Back when the valve cover gasket leaked oil into the spark plug wells, the oil must have carbonized-created a weak track to ground so the spark plug only fired on the exhaust stroke. On the intake stroke the internal cylinder pressure is higher and requires more voltage to fire the spark plug=the higher voltage went to ground.
There was a black line down the porcelain insulator and a faint line inside the spark plug boot.
It took 1 new (used) spark plug and one spark plug wire but I installed a wire set.
Yes, more compression (compression stroke) requires more voltage for the spark plug to fire. I had a spark plug tester that when plugs were tested under air pressure, it took more voltage for the spark plug to fire the higher the air pressure was.
2008 Aveo 5 speed, no A/C, no Cruise control, no electric door locks, just a radio, heater, and rear window heater.
Current Problem The engine bucks under very light load from idle up to 1200/1400 RPM when accelerating. Under moderate load when starting from a stop, the engine bucks from the beginning speed up to about 1600/1800 RPM. If the throttle is opened almost all the way, the engine bucks up to about 2100/2200 RPM.
I had my Zurich ZR-13 OBD-II scanned connected in graph mode and set on various live data points, such as short fuel trim, RPM and timing advance that seem like it might give me some clues as to what is happening when the engine bucks.
1.RPM was just to see in what range the bucking was happening.
2.The short term fuel trim I can't say if it was normal or not but my guess it wasn't out of range.
3.However, the timing advance graph correlated almost perfectly with the bucking. Sometimes the timing was advanced say +10 degrees and then went to -5 degrees, back to +15 degrees, back to 0 degrees and when the bucking stopped, the timing went to over +20 to 35 degrees advanced.
In other words, when the timing advanced changed back to around zero and advanced a little bit, the engine bucked. When the timing went and stayed over +15 degrees, the engine ran smooth.
I have a 2009 Aveo-5 similarly equipped and the timing advance does not bounce around much and the engine runs smooth.
Two years ago I replaced the timing belt, water pump, pulleys, spark plugs (Iridium), valve cover gasket (oil in the spark plug wells) and did some other maintenance. This is when I first noticed some bucking. I also had a crankshaft -camshaft error code. I re-installed new copper core spark plugs and replaced the camshaft position sensor. The wiring and spark plug wires all tested good. The bucking decreased to a point it only happened on wide-open-throttle starts from a complete stop. I have another thread on this forum about that repair and issues. Shortly after the timing kit was installed I got a throttle body code and replaced the throttle body and cleaned the EGR crud from the intake manifold. I also rechecked the timing belt synchronization 4 times and know for an absolute fact it is correct.
Recently the bucking got worse as I said above. there have been NO recent codes since the timing belt kit was installed except the MIL/check engine light never came on when you start the engine, like it is supposed to work. The solder joints in the center cluster were cracked so I re-soldered the resistors and cleared the code for the MIL and seatbelt lights not working. BTW, thanks to the other Aveo members for their auxiliary information center repair information. It was an easy fix.
I sprayed carb cleaner on all of the vacuum joints and hoses with the engine idling, looking for a vacuum leak. No change in engine RPM when I sprayed everything I could get to. If there was a vacuum leak, the RPM should change if carb spray entered the leak. I inspected the large diameter rubber air tube from the air cleaner to the throttle body and didn't find any leaks or holes. I re-tested the sparkplug wires and they test 5K ohms.
Next I filled the tank with no ethanol premium because maybe the engine knock sensor was hearing a knock I couldn't hear. The engine still bucked.
I changed the Knock sensor and still had premium fuel in the tank. It still bucked.
I didn't have any codes but I decided to change the camshaft position sensor again just because it seemed to decrease the bucking 2 years ago. It still bucked.
I have a Tech-2 replacement diagnostic device** that connects the OBD-2 port to a laptop computer and it can do a "re-learn" procedure for the crankshaft position sensor. I did the "re-learn" and it still bucks.
**VXDIAG VCX NANO OBD2 Support GM/OPEL GDS2 & Tech2win Diagnostic Tool Code Scanner
We need a whole area dedicated to this tool. It replaces a very expensive diagnostic tool from the past. It isn't a scope but it is reasonable cost wise.
Here is a link describing what the VXDIAG VCX NAN is used for.
I had a spare crankshaft position sensor so I installed it. The engine still bucked.
I had a used ECM-computer from another 2008 Aveo similarly equipped, installed it and the engine still bucked. I tried a coil from another Aveo and the engine still bucked.
There is no change after the engine warms up. With a cold engine the bucking is less noticeable.
I did a search on the Aveo forum for bucking and manual transmission surges and it seems no one fixed their bucking or power losses.