I recently noticed a driveline clunk on my 97 4R: There is no play on rear drive shaft.
My wife starts the truck and drive off within 10- 15 seconds. At this time the RPM is around 1000 and I can hear the clunk. Once the engine comes to an idle 600ish rpm, I find no clunk so I feel like to close the IAC valve completely and fool the ECU with another IAC unit attached to its wire connector.

Any thoughts?

Thanks

I’m not sure I fully understand your issue? The idle adaptations for the IACV are higher on a cold start due to the cold start enrichment and open loop. Once the engine warms up idle is decreased and closed loop takes over making proper adaptations and utilizing a lower idle rpm.

You description on the other hand makes it sound like you only get the clunk at the higher rpm? So why are you trying to increase idle rpm?

Lastly, why not look into the clunk itself? I have seen U-joints cause a clunk on acceleration that happened at no other time similar to what axle wrap feels like.

Thanks for the reply:
I want the rpm to stay at around 650 at cold start. I think this can be achieved if I can permanently close the IAC valve similar to the the closed-loop.

No excessive play at U joints or at splines. I never hear a clunk when the rpm is at 650 (fully warm up).

Bumping the idle up on a cold start (so the motor doesn't die) isn't all that the idle valve does. It makes up for the A/C compressor kicking on and off, and just generally holds a steady idle regardless of load.

The throttle body isn't really designed to work without the IAC managing the idle airflow. you'll probably need to readjust the throttle valve so it's slightly open. And even then, you're not going to have a nice constant 650 rpm idle at all times, it's going to be hunting up and down some, you might need to keep your foot on it when it's cold (they don't bump the idle up to 1000 just for the hell of it), it might occasionally die when coming to a stop, etc.

It really sounds like you're wagging the dog with the tail here. It shouldn't have a nasty clunk when you put it in drive, if it does, maybe try to figure out that issue. It's likely just going to get worse until it clunks at 650 rpm as well.

the IAC is functioning at all times, even when fully warm. as said above, to remove it completely you will have to manually open the butterfly. do those even have an adjustment screw?






have a conversation with the wife, and convince her to let the vehicle idle for another 30 seconds or so before taking off. or, better yet locate the clunk....

I apologize in advance for this smart ass comment, but if you're just looking to not hear the clunk anymore instead of fixing the clunk, would it not be easier to just tell your wife to turn up the radio?

Play in the drive line doesn't have to be 'excessive' to make a sound. I would check the slip yoke. The 'clunk' on acceleration has been discussed a few times here as I have it too. I've read that re-greasing the slip yoke with some ford grease was the fix.

(no hard feelings either, the volume knob fixes a lot of things on my '78 F-150 lol)

Sorry for the delay, I have seen others chimed in with similar advice to diagnose the clunk instead of Medicate the symptom.

The IAC is pretty actively changing resistance and when the air is controlled by the valve and not a hose to the valve you can’t really restrict it without controlling the B+ terminal voltage and you don’t want a service engine light triggered which would likely require a DLP switch.

I haven’t read up on this specific IAC to be sure but most are similar.

Slip yoke comment was an excellent recommendation as well.

I'd inspect your rear brake drums. You might have an axle seal leaking gear oil on your brake shoes. After sitting a while the brake shoes would sort of fuse to the brake drums because of the gear oil contamination. This happened to me and I would get a similar clunk on start up. Worth taking a look at.

Is it good to shift into gear (R or D) from P or N, while engine is running above idle speed, such as in cold start (around 1000 rpm)? I feel like that's too much force going down the drive train and continuous use could damage the components in the drive train including the flex plate.

On a manual transmission, we can control the engine force down the drive train by the clutch and it is such a relief to drive a manual.

Regardless of high or idle rpm, when AC comes on, I see a slight increase in RPM to compensate the strain on the engine by the AC compressor.

The clunk happens at cold start regardless of the duration it is parked (a day or 3-4 hours)

As was stated before... The two things I would check would be slip-yoke and especially rear brakes. Had the same thing happen to me because of a leaky rear axle seal.

if you completely close the IAC the engine will die. without removing the IAC remove the the two screws holding the coil and remove it, you will see what looks like a black knob this is the armature and will act exactly like a idle control. rotate this to the exact idle you want dab a touch of epoxy on the armature to hold it in place, also leave the coil plugged in and it won't throw a code. I know this will make folks here lose their minds but I can tell you it worked perfectly on my '97 and didn't change any functions.

I will check the rear brakes. Thanks.
I'll also do the method Baja 1 proposed.
Thanks for the replies guys!!