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Fouled Spark Plugs from Bad Catalytic Converter

Can fouled spark plugs be caused by a bad catalytic converter? After only 14,000 miles a set of plugs were bad, one particularly. Can a bad catalytic converter cause the engine to use more oil? My car is a 2001 Jetta w/2.0 liter engine. My mileage is only 55K.

What does “bad” mean with regard to a catalytic converter?

I don’t know how, bad plugs can hurt the converter, not the reverse. Something else is probably hurting both.

Poor combustion–which can result from a variety of causes–can damage a catalytic converter. The reverse is not likely.

With a speed density logic system a restricted converter will cause an overly rich mixture. The Jetta is a mass air system and wouldn’t have the same effect. If partially restricted, the driver might get deaper into the accelerator to overcome the power loss and raise the combustion chamber temperature causing all manner of damage. If it is even partially restricted it needs replacement, regardless.

I hadn’t thought of it that way. What about here, with one especially bad plug?

Some more info would help. Buy the car brand new, as in 4 or 5 miles on it?

You state the plugs are fouled, one in particular. Fouled with what? Sooty black, oily wet black, coked oil black,?

How much oil is the car using? Had any overheating issues?

If the cat were plugged, you would have a warning light on. Most Haynes manuals as well as all others have pictures of bad looking spark plugs. Compare yours and describe what they look like. Oil burning is a common engine problem with many possible causes.

Thanks for your reply. I didn’t do the work. Took it to my VW service place. My question came from the article at The car has been through the standard 40K & 50K service without any issues other than replacement of the plugs, hoses, etc at 40K. I was just surprised that the plugs (as the service statement quoted “spark plugs badly fouled”) were in that shape. I was just wondering if the exhaust flow via the converter, if it were restricted, would create back pressure on the engine.

Thanks for your response. In combo with the info I gave to Rod Knox in his reply above, we got the car used from friends 5 years ago. It had been well taken care of before then. I didn’t get a chance to personally inspect the plugs. And yes, since your question, the car did overheat this past November. A thermostat valve on the bottom side of the engine would not open, hence the overheating. Got that fixed. The issue with the fouled plugs occurred early this month (March) when the check engine light came on. I guess it could be logical to tie the two events together. Even before either of these events, the car had been losing about a pint per month or so for the last couple years. It doesn’t pool under the car nor is it splattered all over under the hood nor blue smoke coming from the exhaust. Where I live in Seattle, I have to drive up one of the steepest hills in town. I usually have to go up only in second gear (manual tranny)at high rpms (no red lining though)to make it up the hill. Sometimes I can get into third but if people are in the car, I just can’t make it at more than 12-15 mph. I’ve always thought this hill climbing caused the oil loss. But I hadn’t observed “badly fouled plugs” till this recent “check engine” incident. Plus, I should say that the car doesn’t get many freeway miles. Used mostly for start/stop city driving.

Thanks for your reply. Please see my replies to Ron Knox and ok4450 above.

Thanks for your relpy. See my comments to Rod Knox & ok4450.

I would always take those well maintained comments with a grain or two of salt. Only a minority of cars out there are well maintained, although the owners of those cars may think they’re taking good care of them.

As to the overheating issue that could be the reason behind any oil consumtion. If a car starts to overheat and you shut it off and walk home or have it towed to a shop, fine. If you continue to operate the engine while it’s overheating then damage can be done to the piston rings and valve seals. Excessive heat can cook valve seals and either seize the rings in the piston lands (grooves if you will) or remove the temper (springiness so to speak) from the rings.
The rings have to constantly expand and contract to conform to the cylinder walls and if they are unable to do so due to the 2 things mentioned then oil useage will occur.

Excessive oil burning can clog a converter and this can easily cause a loss of power along with possibly more engine overheating.
Checking for a clogged converter is very easy to do if a vacuum gauge is used. It only takes a minute.
(What happens is that oil residue accumulates on the honeycomb structure in the converter and starts to clog those small pores. This can occur to some extent even on engines that are apparently running fine)

I wonder if they may have used the wrong plugs, wires on the wrong plugs etc.

my VW service place Is that a dealer, a chain outfit, local outfit ???

What you might want to do is to pull the spark plugs and inspect them every 1k miles, for the next 5k miles, and see if that same one is getting fouled before the other 3.

If yes, then that cylinder is having troubles, and will need to be repaired, pretty soon.