My son drove our 99 Neon Sport with DOHC engine home one day and said the oil light was coming on. The car acquired a knock after that so I correlated the two and decided that the number one rod bearing had gone out due to being ran out of oil.

5 months later and I have the engine torn apart in my shop. The crank and bearings are fine. My original diagnosis of bearing failure due to low oil pressure was wrong and now I'm not sure if the oil pressure was ever low enough to cause any problems.

So why the knock? It was having a pretty bad stalling problem at the end but don't know if that would be related. All I can think to do is put the engine all back together to see if it still does it. Since I didn't change anything important, I don't see what it would change.

Any help is appreciated

 

With a low enough oil level she will knock, especially the lifters. Had this issue with my '96 Caravan and it was a problem with the oil filter gasket, at last oil change a gasket stuck in the mount and when new filter was attached that made two gaskets and that created a leak, and a low oil level thus the knock.

 

how bad of a knock? Neons are known for valve chatter, worse if you've driven it without oil or low oil.

Did you ever add oil to it or just let it sit till u tore it down?

 

I changed the oil and it held the level just fine. The knock persisted. I don't think it was lifter noise because it sounded more like a knock that a clack. The timing of the sound was different that lifters too. At least that was my original thought. Now I don't know.

 

when you rev the car does it knock faster?

 

Yes, RPM does affect knock.

In disclosure, I had talked with my Dad who is an old school mechanic and he told me that when a car runs out out of oil it's the number one rod bearing that suffers the most. If there is a knock after an oil starvation situation, it's usually that rod bearing that is the culprit since it's the last to get oil. His knowledge of engines may be obsolete in this case but I have to admit that I agreed with him or I wouldn't have taken the steps I did to check his diagnosis.

 

I think I'm going to re-examine the bearings. I'm not convinced yet that my original diagnosis was wrong.

 

on the neon, it's usually the #4 or #3 rod bearing...i've gone through 2 motors that way. Also, It wouldn't be a bad idea to look at the wrist pin on the #3 cylinder...that was the point of failure on 2 other motors on my neon.

 

How many motors have you gone though?

It's funny that you mention the wrist pin on #3. I thought it seemed kind of loose but wasn't absolutely sure.

 

I found this in a Google search. It doesn't really change or help anything now but it's information that may have helped.



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 con 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 conrod 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
conrod peak due to no downward pressure on the piston face and high speed
loading of the bearings.

The noise a conrod 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 conrod should make noise
at idle. Also, changing ignition timing, or increasing octane will cause
pinging to go away, but a worn conrod will continue to knock!