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Based upon an age old diagnose of hearing a problem with a carburated vehicle, and other experiences,(the "bog" sound under full throttle), ignition timing is off (late)and is not responding properly contrary to what the base timing light indicates.
Causes:
Ignition wires not in the proper sequence. Certain engines will run but with little power if two ignition wires are swapped. If close enough in the timing cycle to each other, they fire at or near the bottom of the power stroke or on the exhaust stroke and does not backfire. Funny to watch on the "studs" eight cylinder GTO's and SS's as they just suddenly come out to race and have "NO POWER". Shows my age doesn't it? The engine is actually running on two and a third cylinders if four cylinder.
Has the flywheel been removed and replaced lately? Like when the short block was replaced? Flywheel off by one bolt or backwards would cause a lot of problems.
Are there two holes on the flywheel very close together for the crank position sensor and the wrong one is being used? One caveate of multiple application parts (auto and manual trans flywheel with multiple bolt holes for differently mounted pressure plate or torque converter bolt positions and crank position magnet or stud on flywheel ). Part of this: is the pressure plate or torque converter mounted in the correct holes. Or wrong part altogether. The later particularily applicable if parts not obtained from Volvo savvy parts suppliers.
When this Unsolvable Problem showed up may mean you have to pull the tranny to correct the problem.
How does the timing react when throttle opened. Timing light with advance adjustment required.
The plugs with white center indicate a lean fire condition, the black ring indicates a rich, incomplete burn cycle. ie, Fuel being burned late in the power stroke or after the exhaust valve is open. Late ignition timing.
The ECU via the O2 sensors is leaning the mixture out after they enter the cycle because of the excess oxygen from the interupted incomplete burn after the exhaust valve opens. Good gas milage with lean burn indication on the plugs.
If the flywheel and everything else is on the money, the ignition portion of the ECU is at fault. Verification via diagnoses step by step with a Volvo Service Manual would be well worth the 20 or so dollars for the manual. Considering what has been spent so far.
You need to find a service tech who is not a parts swapper to isolate the problem. In other words, they need to know what they are doing. Hopefully who you have been using is not ASE Certified since this is classic timing related and should not be mistook for anything else.
Duane
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