Code P0455, slow to start intermittently rough initial idle?

P0455 - Evaporative Emission Control System Leak Detected (gross leak)

Most likely that code isn't related to this problem.

First of all, how does it run once it's warmed up? Specifically, is the idle speed normal? If it's too low, hold the accelerator pedal down 1/4" when starting the engine. If that helps, this commonly happens after the battery was disconnected. There's a really easy fix for that.

The next suspect is the fuel pressure is bleeding down when the engine is stopped. That should hold for weeks. If you don't have a fuel pressure gauge to monitor the pressure, you can usually borrow one from an auto parts store that rents or borrows tools. If you find the pressure drops a lot when the engine is off, a temporary work-around is to turn the ignition switch to "run", wait a few seconds, turn it off, then turn it back on, then crank the engine. Each time you do that, the fuel pump will run for one second, then turn off until you start cranking the engine. By doing that two, or even three times, the fuel pressure will build up high enough for starting. If pressure is the issue, we'll have to figure out where it's bleeding down. Due to the battery's voltage getting drawn down by the high demands of the starter system, the fuel pump runs really slow during cranking. That makes it take a lot longer to build pressure if you start cranking right away.

Let me know what you find up to this point.

Hold on. If everything else is okay, fuel pressure isn't the problem. I was trying to get too far ahead because most people will reply nothing was done with the battery, then we start looking at other things.

When the battery is disconnected or run dead, the memory is lost in the Engine Computer. Later, when the engine is restarted, the computer relearns the personalities of the various sensors, and it starts to rebuild the fuel trim tables right away as you drive, without you even noticing. The one exception is "minimum throttle". Until that is relearned, idle speed will be too low. The engine may not start unless you hold the accelerator pedal down 1/4", (as you found). It won't give you the nice "idle flare-up" to 1500 rpm at start-up, and it will tend to stall at stop signs or when shifting into gear.

The computer needs to see a very specific set of conditions to know that your foot is off the accelerator pedal, then it takes a reading from the throttle position sensor and puts that in memory. From then on, anytime it sees that same voltage, roughly 0.5 volts, it knows it has to be in control of idle speed.

To meet the conditions for that relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

Coasting from high speed with a closed throttle gives very high intake manifold vacuum. The computer knows the throttle is closed by that. If your foot is on the accelerator pedal, even just a little, the TPS voltage will bounce around due to bumpy roads. Holding perfectly steady, even to the hundredth of a volt, tells the computer your foot is not on the pedal. Tapping the brake pedal during that seven-second coast also aborts the relearn.

For most people, coasting for seven seconds is equivalent to half of an off-ramp, so problems seem to "fix themselves", and they don't know why. Once that relearn has taken place, if you replace the TPS, and the new one reads a lower voltage at closed throttle, the computer will put that new lower value in memory as soon as it sees it.

A totally different way to watch idle speed is with a scanner and viewing the "idle step" number. I have a Chrysler DRB3 for all of my older vehicles, and a DRB2 for my '95 and older models. You can find these on eBay as well as a lot of nice aftermarket scanners. The Engine Computer controls idle speed by running the automatic idle speed motor on the side of the throttle body assembly. That's not a motor with brushes as you normally think of. Instead, it has four coils of wire that get pulsed with varying voltages and polarities. The armature follows the rotating electromagnetic fields. It can rotate up to roughly ten revolutions from one extreme to the other. As it does, it turns a threaded shaft with a pintle valve on the end. As that valve retracts, it opens an air passage around the throttle blade to let in more air. At the same time it increases the number of milliseconds it pulses the injectors open. Increasing air and fuel is how it increases idle speed. This control only occurs until the computer sees a rise in TPS voltage indicating the accelerator pedal is being moved, then it leaves engine speed up to you.

The computer can set the AIS motor to any of 256 steps as necessary, to keep idle speed where it wants it. For a typical, properly-running engine, I find this to be around step 32. For a V-8 engine with a single-cylinder misfire, expect to see it at around step 50. At a Jeep school I attended, the instructor wanted to show us how much control the computer had through that AIS motor. With a running V-8 engine, he unplugged one injector. Idle speed dropped, as expected, the computer saw that, and responded by setting the AIS motor to somewhere near step 50. That got the idle speed back up to normal. He unplugged a second, third, and fourth injector. Each time the computer raised the step number and idle speed stayed correct. At the end he had seven injectors unplugged. Obviously the engine ran very poorly, but it was able to maintain proper idle speed even though it was shaking like crazy, and it had just barely gone past step 200. The AIS valve was only 4/5 of the way open.

The important point to that sad story is with the scanner, we can read the desired idle speed, the actual idle speed, and the current AIS step number to keep that idle speed correct. When minimum throttle has not been relearned, it will sit at step 0 and never change.

