Misfire diagnostics for dummies

Jan. 1, 2020

We all make mistakes. The key is to learn from them so we don't do it again.

The 2003 Ford Mustang that awaited me that morning had been dropped off the night before by its new owner. He recently purchased the car from a private seller and was, at first, thrilled by his new ride. He noticed a rough idle and stumble on his initial test drive but attributed it to the car having sat for the last several months. But when the Check Engine light came on, he decided there might be more to the problem than he originally thought. Now it sat in my bay. Join me as I discover the right — and wrong — ways to diagnose a misfire!

First Things First

It wasn't hard to figure out I was going to be dealing with a misfire issue. The car idled just like one of the six cylinders wasn't home, but the malfunction indicator lamp (MIL) held steady as I drove the car into the shop. There was no flashing MIL that would indicate a severe misfire was present.

As was my habit any time I had a Check Engine light complaint, I reached for my scan tool first. I retrieved both a code P0301 (cylinder No. 1 misfire detected) and a P0316 (misfire detected on start-up/first 1,000 revolutions) from the engine control module (ECM) controlling this 3.8 V6 engine. The next stop was Mode $02 to take a look at the freeze frame data recorded by the ECM at the time it decided to turn the light on.

The OBDII misfire monitor is a continuous monitor, constantly testing and updating the results during any given drive cycle. For the ECM to turn on the MIL, it must have seen the misfire occur at least twice in a row and under the same load/rpm conditions. The freeze frame data will help me determine what those load/rpm conditions might have been so I can duplicate the conditions during my testing. It also might provide some clues as to the misfire's cause.

In this case, all it did was confirm what I already knew. The engine misfired cold, at idle. But at least so far I was doing everything right.

The odometer on this Mustang read a little more than 65,000 miles, and I had no service history. My next step was to do a little visual inspection. It wouldn't be the first time I've found a dead hole caused by something as simple as an unplugged injector or loose ignition wire. The ignition wires immediately caught my eye after I opened the hood.

They looked to be original equipment, bearing the Ford brand and numbered for the cylinder to which they belonged. It also was plain that they had seen their better days. The "dusting" coloration on them near points of contact generally indicates leakage through the insulation. For giggles, I pulled the plug from the lost cylinder. The gap between electrodes was large enough to drive a truck through!

OK, slam dunk. This one is an easy fix. I ordered new plugs and wires, cleared the codes and went on to the next car while I waited for the parts to arrive.

What is a Misfire?

Was I still doing things right? When diagnosing any kind of diagnostic trouble code (DTC), it is imperative that you understand how the computer is testing the system. The computer, not the owner, becomes your customer. If your repair does not meet the computer's test requirements, the code will return. And so will your customer!

In the case of the Ford misfire, the ECM monitors crankshaft speed by monitoring the crankshaft position sensor (CKP) signal. When one of the cylinders produces less than its fair share of power, the crank will slow momentarily. Any condition that results in a cylinder's loss of power could be the cause of a misfire code. That means it could be related to the ignition system as I was assuming, but a fuel issue or even a mechanical one also could cause it. A problem in the CKP signal to the ECM even could cause it. The first mistake was one I've seen many technicians make: assuming the ignition system is most likely to blame.

Now, in my defense, the wires and plugs did require service. But I did not verify that this was the cause of the codes I was charged with repairing. So what do you think? Did this fix the car?

After Lunch

With the new parts in hand and installed, I reached inside and turned the key, fully expecting the 3.8 to purr like a kitten. Well, as much as any 3.8 Ford could. Instead, I was rewarded with the same rough idle I started with.

Now I know that a major killer of ignition coils is overheating from too high a firing demand, caused by worn plugs and wires. I was so convinced at this point that it was ignition related that I tried a new coil on for size. Of course, that didn't fix the car either.

By now, I've added a few more sins to my growing list. One of the first that comes to mind is allowing myself to develop a case of tunnel vision. I was so convinced that the problem was in the ignition system, I was beginning to refuse to believe anything else, in spite of the obvious evidence to the contrary. The next sin I committed was throwing parts at the problem in an attempt to cure the complaint. Luckily, I had a good coil in my parts collection, so this customer wasn't suffering as a result of my sins.

