N2Performance.com - The Automotive Performance Professor, Jim McFarland
Diagnosing a
Clogged or Partially
Restricted Exhaust

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Pro Tips

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Locating Oil Screen Pick-Up

When installing an oil pan, make sure the oil screen pick-up is a minimum of 3/8" off the bottom of the pan, but no more than a 1/2".

Finding Elusive Vacuum Leaks

With the myriad of vacuum lines and hoses found on contemporary vehicles, it’s often difficult to locate pesky vacuum leaks. Also, after servicing or installing induction system parts {manifolds, carburetors, TBI or MPFT systems}, vacuum leaks can impair engine idle quality, throttle response and fuel economy. PMAG mechanics do the following: With the engine idling, squirt small amounts of “quick start” {ether} from an atomized can near any suspected leaks…or even areas where you suspect a leak might be. If the engine changes speed {typically an rpm increase}, you’ve found the leak. Take caution not to spray any of the “quick start” near the engine’s air inlet because any change from applications to this area are not leak-related, and will certainly increase rpm.

Diagnosing a Clogged or Partially Restricted Exhaust
This involves drilling a small hole {typically 1/8-inch o.d. maximum} in the exhaust pipe ahead of the suspected part {catalytic converter or muffler}. Insert a probe connected to a pressure gauge that can read positive pressure {a two-way vacuum gauge works well} into this hole. Bring engine rpm up to a cruise speed {2500-3000 rpm} and note the pressure indicated. Readings on the order of 5 inches are considered acceptable, but if they reach the range of 10-15 inches, chances are good an abnormal restriction exists. When the test is complete, a small pipe plug or self-tapping metal screw will plug the hole, especially when used in conjunction with some form of high temperature epoxy.

Identifying Restricted Radiator Cores
Bring the engine up to operating temperature. With the engine still running, slowly pass your hand around and in close proximity to the radiator core. Unless the surface is excessively hot, actually touching the surface is preferable. Temperature of this surface should be relatively uniform, over the entire radiator core. Any areas that appear noticeably cooler suggest regions where coolant is not circulating, thereby reducing radiator surface temperature. This condition often manifests itself as operating engine temperatures slightly higher, but not necessarily to excess, than normal radiator core temperature.

Diagnosing for Leaking Cylinder Head Gaskets
Aside from unusually high coolant temperature {this condition can be one of gradual increases over time}, bring the engine to operating temperature and check for bubbles in the coolant system…viewed through the open radiator fill neck. Short “blips” of the throttle should increase the intensity and size of any bubbles present. Another check involves running the engine, at operating temperature, for about two minutes at fast idle or cruise rpm. Allow the engine to cool and remove all spark plugs. Starter-crank the engine to observe any coolant displaced out the plug holes.

Tracing Air Flow Around Air Scoops or System Inlets
This check is usually applicable to racing or otherwise high demand driving conditions. Using regular-grade white grease, form small “tufts” of grease around the air entry. The shape of such “tufts” should resemble small Hershey candy “kisses.” Perform the type of driving for which the measurement is intended; e.g., a quarter-mile acceleration, etc. Examine the “tufts” to see how their shape has changed. For example, the grease will tend to relocate in the direction of airflow, while the extent of “smear” created relates to air flow velocity. This same test can also be used to determine the presence {or absence} of air around intended inlet points.

Diagnosing “Noisy” Serpentine Belts
Many times, these belts become embedded with dirt and other residue that finds its way into the grooves of these belts. And because serpentine belts are relatively expensive, the following method of service often works well. Simply remove the belt, reverse it, and scrub it {with a stiff bristle brush} in a solution of water and liquid dish soap. Allow the belt to dry before reinstallation.

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