I am at my wits end. My 95 with the TBI 350 is running rich. It was running rich before the rebuild and is still running rich. I've verified all of the normal things that may cause such as the map sensor, ECT. I haven't checked fuel pressure. The problem seems to only be affecting one side of the engine as cylinders 3 and 5 have sooted plugs. I plug in the scanner and I can watch it slowly trim the fuel back until it reach 95-100 on the BLM. I don't know what else it could be at this point.
TBI 350 Running Rich
- Thread starter 95 Tahoe
- Start date
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Check your fuel pressure and, take a flashlight, shine it behind the injectors, while idling, and look for a nice even spray cone without any dripping.
I should have mentioned I didn't see any dripping or spray pattern issues. However, I went ahead and changed both injectors today hoping my problem would be solved.
"Running rich" is not the same as having the computer in "lean command".
How old is the O2 sensor? Does it produce adequate cross-counts? What is the O2 sensor showing for mixture strength/voltage? I'm guessing that either the O2 sensor is outright defective, or it's showing normal, controlled air-fuel ratios once the fuel trims settle down.
What is the cranking compression pressures of #3 and #5? How do they compare to a "good" cylinder?
I am not an expert so forgive anything I misunderstand. Running rich is based off the visual observation of a black tail pipe. The computer is commanding the fuel to lean out I assume because it is sensing it is running rich. The o2 sensor is cycling between .4 and 800 MV constantly. Which appears to be normal. The 02 sensor was replaced last fall when this issue started. I have since rebuilt the engine due to a knock and a bad cylinder. I haven't checked the compression on 3 and 5 yet. I will do that tomorrow probably. I would expect it to be good since I rebuilt the engine 3 months ago. I guess I could have screwed up something. I was looking at the spay patterns on the injectors and the right bank looks like a denser spray than the left. Both injectors are remanufactured and were replaced today.
Were they flow matched? An original factory injector can vary by over 10% in flow, just saying.
No they were not flow matched. They were Autozone replacements. I really didn't know that was a thing.
Not real reliable.
The computer is in lean-command, trying to get the actual air-fuel mixture to the desired 14.7:1. IF the O2 sensor is showing the expected swing between low- and high-voltage, the mixture is "correct", averaging neither rich nor lean.
.4 mV and 800 mV? Or 400mV and 800mV?
400 to 800 mV would indeed be "rich". I don't think I've ever seen an O2 sensor go down as far as .4 mV, but a swing from very low to very high voltage is normal for a properly-functioning O2 sensor.
Let us know how that goes.
For the record, the "right-side injector" does not feed only the "right bank". Both injectors feed two cylinders on each bank--the two outer cylinders on one bank, the two inner cylinders on the other bank.
One injector feeds 1, 7 (outer left bank) and 4, 6 (inner right bank).
The other injector feeds 2, 8 (outer right bank) and 3, 5 (inner left bank).
Therefore, a TBI with only one O2 sensor, is still sensing the fuel output of both injectors.
Good catch on the EGR.
It's not commanding a lean condition. It's in lean-command, trying to get to a CORRECT air-fuel ratio. Lean-command is reducing the "expected" amount of fuel according to the base programming because for some reason the base programming would be too rich on your engine. For example, excessive fuel pressure. The O2 sensor (if it's working properly) is the best way to determine rich/lean, and yours seems to be showing a proper air-fuel ratio once the fuel trims are in lean-command.
What is your fuel pressure?
