oil-change cause rough idle?

per our '97 volvo 960 wagon, we had the oil changed at a drop-in
mechanic/tire shop chain a few weeks ago and, about three days later, the
engine started idling rough. a mechanic friend of the original owner (he's
out of state) familiar with volvos said that non-volvo guys who change the
oil can and do disconnect or otherwise goof-up connections such as vacuum
hoses or tubing that could cause a rough idle. a non-volvo local mechanic
says he can't imagine how an oil change procedure could produce a rough
idle. before we start shoveling money at the problem, would appreciate some
learned chat on the appearance of a sudden rough idle on an engine that was
running flawlessly prior to the oil change. many thanks. art

 

Unscrew (torx bolts) the plastic cover for the spark plugs, and check
for oil spilled when the engine was re-filled. Spilled oil can seep
under the plastic cover and collect around the spark plug leads/coil
packs/plugs. Also check for loose/unseated cables which may have
been moved when cleaning up such a spill. (Don't mix up any cables if
removing them to clean/re-seat). I've had uneven running occur just
as you describe, a couple of times in the last 2 years on my 960,
shortly after an oil change or inadvertent spill during topup. After
careful cleaning with a paper tissue and re-seating any loose
electrical connectors, the car returns to normal, very smooth, running
(now at 125k miles).

Your source on vacuum tubes is also correct though, they're just harder
to check - there's a junction box of vacuum tube pipes somewhere
(lower-right engine compartment? it's too dark to check here just now),
one tube could be broken/perished after movement during general wiping
up of oil spills etc. I'm told faulty vacuum pipes are harder to
detect and replace.

Good luck. 960s are still great cars when they work, just a few more
things to go wrong than on the 4/5 cylinder models. But much better
than the '98 V70 I used to run...

 

They may have knocked loose a vacuum line, it's not a Volvo specific
problem but if there's lines in the vicinity of the oil filter that's
where I'd start, just a good look under the hood with a flashlight
should determine the state of those.

 

ascott: many thanks your good suggestion. a non-volvo mechanic (nephew of
a close friend) said he'd give it a quick look-see tomorrow and i'll give
him your note. if he can't find the problem, we'll have to move it along to
a guy named olaf with an obdii scope. fyi, i took it out on the freeway
today and then around the town and you'd never know there's a problem except
at idle. smooth as an icy blast of absolut. am

 

The problem only at idle really increases the "vacuum hose" suspicion.

Mike

 

the mechanic is reporting an unhappy coil. not sure how that could
affect only one cylinder instead of all six. we'll see. thanks all for your
observations. art

 

I don't know when they started doing it but my S80 has an individual coil
for each cylinder (connects directly to the sparkplug, no high tension wire)

Allen

 

folks: again, many thanks for your input on this query. it turned out to
be a bad coil. all best, art

p.s. i am unable to determine the root of the coil problem, whether nature
or nurture.