knocking at idle

swimmingly · 10-21-2006, 08:08 PM

I recently did a top engine refresh (didn't touch the head at all) and it seemed like all went fine. The car feels faster and has better response. I also just replaced the sway bars with 968 m030s and installed delrin bushings. For what it's worth, I also took it to an automated car wash place for an exterior cleaning.

However, today I picked up a loud low-pitch knock when the car is at idle. When the rpms are greater than 1200, the knock goes away. When I return to idle at a stop light or before parking, the sound returns.

My fear is that it's rod bearing failure, but from what I read, I think that would have constant knocking throughout the rpm range. Second, I'm thinking it's bad motor mounts. My car shook a lot before at idle and it went away at 1200 rpms, but I've never had this sound before. Could stiffening the suspension (bigger sways/delrin bushings) or even a car wash aggravate the crappiness of my old motor mounts? Any other ideas?

Razorback1980 · Razorback1980's Garage

Of course it's hard to tell from here....what all did you remove on the top engine refresh? Did you change anything on the timing or balance belts? I wouldn't suspect a rod bearing...that knocking usually gets louder with rpms. I've seen timing do that on other cars...I'm thinking a 951 has a knock sensor...check it's connection...maybe it was knocked off if not, check it to make sure the sensor is good.

bigyagi · bigyagi's Garage

easy way to check for rod bearing knock. assuming you have a good handbrake, that is. put car in first gear, handbrake on tight. bring engine rpm's up to about 2k, and let the clutch out, until you load up on the brakes. you will feel the car want to go, and it will dip at the back as it trys to go. if you have bad bearings, then they will show up now, as you have loaded the engine.
i checked my honda 900f this way. i put the front wheel up against the house, foot on rear brake, brought up the rpm's, and let out the clutch, until the grab point. trust me, if your rod bearings are shot, you will hear the most gut-wrenching knocking, and be left in no doubt at all as to their condition.
however, from your description, i dont this this is it!
bob.

many944s · 10-23-2006, 08:44 AM

Agreed. when a rod bearing starts to go out, it will usually be quiet at idle and begin to appear on RPM/load increase and decrease. I would give serious consideration to the motor mounts. I worked on a friends 951 earlier this year that sounded for all the world like the motor was going to knock itself apart. I found that the pass side motor mount was separated and causing the noise. Replaced both mounts, and the car sounded like new. Just my $.02 worth..

Nick

MBEngineering · 10-24-2006, 03:57 AM

HI You have piston slap, try a thicker oil. like 15-50 as this will support the piston better in the bore.

regards mike

swimmingly · 10-24-2006, 06:38 AM

I'm already running 20-50.

Some more observations: the noise is present regardless of whether the car is hot or cold. Only half the time at idle will it make the sound. However, a shift in weight (letting it roll in neutral then pressing the brakes) sometimes will cause the noise to appear. Sounds to me like something's loose and I'm hearing the knocking vibrations when it contacts.

I don't have the time at the moment, but I plan on changing the motor mounts and seeing if that makes any difference. Need to figure out which ones to buy first...

MBEngineering · 10-24-2006, 12:21 PM

HI try the R/H side one under the exhaust as this one is the usual one to collapse, there is a measurement to check to see if it has and a small tool to make, to check it, just out of curiosity check the end float on the crank??

regards mike

swimmingly · 10-24-2006, 02:22 PM

Thanks Mike. I don't have access to the underside of the car right now, but I'll check the motor mounts and probably replace them when I do.

I'm not familiar with what end float on the crank refers to. How do I check this? The name makes it seem as though the process to check it is labor intensive.

MBEngineering · 10-25-2006, 12:37 AM

HI the float on the crank is as it says , it is floating in the bearings in the cylinder block , to check I would run the car up ramps and drop the under tray , remove the small tin cover at the back of the sump, if it has one, you can then lever the flywheel to the back of the car and put a lever behind the front pulley to lever the pulley to the front of the car, if you can move the crank by using each lever then you will need to check with the manual to check how much end float is allowed, I would say the R/H engine mount is the prob . new engine mounting is 70mm- + 1mm between the top face and the bolt holes face, when you check the mount on your car the min' size fitted should be not less than 62mm from the top face to the bottom face, I made a small tool to check this by bending a flat bar to the in side gap of 62mm and filed the out side edges-to clear the mount, you could use a bit of welding rod ?? If some one has a 944 manual you could look at , on page 10-10 it gives instructions to check the mountings, if not PM me a FAX No and I will send it over.

regards mike

Slam · 10-25-2006, 09:41 PM

A really easy motor mount check is to get yourself a baseball bat, jam it between the pass. side strut mount and the exhaust or cam tower and try to lever the engine up. If the mounts are bad that baby will rise quite a bit. Don't use a metal pry bar...for obvious reasons...