Engine On A Roll

In the days of carburetors, if the engine was running rich, it would roll instead of run smooth. Thats what my 2000 Buick Century 3.1l
is doing lately. I have no trouble codes displayed. The intake portion of the engine has so many sensors that i wonder if i have a bad sensor. It rolls often at idle and when cruising at very slow speed. I did have a code 131 low voltage o2 sensor but i replaced the sensor & the code has not returned. I also changed the air filter, plugs & plug wires yet the engine still rolls. I would describe this rolling as a definitely noticeable “rrrruumrrrruumrrruum” sound and feel rather than a steady “rrrrrrrrrrrrrrrrrrrrr” sound. I changed the plugs because 3 of them showed a high resistance conductivity between the electrode & ground, probably fractured porcelain insulators & carbon tracking. Three of the plug wire insulator boots broke in half when i removed them because they were seized onto the porcelain insulators, this is why i replaced them although also, at least one of the wires had an ohm reading that was too high.

I would first pour a bottle of Chevron/Techron in the tank.
Second, look at the fuel trim data.
Third, a compression test.

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Good advice above. It sounds like you’ve done some common-sense replacements already. Never had this on a fuel injected engine but I had similar problem on my carb’d truck, caused by a problematic fuel bowl vent. Subtle, mostly noticed as uneven idle. But glad I got it fixed, annoying.

I’m thinking

  • vacuum leak
  • problematic vapor-emissions system (evap).

If it isn’t throwing a check engine light, it isn’t running rich. The O2 sensor keeps it on trim and if the O2 sensor fails, you will get at least one code and maybe more.

But I agree, put a scanner on it and see what fuel trims you are getting.

Thanks for all replies. And I had thought that a spent vapor cannister being ineffective would only be an emissions control failure but not affect driveability and fuel economy. How does the vapor cannister system affect engine performance? And don’t they only get purged into the intake manifold a brief time during engine warmup? I will buy a new cannister if it is faulty even though they are quite expensive. A while back I did get a trouble code for it but the code went away & never returned. I checked prices on Amazon & didn’t buy another one because at the time I didn’t think a spent cannister would affect engine performance.

If the purge valve is leaking or stuck open.
Then too much air gets drawn through the canister and into the engine intake.
As a quick check you can pinch or plug off the vacuum hose to the purge valve and see what happens to the short term fuel trim number.

I have never heard the term ’ engine roll ’ and have no idea what that means.

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I interpreted roll as lope. Like an engine with a performance cam installed.
Might be a regional term.

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Also consider the possibility of an exhaust system leak. Pay special attention in the area around the pre-cat O2 sensor(s). Do you notice any black smoke coming out the exhaust pipe, other than briefly after initial start-up

If you don’t have a check engine light, I would not change the canister. Do check for a pending code if you don’t have a check engine light. If you have no codes, I wouldn’t worry about it. You might just need a new muffler.