KuHL 951

I'm trying to help a friend out with an 86 951. He let his girlfriend borrow it and now it knocks. Here's the facts:
1. Happened at higher boost (1.8bar)
2. Low oil level
3. Immediately shut down, no overheating, and towed home
4. Knocks only under load not at idle or initial start-up.
5. The car has been started twice since the failure and shut off quickly.

Is it possible that it can knock without spinning a bearing?
Will a spun bearing "always" damage the journal?

My impression from reading here is that once a rod makes a noise the crank pretty much has to come out...True / False?

If all he needs is new rod bearings his life would be so much easier and the work can be done at home.

Any help or advice always appreciated.

Steve

Perry 951

It sure is possible. A failing wrist pin can also create a knocking sound.

Not always, but you will need to pull the crank so the journal can be tolerance checked. You will also want to have the thrust surface on the crank checked as well.

For your diagnosis.. are you are sure that the noise is coming from the motor? Since this noise appears under load, you are obviously in motion when testing.. so it could be coming from somewhere else.

Failing CV joints can make a rythmic knock when under load, and transmit the sound and vibration into the cabin via tranny and torque tube. Check them if you are unsure if it is the motor.

Rod knock (failed bearings) will usually knock under load as a first symptom. Knocking at idle typically means they are spun or broken. If the sound is from the motor, you gotta pull the pan. If the bearings check ok, pull the pistons and check your wrist pins.

cpt_koolbeenz

Same exact symptoms here EXCEPT I have good oil pressure. It only knocks under acceleration or revving with no load. I just had 3 gallons of coolant in my oil pan so I suspect rod bearing failure.

KuHL 951

Thanks for the great info. The noise is positively from the motor. Sounds like a classic deep rod knock. Test method was "Start, idle-no knock; set hand brake let clutch out slowly/slight gas in 2nd gear-knock increases. This is what I've always used as a down and dirty rod bearing test from my early years; probably not the best but it's all I have at this point. Does a failed wrist pin sound the same to an untrained ear? We know we have to drop the pan, thats a given, but it would take place at my garage so I'm cautiuos to avoid a derelict vehicle taking up garage space waiting for major repairs. A complete rod bearing replacement is doable with all the info out there; I'm just concerned about the likelihood of journal damage with the lower end apart. If the bearing hasn't spun and the journal looks clean and unscored would bearing replacement at that point buy him enough time to drive it carefully until a rebuilt engine can be found? Sorry for my ignorance of these issues but this is the first Porsche trauma I've been involved with. Knock on Wood.

Steve

944x-rated

is there a way to fix it w/out pulling the engine? i heard of someone pulling the oil pan off and changing it like that... what do you guys think?

944x-rated

sounds like me and your friend are in the same boat.

KuHL 951

944x-Yes the rod bearings can be installed with the engine in the car. Many good write-ups are available for this. If journal damage is present, all bets are off for a weekend type project. The crank needs to be sent out for machining, oversize bearings need to be ordered...in other words the difficulty of the repair increases dramatically and the car doesn't move for awhile. I hope others with experience will add their opinions.

944x-rated

so, its probably a high likely hood that the bearing messed up the journal?

ibdavid

Yeah, sounds like the boat I was in about a year and half ago. Of course this was reason for the 2.8 upgrade. It's not a pretty picture, at least from my vantage point.

turbinek

1.8 bar - absoulut or relativ? I guess you mean 0.8 bar!
From my personal experience a spun bearing makes a knocking noise at any rpm - at idle it is knock knock knock - higher rpm is knockknockknock...
But little oil is always an issue - I am not sure what I would do if I was in your position...
Frank