I bought a 1997 E320 that had overheated before.I got the car checked out, replaced the head gasket and water pump due to damage from the overheating.Now,when I start the car,it knocks loudly and I have to pump the gas pedal to help it start.It does start all the time when cold with the knocking still there,then when the engine is warmed up the car does not turn on all the time when hot,it will only turn on sometimes.The RPMs bounce up and down and sometimes drop to 500 to 1000,but the car never shuts off by itself it will always stay running,however the knocking sound is still there.The engine temperature stays perfect the entire time.Any ideas on what this could be.

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1998 S500
2006 E320 CDI
1963 220S
1994 S500
1995 S500
2005 BMW 545i sport
2007 GL320 CDI
2007 ML320 CDI
1991 Toyota Previa White over Blue
2000 Lexus RX300 White over Beige
2008 Audi Q7 Black Opal over Grey
1993 Toyota T-100 Silver over Grey
2002 Chevy 2500 Express White over Grey

I would say timimg marks were not correctly lined up on head gasket repair (and head not magnafluxed to check for cracks). After that id sayrings and bearings depending how much water got in oil after head gasket failure. You took a chance with just replacing the head gasket. Depending on whats wrong now a junkyard mototr may be a good bet.

the cost of just finding a junkyard motor vs. spending time on what you have now would most likely make sense.

It sounds like a toasted bearing, not good.

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)

Were it a classic, it would be important to keep that engine to maintain serial number integrity.

Since it is not, finding a used engine is your best bet. It would require a complete tear-down and examination of all internal parts and replacing all the damaged bits. Unless you have the time and tools to do the work yourself -- and have a line on discount parts and machine-shop rates -- trying to fix yours will cost you lots more than a good used engine.

Good luck.

Is it possible it's just the valve timing and that's it?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Not with the knocking. That means a bearing either on the crank or pistion is loose. The play when it fires is from the combustion pushing it fully the other way from the compression stroke. The kind of timing it would have to be out would be herendous and shouldnt ever start to fire after or before top dead center. Its 1 in a million but the torture you would put on the motor driving it that way at all would call for bearing to be redone.