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Can clogged EGR passages foul spark plugs?

Ok here’s an update. I had the car diagnosed and the long and short fuel trims were way out of the normal range. Specifically long term bank 1 was 9 and bank 2 was 19.82. Forget the exact numbers for short term fuel trim but that too was definitely off.

Mechanic took off the upper intake manifold to clean carbon but upon removing it, there was not very much buildup present to actually clean. So he said we might have to continue the diagnostic…however he did notice the IMRC bushing was broken causing the IMRC arm to hang off and the butterfly valve on the drivers side to be stuck open. He put on a new bushing and put the arm back into place ensuring proper operation of the butterfly valve. We cleaned the small amount of carbon there was off anyway from the EGR passages In the lower intake and cleaned the upper intake passages too. Installed it back, and then did an induction service with seafoam to clean cylinders and pistons from carbon. Finally put a new set of oem spark plugs. After doing all that and driving it, the long and short fuel trims were back in normal range fluctuating within 5% of zero. This SHOULD fix the problem right? The fuel trims being off should be what was causing the spark plugs to foul with carbon right?

Did he let the fuel trims stabilize on their own after the repairs? Or did he perform the repairs and immediately reset the fuel trims with the scanner?

If it’s the latter . . .

He did the repairs and then connected the scanner to show the live data. He didn’t reset the fuel trims we took it on a drive and then checked again with car parked and both fuel trims were normal range. Should this fix my spark plug fouling issue?

I guess we will all find out in a few days.

Alright. Great info and responses from you and others on here. It does make a difference in predicting the outcome if he had reset the FT to zero. At this point waiting it out is wise. However, you should be able to narrow down the answers to few possible causes and effects. Let us summarize: Symptom: Dry Soot (carbon deposits on SP#4). Diagnosis: 1. Inefficient Ignition (example: weaker spark, bad connection, bad coil pack) 2. Too much fuel (but ECU compensates by decreasing LTFT up to negative 25% before engine check light kicks in- or worse. However, I think you stated LTFT shows positive 9 and 19 which is inconsistent with Rich condition prior to ECU adjustments). 3. Too much Air (Lean condition in which ECU will compensate by increasing LTFT up to positive 25% before check engine light kicks in. Your new finding confirms ECU positive increase to LTFT. So, Air blockage would not be an issue). 4. Too much fuel (without ECU adjustments due to incorrect sensors output) could result from leaky old injector or new injector cauing inefficient burning. 5 Incorrect sensor outputs or incorrect ECU commands (however, your O2 sensor showed rich condition but that could not have been specific to Cylinder 4 rather to the total fuel trim) which means ECU and sensor need to be tested (albeit, this is unlikely). 6 exhaust valve damage results in the above symptoms but also can be measured by measuring the compression in that specific Cylinder (also some would use the tail pipe or the valve cover oil port to confirm air is noticeably escaping at high velocity through the ports).

What fuel are you using?