924S Low Engine Idle Knock

[srussell]

My son brought home a 87 Porsche 924S. It was running hot only to find no coolant in the lines!!
I've replaced the radiator, thermostat and bleed the system. Engine is cooling okay and don't think we have a head problem (no coolant in the oil).

BUT we have a engine knock at idle. Somewhat loud. As soon as he touches the gas it hums like a sewing machine. No issues.

It does not sound like a lifter to me but it has been years since I worked on a car.

Ideas or suggestions

[wild man]

Engine mount? Pipe banging against sway bar? Then it gets ugly>>> #2 rod bearing? Or worse?

[HondaDustR]

Is it a knock or more like a ticking or clacking sound? Lifters like to go flat on these things as well, but that would sound like a clack way up in the top end of the motor where the cam is.

[John_AZ]

Originally Posted by HondaDustR

Is it a knock or more like a ticking or clacking sound? Lifters like to go flat on these things as well, but that would sound like a clack way up in the top end of the motor where the cam is.

I pick HondaDustRs choice.

1 maybe 2 soft lifters:
Bennington Motorsports, Nick, aka many944s....has a good explanation.
Bennington Motorsports

I chose this instead of the rod bearings because you said it was dry of coolant and this tells me the car has sat for a very long time.

Pull the cam housing---get a gasket---WAKE UP the cam bolts.
Check the lifters. Get OEM used from a forum member ---very reasonable.

GL
John_AZ

[fast924S]

I bet its rod knock, Low rpm means lower oil pressure making rod knock easy.

[wild man]

Speaking of oil pressure, how's that looking? If it's down from "normal" at all, it's most likely a rod bearing. The first 1 to go is "usually" #2.

[srussell]

Thanks for the replies.

The sound is more of a clank than a knock. I tried the Mystery Oil and I think it sounds better (subjective)

Oil pressure looks okay. I did notice a small oil leak on the back side of the valve cover. It looks minor but I am planning on fixing. When I do then I should be able to inspect the lifters.

I might try the sea foam before I go into the engine.

[Tidybuoy]

One more suggestion - check your exhaust nuts at the manifold.

I had a problem once that sounded like a bad knock. I even had a Porsche mechanic looked at it and I was told the head would have to come off to truely diagnose.

A friend of mine (who drives a 944) told me to check the exhaust nuts at the head and sure enough the far rear nut was loose.

The noise that the engine was making did not sound like an exhaust leak in any way and several people helped me with the diagnosis - it was the exhaust nut all along.

Just my $.02

[krystar]

yea exhaust leak sounds like clacking.

if mystery oil actually makes it better, and not just placebo effect, then it's most likely lifters. mystery oil thins the oil and makes it seep into the lifters easier.

engine that's been sitting for extremely long time will drain the lifters of oil.

[HondaDustR]

Yeah, try seafoam. Sometimes it helps. I've got a noisy lifter in mine as well and just haven't gotten around to swapping it out. The sound is quite disturbing if you don't know what it is. It's not a hard job if you are comfortable with re-tensioning the cam belt.

[Rasta Monsta]

Check all engine brackets and accesory mounts. My 2.5 sounded like it was about to blow but a bolt had broken on the AC mount.

[jgporsche]

I had the same odd noise at idle out of gear. Once I let the clutch out it went away in any gear once moving. It was the rubber centered clutch completely shot. At idle it was the fingers chattering on the clutch disk.

[cvriv.charles]

Hey. Dont know if this is off topic or what but,... what is the "knock" that the knock sensor detects?

[jgporsche]

The knock sensor detects spark knock caused by pre-detonation of the Air/Fuel mixture. My 968 engine has it and it sets a check engine light and you lose some performance. There are two on the 968 on the drives side under the intake. Front one is easy to change the rear one is a pain in the ass!

[JonA]

The injectors are quite loud in these cars. It is a clicking noise, not deep like if something was bad at the bottom of the engine.
Good luck.

Jon

[Schumi]

Get a mechanic's stethoscope or even just a long metal rod to your ear and find out where the sound is coming from by poking around the motor. The injectors can be very loud. That is easy to pinpoint. Same with the lifter- poke around the cam tower- it will be easy to tell if it's coming from the cam tower or lower. Then move to the belt area. Belts loose and slapping the covers?
Then down the block. Worst case would be rod knock.

[Slam]

No coolant? Check the dipstick for milkshake sludge. Does the coolant overflow tank pressurize too much? Do the hoses become hard? IIRC these 924S motors like to blow their head gaskets. Probably wouldn't hurt to drain the oil and check it out.

HTH and good luck!