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I am puzzled by my rough-running 1988 740 turbo wagon. The first mechanic that I brought it to diagnosed it as having a thrown rod. Not satisfied (because I really like the car) I took it to another mechanic who proclaimed "bad headgasket." Thinking I had nothing to lose, I bought a couple of bottles of Bars Leaks to see if that would help. First, I drained out about a gallon of coolant so that I could be sure that the stuff would actually get to the engine and not just sit in the expansion tank. I shook it up, poured it in and then filled the system back up with new coolant and proceeded to drive it home very slowly (that is as fast as the car will go.
Well the Bars Leak had no effect at all that I could tell. The car still ran rough and had next to no power. Next, I decided to install the new set of spark plugs that I bought several months ago and never got around to installing. When removing the old plugs, I noticed that the electrodes of each one was covered with carbon. I put the new plugs in and the car idled smoother but still has no power. When I floor the engine it goes faster but knocks and puts out dark smoke from the exhaust.
Any ideas? It seems like the thing is running rich. I don't know that I buy the head gasket theory because the car really hasn't lost any coolant (before or after the bars leaks) and because none of the plugs showed any evidence of exposure to coolant (I understand that a spark plug exposed to coolant looks "steam cleaned"). I don't buy the thrown rod theory because by now (I have driven it at least ten miles since the first diagnosis) I figure something would have pierced through the block.
I was tempted to buy the gasket and install it but don't want to spend a whole weekend fixing the wrong problem.
Your advice is greatly appreciated. I would love to keep this car.
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