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  Tech to Tech

Dynamic Testing Finds the Root Cause

Posted 2/2/2007
By Brian Manley

Drawing on experience pays off when trouble tree leads author down a dead-end trail.

I was intrigued by the details of a '67 Chevy truck: It had failed our state's tailpipe emissions test; it had been to several shops and came with several differing repairs and diagnoses. It originally had a six cylinder from the factory, which had been yanked in favor of a 350 cubic inch displacement (CID) And it had been passed down to a third-generation owner in the same family.

I pulled out my camera because this was a sweet, old, step-side pickup with side pipes and a factory, hood-mounted tachometer - very cool. I took many ensuing pictures because of the fun diagnostic nature of this old Chevy. The owner had given me the details of how he just wanted it fixed so it could pass our emission test - which for a 1967 is quite liberal - and get it plated and on the road.

We could feel a "seat-o-the-pants" misfire right away after a startup that never seemed to go away at idle. The performance on acceleration was so-so for an old school, small block V8. Definitely not ready for the quarter-mile track.

We inserted our emissions tester's probe into the tailpipe to confirm what we could feel from the idle shake: This thing was emitting 2000 parts-per-million (PPM) of hydrocarbon (HC) and had all other gases well below their limits. Next, we needed to narrow our focus by performing a power balance test, which would pinpoint the cylinder(s) responsible for our high HC. So, out came the old, blue, roll-around Sun Engine Analyzer (perfect for our 40-year-old pickup), and a balance test was performed with the following results:

Now that we found two cylinders that were not contributing to the performance of our V8, it was easy to see where the low power and massaging action of the front seat came from. The interesting thing was the side-by-side nature of our cylinders; this engine has 2, 4, 6, 8 on the driver's side, and 1, 3, 5, 7 on the passenger side. The entire time our "old blue" oscilloscope was attached to the running engine we observed the ignition patterns for each cylinder, and although the pattern for cylinders 1 and 2 varied slightly from the others, they both had good ignition patterns and produced good spark with spark testers. This engine has a high energy ignition distributor (HEI) that made some wicked lightning! Yes, it bit me while I was wiggling wires - ouch! We proceeded to check for vacuum leaks around the carburetor (how often do you get to do that anymore?) and from vacuum lines running from ports above those runners, but found none.

In the back of my mind I constantly entertained the fact that this was a 1974-ish 350 engine in a 1967 truck that originally had a six cylinder, and that plenty of modifications had been done. It was, however, still a clean Detroit small-block that has the most basic of fuel and ignition systems.

Hmmmm ... time to pull the plugs and begin thinking about mechanical issues. We found it odd that our two suspect cylinders had two different plugs from the other six, especially since one of the operations recently completed was a full tuneup. I suppose shop No. 2 or No. 3 tried new plugs after discovering the weak cylinders. Time for a full battery of compression testing to determine if our "air pump" was mechanically sound, so we performed cranking, running and snap-throttle tests:

Bingo! We had isolated a mechanical issue with our engine: The reading of 120 psi indicated that the air and fuel mixture was entering cylinder No. 3, but it was having trouble getting out. It was time to pull the passenger side valve cover and look for valve opening issues. Once the cover was off, we disabled the ignition system, and then cranked the engine over. Immediately, we saw one of the rockers only moving about half as much as the other ones, and it was the exhaust valve for cylinder No. 3! We could also see that the problem rested in the camshaft end of the valve train, and not in a worn rocker arm. We received authorization to pull the camshaft, and we found the worn exhaust lobe.

This was a fun trip through a diagnostic sequence that led to a camshaft failure that I don't see very much anymore. Like a trip through the ASE l-1 Advanced Engine Performance Test, I had fun moving through data results and funneling down the diagnosis. After completion of this job, we had great gas numbers from the truck's tailpipe, and the idle was nice and smooth.

This running compression test was included as part of our state emission test training program, and I have used it with success many times. Here's how.

How to Perform a Running Compression Test

  1. Start with a normal ("static") compression test. Eliminate rings, valves, holes in pistons, etc. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm.
  2. Put all but one of the spark plugs back in. Ground that plug wire to prevent ignition module damage, and disconnect the injector on a port fuel system.
  3. Put your compression tester into the empty hole.
  4. Start the engine and take a reading. With the engine running, "burp" the gauge every five to six "puffs."
  5. Now snap the throttle for a "snap acceleration" reading. Reading should rise. Write it down. Note: Don't use the gas pedal for this snap acceleration. The idea is to manually open, then close throttle as fast as possible without speeding up the engine. This forces the engine to take a "gulp" of air.
  6. Write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe two to three good ones.
  7. Analysis: Running compression at idle should be 50-75 psi (about half cranking compression). Snap throttle compression should be about 80 percent of cranking compression.

Example: Restricted Intake If snap reading is low (much less than 80 percent cranking compression), look for restricted intake air - severely carboned intake valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the runners. (Toyota, Vortec, etc. with variable runner length). Comparing measurements between cylinders is important.

Example 2: Restricted Exhaust If snap measurements are significantly higher than 80 percent of cranking measurements, look for restricted exhaust on that cylinder, such as worn exhaust cam lobe or collapsed lifter. If they are all high, look for a clogged catalytic converter.

Craig Van Batenburg Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.


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