3.9l One Bank running rich

545 Views 13 Replies 5 Participants Last post by WeBeCinYa, 2 mo ago

WeBeCinYa

Discussion Starter · 4 mo ago

Chasing an issue where my passenger side bank is running rich, fouling plugs and getting overall bad mileage.
 Start from beginning. Bought 93 RRC 3.9 couple years ago, upon delivery it smelled like running rich. Had CEL for O2 sensor. Connected Roverguage and it shows even bank pegged out on fuel trim. Other side shows normal operation.
 Of course I start throwing parts. First replaced O2 sensors, no change. Swapped in 2 good MAF sensors, no change. Replaced entire exhaust thinking cats were clogged, no change. Swap out all fuel injectors, no change. Swapped plugs, wires, cap, no change. Swapped in known good ECU, no change.
 Truck runs/drives amazing at 75mph missing no beats. At idle it’s got a mini hesitation that feels like a misfire, but ever so slight enough to just make the idle not clean. There is a rich smell from exhaust naturally since it’s running rich.
 I’ve checked all the wiring at ECU and O2 sensor, nothing obvious. I’ve checked all hoses for vacuum leaks, none found with spraying around.
 Not sure where to go. The truck has 140k miles on it. Hard to believe cam shaft with just one side acting up.

Any other pointers or help appreciated.

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o2batsea

· #2 · 4 mo ago (Edited)

The purpose of the O2 sensors is to detect rich/lean condition and adjust bandwidth on that bank to account. If there is no signal being processed there will be a full bandwidth solution to prevent leanness and possible damage. I would try a known-good ECU and see if that is part of the problem. 14CUX ECUs are rarely "bad" but it happens. I assume by checking the wiring that you unhooked the AMP plug from the ECU and tested the wiring to the o2 sensors with a meter.
EDIT: I see that you tried a known good ECU. There's not much else to be done. Clearly the signal from the o2 sensor on the right side isn't being seen. A vacuum leak would create a lean condition, making it buck and stall at low RPM.
It must be sucking fuel like a fiend

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WeBeCinYa

Discussion Starter · #3 · 4 mo ago

o2batsea said:
The purpose of the O2 sensors is to detect rich/lean condition and adjust bandwidth on that bank to account. If there is no signal being processed there will be a full bandwidth solution to prevent leanness and possible damage. I would try a known-good ECU and see if that is part of the problem. 14CUX ECUs are rarely "bad" but it happens. I assume by checking the wiring that you unhooked the AMP plug from the ECU and tested the wiring to the o2 sensors with a meter.
EDIT: I see that you tried a known good ECU. There's not much else to be done. Clearly the signal from the o2 sensor on the right side isn't being seen. A vacuum leak would create a lean condition, making it buck and stall at low RPM.
It must be sucking fuel like a fiend

Yeah only thing left is wiring/communication between O2 and ECU. I have not tested wiring from AMP plug to O2 sensors.

I find it hard to believe engine related with how well it runs while on the gas.

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o2batsea

· #4 · 4 mo ago

It's running crappy on the gas, you just can't tell as much

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WeBeCinYa

Discussion Starter · #5 · 4 mo ago

I’ll try and check communication between pin and pigtail connector next. Hard AF to get any insight to pigtail as it runs behind engine and they gave all of 2 in to work with on the furthest point from reach.

Thanks Bill

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carlosz

· #6 · 4 mo ago

Just replied on the other forum

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WeBeCinYa

Discussion Starter · #7 · 3 mo ago

Seems you have covered much of the basics and bases, how many spark plugs of the offending side are fouled? Have you performed a cylinder leak down and or compression test? What is the lobe condition of your cam?
Normally 1 side running rich is due to false air on that side, how old is your valley pan gasket? Have you checked intake manifold bolts? They tend to work them selves loose .

Reply to ^^ from RR.net

No compression test yet but on the list. No idea on cam. Bolts seem tight on manifold previously but I’ll check again. Seems more like communication issue so I’ll dive into wiring diagnostic soon.

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Pbfender15

· #8 · 3 mo ago

For what its worth, I had a Jeep back in the day with similar symptoms. MPG went from 17 to 7, and would get periodic hesitation, occasionally followed by a massive backfire (with flames coming out the tail pipe which got great looks from people on highway). Anyway, was faulty fuel injector running it really rich. But you've addressed that... Wish I had more to offer. good luck -

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WeBeCinYa

Discussion Starter · #9 · 3 mo ago

Nothing like 14Sux

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SC110

· #10 · 3 mo ago

Pull that junk out and convert to carbs!
(Sorry, I know that wasn't helpful. When I was chasing a rich mixture issue with my SU's everyone told me to convert to fuel injection...)

What other sensors are feeding info to the brain? Could it be a timing issue? (Check TDC the old fashioned way as we know the timing markers are sometimes way off.) Did the PO install a different fuel map? (Is that a thing?)

14CUX diagostics

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carlosz

· #11 · 3 mo ago

yes a stuck open injector is possible, an old 87 I owned long time a go did suffer from said malady, it used a full tank of gas on a 12 mile trip. fuel was physically dripping out of tail pipe. it did have a horrible misfire thou.

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WeBeCinYa

Discussion Starter · #12 · 3 mo ago

Tested all wiring from O2 to CPU, no issues. Swapped O2 sensors from each side to rule out, no change. New cap/rotor/wires. No change.

Smoke tested and no vacuum leaks noted although that would typically be for lean issues.

Compression test next I guess.

Could it be dizzy or air gap? I noted that is factory set and advised not to adjust.

Running out of ideas.

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carlosz

· #13 · 3 mo ago

Visit britishv8.org, they have a great article on the 14 cux and inner its workings

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WeBeCinYa

Discussion Starter · #14 · 2 mo ago

Broken O2 ground wire on connector behind engine. Jimmy figured it out and fixed. Sensor reporting normal readings now.

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