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I knows sumpthin
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We've all had it and all been scared to death that we've drastically damaged out motor, and it usually comes out of the blue...all the more reason for worry, well in all likelyhood it's a relatively simple fix and most likely a cheap one too. It's sputtering / misfiring under boost. The car may run perfectly fine in vacuum, but once you get preassure, the problem presents itself. Here's what to do...

First, Visually inspect your spark plug wires...any wire exposed will be a big problem. Next take a solution of Salt Water (table salt will do) and spray it on the plug wires with the motor running...it doesn't take much. And it's better accomplished in the dark where you'll be able to check for arcing much easier. If you do see lots of arcing, and every time you do see one you also hear the motor's idle slightly change, you know it's time for new wires.

If no problems there, pull your plugs, and using a Spark Plug Gapper, check the gap. It should be between .039 and .044 If they're fine, good gap, no damage, no abnormal build up then check for cracks on the ceramic portion of the plug...this could also cause a problem.

Next, if no problems...Check the Cap and Rotor. Contact points should be relatively clean, free of oxidation and build up. While you're at it be sure all wires are secure and cliped in, including from the coil to the cap.

Next, boost leaks. Most likely is the TB boot itself. Take it off and inspect it. Cracks often happen right at the clamp where they're near impossible to see with the boot installed. Flex it a bit, and look carefully. Even a relatively small crack will cause problems. And dont' try to fix it...won't work...I've tried many ways of doing this...Electrical tape...duct tape...RTV...combinations of all...They all fail.

Continue checking for leaks...carefully check all rubber hoses and clamps including turbo inlet and outlet hoses. If the inlet hoses are loose this can also cause a problem.

If no problems found, further diagnostics are in order...CEL? If so, what code....If none...Compression test may be in order, as well as possibly checking out the Cat. :)

Laters
Ryan
 

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I had this problem for ages, I have it fixed now. It was ignition related.

My other suggestions would be
1. Dont throw away your old original leads if you fit new ones which you think are better unless you have evidence to suggest the old ones are no longer functioning correctly, e.g. high resistance. I bought a set of new leads just because the originals were old, it then developed this misfiring problem.

2. Check that your leads are not in contact or will not come into contact with anything metal. All the clips which hold the leads got broken by the new leads mentioned above so the leads would hang slack, but not visually touching anything. When the bonnet was down and the car came on boost the engine would move on its tired mounts and cause the leads to contact with various metal objects, these included the heat shield, bypass bracket and the coil lead on the IC piping. Im not sure which of these caused the problem, but replacing the clips actually prevented the misfiring and it has been fine for the last 6 months.
 
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