SPARK KNOCK CAUSED BY BRAKE PEDAL PRESSURE/SWITCH ADJUST.

Subject: ENGINE SPARK KNOCK - 1981-1982 3.8L V-6

To meet imposed regulation on exhaust emissions, engine designs have been modified. These modifications include fuel delivery, ignition timing, electronic engine controls, etc.

It is not uncommon that an occasional engine ping or light spark knock be noticed under certain driving conditions. Conditions such as light acceleration under load, driving up hills, high operating temperatures, or even very dry air may cause a light spark knock. Engine spark knock other than described above is abnormal and can be corrected. Normally, three conditions contribute to engine spark knock.

1. Fuel Octane Rating Below Requirement 2. Incorrect Engine Timing 3. Lean Fuel Mixture

Although not a common condition, abnormally high compression ratio will cause engine spark knock. The high compression ratio may be caused by carbon build-up on the piston or cylinder head. To correct spark knock caused by carbon build-up, use a can of GM Top Engine Cleaner, following instructions on can. If the condition returns, determine what is causing the carbon build-up; leaking valve stem seals should not be overlooked.

FUEL

The 3.8L V-6 engine is designed to operate on fuel with an octane rating of 87, anything less can cause engine spark knock. (See Owners Manual for an explanation of octane rating and high altitude octane rating.)

ENGINE TIMING

Excessively advanced engine timing can create an engine ping or spark knock condition. This knock is the result of premature combustion working against engine rotation.

ITEMS AFFECTING ENGINE TIMING

o Engine base timing too far advanced.

o Park neutral/back up light switch out of adjustment.

o Abnormally high engine operating temperature.

o Check for proper operation of: Coolant sensor. MAP sensor. BARO sensor.

Correct distributor wave washer.

Windshield EST harness must be routed at least one inch from any spark plug wire.

Check for correct camshaft end play, button and spring must be in proper location.

LEAN FUEL MIXTURE - EGR SYSTEM

The EGR system is designed to introduce exhaust gases through the intake manifold into the engine cylinders. These exhaust gases reduce the combustion temperatures which in turn reduce the oxides of nitrogen. Improper EGR operation can cause a lean air/fuel ratio which can cause high combustion temperatures. These high temperatures cause the mixture to ignite prematurely which creates spark knock.

When diagnosing the EGR system, the brake switch should also be considered as a cause. Misadjustment of the brake switch may activate the EGR cut-out relay and prevent proper operation of the EGR valve.

A complete EGR system diagnosis can be found in Section 6E1 of the 1982 Pontiac Service Manual.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.