zvolv

:nod:
 
That is because this two wire connector is just for the heater. Power and ground. Once the O2 sensor is warm, it doesn't need the heater. The heater is just there to warm up the sensor on a cold start and get





That's not right.

Yes, I?m aware. I checked the diagrams after checking it.
 
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