I just bought an '89 745 turbo. At present, the car runs quite rough - the plug wire to the 3rd cylinder (right above the turbo's connection to the exhaust) will burn up after 5 minutes of driving. Bosch Plugs looks ok, wires are the cheap "carquest" brand.

By "burn up" I mean that the rubber socket at the spark plug end will get really hot, and then combust. I'm planing on buying a good set of plugs a nd wires from NAPA tomorrow, but this seems like a really wierd failure. Any thoughts?

NT Moore

From the posts, you have an exhaust leak around the turbo that is blowing hot exhaust on the wire to melt it as it is the only one being melted. Your coil wire gage on the high voltage side cannot sustain enough current to melt it, and again, only on three, why not the others? Thus if it is number three, it is about where the turbo bolts together, so my prediction is you have a crack or pinhole type hot exhaustleak from the turbo assembly leaking with more pressure as the engine revs.
You can test this with a donor wire from your burned up set by putting foil around the turbo area under the plugs and see if it melts. If the foil melts, which an exhaust leak can do, you'll know for sure.
Good luck

The bit about the exhaust leaks sounds reasonable - the car sounds like an old tractor when it runs - lots of loud, un-muffled puffing whose frequency changes with the engine RPM's. Unplugging eack spark plug wire individually while the car runs demonstrates clearly that all 4 cylinders are firing.

Here are two twists - first, the 3rd sparkpluge doesn't twist into the socket in a smooth way. The other 3 plugs are easy to screw in and have a very definite "all the way in" position. By contrast, the third plug, once seated, doesn't seem to reach a fixed point where easy turning stops (although it becomes progressivle harder to twist, to a certain extent it feels like I could continue screwing in the plug indefinitely).

So, it seems to me like the 3rd spark plug socket is at least partially stripped (and could perhaps be the source of the blow by, racket, and high-resistance/high heating - like a toaster element - that's melting the spark plug boot). Does this explaination seem at all reasonable?

Here's the second twist, while I have a garage to do these repairs in, that garage is 150 miles away (in Winona - the car and I are with family in Minneapolis). Given the condition, how bad of an idea is it to drive the car back to Winona before starting to pull the head, replace the exhaust gaskets, check to see if the turbo needs to be rebuilt etc.

Finally, at my local junkyard, there's an '88 740 turbo in pretty good condition. Could I pull the exhaust manifold, turbo, and head off of the 88 and have them fit on my 89 740?

NT Moore

Sounds like you're correct about the spark plug threads being stripped. The plug could pop out at any time so your proposed tip is very iffy.

I pulled the plug today and a friend put a Napa "heli-coil" replacement sparkplug socket in the head. A second problem - along with the plug socket is that there's a leak in the exhaust manifold gasket above the 3rd cylinder - thus the tractor noise and burned plug wire. Next on the list I suppose is to replace the exhaust manifold gasket.

Thanks for all the coments!

NT

Is it melting from the inside out
or outside in.
I would think if the core was the start of the heat it would be disintigrated.
however if an outside sourcr of heat is melting the insulation and the core is reativlly in tack then you know where to start looking
maybe a plug that is creating too much resistance.
mho

I assume the heat is coming from the electrical current. Try changing the plug, and the wire for the #3 cylinder, and see if that makes any difference.

jp

Were the heat shields removed by PO?
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Post Back. That's whats makes this forum work.

No, the heat shield between the turbo and plugs is still there. It was only the 3rd cylinder (counting from the radiator) that had the problem which seemeds really wierd. Last night I did a compression check and all cylinders were 115-130psi. Surprisingly, the car will run with one of the plug wires completely removed.

Are all of your manifold studs present and accounted for? On my 760 turbo (same under the hood as your 740), one of the studs broke, and the resulting leak melted the wire. Hope this helps. Though, it may just be an exhaust manifold gasket leaking.