I recently replaced a blown engine in a coupe I bought. I put the stock 1.5l back into it but there's a weird surging idle, but only sometimes.

The only time it happens is when I'm in gear and press the clutch in. Like if I'm coming up to a stop and I press the clutch in to go to neutral it starts jumping between 1k and 1.5k but it will only do it for a few seconds. I've searched and couldn't find anything on this particular problem. Any help would be great.

 

The surging usually occurs when the air/fuel ratio is off and the computer is having trouble getting it right. Try cleaning the TB and idle valves. Check the MAP sensor signal voltage at idle after the engine has reached normal operating temperature. When checking the MAP sensor, try squeezing the PCV line closed with some kind of pliers or vice grips, being careful not to damage the line. See if the MAP voltage changes.

 

I'll try that out when I get home tonight. Thanks a bunch, hopefully I can get it fixed. It's more of an annoyance than anything else.