"news.redshift.com" wrote in message
...
I have a Volvo 170AQ. It is an older model from the late 70's. Last week
on the lake it over heated and started making a knocking noise. I took it
home, replaced the impelller, but it idles real rough.

If the rough idle just started happening, then I would expect that the
overheating damaged something. A likely possibility is a blown head gasket
(not too hard to fix) or a warpped/cracked head (major bummer).

First step would be to buy/borrow a compression tester. To use this you
remove all the spark plugs and thread the tester into each spark plug hole,
one at at time. Crank the engine with the starter a few revolutions and the
tester will indicate the maximum cylinder pressure. You should see
pressures in the 110 to 140 PSI range, and what is critical is that all
8(6/4) cylinders be roughly the same. A range of 120 to 130 would be fine,
but ff you get one cylinder at 40PSI while all the others are at 120 then
you know you have a problem.

If compression checks out okay, then I would try a standard tune up, and
then I might try just using it. If the rough idle is an indication of a
more serious problem then actually running might make it obvious.

Rod