Strange knocking

[FormulaNascar]

Hi guys. My 2.3 1989 is making a strange knocking. It is too fast to be a rod knock. Also, it doesn't knock under load. It's only noticeable at low RPM. Any ideas? I have experienced a somewhat high idle recently but other than that and the knock it drives great. Thanks.

 

[tomw]

If it is 'too fast to be a rod knock', then it has to be an accessory driven item, or the flywheel/flex plate/torque converter flopping around.
To check accessories(that rotate faster than a connecting rod==crankshaft speed), try using a piece of tubing stuffed into an ear to localize the sound. Move the open end around, avoiding things that will amputate fingers, noses, ears, etc, and listen for the noise to maximize. You can also remove the serpentine or v-belt and determine if the noise goes away, or rotate the accessories by hand to see if there is something loose or something with a bad/dry bearing.
The valves operate at 1/2 engine crankshaft rpms, the connecting rods at crank rpms, so all that's faster is driven stuff, at either end of the crank. I think.
tom

 

[FormulaNascar]

The strange part is that it's intermittent. I just drove the truck home from school and the noise is gone. But when I drive to work or school in the morning it will likely happen again.

 

[RonD]

manual or automatic?

I assume the knock frequency changes with engine RPM regardless of speed RPM, i.e. shift to neutral and coast doesn't change knock frequency

So drive to school is uphill there and downhill back, or.................??

Engine and outside temp is colder on the way to school than when coming back?

 

[FormulaNascar]

My truck is a manual. Also the knock doesn't seem to be dependent on the weather. Although it is a bit more frequent once warmed up. The strange part is that it doesn't occur under load. It happens randomly. More often once I come to a stop.

 

[tomw]

A lifter/support that has too much clearance or has a flake of carbon holding the check valve open will knock and make more noise than you expect on a Lima. They really RAP when the lifter/support is collapsed or semi-collapsed.
One fix that old timers used for sticky lifters was to do an Italian Tune Up. Rev the engine to hopefully dislodge the chunk of carbon or ???. If a manual, what rpm do you upshift at? If low, you might consider running it up a bit now and again.
tom

 

[FormulaNascar]

I'm pretty easy on this truck considering it has 300k on stock engine and trans. I'll run the guido tune up tomorrow and see if the noise lessens. I plan on rebuilding the top end with all new gaskets as well as a manifold gasket too