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Small Block Chevy Angle Plug Combustion Chambers - Ask Marlan

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• Sources:
Air Flow Research
Dept. CC
10490 Ilex Ave.
Pacoima, CA 91331-3137

Isky Racing Cams
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16020 S. Broadway St.
Gardena, CA 90247-0803

Check Engine or Checkmate?
I have an '86 Buick Grand National. When I step on the gas pedal the car stalls or stutters. It does not happen when the car is cold, just when the temperature is at or about 160 degrees. It has nothing to do with the transmission. I changed the spark plugs, the wires, the coil pack, and also the temperature sensor. I also checked the fuel rail and it is getting sufficient fuel pressure. I noticed that the "check engine" light would come on, and it returned a code which translated to the "boost control solenoid." I changed it for an AC-Delco unit and it still continues to do the same thing. The "check engine" light still comes on only when I drive on the highway, when the turbo is building up boost. Could it be the module? Why is the "check engine" light continuing to return the same code?
Mohendra Ghamandi
Brooklyn, NY

First, a rule of thumb: When a code is set, it's not always the device itself that's faulty-the hoses, electrical connectors, and wires that comprise the entire circuit controlling the device could be the actual culprit. For example, there are lots of vacuum hoses on a turbo car, and with age they eventually crack.

A bad solenoid, or leaky vacuum hoses from the turbo wastegate to the solenoid, would cause an overboost condition, in turn resulting in momentary detonation-then the car falls on its face. But that does not sound like the symptom you describe.

So, chances are the primary performance problem is not caused by the code your diagnostic scan is reporting. There is a MAP sensor that monitors boost pressure and also displays boost on the cluster. This sensor doesn't set codes when faulted, but can cause trouble when it does. Possibly it is skewed and feeding the ECM the wrong boost data.

If the stutter is only a transient problem (just a second or so) the TPS (Throttle Position Switch) could be the culprit. It may be slightly out of adjustment, but not enough to throw a code. On a digital voltmeter, the '86 turbo Buick V-6 TPS should read between 0.35-0.45v throughout the entire throttle range, with a 0.40v plus-or-minus 0.05v curb idle adjustment spec.

If the TPS is OK, check the fuel pressure again under a load. It should increase by about 1 psi for each pound increase in boost.

Inspect the EGR valve. It does not deploy until 160 degrees F, which is why you may not have a problem until the vehicle warms up. Once again, a slight EGR valve malfunction may not necessarily throw a code. For example, a diaphragm spring going "soft" could cause premature valve opening, resulting in a tip-in stumble. Disconnect the hose going to the EGR and see if the problem goes away. There is also a serviceable EGR valve filter; it can't hurt to replace it.

If a scan tool is available, check the O2, coolant temperature, and Mass Airflow sensor outputs. Also recheck the coolant sensor (ECT); that circuit may still have a problem. If the O2 sensor is sending a false rich readout the ECM will continue to lean-out the fuel curve when in closed loop. Disconnecting the O2 sensor can disable closed loop long enough to see if the problem takes a hike. If a scan tool isn't available, disconnecting the ECT or MAS sensor will send the system into back-up mode, possibly rectifying the condition temporarily.

If the above doesn't cure the problems, then yes, it's probably a bad module.

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