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A few weeks ago I had a Ford Explorer in the bay with the 4.0 SOHC engine, and an idle quality complaint. The idle would roll when cold, and then get a little better. It also felt “harsh”…like it was misfiring on various cylinders, but not to the point of creating a dead hole. Another tech had cleared the ECM, so Mode $06 was of no help.
Testing cranking compression with my scope and a pressure transducer told me quickly that there was a mechanical issue. The patterns showed weak cylinders, but they were never consistent. Sometimes 3 would be weak; sometimes 2, and sometimes 4 would show up with weak peaks. Ever deal with an intermittent valve train issue that led to loss of compression? I’ve seen excessive carbon cause this issue, as well as broken valve springs and sticking lifters. A running compression test would show these problems, but only if the fault occurred at the same time the test was made. I sure wish I could perform a running compression test “live” and watch for the fault to occur.
Thanks to our own Jim Garrido for introducing the idea of a “live” running compression test to me in the pages of Motor Age (August, September, and October 2008 issues). After reading his articles I was dying to try this out but hadn’t had the chance until now to do so. This method uses a pressure transducer attached to the cylinder you want to test in place of the plug. I used the hose from my compression tester, with the Schrader removed to capture the illustrations below.
A normal running compression test, for those of you who don’t know, is a good way of checking the cylinder’s ability to flow. Any “restriction” to flow will show up using this test and that includes valves out of time, not opening or closing fully, and physical restrictions in either the exhaust or intake tracts. A live running compression test shows even more…every pressure change occurring while the cylinder is actually pumping, making it an excellent way for catching those intermittent issues.
The Ford fault was caused, in part, by sticking lifters and I only found this out by physically accessing the valve train and performing a leakdown test on the worst cylinder (found using the ACE Misfire Detective software). The first few times I pressurized the cylinder I got lucky, and air came pouring out of the intake valve. A quick shake of the rocker yielded no play, telling me the valve was being held open. After testing the rest of the cylinders, I came back to #1 only to find that now it sealed just fine.
This was a lot of work. Had I hooked up the pressure transducer and fired up my scope, I would have seen the peak cylinder pressures vary and possibly even seen the variance in pressure where it would normally fall into a vacuum. In addition, this test can be used to quickly verify cam timing. By overlaying the opening specifications onto the pattern using Microsoft “Paint” or a similar program, the opening times should match pretty closely to the changes in pressure that indicate the valve actually did open.
It is also a means to check for a clogged exhaust. Since an exhaust restriction will build up back pressure, the peak of the exhaust ramp will increase as the gas pressure builds in the pipe.
I encourage you to visit the MA archives and read Jim’s articles on this subject. He includes additional references that will help speed up your learning curve on this testing method, and allow you to add this powerful diagnostic approach to your arsenal.
Edited by Peter, 5 years ago
VERY HELPFUL ARTICLE
I was taught this method a while back in an AC Delco class. It is very effective to "cut to the chase" on cranking and running to isolate a more in depth look at the cylinder's
Thanks to all who offer their time and knowledge to all of us learning everyday. As the instructor
Alford's Signature Auto Care
Edited by Peter, 5 years ago