By Any Means Necessary
I am working on a 2005 Ford Taurus that has about 125,000 miles on it. The engine runs pretty well, considering the accumulated mileage, but the idle quality leaves something to be desired. While the idle isn’t rough enough to indicate a dead cylinder, or one with significantly lower-than-normal compression, it isn’t what you’d call smooth, either. The engine will idle pretty well for a time, followed by a flutter of roughness. It’s not bad enough to flag a misfire code and the engine runs as well as might be expected the rest of the time, but the customer is still concerned about the idle quality.
Thanks for your question, Rick. Not that long ago, an engine with about 125,000 miles on it would be considered a candidate for overhaul or replacement. Either that or the rest of the vehicle would have been so badly rusted that the whole thing would have been ready for a last ride to the crusher. Today, an engine with 125,000 miles on it—at least one that has received proper maintenance—is considered “broken in.” So your customer’s expectations aren’t entirely unrealistic or out of line. It’s just a question of how much he’s willing to spend in the pursuit of a smooth-idling engine.
Your diagnosis should begin with an evaluation of the fuel and ignition systems. What’s the condition of the spark plugs and other ignition components? The problem is occurring at idle speed, so it’s unlikely that a fuel pressure or volume deficiency is to blame. You can relatively quickly determine the overall health of both systems with a secondary ignition oscilloscope. The secondary pattern should reveal if one or more of the cylinders is misfiring slightly. Fuel trim readings may help you determine if the engine management system is attempting to compensate for one or more cylinders that are running too rich or too lean.
You indicated that the symptoms seem to be intermittent, so none of these tests may reveal the actual cause of the slightly rough idle. At least they’ll help you determine what isn’t wrong with the engine. The next thing to check is the engine’s mechanical condition.
If you have a diagnostic oscilloscope, a relative compression test can be conducted without disassembling anything. A relative compression test isn’t an actual compression test, as you’re not taking a real compression reading of each cylinder. Rather, this test measures the starter current draw during cranking. A cylinder that has lower compression than its companions will demand less effort from the starter motor, so the current draw will be lower relative to the other cylinders.
To test relative compression, attach an inductive current probe to a battery cable and attach an ignition trigger to cylinder 1 ignition (either primary or secondary). Disable the fuel supply, then crank the engine for 10 seconds or so while capturing the data with your scope. Some scopes will automate this process for you, and will present the results in graph form. Others will require you to evaluate the waveform you’ve captured. Use your ignition trigger to determine where cylinder 1 is repeated in the waveform, then follow the firing order while checking the current draw for each cylinder. You’re looking for a cylinder (or cylinders) with noticeably lower current draw as the starter motor turns the crankshaft through each cylinder’s compression stroke.
If you still haven’t seen anything that points to a problem in an individual cylinder, you can always do a conventional cranking compression test with a compression gauge. This is also referred to as a static compression test because the engine isn’t actually running during the test—just cranking. It’s a measure of each cylinder’s ability to seal. Just about everyone knows how this test is conducted, so I won’t repeat the steps here. The results of the test are what’s important.
All spark plugs must be removed and the throttle must be held open. Make sure the battery is up to snuff so the cranking speed will be the same for all cylinders. You should be able to capture a peak compression reading on your gauge after four or five “puffs” while cranking the engine. Each puff equates to one compression stroke. We’re looking for consistency here. An engine with higher mileage on it may not be able to reach the peak compression it was able to develop when new, but all cylinders should have nearly equal compression. A good rule of thumb is a variation of no more than 10% between the highest and lowest compression reading.
An intermittent rough idle might not show up as a lower-than-normal compression reading during a cranking compression test. This leads us to our final test of engine mechanical health—the running compression test. This test may also be referred to as a dynamic compression test and its purpose is to measure the engine’s volumetric efficiency—the ability of the cylinders to draw air and fuel into the combustion chambers. A vacuum test determines an engine’s overall breathing. This test measures the volumetric efficiency of one cylinder at a time. It also may help you find the source of an engine miss that’s too subtle to be captured by any of the previous tests.
To test running compression, reinstall all of the spark plugs except one. Install a standard compression gauge in the remaining hole. Leave the compression gauge’s Schrader valve in place. Start the engine. Use the compression gauge’s pressure relief valve to bleed pressure and allow the reading to stabilize. It should be bouncing around at about 50 to 60 psi. Snap the throttle wide open, then return to idle. With the Schrader valve in place, the gauge will hold the peak reading. This reading will be higher than at idle because the peak comes at the instant the throttle restriction is removed and the piston speed is still relatively slow.
Record your readings for the running snap throttle compression of all cylinders. The running snap throttle compression reading should be approximately 80% of the cranking compression reading. For example, if a cylinder’s cranking compression reading is 150 psi, running compression should be approximately 120 psi.
If the running compression reading for all cylinders is below 80% of the cranking compression reading, the intake system is causing a restriction. If the running compression reading for all cylinders is above 80% of the cranking compression reading, the exhaust system is causing a restriction. If only one cylinder is affected, look for a problem with a worn camshaft, a broken valve spring or rocker arm, carbon buildup, etc. If the problem is in all cylinders, look for a restricted intake system or a clogged catalytic converter or muffler.
Copied from the Motor Magazine July 2016 Issue, if you like what you see please subscribe to the Motor Magazine by clicking here.