This truck is in Belgium and this is a 4.7 HO? (from your owner id info listed here).
Jkelly is correct in mentioning in cleaning the throttle body and the IAC
which looks like a solenoid but actually is a stepper motor adjusted by the
PCM to provide the correct idle when the engine load changes while in
PARK and the TPS sensor is closed.
What is the idle rpm normally? at 600? or lower than that and fluctuating
up to ?
You mentioned that you already changed "all" the sensors and the sparkplugs.
This engine design is COP (coil on plug) with individual coils for each plug.
I presume that other than the surges at idle, it runs fine otherwise when in D
Since you might be cleaning the throttle body as has been suggested..
You can test the TPS sensor, plugged in on the vehicle with a digital
voltmeter backprobing the center wire (through the insulation) with the
positive probe of the voltmeter and the negative probe on a suitable
battery or engine ground.
(With the throttle body air duct off, you can see the two throttle plates
connected to the throttle shaft which has the tps sensor on one end of it.)
Measuring the output of the TPS.
closed throttle plates should be 0.5 volts
fully open throttle plates should be somewhere between 3.75 and 4.5 volts.
It is a 5 volt sensor. The 5 volts regulated is supplied by the PCM on one
of the 3 wires plugged into it, the other is sensor ground.
In between there should be a smooth transistion of voltage as the throttle
plates are moved by hand ( NOTE: engine NOT running but ignition key is on)
from a fully closed ....to fully open position, with the voltmeter attached to centre wire of the plug reading the actual voltage while you do it. If it jumps around
and is not smooth from 0.5 to 4.5v approx, the sensor is faulty and that
will definitely affect idle as well as other operational rpm requirements.
The IAC located at the back of the throttle body is held on with 2 screws
and can be removed for cleaning. It can get carboned up at the tip
and that tip controls the amount of IDLE AIR at the air passage that
bypasses the closed throttle plates at idle.
If it gets restricted from engine carbon, the air going through can become restricted and the PCM (computer) will try and constantly compensate
by increasing or decreasing the idle rpm.
Normally it only increases the idle on heavier engine loads such as
A/C on or power steering pump working hard with the wheels fully cranked
in one direction.
All good now guys thanks for the help. I think the new IAC got some bad carbon from a dirty throttle body. Cleaned both out and idle is back to normal. Just another case of overthinking a simple problem, I appreciate the help and the common sense.