How to diagnose a bad main bearing?

Hi everyone:

A mechanic that recently replaced a starter in my '94 Nissan Sentra (105k miles) told me that he heard the engine knock at high RPM and concluded that the main bearing is bad. He said I need to replace the engine ASAP. I'd like to get a 2nd opinion, but also wanted to educate myself a bit.

o What are good ways to diagnose a bad main bearing? o Does a bad main bearing deteriorate quickly, or can it last for a while before totally failing? o Is there a less expensive way to fix it than to replace the whole engine?

Thanks,

-- snipped-for-privacy@fastELEPHANTmail.com (remove the pachyderm)

Reply to
remove the pachyderm
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A few things spring to mind.......

I've never heard a Nissan main bearing let go, ever, but on engines that do it's usually more of a rumble for quite a long mileage. I had a non-Nissan crankshaft crack in a big end journal once and that was a high rpm knock for a short while before it sheared in half rather explosively. Worn big end bearings knock loudly especially at low rpm, worn pistons and small ends rattle at most rpm, but you can usually 'tune in' the sound by altering the engine load, often being worst as you back off a bit. Why was the mechanic running the engine at high rpm, hardly necessary for a starter motor swap? Have you heard this noise yourself?

Reply to
Steve B

Dunno. I think this shop always fishes around for more repairs to do....

No, I wasn't sure what to listen for. I'll see if I can hear a rumble at low RPM.

Thanks, good to know!!!

-John

Reply to
remove the pachyderm

Typically, a bad main will knock briefly (kind of a dull clunk 3-4? times) when starting the engine. It'll go away as soon as oil pressure has built up.

Reply to
Bill Kapaun

The GA16DE engine in your 94 Sentra is practically bullet proof, it is one the best engines Nissan made. In all my 16 years as a Nissan Technician I have NEVER seen a main bearing failure in a GA16 that was maintained properly.

I would run, not walk away from that shop right now.

Reply to
NissTech

eriorate quickly, or can it last for a while

It sounds like your mechanic was outright lying to you. If it were that bad, you would have heard it with an untrained ear. I'm assuming you didn't bring it to him because of a loud engine noise. Of course the bearings can be repaired, but there could be a lot more wrong with the rest of the engine if the mains are that bad at 105K. Sometimes it is better to move to another engine at that point. So tell us more about what brought you to the mechanic.

Reply to
MaxAluminum

My car wouldn't start -- didn't even turn over -- but I was able to push-start it and drive it to the mechanic. He replaced the starter and it's been fine since.

Good to know. So far no unusual noise, as far as I can tell. I'll try to listen for the dull clunks.

105k miles. Bought it at about 85k from a seller who claimed it was well-maintained, and I've been changing the oil every 3k miles, etc.

I think chances are good that my mechanic is wrong (or lied) about the main bearing. He also told me the clutch needs to be replaced immediately. I think he's generally right about the clutch, but I don't think it needs to be done urgently. Thanks to all those replied so far. It's been useful.

-John

Reply to
remove the pachyderm

Either

1: get a warranted exchange reconditioned engine. Usually the engine you send back should be worn but undamaged or you may be asked to pay for the parts they don't normally replace like con rods with oval big ends or bent rods after cam belts have snapped. 2: second hand from breakers / import, maybe warranted for up to 90 days but will not pay for labour to swap it if anything goes wrong. 3: Engine out, strip, check and replace shells, rebuild. This is essentially a DIY option as few shop mechanics will take this sort of work on. That's why engine re-conditioners get work. You need to locate a motor engineers with crank grinding equipment.

It will last a bit if driven gently. Light throttle and try to keep the revs over 2000 and under 3500. Slogging is as bad as thrashing. If left or driven hard it will go big time and take out valves, possibly throw a con rod though side of case, then you have nothing to exchange or rebuild. Another good (bad?) sign of imminent failure is the oil light coming on at tickover when hot - the bearings are so worn the oil is leaking out too fast or the pump is worn and bearing failure will soon result from that too. Once you can hear it while driving it's not got long left, tends to knock at cruise and go quite under load or lifting off. If done asap it may not need a crank grind, just new shells. New oil pump is a very good idea if rebuilding, or at least split it and check clearances.

Is there water in both header tanks - radiator and expansion? Lots of people make the mistake of thinking the system is sealed because there is an expansion tank. Sealed systems have pressure caps on the expansion tank. The pressure cap is on the rad and if the system looses coolant it will not always show as a drop in the header tank level. I just bought a 200SX turbo S13 CA18DET as a parts car for £150. Claimed to have a cam rattle. Chap said must be full of coolant as the the header tank is full and the temperature gauge sits just below the half way mark all day. Took over 1L of water to dilute the red/brown mud. Big ends are knocking quite well.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Reply to
Peter Hill

Normally a bad main knocks when first started cold.

Also normally the oil pressure takes a serious dive with a bad main because the pump can't build up pressure at the gap that is making the noise.

Nissan engines are built really well from what I have seen. The car will disintegrate around it, but the engine will keep on purring.

A high rpm knock can easily be exhaust or a slightly sticky lifter or maybe piston slap, but if a main knocks at high rpm, the next sound you normally hear is BAM! Then 'oh shit' as it comes to a screeching halt leaving a trail of pieces down the road.....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

remove the pachyderm wrote:

Reply to
Mike Romain

I doubt that its *failed* in the sense of having spun in its bore- you'd certainly hear that. It may be worn, in the sense that it gets a little noisy at high RPM, but that isn't necessarily cause for replacement. How is the oil pressure? If the main bearing clearances are opening up too much due to wear, you will see lower than normal oil pressure, especially when the engine is hot.

Reply to
Steve

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