I'm looking at buying a 1998 Bonneville. The price is really good and everything is in good shape, except the engine.

The story from the previous owner is that it was running rough, but no knock.

So he replaced:
* crankshaft sensor
* coilpacks
* valve cover gaskets

Now it knocks, the knock increases in volume with the revs, but is pretty mild in the low range 1-2krpm.

He'* asking $750. I just need semi-reliable transportation, nothing else.

Is this worth it? Any theories?

Can you tell if it'* a mechanical? Like a bearing, connecting rod, piston? Or is it a ping like detonation?

I would say mechanical. It does it even at idle. Would it ping at idle?

He'* a guide to help you out. It sounds like there'* a issue with a piston or bearing. If your handy it can be fixed. If not, it can get expensive to have a shop do it.

https://www.gmforum.com/t279317/

If you have any other questions, feel free to ask.

If this is a naturally aspriated motor, it has the potential of having hurt the bearing with an upper intake failure. The typical failure is the area around the EGR stovepipe inside the upper manifold cracks and fills the intake with coolant. This fills the cylinders and then a hydrolocked motor is attempted to be started. As we know..starting there'* no oil protecting the bearings and hitting the brick wall that hydrolocking does is bad.

Being that it'* not running, I'd negotiate. Most non-running cars sell for $500 or less.

It starts right up and runs quite well. That'* what makes this such a dilemma.

It doesn't smoke, doesn't overheat and accelerates strongly. I drove it for 20 minutes this morning. The only apparent symptom is the knocking noise.

Danthurs mentioned pinging. That troubleshooting guide mentioned that pinging sounds like a sharp metallic knocks. That sounds accurate to the sound of this engine.

The PO replaced the coilpacks and it fixed his rough running issue, but revealed this new knocking/pinging issue.

Could this be caused by a timing issue or fuel issue that was masked by the bad coilpacks?

Does it feel like it lakes power? There could be corrosion under the coil packs. There could also be a bad coil in there, testing them would show this. What you really need to do is pull each spark plug and mark each one as to the cylinder it came from. Look for differences in the coloration. Even small differences can tell you a lot. They should all look very much the same, a light gray color.


Look for plugs that are darker or damaged. Post pictures if you can.

Also, when the coils were replaced, did the wires get replaced?

You know..I'd start by checking the wiring.

Starting by facing the car. the coil packs should go in order from radiator to firewall in the order 1/4, 2/5 and 3/6. Check the wiring from each to the cylinders. The below info is where the cylidners are located.

Firewall
2, 4, 6

1, 3, 5
Radiator

Try spraying a light mist of water on the plug wires while the engine is running, then turn off the lights. In the dark you may see arching to ground. A very easy way to find grounding problems.