Productive engine service tips

Being a specialized service, engine repair work challenges change every day.

A: The flatness of both the head and block must be within limits for an effective seal. To check the head, lay a straight-edge lengthwise on the valve side of the cylinder head and stick the largest feeler gauge possible between the mid-point of the head and the straight edge. Check the deck by laying the straight-edge lengthwise on the block. Generally, a maximum of 0.003” warpage is allowed for 3-cylinder heads; 0.004” for 4-cylinder heads; and 0.006” for (inline) 6-cylinder heads. Warpage specs are for the total amount of warpage for the head and block, not just the head. The block is frequently forgotten when making this check, because blocks usually don’t bend, heads do. Excessive warpage means the head, the block or both may need to be resurfaced.

Q: The techs in our shop have an ongoing debate about how to best find a a weak cylinder. What’s best?

A: A compression test, whether done with a gauge at each cylinder, or electronically with an engine analyzer, reveals each cylinder’s ability to compress air. Car makers differ on the amount of pressure differences they allow, but as a general rule of thumb, the remaining cylinders should read within 10 percent (or about 15 to 20 psi) of the highest cylinder. A power balance test is similar to a compression test, but a power balance test really compares the relative power output between the engine’s cylinders. The test really serves as a quick way to identify a problem cylinder. When you do find a weak cylinder with a power balance test, you still don’t know if the problem is ignition-related (like a fouled spark plug or bad plug wire), fuel-related (like plugged or dead fuel injectors) or a mechanical problem (burned valves, blown head gasket, cracked or burned pistons, cracked head or block, or rounded cam lobes). Sometimes a compression test can give you a clue to what’s wrong, but additional detective work is usually needed to pinpoint a problem. Compression readings alone won’t tell you if the engine has worn rings, worn cylinders, or leaky valves. Even a “wet” compression test, where you take a second set of readings after squirting oil in the cylinders, isn’t exacting enough to confirm the source of the leak.

Q: Since a compression test isn’t very conclusive as to the root cause of a cylinder problem, what’s the next step?

A: If you were to look at a diagnostic flow chart, a cylinder leakage test would represent the last block in the chart. For instance, in the case of an engine misfire, you would test the fuel and ignition systems first, then go to a compression test and finally, end up at the cylinder leakage test. How does a leakage test differ from a conventional compression test? Instead of checking how well each cylinder pumps air, the cylinder leakage test checks how well each cylinder holds air. You do this by applying a controlled amount of compressed air into each cylinder and watching a gauge to see the relative percentage of leakage. Because you’re filling each cylinder with air, you can also listen at different points to find the source of the leak. Since these air leaks are really compression leaks, the signature hissing sound leads you to the source of the cylinder’s problem.

Q: Our shop replaces timing belts on some makes and models right now, but we’d like to go after this market more aggressively than in the past. What do we need to consider?

A: First, consider the vehicles that would represent the highest demand for service. With volume on your side, you can more easily justify the tools you’ll need to do the job. Once you’ve defined your spectrum of vehicles, start looking for the timing belt tools you’ll need to handle that group of vehicles. In some cases, you may find that certain tool companies have timing belt “master kits” that provide a wide range of vehicle coverage, rather than buying multiple individual kits. Like all areas of service, don’t stop with just the tools. You also need to account for the breadth of service information needed to handle that spectrum of vehicles. Consider expanding or rethinking your current subscription if needed. Remember to continue to evaluate during the discovery process, as you may learn that some vehicles may not be worth your while if tool costs are too high and the volume of vehicles you’d see would be too low.


Thanks for checking out this month’s Tool Q&A column. Remember, this is your column, because it’s based off your questions. So, let PTEN know what’s on your mind when it comes to tools and equipment.

We Recommend