When you do the drive cycle with the seven-second coast, it will not work about two or three times out of a hundred. Do it again while holding the brake pedal up with your foot. If that works, the brake light switch is out of adjustment. There's always two, and often three separate switches in that assembly. One is for the brake lights. That part may not be switching on when it shouldn't. That would cause flashing brake lights while driving. Bumpy roads can cause the brake pedal to vibrate, allowing the other parts to turn on briefly. The part in question causes the lock-up torque converter to unlock, (in preparation for coming to a stop, the computer thinks) and it cuts out the cruise control if it's engaged. That part of the switch also aborts the minimum throttle relearn. Holding the brake pedal up will overcome that bouncing, but the proper fix is to reset the brake light switch. For fun, you can even watch the operation of the brake light switch on the scanner. It will be listed with a "Pressed" or "Released". When a customer complains of the cruise control cutting out intermittently, the DRB3 even shows the "Reason for last cutout". You have to be driving when reading that, otherwise it will show the last cause to be "road speed too low" or "ignition switch turned off". When those pop up while driving, it tells you to look for intermittent wiring or electrical problems with the vehicle speed sensor or ignition switch wiring. If the cruise control cuts out at highway speed, and the reason for last cutout is "brake pedal depressed", you would expect to find the switch out of adjustment and the brake pedal was vibrating on bumpy roads.

All of that stuff is related to idle speed. I hope that takes away any mystery. Let me know if there's anything else related to this problem.

Yes, a TPS can get a bad spot on it, but that will be detected, a fault code will set related to it, and the Check Engine light will get turned on. If the idle speed is still too low, there's two things left to look at. The first is reading the actual idle steps on a scanner. If minimum throttle hasn't been relearned, it will be at step 0.

If you find it's at a rather high step but idle speed is too low, either the automatic idle speed motor is gummed up, or the air passage is blocked with carbon. You can see that passage when the AIS motor is removed. Be sure to use a well-fitting Torx bit when removing the two screws. They can be rounded out pretty easily. Being blocked by carbon was real common on the 3.0L Mitsubishi engines like I have in my '88 and '89 Caravans. I never ran into that on any of the other engines, and I haven't seen that on any engine for a long time, I assume due to better additives in the gas today.

You can also remove the automatic idle speed motor and watch if it's operating correctly. The screws are going to be tight with threadlocker on the threads, and they'll round out easily. With it removed but still plugged in, have a helper turn the ignition switch to "run", then turn it back off. You'll see the pintle valve extend, then retract part way. Some engines, particularly GM engines, do that when the ignition switch is turned off. That's setting its position in preparation for the next engine start and will provide the "idle flare-up" to 1500 rpm. Other engines do that when the ignition switch is turned on. You'll hear the buzz of the motor too as it runs the valve in and out. The position of that valve is not monitored. The computer pulses it to move it the desired amount, then it just assumes the valve did what it was told to do. To avoid losing track of where it is, the computer runs it closed way more than enough to ensure it gets there, then it opens it the predetermined amount each time the ignition switch is turned on or off.

This same physical part was used by GM and Chrysler. If you put a lot of pressure on that valve, you can get it to retract. If you install it that way, and it's sticking, idle speed will be too high.

If minimum throttle hasn't been relearned, but the motor is good, and you install it partially retracted, the computer will either close it at the next engine start, or it will leave it wherever you put it. I don't know which is correct because I never did this experiment without minimum throttle being learned. I hope that didn't cause your eyes to glaze over from confusion.

Most scanners also allow you to perform an idle speed test on the entire system. With the DRB2 and DRB3, by pressing a button repeatedly, idle speed will increase by 200 rpm up to a maximum of 2,000 rpm. That shows if the AIS motor is working smoothly and the air passage around the throttle blade is clear. Aftermarket scanners should off a similar test.

To finish up the story on idle steps, when reading them on the scanner, if it stays on "0", minimum throttle hasn't been relearned. Normal is to find it around step "32", give or take a half dozen steps. It's when you find it's real high, as in perhaps 50 or higher, but idle speed is too low, that the computer sees it's too low and is trying to raise it to where it should be, but isn't having success. That's when you look for that carbon build-up or a sticking AIS motor.

This system causes very little trouble so hopefully one of these suggestions should lead to a solution. The only other thing I can think of is once minimum throttle has been learned, the computer will control idle speed, but only when it sees your foot is off the accelerator pedal. Lets say, for example, it sees the TPS signal voltage is 0.55 volts the instant minimum throttle is learned. From then on, any time it sees 0.55 volts, it will adjust idle speed as necessary. If it ever sees a lower voltage, say 0.52 volts, it immediately puts that in memory as the new value to watch for. This can include installing a new TPS. No two sensors are ever exactly alike.

On the other hand, if the TPS voltage never gets back down to 0.55 volts, the computer assumes you have your foot on the accelerator pedal, and it leaves engine speed up to you. Look for a floor mat holding the pedal down, or a tight throttle cable or cruise control servo cable that don't have any slack in them. In rare cases, also look for carbon build-up preventing the throttle blade from closing fully. A tight throttle cable can also cause the TPS voltage to bounce around during the coasting procedure. To work, that TPS voltage has to remain rock solid during the seven seconds.