It was time to get back to the basics of fuel, spark and compression. I could be pretty sure at this point that spark was OK, right? Let's go with a quick check of compression. I had my old UEI scope on the workbench, so I connected a high amp clamp to it and checked the relative compression using the starter current draw as my indicator. The pattern on the screen was painfully obvious, with five nice little peaks followed by one missing one. One of this engine's cylinders was not making any pressure! I know better; I should have started here first!

Diving Into Number One

I removed the plug from the suspect cylinder and threaded in a manual compression gauge. After disabling the fuel system so I wouldn't wash down the cylinders and to keep the engine from starting, I cranked it over to get a compression reading. The result was a measly 54 psi. Next, I grabbed my cylinder leak down tester to isolate where the loss was occurring. A cylinder leak down tester is just a tool that allows the user to pressurize the cylinder in question with shop air, with two gauges fitted.

One is the air pressure entering the cylinder, and the other is the pressure being maintained in the cylinder. If the cylinder is tight, the two pressures should agree. If there's a leak past the rings or the valves, the second pressure will be lower than the first. That difference can be considered as a percentage loss, with approximately 10 percent being the maximum loss acceptable. The primary advantage of this tool is the ability to listen to the air escaping to pinpoint the cause. Air streaming from the tail pipe, for example, means the exhaust valve is leaking.

In the case of the Mustang, I had 90 psi going in and roughly 70 psi staying in, for a 20 psi (or 18 percent) difference. The air was clearly heard coming from the throttle body, and I knew I had a leak coming from the intake valves. I wasn't happy about missing this the first time around, but at least I had found the problem and could get on with the repair. With the new evidence in hand, and after a conversation with both my service advisor and the customer, I removed the head and sent it out for reconditioning by our local machine shop. Now I'm doing it right!

A few days later, the head was back and it was time to put the Ford back together. The customer had been extremely patient and understanding so far, and I didn't want to let him down further. He was expecting the car the following day, but if I can meet an expectation I certainly prefer to do so. My schedule that day was light and all looked promising as I reassembled the engine.

Until I started it up, that is. You honestly didn't expect it to be fixed, did you? I did.

Now What?

The head is on and it still is running rough. A quick check of compression shows the same lousy 54 psi I had before! I hooked up the leak down tester to check the valves. Maybe the machine shop had messed up, though they never had in the past. With the air flowing in, the cylinder was holding tight as a drum, no loss anywhere.

If the engine is turning over and there is no loss of seal, then it must be a matter of air getting into the cylinder in the first place. What if the valves weren't opening fully or when they should? Could I have a worn cam lobe on that cylinder? A valve train issue that was keeping the valve from moving to its full lift? All I knew at the moment was that I was going to be doing this job all over again.

As I began to remove the head, I paid close attention to everything I could think of. I watched the valves open and close while comparing them to the companion cylinders on the left bank of this V6 design. I measured installed height to see if there were any variations. I checked the push rod length to make sure there wasn't a problem there. I didn't find anything.

That is until I got the head off. Looking closely at the front two cylinders, I saw what I should have seen the first time around. The ridge wear on the failed cylinder extended at least a quarter of an inch into the cylinder while its companion cylinder's ridge was barely visible. Rotating the engine over with the head off made the problem even more apparent. The No. 1 piston was not reaching Top Dead Center (TDC). This wasn't a matter of compression pressure, it was a matter of compression ratio!

I removed the piston and rod from the engine to find the rod bent almost perfectly along its axis. This allowed it to continue to move without any noise or vibration, and effectively shortened the stroke on that cylinder. Because the piston had a "new" TDC, the compression ratio for that cylinder was almost non-existent. And I should have known that a small 18 percent leakage from the valve wouldn't have that dramatic an impact on the measured compression reading.

I'm not sure what caused the bent rod, but I suspect a prior coolant leak into that cylinder resulted in a hydraulic lock up. When the head gasket/head was repaired, the bent rod went undetected.

Yes, I made some mistakes on that job. I hope you'll learn from mine so you won't make them yourself. But honestly, would you have seen that the first time around?

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