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Question
I have a 99 GMC JIMMY 4x4 with the 4.3 V6. About 6 months ago I put new plugs in. It got to running rough again and I'm driving it on vacation in a few days so yesterday I replaced the belt and put on new plug wires. Now it's missing, backfiring, running rough and spitting water out the exhaust pipe. After driving it just a couple miles I stopped left it idling and was checking the exhaust pipe and it was burning my hand roughly a foot or more away from the tip. Any idea what is going on?

Answer
Dear Tony,

It's normal for a small amount of water to come out the exhaust pipe if at least some of your cylinders are burning the fuel correctly.  If you are missing and backfiring immediately after changing the plug wires, you should double check the order you put the wires back on.  The symptoms you describe are exactly what it would do if a couple of wires are mixed up.

If it's putting a lot of water out the exhaust pipe and you see some white steam coming out even on a warm day, then the 4.3 may have blown a head gasket, which is a more serious problem.

While driving, the exhaust pipe should get too hot to touch, even near the tip so that's normal.

So the verdict is first check the order the plug wires are installed in, and once that's corrected, check for symptoms of the blown head gasket as follows:

1.  Check the oil and see if there is a muddy look to it on the dipstick, kind of light color and possibly some bubbles, but not necessarily.  Just a lighter color than normal and more opaque, and a little muddy looking.

2.  Open the radiator while the truck is cold (or you know there is no pressure built up inside) then start the truck with the radiator open.  Look for oil floating on top of the coolant, or something similar to the same muddy look you saw in the oil.  Sometimes the coolant remains clean but you just see bubbles coming up through the coolant.  When that is the case, you can rev the engine and you should see a lot more bubbles coming out, bigger and faster.

3.  Look for water coming out the exhaust pipe.  Again, a few steady drops coming out is just a sign of a clean burn in a good-running engine.  A lot of water dripping out fast and white steam coming out is a sign of the blown head gasket.

Look for those three symptoms.  If the oil is clean and light, or clean and dark and looks normal, the coolant is only coolant and is not bubbling when the engine is started cold, and the exhaust looks normal with only drops of water every couple seconds, then I would say the head gasket is good and we will have to talk more to find the initial rough-running problem that happened before you changed the wires.

If you find that you do have a blown head gasket, you will want to cease running the engine at all until you get it fixed, because the water in the oil will mess up your bottom end (rod bearings and main bearings).

The missing and backfiring is a definite sign of a mistake in the firing order when installing the wires.

I'm sorry this took so long.  I took a steady job with long hours and I do get to these questions as often as I can.  I appreciate you choosing me, and I wish you the best of luck.  Please, ask as often as you need.  Thanks again.

GMC Repair

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