Need some creatiave feedback - PeachParts Mercedes-Benz Forum

cadurning · #1 04-12-2010, 02:25 PM

My '72 280SE with 2.8 engine has a perplexing noise upon startup. The car is a barn find that sat for 6 years before I got it. The car ran surprisingly well but would starve for fuel after a few miles. Repairs that I have made within the last few months are, refurb the gas tank, new sender, new elect fuel pump, changed fuel hoses, changed fuel filter, change oil & filter, adjusted the valves, replaced cap, rotor, plug wires & converted to Pertronix. After doing that work the car ran better with plenty of power.

The problem is that the engine will make a knocking noise, like a rod bearing knock, and will run rough for a second or 2 upon startup after the car had sat for 30 to 45 minutes after a spirited drive. In contrast the engine will start and run smooth and would not make the noise if started cold or if restarted within 10 minutes or so when warm. On every start the oil pressure goes immediately to 40lbs + hot or cold and the pressure is usually 40+ before the engine lights off.

At first I suspected a leaky injector was loading up a cylinder. I pulled the plugs and sure enough 1 was black while the remaining 5 had a nice color. I pulled all of the injectors and had them cleaned & pop tested. The pump shop confirmed that 1 injector did leak down over time but the cleaning resolved that problem. They further confirmed that the pop pressure on all of the injectors was below 200lbs. The cleaned injectors were reinstalled. The engine ran much better but the starting rough running & noise problem persisted but all of the plugs are now the same color. I then eliminated the cold start injector by removing and plugging the fuel line. The problem persisted.

I finally broke down and installed 6 new injectors since I wasn't pleased with the report about the original injectors. WOW what a difference in the way the car runs now. It's smoother, quieter and has a bunch more power. All of the plugs now have a light tan color. Unfortunately the noise at start up is still there.

I have considered the possibility of leaky valve stem seals but the car does not display the usual signs such as blue smoke out the exhaust upon deceleration. The car does not use or leak oil and there is no evidence of oil on the plugs. I have also considered carbon buildup in the combustion chamber or the top of the pistons.

Any input would be greatly appreciated.

Pooka · #2 04-12-2010, 04:12 PM

Does this engine have hydraulic lifters? It sounds like a lifter is leaking down when the engine is off and then loading up when it is started.

A lot of GM cars from the 1960's had this problem and there was an easy way to tell if the problem was a lifter or not. You can find out by going to the auto parts place and getting any can that says 'Quites noisy lifters!', and pouring the contents of the can into the engine.

This is not a long term fix, but it is a cheap temp one.

cadurning · #3 04-12-2010, 05:05 PM

Pooka,

Thanks for the reply. No it's mechanical lifters. The noise is deep inside the engine. Right now I suspect that it could be carbon buildup. Back in the dark ages cars would get carboned up so badly that they would make the same knocking noise while running. I would just slugg the engine with ATF or the garden hose and that would usually solve the problem.

Unfortunately in recent times my memory does not serve me well. Since the problem is intermittent I was hoping that someone else had run into the same problem.

When I had the injectors out I could get my proctologist light in the hole and the intake valves looked clean so I discounted carbon at that point. Unfortunately the light isn't much help looking into the spark plug hole.

Perhaps I'll just let the cylinders soak in ATF for the week and see what happens on Saturday. BTW does Sea Foam work to get rid of carbon?

wolf_walker · #4 04-12-2010, 06:03 PM

Sounds exactly like one of the being-a-jerk modes of the Solex carb on my M110, I was all ready to replace the chain tensioner before I figured it out.

JohnM. · #5 04-16-2010, 10:04 PM

Quote:

Originally Posted by cadurning

BTW does Sea Foam work to get rid of carbon?

Sea Foam does a great job of removing all sorts of nasties out of the engine. You can put it in the oil, gas, or intake. My favorite method with the older carburetor cars; you pour it directly down the carb barrel with the engine idling, but slowly enough as to not kill the engine. Then you shut her down for 15 minutes and start it up again. It will smoke out the exhaust like a MF'er for awhile and then clear up once all of the crud is gone. Buy some and read the can, you can use it more ways than a Swiss Army knife.

cadurning · #6 04-19-2010, 12:33 PM

Here's what I did and how it turned out. I let the ATF soak in the cyls for the better part of a a week. Started her up and killed a bunch of mosquitos. Same problem but to a lesser degree.

A while back I had a problem with the vac advance sol clicking on & off even with direct battery power. I was advised by a respected member to just eleminate the vac sol circuit from the vac circuit and hook the vac line from the dist directly to the throttle body. Then set the timing with the vac hose hooked up. I was advised that by doing that the car would then have a "euro spec" ignition. That didn't seem right but I took the advise.

In retrospect that's when the problems began. I suspect that the engine was way too far advanced at startup causing the hard knock and as soon as enough vacuum was built up the vac retard would bring the timing back to normal. I disconnected the vac retard line from the throttle body, capped the port, and set the timing to 7 BTDC and left the vac retard line disconnected. Now the car starts without any problems and runs better than ever.

While fiddling this weekend I found that the cold start sol in inoperative. Any hints on where to find a cold start sol?

wolf_walker · #7 04-19-2010, 07:04 PM

I despise the timing crap the US cars got. Ditching the retard is unfortunately half the battle.