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92 - 626 - 2.2L cold idle high and fluctuating

5003 Views 2 Replies 3 Participants Last post by  megalomaniac32x
Cold idle initially @ 2000rpm
Increases up to 2500rpm after ~ 15-30secs
Then starts to fluctuate - up and down 1800~2500rpm every 2 seconds
Fluctuation quickens to under 1 second
Revs drop to around 2000rpm after 1 minute approx.
Revs continue to drop slowly until completely warm
Perfect idle when warm.

(Engine has only 88500 miles)
Checked so far:-
Timing 6deg BTDC - OK
Tried another ECU - no difference (also, my ECU was good on another 626!)
Throttle closed switch OK
Throttle position sensor - (I think) OK
All vacuum hoses OK
Rubber Manifold to Airflow Sensor hose OK
Idle @ 750 rpm with 'Check-Connector' green wire grounded. (engine hot)
EGR valve closed when cold - OK
PCV valves & hoses appear to be OK

The BAC (bypass air control) system seems to be doing it's job. What I don't understand is how to control the amount of bypass air injected when cold by the wax filled valve! It's almost as if this water temperature controlled air valve is opening too far. I cannot find anything about adjustments to the throttle body apart from the closed switch and position sensor.
By way of coincidence, my buddy has a B2600i (1990) which has developed EXACTLY THE SAME PROBLEM. This truck has many similar Mitsubishi parts. We can't find the problem on his truck also.

Is this a common problem perhaps???
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I had the exact same problem on my 1990 626 2.2L (non-turbo) and was going bonkers trying to figure it out. With over 240000 miles, I feared that it might be a lost cause.
I checked everything like you did, but what it turned out to be was the engine coolant was almost bone dry. Filled up the radiator and it stopped fluctating almost immediately.
That was an easy fix, or so I thought until discovered the root cause of the low coolant.
Yep, you guessed it! The water pump was shot and leaking like a faucet. Fortunately it wasn't too expensive or difficult to replace and I'm just a shadetree mechanic. It was just time consuming job.

Its still on the road and running good.
u also might tap on the iac when the idle is erratic and see if it smooths out. it could be a carboned uped iac valve. check the coolant sensors and amke sure they r plugged in and the connectors have no corrosion on them.
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