My 91 RS (305) does the weirdest thing ... When sitting at a light, the idle will randomly switch between two different RPM's (from the nominal 1500 to about 2500). While in the higher rev state, she runs very poorly. However, when she drops back down to normal RPM's she runs great ...

I've been to a couple shops (and of course she runs GREAT in front of a mechanic) trying to track this down ... but as of yet have gotten nowhere.

Any ideas ?

 

Have you ever given your engine a tune-up? My RS used to do the same thing when I first got it. Replacing the plugs, wires, distributor cap and rotor cleared it up. I'm certain mine was either a bad wire or a carbon tracked cap.

 

Yeah, she has new plugs, wires, cap, and rotor ...

 

nominal 1500?!? it shouldnt idle higher than 1000 unless its cold. anything that high is either a stuck idle air control valve or a large vacuum leak

 

The Idle Air Control value has already replaced ... No vac links, checked that too.

Nevertheless, neither of those would cause the idle to oscillate to that extreme ... the RPM's almost double instantly, and in after a few minutes it goes back to normal.

 

Check all your vaccum lines for cracks. sounds like when one of the vaccum switches opens you get the problem, also check brake booster valve

 

i still think its crazy that it would do that. are you sure the engine is going that high and not just the gauge?

 

Crazy, but rather discouraging. I'm not going to swear to the accuracy of my stock tach ... but the running idle (whatever it may actually be) does in fact almost double. You can hear and feel the change.

Now what about Air/Fuel mixture measurements? Could a faulty sensor have that much effect on the idle?

 

... the brake booster value? I wouldn't have thought to look there ... ?