Engine/car Shaking over 90 kph (different from previous posts)

[Ed Li]

My car is 1984 W123 2.0 liter gasoline, 4 cylinder running on carburettor. Recently I have experienced some "shaking" problem. Details as follows:

(1) It is shaking, not vibrating. The shaking comes from the engine, not the type of vibration that comes from tyres/wheels.

(2) It is come-and-go. Sometimes I cruised for 20 km at 120 kph, no problem. Sometimes I drove at 90 kph for 10 km, the problem kicked in.

(3) When the problem happens, the whole steering wheel and gear level are shaking. I have to naturally slow the car down to below 80 kph. Then it is OK. But pressing accelorator again will tricker the problem again.

(4) I have checked and repaired all electric wires, spark plugs, and even the air filter. No help.

My workshop seems to be of no clue too.

Any advice would be appreciated.

[Benzmac]

Two things to check. You could have a fuel starvation problem. Also, it could be a driveshaft vibration.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

[Ed Li]

Thanks for the advice.

I want to add that the shaking is really very rigorous, as if the engine wants to die.

I have also changed the spark plugs, thinking that could be the source of the problem. No help. After three days of use, my workhop and I inspected the (new) spark plugs and found that they are "blackened", only after three days of usage. It looked like it was due to fuel flooding.

So, the question is, could the "shaking" be due to a shortage of fuel, but only at high speed cruising after some time, while at normal speed and driving time of the car there is a oversupply of fuel?

We suspect that there are two parts to the problem:

(1) The over-supply of fuel at low speed could be due to a malfunction of the carburettor; while

(2) The under-supply of fuel at high speed could be due to a malfunction of the AC fuel pump.

Are we on the right track? Any more insights?

I had a similar problem in my '82 W123 200 not too long ago. Check your fuel line, the ones that are connected to the Stromberg Carburettor, the engine will rock at high speeds if any of the flexible hoses are pinched or folded due to age, etc.

My problem was solved when I changed the damaged hose. An obvious case of fuel starvation.

Hopes this helps!

Jimmy Ng
'82 200 W123

[Ed Li]

Sigh - problem still persists.

In addition to all the work done mentioned earlier, I have in the last couple of days (1) replaced the fuel pipe leading to the carburettor, (2) replaced upper arms and the bushes under the carriage (3) done another round of wheel balancing.

What I am going to do next is to replace the engine mounting, gear bushes, etc.

Could it be due to the engine valve problem, which means one (or more) cylinder is leaking? And, why the problem does not happen all the time?

HELP !!!

[jeffsr]

Well, if you keep replacing things, sooner or later you'll get around to the carburetor. If it's intermittent (comes and goes), then it's not your suspension. Black deposits on plugs = running rich. If it was consistently lean but occasionally rich, then the plugs would tell you a different story. Get the carb repaired or replaced. Could be main float, metering rod internal vacuum leak even a cracked fuel bowl. 4 cyl engines are not famous for being smooth anyway. If there is a problem with fuel, they can really rock and roll.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 06-19-2000).]

[Ed Li]

Thanks for all the advice above. But, the problem still there.

I have just changed the carburettor to a refurbished one. Everything seems working fine but the shaking problem is still there.

I have discovered one thing though. While I was driving and the shake came on (still "come and go"), I pressed the clutch and pressed the accelorator pedal, the engine rev went up smoothly. So, the shaking did not seem to come from the engine itself - hence it did not seem to be the (occassional) fuel starving problem.

When I pressed the brake pedal (while pressing also the clutch), I could feel the shaking from both the clutch and brake pedals quite severely. The shaking is like horse gallopping.

I have re-checked all the wires and spark plugs and confirmed that they were all right.

So, anymore insights? Someone mentioned "driveshaft" problem earlier. What is it and how to diagnose it? I still don't understand why the problem is "come and go".

[jeffsr]

Benzmac mentioned driveshaft vibration. what you are describing sounds more like what he suggested. May be a joint, flex coupler, or support bearing. Your description would also suggest something in the suspension. sounds like the engine can be ruled out. I really think you need to have a compentent tech look this one over. You may have more than one problem. Whatever it is, you had better get it figured out before throwing any more money at it. Do you get any shaking or vibration thru the steering wheel?? Coming and going could indicate a flex disc about to head south.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 06-21-2000).]

[Ed Li]

Thanks jeffsr and all the folks for the advice.

Yes, the shaking appears on the steering wheel and indeed the (manual) gear lever too. I can feel the shake from the floor as well.

HELP ....

[jeffsr]

You've got a steering problem, or at least an alignment problem. See if your steering damper is broken or missing. if you're getting that much shaking in the steering wheel, then your car is not safe to drive. Please get the front end checked by an alignment shop. Take a look at the damper. That can cause shimmy or shaking. Don't wait until something really nasty happens to you or your car.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[Ed Li]

Eventually I think I have cured the problem. Thanks for all the invaluable advices offerred by all of you.

I have been working very closely with my mechanic. I have seen for myself every single piece of component/part that has been replaced. I have come to the conclusion that the problem was due to two parts.

The first part is that (this is the latest discovery) my front right brake disc rotor was distorted/deflected. Therefore when it was heated up it curved, thereby jamming the brake pad and caused the wheel to shake.

The second part is that the left upper arm of the (left) wheel which joined the lower carriage was damaged. This was diagnosed and had been fixed earlier.

Now my conclusion is that the first part was the main cause of the shaking, and with the second part, the shaking was simply magnified.

The breakthrough came from my diagnosis of ruling out the engine/fuel/carburettor problem. When the shake occurred, I stepped down the clutch and brake pedals and I could feel the same type of vibration came from them (and the engine worked perfectly). So, we have zeroed into the clutch and the brake system, with the latter at a higher priority. Moreover, I gradually believed that the problem was "driving time dependent" rather than "speed dependent" - or to be exact, "heat dependent" which explained the heating up of the distorted rotor. The "come and go" phenomenon happened probably because the rotor was heated up to different extends at different conditions, etc.

This is a good learning process, well at least for me. Again, I would like to thank all of you who have offerred help. I do hope (touch wood, who knows) I have genuinely conquered the problem and the next time I put up a post would be offerring help/advice.