This month’s story features a new guest columnist, Edwin Hazzard. He is the owner of Automotive Tech Systems, a mobile diagnostic and repair service serving shops in the Hudson Valley in New York. He holds ASE certifications T1 and T2 and is a Certified Master Automotive Technician with more than 25 years of experience. I encourage you to check out his website at AutomotiveTechSystems.com.
When a Miss Isn’t
When a shop calls me and wants me to diagnose a problem vehicle I treat it as a “call to arms.” Usually the shop has exhausted all possible means of repairing the vehicle based on the capabilities that it has. When I say, “call to arms,” that means that the enemy (the vehicle) has reared its ugly head and I must arm myself for battle. Working on today's vehicles really can test your wits and your patience.
Using the OEM tool is one way to be sure you get to see everything the factory will let you see.
The vehicle in question is a 2002 Chevrolet S-10 4WD with a 4.3L (vin X) engine. It has 120,000 miles on the clock. The customer complaint is a “Check Engine” light that is on steady and flashes while driving. But on the test drive, the vehicle exhibited no drivability issues at all. Based on this preliminary information, I hooked up my scan tool and proceeded to see what code or codes were stored in the Powertrain Control Module (PCM). I retrieved a P0300 and P0304. Code P0300 is a random misfire and code P0304 is a misfire on cylinder number 4.
Based on my experience and training, an engine misfire is usually one of four possibilities. It’s a fuel delivery problem, an ignition (spark) problem, an engine mechanical problem or an electrical problem. Sometimes it’s a combination of those mentioned, but it is definitely one of those. Because code P0304 is a cylinder specific code, I'm going to focus on that code first. I asked the shop owner what was done to the vehicle so far. He explained his guy had replaced the distributor cap and rotor, tried an OEM cap, then a complete distributor assembly. In addition to all those parts, they also had installed an ignition wire set, new spark plugs, an injector spider kit (one aftermarket and one OEM), a Crankshaft Position Sensor (CKP) and intake manifold gaskets. The shop performed a cylinder leak down test and a compression test as well as a running compression test. None of these tests revealed a mechanical concern.
So with a lot of parts replaced it doesn't leave me much left but a lot of questions. Why were all these parts changed and what tests were performed to warrant the parts replaced?
I hooked up my scan tool and went for a test drive. This truck ran very well, but after a few minutes driving the Malfunction Indicator Lamp (MIL) started flashing. Strange, I didn't feel any engine miss and it didn't skip a beat. I looked at the recorded scan data when I returned to the shop and nothing looked out of the ordinary. The only thing I saw was the misfire counters ramping up for cylinder 4 and a couple of other cylinders that were barely showing any misfires. That would explain why I have a P0300.
I asked myself, “If a cylinder was misfiring, wouldn't I see an issue in either the front oxygen sensor voltage or a skewed fuel trim reading for that bank in either a lean or rich direction?” After all, if a cylinder isn't firing then the combustion process would be inaccurate and the lack of or excess of fuel would show up, right? Now I’m thinking this isn't going to be as easy as I hoped.
Who knew a failed bearing would cause a misfire code to set? (Sample failure shown).
I decide to break out the big guns. I'm going to look at this cylinder up close. I grabbed my scope and proceeded to hook up to the PCM connector to check the CKP signal, the Camshaft Position (CMP) signal and the number 4-injector signal. Remember, the CKP sensor is the one the PCM monitors to detect misfires (on most vehicles). Based on the waveforms that I saw, though, there doesn't appear to be any erratic signal or signal drop out. Well, this doesn't make me happy. I hook up to the No. 4 plug wire and look at the ignition waveform. That, too, looks fine. Now what do I do? I did perform my own compression and leak down test as well as a visual inspection of the ignition components and injector connector pins. Even though the shop did those tests, I like to do them myself for my own peace of mind.
But now my mind was getting a little frazzled. Time to take a much needed break and recoup my thoughts.
Let’s see, the CKP sensor reports to the PCM, right? Could the PCM not be reading the data correctly? (I had already verified the CKP signal was OK). I removed the CKP sensor and visually inspected it to see if any damage was evident. As expected, it looked good. The next thing I tried was to check crankshaft free play. Maybe the crankshaft reluctor wheel is moving too far away from the CKP sensor? Nope; no excessive end play was found. This was starting to get on my nerves and it was becoming personal. How could a simple misfire problem be kicking my butt?
Most OEMs use crank speed as a way to detect misfires. Anything (key word!) that causes that signal to vary in speed can set a code. (Toyota example shown.)
The one tool that everybody has is the tool to be able to see and to hear. Granted, as you get older they seem to not work as well as they did when they were brand new but still they are necessary tools to have in your arsenal. When I was running this engine at 2,000 rpm, I didn't hear anything that sounded out of the ordinary but with the accessories on the front of the engine it does add some extra noise to the engine noise. I wanted to hear the engine without the belt noise and the accessory noise.
I removed the belt and started the vehicle up. Wow, the engine was quiet and the misfires were no longer counting up on the scan tool! Are my eyes playing tricks on me? I reinstalled the belt for my own sanity and the misfires returned. How could this be? I removed the belt again and sure enough, the misfires disappeared. Increasing engine speed to 2,000 rpms with the belt off, I could here the sound of a rod-bearing knock. Now there seems to be some light at the end of the tunnel. I had the shop remove the oil pan and inspect the bearings for excessive wear. They called me up and told me that the No. 4 rod bearing journal was wiped. It all started to make sense. Misfires are detected by monitoring the speed of the crankshaft via the CKP signal to the PCM. I can only think that the worn rod bearing play was enough to impact the speed of the crank whenever the No. 4 cylinder hit its power stroke and that effect was more noticeable when the crank was additionally loaded with all the accessory drag.
Based on what I said earlier, a misfire is either spark, fuel, mechanical or electrical. This one happened to be a mechanical issue but not your normal mechanical issue! Sometimes in battle you exhaust all your options and you need to fall back and reload. Look at the system your working on and know the system your working on. Think outside the box just might help you win that battle so you can move on to finish fighting that war.
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