Civic SI idle problem

For some time my daughter's 1985 Civic SI has had an intermittent idle problem, and now it has become much worse, to the point that it can't be ignored. At idle the engine speed cycles up and down between about 1000
rpm and 2000 rpm, about twice a second. It is almost as if the engine cuts out when the revs get up to 2000, then cuts in again when it drops below 1000 rpm. Now the condition seems to have become permanent (funny how problems like this never seem to get better) and it affects low speed driving, e.g. in heavy traffic. While driving if the revs are about 2000 with little throttle the engine does the same cycling, making driving very difficult (once was enough for me to convince myself not to do it again).
Perhaps someone can tell me what is likely to be the problem. Does something need to be cleaned, or is it a matter of replacing a sensor or valve? Otherwise this car still runs very well - very sporty - and I'd hate to junk it. Thanks in advance for all suggestions.
Gib
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Gib Bogle wrote:

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First thing to check for AIR in the cooling system. Top up the reservoir and the rad, then do it again next morning (cold engine). The engine uses hot coolant so set various functions and the idle is one of them. Use premix coolant, not tap water.
Report back.
'Curly'
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motsco_ wrote:

Thanks Curly. I will report back, but it will be a few days before I have time to try this.
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The next thing to look for is bad vacuum hoses or components. Vacuum leaks cause a lot of higher than normal idle problems.
-SP
Gib Bogle wrote:

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speedy wrote:

I'll check the vacuum situation too, as well as I can.
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