I have a 1999 4.2L 6 cylinder that started shaking a few days ago. It shakes a little at idle, really bad during acceleration. Check engine light flashed then turned off initially. I replaced my cop today but it didn't fix the problem, and afterwards check engine turned on and stayed on today with code P0171 and one other one that I can't see because my scanner is broken, I assume P0174.

Some people are also saying spark plugs, I replaced those 10,000 miles-ish ago, could that be it? Any help would be appreciated, trying to avoid taking it to a mechanic as last time I got raped with a rusty oil dipstick.

 

Bad or dirty Mass airflow sensor or upper intake. Pretty common in the 4.2's.

 

Could that cause the shaking, or would it just set off my check engine light?

 

Yes, shaking.

 

Check the vacuum hose at the back left of the engine (pass side). You can't see it, so reach back and feel if it is soft. If yes, remove it and look for cracks. I used fuel line to replace mine.

 

Ethanol is vaporizing oil if you don't install Air/Oil Separator on the PCV side your intake could probably very dilluted with oil by now. It will more looks like exhaust manifold than intake manifold.



Try Driving the Vehicle with the MAF Sensor or harness disconnected. If this did not fix the problem, it could be a problem in your fuel system.

Before doing this make sure the Brake Booster is not Leaking. Are you sure the Engine is reaching
normal operating Temp?

 

It turned out to be a spark plug after all. The code that I couldn't read was actually a misfire on cylinder 2 from a fouled spark plug. Looks like there's some oil leaking into that cylinder and fouling the plug in a hurry. In any case, the truck is running fine for the moment. Thanks for the help.

 

Let me Guess where the Oil comes from. Remove the Intake Tube connected to the throttle body and open the butterfly valve to see if there are traces of oil inside the intake manifold???

If the Spark Plug is not firing correctly the code will be P0172 or P0175 not P0171. A single Loosely Installed Spark Plug cannot caused both side of the bank to be lean. Did the Mechanic Loosely installed some of the plugs? I believe 4.2 are using Ignition Coil Pack & Spark Plug Wire not CoilOverPlug.

What Spark Plug Company are you working for?

 

I apparently have a leak on the lower intake gasket too which caused the p0171, but the check engine light didn't turn on until the spark plug fouled on cyl 2 and misfired a bunch of times. I promise I'm not a rep from "big spark plug", just a kid that doesn't know a ton about engines haha

 

I would not recommend driving the truck without a maf. Check your sparkplugs and pull the injectors and make sure they are clean. Your description of the problem sounds like a misfire. Probably want to change your fuel filter too if you haven't done that in a while.