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I just completed an engine swap on a 5.4L 2000 Ford Expedition (2W drive). New engine works great except it won’t idle properly. It will startup and rev up to a bit over 1000 rpms and then gradually come back down to about 700 rpm. Then the engine starts to gasp a bit and then it stalls. I am going to check the usual suspects (vacuum leak, IAC) but wanted to find out if anyone else has experience with the Ford MOD motors.
A secondary question – the IAC hose has a rectangular box in it at the midpoint between the intake and the throttle body. Anybody know what this box does ? My mustang has the same thing on the 4.6L version of the motor.
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I think you are on the right track with checking the IAC and vacuum leaks. Did you reuse the old IAC or did the swap motor come with it? Also I’m not too sure what you mean by “IAC hose”. The IAC shouldn’t have a hose. It’s an electronically controlled duty cycle motor.
I don’t have alot of particular experience with that engine, but I think you are on the right track with vacuum/IACV issues.
By IAC hose, I’m guessing you mean the snorkel between the airbox and the TB. It’s probably a muffler to help mitigate the noise caused by the air flowing. Customers are picky about noises.
Yes – ford uses a hose that is attached to the IAC valve on one side and the intake snorkel on the other side to tap off air flow after it has been measured by the MAF. This hose has a small black box installed in it. The best guess I have is that it’s used to reduce noisy airflow in the idle circuit.
I did not replace the IAC after the engine swap because the one that I have is not that old. I took it off yesterday and it is spotless inside. I was careful with all the vacuum lines when I put the motor back together so I think vac leaks are not the cause of my problem.
The symptoms scream “IAC”. I wonder if the aftermarket IAC valves have a pretty high failure rate? I am headed to NAPA to pony up another 50 bucks on this project! I’ll report back if this fixes my problem.
I solved my rough idle problem on the Expy yesterday. The biggest clue was that it was throwing a P0174 code (System to lean – bank 2) which got me looking for a vacuum leak. I discovered that the Dorman replacement intake manifold that I put back on requires that you install an o-ring on the PCV tap off point. This is not in the instructions for the manifold! The PCV tap installs into the manifold and then you rotate it 90 degrees to lock it down. I got looking at the bag of parts that came with the manifold and noticed an o-ring was left over after I installed everything. So I had a giant vacuum leak without it. So the IAC valve was not the problem in this case.
Well done finding the problem. I agree with the other posts that the black box is probably there to prevent noise. Thanks for the update and for using the ETCG forum.
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