knocking at low rpm

PONTIAC TRANSPORT
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LIL_LARSON77
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97 pontiac transport, 145,000 miles, 3.4L. there is a loud knocking at low rpm. when you rev above 3000rpm, it smoothes out. we have checked the plugs, wires, distributor, coil pack, computer, ignition module, starter relay. everybody i talk to has a different diagnosis-even mechanics. i am thinking we are either on a thrown rod or stuck valve. any other ideas? i am trying the small things first before i replace the whole motor. about a year and a half before this problem started, we sent it to the shop to repair a blown head gasket. this problem started in a instant, it was not a slow progression.
Nov 4, 2007 at 7:03 PM
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BIGO
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If I were you id go to a place that works on deisels like a smaller outfit.
youve got something fueling it too much than it should black smoke is unused fuel. not sure what is loose or is sticking. just a guess
Aug 23, 2007 at 7:43 PM
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JWALTH02
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Couple of things first;

Is it automatic or manual?

By in gear do you mean (stick shift) when you shift from neutral into 1st or whichever gear? And if so is it only certain gears or any and all?
(automatic) does it do it while in park and also when you switch to drive? Does the car lurch forward when this happens?

You said after about ten minutes of being on, does it matter if it's hot or cold? Does it happen everywhere you go? (elevation)

Just need a little more info about the problem and we can narrow it down.

And yes bigo you are correct, the black smoke is unburned fuel coming out the exhaust due to the over-revving and quick decelleration of the engine.
Aug 24, 2007 at 2:18 AM
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MIKE H R
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What it sounds like is that with the engine ideling a lifter is not getting enough oil and when you rev it the oil pumps up to correct pressure letting the valve work properly.
Nov 6, 2007 at 9:51 PM