Don't go guessing yet until we have some codes to look at. When a misfire is bad enough to feel, it has been bad enough for a long time for the Engine Computer to detect. The most common fault codes start with "P0300". That one indicates a random cylinder misfire. The next codes get specific, as in a code "P0305" for example, means cylinder number 5 has a misfire.
Up until a few years ago, a fellow by the name of Jim Linder had a fuel injector rebuilding company close to the Indianapolis Speedway. He put on injector-related classes, including one at my community college for just 50 people. That's where we learned about common injector problems. Eight of the top ten failures listed on his web site involved GM vehicles. The other two involved import models. According to him, when Chrysler buys injectors, they buy them from Bosch in flow-matched sets. Injector problems are just about unheard of and they easily last the life of the vehicle with no or little maintenance. I can't say what other manufacturers do, except for GM, as that's what most of the day-long class revolved around. When GM installs injectors on the assembly line, they grab a handful out of a large bin, and pop them into an engine with no regard to flow-matching. Being new, they all tend to flow roughly the same volume of fuel each time they fire, up to around 100,000 miles. By that time, with a little normal wear and maybe a little varnish buildup, one or two start to flow a little less volume than the rest. Eventually this results in a lean mixture misfire that you will rarely feel. This is why it is real common to get a Check Engine light, with one of those P03xx fault codes, but with no noticeable engine performance problems. This has been an elusive problem for years. The solution is to install a set of rebuilt injectors that have been matched and are sold in those matched sets.
Experienced engine performance specialists can look at the fuel trim numbers to get some clues to this. For the rest of us, a common plan of attack is to switch two injectors, including the one from the cylinder with the misfire code, erase the old code, then see of a misfire code sets for the cylinder the suspect injector was moved to.
Cleaning the throttle blade of carbon would not be my first stab at a solution. That used to cause hesitancy or surging when trying to accelerate, and it affected all the cylinders equally, so it also caused idle speed problems. When you can feel the misfire, it is affecting the cylinders unequally, so we have to look for things that can vary or be different between them. I don't see much reference to any carbon-related problems today like we used to see, due to better additives in the fuel.
Since you started out asking about the IAC motor, allow me to add another chapter to that story. That motor used to be the exact same part used on GMs and Chryslers for many years. On the Chrysler engines, you can look on a scanner to see the number of "steps" the Engine Computer has placed that motor at. They go from step 0 to as high as step 256, that being the air bypass valve is wide open. For a typical, properly-running engine, step 32 is common. With a V-8 engine with one dead cylinder, it will be around stwp 50 to keep the engine at its target idle speed. To show how much control the computer has over idle speed, an instructor unplugged one injector at a time on a V-8 in a Jeep. With each disabled cylinder, idle speed would of course drop, then the computer bumped up the idle steps to get it back to the desired idle speed. The last straw was to have seven injectors unplugged. Obviously the engine did not run even close to "well", but it did keep up to the desired peed, and it hadn't yet reached step 200.
GM controlled their idle speed the same way, but I don't think they called them "steps". Most other manufacturers display the percent that valve is open, from "0" to "100".
Also be aware the computers don't monitor that valve's position. They pulse the electromagnetic coils inside the IAC motor, assume it responds, then they look at actual engine rpm to determine the need to make another correction. That is done continuously as the engine idles. On a scanner's display, you'll see those numbers bouncing around until you press the accelerator pedal.
As a point of interest, when you disconnect a battery cable on an older Chrysler, or run the battery dead, the memory is lost in the Engine Computer. All the sensor personalities and fuel trim numbers are relearned automatically as soon as you restart the engine, without you noticing, except for "minimum throttle". Until that is done, the idle speed will be too low. The engine may not start unless the accelerator pedal is held down 1/4". You won't get the nice "idle flare-up" to 1500 rpm at start-up, and it will tend to stall when shifted into gear and when approaching stop signs. There's a very simple procedure to make that relearn take place that just involves driving the vehicle. The clue in this case if you're looking on a scanner is the idle steps will remain at "0" until that relearn takes place. Basically, the conditions that must be met are intended to let the computer know 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 signal voltage from the TPS, it knows it must be in control of idle speed.
To my knowledge, that has never been an issue with GM engines. This is a real common complaint with Chrysler engines with a real easy solution, but I don't recall ever reading about that complaint on other vehicle brands.
Since more engines have been using a single ignition coil for each cylinder, failures have become rather common. That seems to affect Ford truck engines more than anything else, but you might also consider switching two coils to see if the misfire moves to a different cylinder. Send the coil from the misfiring cylinder to a different cylinder than the one you switched the injector to. That way, if the fault code does come up with a different misfiring cylinder, you'll know which part is responsible by which part you moved to that cylinder.
Sorry that this is starting to sound like a legal contract. That should give you some more ideas, but it's really the fault codes we want to see first. If necessary, there are a couple of other experts I can get involved to help find a solution.
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Monday, February 13th, 2023 AT 4:50 PM
