1990 Legacy, alternately revs and almost dies on cold starts

[zyewdall]

So.... it just started doing this a few weeks ago -- when you do a cold start (anytime engine has sat overnight, doesn't have to be a cold morning, though it's a little worse on days below freezing) it will start immediately, like always, and rev up to maybe 1500-1800rpm. Then it will cut out and almost die (400rpm) then, rev back up, then almost die, then rev up. It does this alternate revving up and almost dieing every 2 or 3 seconds for a few minutes till it warms up enough. It runs great other than that (30mpg highway, 25mpgish around town), and has never failed to start... just can't warm up very well. Is this related to the idle air control maybe (what exactly is that anyway??) or cold start enrichment (which is what I'd suspect if it wouldn't warm up at all, but the on-off nature of it doesn't seem to be a failing temp sensor).

 

One other piece of info that may or may not be relevant is that the y-pipe has a fairly good exhaust leak, so the oxygen sensor in the cat may not be seeing everything... but I'd think that would affect it more in closed loop after it warmed up, rather than open loop when it first started ??

 

EDIT: more info: No check engine light (except once when I forded a deep stream, and it went away when the engine dried out). manual transmission. About 140k miles on the engine (190k on the body). New air filter, but I haven't looked at the plugs or wires or anything -- runs great once it's warmed up so I haven't suspected them.

 

Thanks

Edited October 16, 2009 by zyewdall

[Jerry DeMoss]

I would start by replacing the idle air control and then if that doesn't help move on to the temp sensor on the water cross over pipe located below the intake manifold.

[lmdew]

You can try cleaning the IAC as well. Seafoam works well

[subaru360]

You can spray carb cleaner into the big hose that leads to the iac valve to clean it out. Do it while its running. You may have to work the throttle to keep it running.