I would like to hear your opinion on diagnosing an ICE having a RANDOM rough idle (800 to 900 rpm). From my knowledge, can't be worse than that, because it is a time consuming process to get to the bottom of it.

I will start from the allegation that in the absence of fuel and ignition, an ICE is just a big air pump.

To keep things simple, let's say the engine is a basic 4 stroke ICE, with carburettor and no ECU. If you connect a vacuum gauge at brake booster port, you get a good vacuum absolute value, except it varies IN SYNC with engine idle speed. Speed goes up -> more vacuum and speed goes down -> less vacuum.

So my question is: WHAT conclusion can we gather from this reading and WHY?

Is it A) a mechanical problem, an ignition problem, or C) a fuel problem ?

Edited 12 November, 201114 yr by masster

I'd expect the vacuum to increase as you say - the engine is trying to pump more air, and meeting a constant resistance (the near-closed throttle valve), so that makes sense.

Varying ignition timing can alter idle speed. It's a common trick in mapping for competition cars (which often have idle stability problems) to map more advance into an RPM range below the desired idle so if the engine slows down and starts to stall it will 'kick' it back up to a higher RPM.

I'd think either the ignition timing is altering, or the fuelling is - on a car with a carb, it could be either, although if it has a distributor my money is on ignition.

what about a random vacuum leak?

vacuum leaks are usually constant tho as if it was sucking past a gasket then the gasket will not seal itself up will it.

I would second DJ here and also say most apparent fuel problems turn out to be electrical in the end.

how about poor valve sealing , that would effect the inlet vacuum too and possibly more random due to engine warming up

how about poor valve sealing , that would effect the inlet vacuum too and possibly more random due to engine warming up

from my knowledge, a poor valve sealing is not random and makes the vacuum gauge needle oscillate rapidly.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

http://www.secondchancegarage.com/public/186.cfm

Is this a variable venturi carburettor (eg SU or Hitachi ) or a constant venturi type (eg a Weber)?

If it's a VV type, I'd suspect an empty dashpot or leaky diaphragm, depending on make.

With a CV type,, if it's a side-draft then the mounts could be too soft.

Beyond that, I've no idea.