Rod knock, valve tick or ??? - PeachParts Mercedes-Benz Forum

250 Coupe · #1 11-18-2007, 12:19 PM

I've got a rhythmic noise, more of a quiet knock than a tick. It's absent when cold but shows up when fully warmed up. Bringing the RPMs up might make it go away but I can't say for certain or if it is masked by other engine noises.

Oil pressure is 15 PSI at idle, 35 or so at highway speeds and will peg when cold or at high revs.

Is there a definitive sound that pins it down or do I need to do some internal investigating?

Michael

wbain5280 · #2 11-18-2007, 06:44 PM

Check for exhaust leaks. Re-torque the manifold bolts.

Tomguy · #3 11-18-2007, 07:11 PM

If you'te talking about the 450, oil pressure should be pegged at highway cruising speeds. It should be pegged at pretty much anything decently above a coast. What grade of oil are you running and have you checked for cam wear or clogged oiler tubes?

250 Coupe · #4 11-25-2007, 05:58 PM

It is the 450SEL I'm working on.

I have pulled the right hand valve cover and checked it out. The service manual says to press down on the ball studs to check for a collapsed or leaking "lifter". I didn't find any that felt loose. The cam looked good. I have the tool to check the shim height but have not used it yet.

I took it into work one night and used the ultrasonic gun on it. The drivers side valve cover is quiet, just a bit of whirring, but the passenger side is quite a bit noisier. At least one valve is ticking and there may be two or three that are a bit on the loose side.

I listened to the exhaust manifolds on both sides and they sound fine in non-contact mode, (no exhaust leaks) but the passenger side is rattling away with the contact probe. The chain tensioner is very noisy.

The oil pan is also quiet in contact mode.

I'm beginning to think I need to check the chain, the tensioner and the shim height.

I think I'm running 10W30, it was changed about 1000 miles ago. The oil pressure is what we normally expect unless it has warmed up to 185-195 then it drops off and won't peg till rather high revs. My trip to work includes about 3 miles of highway driving and it never drops off the peg but stop and go traffic will warm it up enough to bring it down.

This is the car the diesels when shut off. A tank of premium combined with SeaFoam helped a lot but it still wanted to stumble just a tiny bit.

It otherwise runs fine, gets maybe 14MPG in mostly short trip city driving.

Michael

t walgamuth · #5 11-25-2007, 09:33 PM

It sounds like you have an oil pressure issue.

It should be pegged at highway speeds with any type of oil.

I would check pressure with a separate oil pressure guage to be sure that yours is accurate, then I would pull out the oil pump and have it checked by a competent machinist for wear.

You don't want to be driving it with such low oil pressure.

Tom W

MunichTaxi · #6 11-25-2007, 10:05 PM

Oil pressure is 15 PSI at idle, 35 or so at highway speeds

Is that oil pressure at idle with the car in gear, or in Park?

Tomguy · #7 11-25-2007, 10:16 PM

Quote:

Originally Posted by t walgamuth

It sounds like you have an oil pressure issue.

It should be pegged at highway speeds with any type of oil.

Indeed.

I run 10w30 myself usually (or 10w40), and mine is pegged at full operating temp at anything above ~1000 RPMs. And my 4.5 runs a lot warmer with the 3.5 heads.

The passenger side (right side) cam tends to wear on the m116/m117 before the left. Not sure why but that's what I've seen on EVERY junkyard m116 and m117. That can cause the noise. But with oil pressure that low, you should expect noisy lifters. Check your oil to see if it's diluted with gas. On older m116 and m117 engines, if you run rich or miss, your oil can become thinned by gas, causing low oil pressure, oil pressure taking a prolonged time to peg on starts, and a noisy engine due to poor cam lubrication or stud pressure.

250 Coupe · #8 11-25-2007, 10:27 PM

Quote:

Originally Posted by MunichTaxi

Oil pressure is 15 PSI at idle, 35 or so at highway speeds

Is that oil pressure at idle with the car in gear, or in Park?

At idle in gear, it is much higher when cold and running at fast idle.

Michael

t walgamuth · #9 11-25-2007, 10:53 PM

That is to be expected.

I once had a bugeye sprite which had decent oil pressure when cold but when warm would rattle all the bearings.

As the oil warms up it thins.

Tomguy's suggestion of diluted oil from too much fuel is a good lead. Start by changing your oil. I suggest putting in some 20w50 castrol dino to start with.

Then check the other things suggested here.

You don't want to be driving it with the op that low.

Tom W

250 Coupe · #10 11-25-2007, 11:56 PM

Quote:

Originally Posted by t walgamuth

That is to be expected.

I once had a bugeye sprite which had decent oil pressure when cold but when warm would rattle all the bearings.

As the oil warms up it thins.

Tomguy's suggestion of diluted oil from too much fuel is a good lead. Start by changing your oil. I suggest putting in some 20w50 castrol dino to start with.

Then check the other things suggested here.

You don't want to be driving it with the op that low.

Tom W

I'll give it an oil change and if it still seems low, I'll probably park it for a while and drive the Coupe till I can get to the pump.

Michael

250 Coupe · #11 11-26-2007, 12:42 AM

Research shows that the oil pump is chain driven and has a spring loaded tension rail made of plastic.

I'm buying a replacement before I drop the pan as I'd be willing to bet it's never been changed.

Michael

t walgamuth · #12 11-26-2007, 01:00 AM

It can't hurt.

Good luck.

Tom W