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I have just swapped two EJ22 NA engines , one in a 8/91 Legacy and one in a 11/92 legacy when i start the 11/92 engine from cold it idles at 2000 rpm to start then after 1 minute the idle drops to 800rpm for a second or two then up to 2000rpm and back down to 800 then up to 2k not sure what this surging is.

I did clean the intake / throttle body with carby cleaner quite vigorously when it was out of the car and also swapped the fuel injectors from the other motor but they worked fine before with the other donk so i don't think they are at fault.

if i disconnect the idle control sensor the motor stops surging an the revs jump to 2000 rpm and stay there with no surging.

Both Cars were running properly just before i did the swap.

Any idea of things i could try to narrow down the problem?

cheers in advance

You traded engines between the cars?

What kind of transmission does the 91 have?

both are manual 5mt

yes I traded engines one has done 100k the other 300 k!

The two IAC valves are of completely different designs.

Swapping the injectors was the right idea, but you really should swap the entire intake manifolds.

Or, swap the injectors back, swap MAF sensors, and swap ECUs.

Im a little confused IAC valve ? i can only think of the BAC valve
All the sensors look and connect identically
If either the BAC valve, PCV Valve or Throttle position switch is faulty what faults will this cause?
The reason i say this is i have flooded the intake with throttle body/fuel injector clear when it was removed from the engine, would this have had any effect on the valves sensors attached to it, as they were still attached?

BAC meaning Bypass Air Control valve? That's what I'm calling the Idle Air Control valve.

The IAC valve is of course responsible for maintaining idle. Each ECU knows how to drive its corresponding IAC valve. When you mix and match the two, you can end up with exactly the situation you have right now. Douglas Vincent recently had this exact problem.

A bad TPS or PCV valve could cause driveability problems also, but carb cleaner won't hurt either of them.

Had another look at bad idle situation, replaced BAC valve, still surged, replaced idle control sensor, still the same, replaced ecu still the same hmmmmmmmmmmm
then i pulled the hose off the egr valve blocked up the both hose end and manifold end then reved car 3 k rpm then put hose back on and hey presto no more probs, possibly the egr valve was stuck i think.
Just to be sure I swapped the ecus back and the car was still idling ok.
Incidently both ecu's and valves all had the exactly the same part number on them.
Cheers and thanks for the tips.

What is the BAC valve? What is the idle control sensor?

There should be no EGR valve on either of these motors.

I'm so confused.

sorry used the wrong names
by idle control sensor i mean TPS
and by egr i mean pcv ( i was thinking pre fuel injection days)
According to the sub serv manual its a BAC valve.

Hehehe. Ok.

BAC meaning bypass air control?

Bypass Air Control (BAC)

How do I adjust the BAC and TPS ?, i could'nt find any reference to their adjustment in the manual.

The reason I ask is they are (after being changed) in different positions so they need to be reset again.

The BAC valve is also often called the IAC valve. Around here, it's almost always called the IAC valve.

Armed with that knowledge, you should be able to find what you need to know about adjusting the BAC valve and TPS just by searching.