Rough idle and stalls

Forget the automatic idle speed motor. That simply is used to adjust idle speed. It has nothing to do with how the engine runs. The best suspect at this point is debris stuck in the EGR port holding the valve open a little. The clue is the engine will run fine at higher speeds. The next suspect is spark plugs and wires.

The next step is to read and record the diagnostic fault codes. Do you know how to do that yourself? Also look for a vacuum leak. In particular, check the hose going to the MAP sensor.

Yes you do. The flashing Check Engine light. The Engine Computer tested the sensors for you.

37 - Part throttle unlock (PTU) circuit, open or short detected.

The only problem is in the way the fault codes are set. It is easier to describe the throttle position sensor. It is fed 5.0 volts and ground. (Actually there's 0.2 volts on the ground wire, but that is not critical to this sad story). The movable wiper picks a voltage between those values, lower at idle and higher at wide-open-throttle. The important detail is there are mechanical stops inside the sensor that prevent it from going below 0.5 volts or above 4.5 volts. Anything outside that range is what triggers a fault code. The TPS is a mechanical sensor, and a signal voltage of 0 or 5.0 volts will only be seen if there is a break in one of the wires or one of the connections in that circuit.

The MAP sensor sets a fault code if the same readings are seen outside the acceptable range, but there are some important differences. This is an electronic sensor with a lot of circuitry inside it. That makes it susceptible to more than just wiring problems. A circuitry problem can cause it to develop the wrong signal voltage.

Understand that the 0.5 to 4.5 volt range in my story is for explaining theory. In practice, if you were to measure the voltages with a digital voltmeter, or look at them on a scanner, you might find 0.38 and 4.58 volts, for example. The actual numbers are not that important as long as they do not get close to 0.0 or 5.0 volts.

With a vacuum leak or an internal circuitry problem, the MAP sensor can send the wrong signal voltage, but as long as it remains within the acceptable range, no fault code will be set. At that point the mechanic has to view the signal voltage and try to determine if it looks right based on the readings from the other sensors. Chrysler is the only manufacturer that has been able to make their engines run right with just the MAP sensor. Other manufacturers need a mass air flow sensor for their main fuel metering calculations. The MAP sensor is the most important sensor on your truck.

One test that might provide some insight is to unplug the MAP sensor. The Engine Computer will see 5.0 volts, set a fault code, and since it knows it can't use that signal, it will inject an approximate voltage to run on, based on the other sensors' readings and operating conditions. The engine will not run well, but if the symptoms improve, the sensor and its circuit become good suspects.

Also, consider removing the EGR valve and reinstalling it with a thin metal shim to block the ports. If the rough running clears up, suspect the valve is not closing fully. Remember the EGR is supposed to open at highway speed, so it will not cause rough running then.

I've never paid attention to that fault code because it typically doesn't set again once other problems are solved. As with almost all innovations that benefit car owners directly, the lock-up torque converter was developed by Chrysler. It was first used in the mid '70s, and copied by other manufacturers a few years later. The system lowers engine speed to increase fuel mileage. It was hydraulically-operated and hydraulically-controlled. By the mid to late '80s the system used electronic controls to operate the hydraulic circuit.

The engine spins a fan. That fan spins transmission fluid. That spinning fluid spins a fan that's hooked to the transmission. All of that is placed inside a metal housing that looks like a giant donut. The slippage between those two fans is what allows the engine to run at idle speed while the transmission is in gear and the car is standing still. The engine can idle at 800 rpm while the transmission is going 0 rpm.

At higher speeds the torque converter becomes more efficient, but the engine will still be running at about 200 rpm more than transmission speed. Chrysler put a lock-up clutch inside to lock the engine and transmission together, similar to what the clutch for a manual transmission does.

One of the characteristics of a torque converter is they double the torque at the wheels compared to the same engine with a manual transmission. That feature is lost when the lock-up clutch engages. The torque doubling is not needed or missed when cruising at highway speeds. The clutch is supposed to unlock near wide-open-throttle when the doubled torque is needed for passing that freight train or going up a steep hill, and at closed-throttle, in preparation for coming to a stop. Also, the clutch never locks up below a specific speed, commonly around 35 mph, or when the engine is still cold or the transmission is in a lower gear.

To verify the system is working, drive at highway speed with the engine warmed up. Hold the accelerator pedal perfectly steady, then lightly tap the brake pedal with your left foot. Engine speed should immediately increase about 200 rpm, then drop back down two or three seconds later. If you see that, the lock-up clutch is working.

This fault code relates to an electrical problem with the clutch circuit. Observing if normal clutch operation occurs and if the code sets again after being erased will show if there is an intermittent problem or a definite break in the circuit. Either way, electrical connectors are the best places to find corroded terminals and the cause of this code. The additional clue is engine speed will be surging intermittently at highway speeds. Also, if you are able to start a cold engine, then immediately get out onto a highway without having to drive through town, you'll observe the transmission up-shift to its highest gear, then, quite a while later you'll notice engine speed and sound drop a little when the engine reaches about half of full normal temperature. For me that is about a half mile down the road on a cool summer day, and as much as five miles on a cold winter day.

It sounds like the EGR valve was leaking internally. The misrouted hot exhaust gas can melt rubber diaphragms and sensors. There must never be any EGR flow into the intake manifold at low engine speeds.

The most common problem is caused by a chip of carbon that breaks off and gets stuck in the valve. The symptom is rough idle. EGR gas gets into the cylinders at low engine speeds where it will cause that symptom. EGR should only occur at higher speeds where its effects won't be noticed.

I've never run into a defective new EGR valve. The biggest problem Chrysler had for a while was the sensor on top of the valve would fail due to moisture condensing in it. The only way to get that sensor was to buy a new valve assembly.

Yah, ... I got a lot of people fooled. You're the reason I drive 20 miles round-trip every day to get on the internet!