Engine Knocking and Idling Rough

My truck: 1996 Chevy Z-71, 5.7L 350 Vortec, 213,000 miles

First time poster here. I was driving down the highway a couple of days ago, after having driven the truck for over an hour, when all of the sudden I start to hear a bad knocking sound coming from the engine. At the same time I lost a lot of acceleration power. I was able to drive it home and park it in my driveway. I turned the truck off and popped the hood. Since I had just replaced the belt, I thought something had slipped but when I looked, everything was fine. I got back in, it started up just fine and would idle in park just fine. However, when I put it in Drive or Reverse, it idles very rough. All of my gauges are reading normal (oil pressure, temp, etc). I let the truck cool down and checked all of my fluid levels and everything is normal. I started it up again and one thing I noticed right off the bat was the smell of fuel.

So, with my experience, I began to believe this may be a problem with combustion. The guy I bought the truck from had just replaced all of the spark plugs, but I am not sure he knew exactly what he was doing. I removed each wire and there is no noticeable scorching on any of the plugs. I plan on replacing all of the plugs today as soon as it warms up.

Do you guys have any other thoughts? From everything I know and have read, it is pointing to the spark plugs and the knocking is the spark jumping when it misfires. Like I said, it doesn't knock when it is idling, only when I start driving it and push on the accelerator.

Well crap. It wasnt the spark plugs. Changed them out and still happening. Any ideas?

you might look into changing the distributor cap and rotor, i've heard of a cracked distributor cap causing a problem like that before. if those turn up fine, i would do a compression test.

Well, it wasn't the cap and rotor.

A good friend of mine who is an oilfield mechanic came over to take a look at it. He drove with me and a couple of things he noted:

1. Only does it when in drive or reverse and when I push on the accelerator.

2. If I put it in neutral and push on the gas, it doesn't knock.

3. Prior to this all happening, I noticed that the tranny was slipping a little around 2nd and 3rd.

He seems to think that based on where he could here it coming from and the symptoms, that it may be the transmission causing this. Possibly a broken flywheel that could be causing the knocking and the loss of acceleration power.

Well, it wasn't the cap and rotor.

A good friend of mine who is an oilfield mechanic came over to take a look at it. He drove with me and a couple of things he noted:

1. Only does it when in drive or reverse and when I push on the accelerator.

2. If I put it in neutral and push on the gas, it doesn't knock.

3. Prior to this all happening, I noticed that the tranny was slipping a little around 2nd and 3rd.

He seems to think that based on where he could here it coming from and the symptoms, that it may be the transmission causing this. Possibly a broken flywheel that could be causing the knocking and the loss of acceleration power.


you may have transmission failure with your symptoms ...

you also have a poor combustion with the smell of fuel when running..

with the plug removal you must check the plug wire resistance..also the cap/rotor/coil as well as the distributor bearing....

you just bought this truck for cheap correct ?

The knocking sound very well could be a flywheel/flex plate but it should rap at idle. Mine was rapping and thought is was the bottom end of the motor. Sounded very similar. Whats stumping me is it don't do it at idle. I'm guessing the torque converter or transmission. The check engine light never came on?

Yes, I just bought this truck for about $3K. I bought it because it already had most of what I wanted (Z71, 350 Vortec, 4" lift). I knew it was going to be a project, so I am not pissed that I am having to work on it.

I haven't tried it in drive with the brake, but I will and see what happens.

Things I have done thus far:

Changed plugs (AC Delco Plats)
Changed wires
Changed Cap and Rotor
Changed oil (with a bottle of Lucas)
Changed belt (gator grip)
Changed tensioner
Changed idler pulley
Flushed radiator
Changed thermostat

The thing that stumps me is that it was running great aside from slipping a little in 2nd and 3rd. Then as I am driving down the highway, it starts the knocking and loss of acceleration power. It seems as though something "broke" though I didnt actually hear anything break. It went from running great to the problem I have now.

So I did what someone suggested and put it in drive, held the brake and pushed on the accelerator and when it gets up to 2K RPM, it starts the knocking. If I put it in park or neutral it doesn't do it. Hmmmmm.....does that change anything or tip anyone off?