The magic computer? We still don’t have one.

“Do you have one of those computers that tells you what’s wrong with the car?” This has got to be one of every mechanic’s least favourite questions to get from a customer. As we’veĀ explained before, while scan tools can provide a starting point for a proper diagnosis, it’s very early in the process that the human brain must take over. As we often tell people, there is a big difference between pulling out a trouble code and actually diagnosing a problem.

Here’s a great example from this week. This little Honda CR-V came to us with a “check engine” light on, and the engine idling rough.

airdrie check engine light

 

We connected a scan tool and retrieved the stored trouble codes from the engine computer, or ECM. As we often see, the stored codes were of no help, as the engine had set misfire codes for all four cylinders. We already knew the engine was misfiring, and the trouble codes don’t tell us why the misfire is happening.

airdrie car scan

 

Even though this vehicle’s rather simplistic engine computer is flagging misfires on all four cylinders, the issue really felt more like a consistent misfire from a single cylinder. Removing and shorting out the spark plugs wires one by one, we determined that the engine was misfiring on cylinder #4.

airdrie spark plugs

 

OK, now what? We’ve got lots of possibilities here: the issue could be a bad spark plug or plug wire; a problem with the distributor (yes, this car still has one); a faulty fuel injector; or about a dozen other things. Removing and inspecting the cylinder #4 spark plug seemed like a good place to start. As it turns out, the spark plug and wire were both in good condition.

airdrie mechanic

 

We noticed, however, that the spark plug was a bit wet with fuel. Having already ensured we had a strong spark supply to the plug from the distributor and coil, this could only mean one thing: The spark plug was firing; the fuel injector was firing; but the combustion event was not taking place inside the cylinder. The next logical step seemed like performing a compression test. We installed our compression tester in the #4 spark plug tube; disabled the ignition system; and cranked the engine over. As it turned out, cylinder #4 was only making about 40PSI of compression! (A good cylinder on this engine measured around 160 PSI.)

airdrie compression test

 
The low compression was definitely the cause of the misfire. Every engine needs at least 100 PSI per cylinder to “get the fire going”, so to speak. Now it was time to determine why cylinder #4 had low compression. Like before, there are lots of possibilities: it could be a burnt/bent valve; worn out piston rings; or a handful of other things. How do we determine where all that lost compression is going? We install a cylinder leakdown tester. Out came the compression tester, and in went this next tool.

airdrie car repair

 

We rotated the engine until cylinder #4 was on its compression stroke, with all the valves closed. Using the leakdown tester, we filled the cylinder with compressed air. As you can see, this cylinder has about 85% leakdown. (20% is the most we’d ever like to see on a good engine.)

We can also use the leakdown tester to determine where the leaking compression is going, by listening for air leakage at different points on the engine. Air coming out of the intake manifold or throttle body indicates a leaking intake valve on this cylinder. Air hissing from the tailpipe indicates a leaking exhaust valve, and air leaking from the oil cap points towards a leak into the crankcase via worn out cylinders and/or piston rings. This vehicle had none of these leaks. The compressed air was actually leaking from the cylinder #3 spark plug hole, indicating there is a blown head gasket or other combustion leak between these two adjacent cylinders.

The next step in diagnosis will be to remove the cylinder head for inspection, and likely replace the leaking head gasket.

 

Almost every warning light diagnosis works this way. The trouble codes (sometimes) provide a starting point, and then there are usually many other tests that must be performed – using even more specialized equipment – to “zero in” on the route cause of the issue. With some issues, there are no codes stored at all, and the technician must let the symptoms and their experience lead them in the right testing direction.

Every good diagnosis goes like the one on our Honda this week: A well-trained technician knows exactly what test to perform next based on the symptoms at hand, and lets the results of that test tell them what test should be performed afterwards. There is no wasted time troubleshooting parts that don’t need to be checked – and more importantly, no money wasted replacing parts that won’t fix the problem.

This is the value of a good diagnosis by a qualified professional, and it’s what we work to bring you every day.

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