[Hpr] John Kulp's truck

Patrick Dailey pdailey03 at gmail.com
Fri Jul 29 05:45:22 PDT 2016

If your rough idle symptoms persist, it may be a stuck EGR valve, the Idle
Air Control (IAC) valve, or a coolant temp sensor (CTS). Cleaning, or
replacing the EGR may do the trick. There may be a way to test the IAC and
the CTS with a scan tool. Often the parts stores have those to lend out.
I'm not sure if the Bluetooth OBD2 testers can perform diagnostic tests,
but its worth a shot if you have one. If its none of those things, then it
may be a vacuum leak, or worn out plug wires. A vacuum leak can be found by
listening for a hiss, and then spraying the area with your throttle body
cleaner. If the idle smothes out while you're spraying, then you've found a
leak. Just don't spray too much at once. Its highly flammable, and engines
are hot. Stay away from the exhaust with it. Vacuum leaks can come from
cracked, or disconnected vacuum hoses. Don't mess with the Mass air flow
sensor, except as a last resort. They are very sensitive, and often fragile

Because it happens after driving for a while, I'd start with the EGR valve,
and the Coolant temp sensor. Some cars have EGRs that just get dirty, and
stick open, and cleaning them fixes the problem. Other cars have EGRs with
weak solenoids, and need to be replaced. A little research will probably
tell you which is more likely for you.

The throttle body cleaning was a good start, and you probably did a good
enough job to eliminate it. The butterfly, and the side passages are the
most important. If those got cleaned, then you're done there. Dirty plenum
walls are relatively unimportant. You may wish to pull the IAT, and clean
it, and its passages.

You could also look to see if there is an adjustment procedure for your
Throttle Position Sensor.

Bad plug wires can be diagnosed by misting them with water from a spritzer
bottle in a darkened garage, or in the evening. If you see little blue
sparks, then its time to change wires. Be sure to do them one at a time so
that you don't mix them up. If you change wires, do plugs, cap and rotor
(if you have them) too.

The last thing that I can think of is dirty fuel injectors that are not
atomizing fuel properly. Sadly I don't know of a cheap DIY remedy for that
(or diagnosis for that matter). Black smoke (even very faint) from the
tailpipe as the car dies would hint at injectors, the IAC, or a faulty
coolant temp sensor.

Best of luck. Let us know how it turns out.
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