Idle revving 1-2 k after engine swap

[Aanic]

Ok, update. New MAF, genuine Bosch unit, numbers match, still having the issue. Everything fully connected, on cold startup the engine shoots up to 3000 rpm then slowly backs down as it warms up. This is on a cable throttle body, so that seems wild to me.

Once warm the engine hunts between 1000-2000 rpm. If I tap the throttle it jumps up and then still just sits high rpm, say 3000 again for a bit. Throttle has returned to normal position at that point but still screaming high rpm.

I've cleaned and swapped the IAC, brand new MAF, coils/plugs. No DTC codes present or pending. This one is getting me stumped. When its sitting at RPM its smooth, no misfires or pings.

If the throttle is closed and the idle is hunting I'd think it woudl be the IAC, maybe its the throttle-valve switch below the IAC? Would O2 sensors cause a wild idle?

[blue62]

Originally Posted by Aanic

Ok, update. New MAF, genuine Bosch unit, numbers match, still having the issue. Everything fully connected, on cold startup the engine shoots up to 3000 rpm then slowly backs down as it warms up. This is on a cable throttle body, so that seems wild to me.

Once warm the engine hunts between 1000-2000 rpm. If I tap the throttle it jumps up and then still just sits high rpm, say 3000 again for a bit. Throttle has returned to normal position at that point but still screaming high rpm.

I've cleaned and swapped the IAC, brand new MAF, coils/plugs. No DTC codes present or pending. This one is getting me stumped. When its sitting at RPM its smooth, no misfires or pings.

If the throttle is closed and the idle is hunting I'd think it woudl be the IAC, maybe its the throttle-valve switch below the IAC? Would O2 sensors cause a wild idle?

O2 sensors are just give feed back info about something that has already happened.
So they would not be the cause of the issue.

I am not familiar with the throttle-valve switch. What is it's function???? Does it kick the IAC in at idle??? If it is IAC related it could be an issue.

Try unplugging the MAF again and do a dead cold start let the engine warm up with MAF unplugged. If things act normal with the MAF unplugged then My mind says it has to be intake air related in some way. Wrong MAF, MAF wiring Issue, Something MAF related. Or perhaps this throttle-valve switch you mentioned.

The only other air intake thing I can think of would be a vacuum leak.
But I would think it would have to be rather big to cause these issues.

[Aanic]

Just tried a statup with the MAF unplugged. Real rough idle but stayed under 1k. After a minute or so got the flashing CEL and when checked showed misfires on cyl 4, 5, 6.

[blue62]

Interesting that when you unplugged the MAF before you reported a steady idle at 700-800RPM but now if you unplug the MAF it is a rough idle.
Also now you have miss fires that you didn't have before.

I am still leaning towards intake air issues.
vacuum leak, MAF, IAC, or the throttle valve switch you mentioned.
Vacuum leaks can cause misfires but they are usually random not just one bank.

The only other thing I can think of is since this is a replacement engine.
Is the replacement engine the same year as the original engine.
ECU/DME compatability issues????

[911monty]

Have you confirmed that the throttle body plate is fully closed when you are idling? Do you have a durametric or a scanner to observe the airflow at idle? If you are misfiring you need the codes.