Could be vacuum leak, EGR valve that won't close properly, bad plug wire, or
a couple dozen other reasons. The first step is to read the stored trouble
codes and run on-demand diagnostics. A five minute affair, which on your '85
can be performed with such sophisticated tools as a test light and a paper
clip (I am assuming that yours is the domestic, throttle-body injected
engine, not the carbureted export model). This would at least give some
clues. Doubt that at this point anyone could tell you how serious this is
and whether it will leave you stranded. Let us know what your mechanic
Hope they figured out your shaking problem by now. As a former owner of one
of those and a current owner of an '85 LTD (same thing, except for the
grill), I am just curious about your car. Is this really a Marquis, or the
'Grand' (Crown Vic's twin)? The common V6, 3.8L liter engine or something
else? Is this the original engine? How many miles? Oil consumption?
Transmission still shifting properly? (or perhaps your is
manual...)Everything else still working (A/C, electric windows, etc)?
EGR valve stuck partly open by carbon deposits? The valve should be
on the back of the manifold, with a flying-saucer shaped vacuum diaphragm
on top of it. While the engine is still cold, see if you can get your fingers
under the edge and work the diaphragm/valve stem up and down a few times.
It could also just be a vacuum leak. Any strange hissing from under the hood?
Any vacuum hoses that have split or come unhooked?
I vote for the EGR too. I used to keep a few feet of vacuum line in the car
at all times for my '84. I would attach it to the egr, then suck on the
other end to open the valve a few times with the engine running. The hose
needs to be long enough so you can reach the throttle lever and keep the
revs up while you operate the valve.
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