Idle revving 1-2 k after engine swap
 

Ok team

I just put in a used engine into my 99. I have some weird idling issues. On initial startup the engine revved straight to 4k, then slowly backed down as it warmed up. Warm its fluctuating between 1 and 2k.

I cleaned the ICV, then replaced it with another one I have. Same issue. I unplugged the maf and idle stayed steady but I doubt its a maf issue. I've got one of the ICVs soaking with some maf cleaner and will try swapping it it again tomorrow.

On the plus side, it moved for the first time under its own power in 3 years so that's hip.

Aaron

When you unplugged the MAF and the idle stayed steady what RPM did it steady out at???

Sorry should have stated that. Around 700-800 rpm

I would say it is a MAF issue.
You unppluged it and the idle was right where it should be.
If you plug the MAF back in and the idle is moving between 1000-2000.
That tells me that the MAF is the issue.
I would replace it with a Bosch not an off brand.
These cars don't like off brand MAF's and other sensors for some reason.
******************************** has them for a very good price.

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.

Doubt it's a maf issue. Sounds like a vacuum leak. could be the hoses, connectors, plenum , check valve, aos, reservoir. if you can get a vac gauge you can hook it up to any of the vac. lines and see what you are getting. should be 12-18 in.

Since it's doing this after an engine swap it's also possible you dont have something hooked up right.

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.

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????

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.

All fair questions

With the MAF unplugged it ran rougher last night compared to the original post. Maybe I hadn't let the engine warm up the same time as before.

The engine is a 98 vs my car a 99. For thebthrottle I have the cover off the engine and can see the throttle fully seated by the position of the spring and cable. I have verified that its not moving while the engine is revving.

I do not have those tools available right now.

Aanic,
Look at the throttle valve switch you mentioned make sure it and any wiring to it are good.
If it is related to the IAC and it is not working it could be the issue.

Blue62,

I appreciate the help. I'll be working on it through the weekend and will update if I find anything.

So to close this one out, I checked all over for vacuum leaks, but couldn't find any. I decided to pull the entire throttle body assembly out, I have a spare full assembly from the engine I swapped in. Did a clean on it, reseated all the gaskets and suddenly no more vacuum leak.

I must have a hairline crack I can't see on a boot or on the TB itself. Either way, back to running. Thanks for the support guys!

So it looks like it was intake air = vacuum leak related as I thought.
Good to see you got it sorted;)