Hi guys I have a 99 accord ex 2.3L .

Mechanic replaced head gasket. Now I have a check engine light and there's a frequent knocking sound- you hear a distinct "plop" every few seconds. The sound is accompanied by jolting feeling (when you're sitting in the driver's seat) and you can see the actual engine jerking. This happens while it's on park and especially when you hit the gas pedal lightly. I don't know if it happens on Drive, since I haven't driven it around since the car started acting funny.
Let me also mention that while it's on Park, the engine sometimes revvs up and down, bouncing between 1k and 2k RPM's endlessly. Mechanic says it's due to bad battery, and I agree it is bad. Also, the flex pipe is bad- it has a hole and it has a leak. But that might be beside the point.

Mechanic says the check engine light is natural for any car which had engine parts changed (I find this hard to believe). Last week, I took the car to AutoZone and did a free diag. I got 4 codes, all engine related, and any one of them can be causing the code.
He says the knocking/jolting is coming from old spark plugs. I have no doubt that the spark plugs are old, but what concerns me is that the knocking only started coming after he replaced the head gasket. (Again I find what he says hard to believe)
He is charging me $95 (parts+labor) to change the spark plugs.
Also I can't do any DIY that you guys recommend on the car right now, because he has my car right now since he is gonna replace the spark plugs tomorrow.

I'm getting second thoughts right now about the cause of the knocking, and I would hate to spend so much money on something which won't fix the problem. Again, I want to stress that the knocking came only after he replaced the head gasket, so please help me from that perspective.

Guys what are my options? Please help!

 

Your mechanic sounds like an idiot for one, and two - a thief. $95 to change plugs is ridiculous since they're probably going to be aftermarket OEM replacements...probably about 10 dollars for the whole set, and it'll take him 5 minutes to switch them out. Just my opinion...

If you could list the codes, that would help. But, the jerking, jolting and the sound you're hearing seems like a misfire from the way you described it, but it's hard to tell without having the car in front of me. Numerous things can cause that(bad plugs, bad plug wires, basically anything that can cause a loss of fuel/compression/spark in a cylinder), but a bad battery isn't going to do that since it's only used to start the car(seriously, go outside, start it and take the positive terminal off the battery, car will run perfect off of the alternator if it's healthy). As far as the 1k - 2k idle surging, sounds like a vacuum leak, which there's a pretty good possibility of that if your mechanic is as retarded as you made him sound. Probably left something unhooked somewhere. Also, I'd go pull the dipstick and look at the oil...if it's milky looking, he didn't do the headgasket right.

Edit: Just saw the line where you said he still has your car. Do you trust this mechanic? Have you used him before?

 

95$ Per Hour Standard Shop Rate... That's book time typically.

Sounds like when he put the timing belt back on hey may not have aligned it correctly. Could be out a tooth. Which leads to timing be completely off, or just off enough to be intaking, and exhausting at incorrect times.... Firing incorrectly, and if you don't get the timing checked, you may just blow the motor up, i've seen it happen more than a few times. With stretched belts, timing belt not correctly timed.

That's the first thing to look at after replacing a headgasket.

 

I'm guessing book time for changing plugs is 15 minutes. So $25 labor plus $20 for the plugs, if he charges double his cost. Less than $50 in any case.