I have a 1990 GMC Sierra with a professionally built 350 TBI/700R4. When engine is at operating temperature I have a knocking sound that seems to come from left side of engine. When truck is placed in gear the knock goes away even when RPM's are raised. If it is in idle in Park or Neutral it makes the noise. When in gear under a load it does not make it. I have great oil pressure and I have checked torque converter bolts. Any help is greatly appreciated!!
I have a 1990 GMC Sierra with a 5.7 350 20,000 miles that knocks!!!!!?
- Thread starter Donald M
- Start date
Molson02536
New member
There are two distinct types of rod knock. The gudgeon pin end or wrist pin or little end bearing (as you prefer) makes a light metallic clack. Often you can hear the double clack as the connecting rod reverses the piston direction.
The main connecting rod bearing or big end makes a loud deeper toned knock and is very bad news. Engines can run for a surprisingly long time with little end knock. Big end knock is usually rapidly terminal.
Disconnecting the spark plug wire (and carefully grounding it in the case of high energy ignition!) and then running the engine will cause almost all little end knocks to disappear. The main source of little end knock is the combustion event hammering the piston down onto the bearing taking up the excess clearance with a clack. If the wrist pin is really loose then disconnecting the spark plug can change a double clack to a single as the lightly loaded piston reverses direction.
Disconnecting the spark plug rarely makes big end knock disappear altogether.
The really troublesome thing about connecting rod problems is if it does let go you need a new block so the risk of continuing to run the engine is quite high. Always run an engine at low rpm, 1500 to 3000, if you suspect rod knock of either type. Not generally known is the fact that inertia loads on the TDC reversal of the piston on the exhaust to intake stroke is when loads on the connecting rod peak due to no downward pressure on the piston face and high speed loading of the bearings.
The noise a connecting rod makes can be similar to detonation (the cause of the sound is similar, the cylinder rings with the banging of the piston on the rod bearings in the one case, and with the sound of the detonation wave hitting the piston face in the other) but a shot connecting rod should make noise at idle. Also, changing ignition timing, or increasing octane will cause pinging to go away, but a worn connecting rod will continue to knock!
Have your engine oil anylized (most Lube shop like shell)to determine any internal engine bearing wear from a dry start or wrong torque to the bearing caps.
