It sounds like.....coconuts.....

[MagicRat]

Hi, folks,
1987 Ford Mustang, 2.3 four cyl. auto trans, 242,000 km.

I know it's difficult to diagnose noises here, but I have a strange one.
This engine makes a moderate knocking sound when warm. This noise has been present for the last 6000 km, and has only gotten slightly worse.

It only occurs between 1000 to 2000 rpm, when under load at light throttle.
I hear it most when driving at low speed around town. If the revs pick up or I give it more gas, the noise goes away. Since it's load dependant, I think its something in the reciprocating assembly only.

The knock is at crankshaft speed, so it is not preiginition.

Also the knock sound unusual, it sounds like someone tapping 2 hollow coconuts together, it does not sound like a bearing knock or piston slap.
I have had engines with those kinds of noises before.

What is does sound like is my old Chevy 350 with loose rod bolts where the piston started to slap the underside of the head. Of course that engine destroyed itself in minutes. This noise has been around for a long time.

It is also not present when the engine is completely cold. It takes about 5 min or running before it appears.

I wonder if its either a loose wrist pin (too much clearance in the pin bore)
or carbon knock, where a lump of carbon is causing a clearance problem. However, wouldn't those make noise when cold too?

Any thoughts?

[HeavyJ]

While running at idle, pull one sparkplug wire at a time and see if the knock goes away. this usually works on a bad wristpin at idle, but could tell if you have one at your rpm range or at least narrow it down to which cylinder is making the noise. Could possibly chock the wheels, set brake and put in gear, have someone power it up to the rpm range while you listen with a piece of hose or mechanics stethascope to pin it down.

[Steel]

Originally Posted by MagicRat

Hi, folks,
1987 Ford Mustang, 2.3 four cyl. auto trans, 242,000 km.
Any thoughts?

There's your problem! You somehow lost half if your motor! Find the other 4 cylinders and you'll be straight

[MagicRat]

Originally Posted by Steel

There's your problem! You somehow lost half if your motor! Find the other 4 cylinders and you'll be straight

It has occurred to me that as soon as this thing ventilates the block with the rod, I can get my 351 V8 installation project going!!

[mazdatech177]

ive had a thrust bearing make a similar sound on an engine before. well a lack of a thrust bearing.... too much clearance

[curtis73]

I've had a damaged piston skirt sound like that before; like a really low-pitched piston slap.

[Moppie]

Originally Posted by curtis73

I've had a damaged piston skirt sound like that before; like a really low-pitched piston slap.

Iv had a broken ring land make the same sound, but it was also accompanied by a chuffing sound as it sucked air around it.

[MagicRat]

Thank you for the help, guys. Excellent suggestions.

[GreyGoose006]

check that the flywheel is bolted on securely.
i had an engine knock that i suspected was due to a bad lifter.
turned out the flexplate wasnt on all the way.

whew. that was easy.