Troubleshooting A Hard To Find Misfire. Toyota Corolla

One of the most common automotive diagnostic issues I'm asked for help with, is diagnosing a car or pick-up that has developed a misfire (rough idle) after a tune-up was done (to it).

In the vast majority of cases I've been involved in helping troubleshoot, the problem always boiled down to either a defective part being installed or human error in the installation process.

Unfortunately, in all of those cases the owner of the vehicle never bother to double check his (or her) work. Thus, he or she ended up looking for the problem (or its solution) every where else but in what was installed/replaced... and ended up replacing parts that weren't needed.

In this article, I'm gonna' spin a yarn about just such a case. This real life story is about a friend's 2002 Toyota Corolla that developed a misfire condition, that wasn't there before he replaced the spark plugs. In trying to solve the misfire, he ended replacing a whole lot of things before he yelled ‘uncle’ and called for help.

Although this case study is about a 02 Corolla... the info applies to just about any type of four cylinder on the road. Also, this real life case study supplements the following tutorial:

  1. Misfire Codes: How To Troubleshoot And Repair Them (Toyota 1.8L).

These other two rough idle (misfire) real life case studies may be of help too:

  1. How To Diagnose A Hard To Find Misfire Case Study (GM 4.3L, 5.0L, 5.7L).
  2. Carbon Tracks Are A Common Cause Of Ignition Misfires (at: easyautodiagnostics.com).

COMPLAINT: Misfire Started After Doing a Tune Up

To give you a little background information... my buddy is no stranger to working on cars. Although he's not a professional mechanic, he's a serious DIY'er that can turn a mean wrench. If he does take his car or pick up to a shop it's for stuff like a wheel alignment (the things he can't do himself).

So, I was a bit surprised when he called and asked for help. He mentioned that his 02 Toyota Corolla had developed a misfire after he had replaced the spark plugs and that he could not figure it out.

He went on to mention that he had replaced the spark plugs as part of his Toyota Corolla's maintenance and not because the vehicle was giving trouble.

Over the course of one week, he had already replaced:

  1. One ignition coil.
  2. One fuel injector.

Having seen this type of problem before (where a problem starts after something was replaced) I mentioned to him that all he needed to do was to double check his work... since the problem and the solution were in those things he replaced.

But he was just plain frustrated with the whole thing that he didn't want to mess with it anymore.

Since it was a Sunday afternoon when he called, I went over to see what I could do to help him out.

Road Testing and Checking For Misfire Codes

Over at my buddy's house, the first thing I did was verify the misfire condition by starting his Corolla and going for a road test around the block. The road test confirmed his Corolla was indeed misfiring at idle and under load and that the check engine light was on.

Getting back to his house, I connected my little generic scan tool and checked for misfire codes. The scan tool registered two codes:

  1. P0300: Random Cylinder Misfire.
  2. P0303: Misfire Cylinder #3.

Since my buddy was standing next to me... he mentioned that he'd already replaced the ignition coil and the fuel injector for cylinder #3... but those parts hadn't solved the P0300/P0303 DTC.

He was getting ready to just replace them all (ignition coils and fuel injectors)... the only thing that had stopped him was the amount of money he was gonna' spend on all those parts just to see.

Doing a Cylinder Balance Test

Now that I had a specific misfire code (P0303), I now needed to confirm that it really was cylinder #3 that was misfiring (you should never take for granted that the scan tool is telling you the truth).

The easiest and fastest way to verify a ‘dead’ cylinder is by doing a manual cylinder balance test.


This simply involves:

  1. Starting the engine and letting it idle.
  2. Disconnecting one fuel injector (from its electrical connector) at a time.
  3. When you disconnect the fuel injector of a cylinder that IS NOT misfiring, the engine's idle will get worse.
  4. When you disconnect the fuel injector of a cylinder that is ‘dead’ (misfiring)... the engine's idle will not react. It's like you didn't unplug the fuel injector at all.

That's exactly what I did to my bud's Toyota Corolla. The end result was that the cylinder balance test confirmed that cylinder #3 was ‘dead’.

Although the manual cylinder balance test doesn't tell why the cylinder is ‘dead’... just confirming which cylinder is the one misfiring is always half the diagnostic battle. Finding out why the cylinder is ‘dead’ is now a matter of seeing what's missing. It's either gonna' be a lack of spark, or a lack of fuel (and in some cases a lack of compression).