High Idle Modified 4.0

[SleeperBronco]

Okay, I just got everything together. Thanks to the many postings on this site for 4.0 swaps. Now I am idling at 1100 rpm-Is this a function of the build or something afoul. '92 4.0 EQ 90 tm heads, Comp cam 410, smith bro pushrods, shorty headers etc. No codes and idles very smooth. Has been running now for 5 days. Every hole is plugged(vacuum line that is). This is in a 1990 BII 5 spd M5R1. I don't mind checking the TPS and IAC but am wondering if the computer has some problems with the cam and headers.

 

[FR-425]

Are you using the bii ECU or the 4.0 ECU?

The 4.0 needs 2 O2 sensors or you will be lean on one side.
Do your headers have locations for both O2's?
The lean situation will hold a high idle.
Don't drive it like this or you'll bust up your new engine!

Are you running your EGR equipment?
If not that will further complicate running lean.
And the computer will never be able to learn the new timing curve.

Once you get the computer control sorted it would be best to have your ECU re flashed for the cam. It'll run with stock programming but won't really be badassenough

 

[SleeperBronco]

The 1992 4.0 came from a single 02 sensor Ranger(for this year 1 O2 sensor is correct) and in 92 there was no EGR also. I am running the 4.0 ECU from the donor vehicle. The O2 sensor is located after both y-pipes collect.

 

[arco777]

Check for vacuum leaks. Also check that the throttle body stop screw has not been messed with. If it has, you'll have to reset it and recalibrate the TPS. Might want to check the TPS anyways; if you drill out the inserts it can be calibrated easily with a multimeter. Also check if the IAC is properly functioning; if carbon gunk is preventing it from closing all the way it might be causing the high idle.

 

[FR-425]

OK good got that sorted then.

Hey arco! fancy see'n you here. I agree with arco just some tuning required here sounds like.

Maybe just as simple as swapping to the 4.0 throttle cable. the old cable may be com'n up short.

 

[SleeperBronco]

Thanks guys. I checked the vacuum lines-okay. I'll do the TPS, IAC, and idle screw check.

 

[SleeperBronco]

Cleaned the IAC and not sticking but not functioning.