On a 98 Mazda 626 GF 2L I'm working on the engine seems to shake / wobble for about a second / second and a half when going to WOT from any starting rpm. It also almost sounded like maybe one of the belts was even momentarily slipping. If I accelerate gently everything is smooth and there is no shake or wobble. It's an automatic and I did this all in park. Not sure if this is related or not, but the idle seems to be low and a bit shaky / erratic. Adjusting the idle air screw doesn't seem to have much effect one way or the other.
I'm assuming this to be abnormal, as I don't experience any shake going to WOT on my other car, a 99 Almera which also has a low occasionally erratic idle and even stalls a few times a day ( haven't had time to work on it yet ).
So I just did a cigar smoke test, and I noticed that smoke looked like it was coming out the number three injector. On closer inspection, it looks like the smoke was coming out where the grommet holds the injector in to the manifold. I also noticed what looks like a dark stain around one side of the grommet ( I shined a light on it and it's definitely not a shadow ):
If smoke can get out at the injector grommet, I'm assuming air can get in.
I also took out the number three plug, and upon close inspection one side of the plug seemed to have a yellowish hue to it.
Could this be the source of my low rough idle and the shaking / wobbling when pressing down hard on the gas? Maybe the excess air is causing that cylinder to run lean and hot, and causing the yellowish tint on the plug?
So I got a scan tool. Yippie! I fixed the small air leak from the #3 lower injector seal, and couldn't see any other air leaks ( doesn't necessarily mean there aren't ), but still have the same problem. I reset the ECU and logged a 16 minutes drive. During idle and cruise STFT stayed at zero, but during moderate or heavy acceleration ( which is mushy at first, then solidifies after a second or two ) I see STFT go around 9% - 14% then return to zero at cruise or light acceleration. Engine load seems to be over 60% during these injection spurts. The engine seems smooth at higher rpm while cruising. LTFT stayed at zero the whole time.
At the end of the drive I ran the car's internal "Key On Engine Running" self test which didn't show any errors
The 98 US WSM lists the following possible causes for "Idle speed too low and excessive engine shake":
• Air leak from intake air system parts • Spark leak from hightension leads • Improper operation of IAC valve • Erratic or no signal from camshaft position (CMP) sensor • Erratic signal from crankshaft position (CKP) sensor • PCV valve malfunction • Restriction in exhaust system • Inadequate fuel pressure • Fuel leak from fuel injector • Engine overheating • A/C system improperly operation • Purge solenoid valve malfunction • EGR valve malfunction • Low engine compression • Poor fuel quality • Air cleaner restriction • Electrical connector disconnection • Fuel pump mechanical malfunction • Fuel injector clogged • Vacuum leak