What would cause the engine to shake but idle smoothly? I have replaced the motors mounts and engine still wants to shake. I am totaly stumped. Anyone have any good ideas???? Thanks

Mark
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1981 245 GLT (NON-Turbo) 2.1L B21f 95,000 Original Miles, Borg Warner 3 Speed,(soon to be M46) Stock Virgos with 195/60/15's

I'M HAVING THE SAME PROBLEM ON A '90 240. I CHECKED THE VACUUM LINES, REPLACED THE MOTOR MOUNTS TIGHTENED VALVE COVER, TIGHTENED INTAKE MANIFOLD, CHECKED DIST. ROTOR WIRES AND PLUGS, CLEANED THROTTLE BODY AND IDLE VALVE. ENGINE STILL SHAKES AT IDLE. SEEMS TO RUN FINE AT HIGHER THROTTLE SETTING. I HAVE ORDERED A NEW THROTTLE SWITCH AND INJECTOR SEAL KITS FROM FCP GROTON, SHOULD RECEIVE THEM ON FRIDAY. I WILL REPORT BACK WITH RESULTS. THE HAYNES MANUAL STATES THAT " A BROKEN OR LOOSE THROTTLE POSITION SWITCH CAN CAUSE INTERMITTENT BURSTS OF FUEL FROM THE INJECTOR AND AN UNSTABLE IDLE BECAUSE THE ECU THINKS THE THROTTLE IS MOVING". MY TPS CHECKED OUT BAD SO HOPEFULLY THIS IS MY PROBLEM.

I had the same type of problem, thought it was the mounts. They were starting to deteriorate, so I changed them, that didn't solve the problem.

It turned out to be a leak in the hose for vacuum advance unit.

So check all of those vacuum lines as others have mentioned.

Good luck,
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Bruce S. near D.C.

My first guess would be to verify that you're running good plugs (non-platinum), good wires, and a good cap and rotor. Make sure that the valve assembly in your Idle Air Control Valve is clean (shiny clean) and free of oil, varnish, contaminats, and responds quickly. Run two bottles of Techron through the fuel system to try and eliminate any deposits in the injectors. Hook a meter up to the O2 sensor in parallel and make sure that it's jumping nice and quick, and if you haven't replaced the sensor in 5 years, now would be a good time.

If all of these are ok, I would (on a hunch) check the compression in the cylinders. I would speculate that if you're up in the 190-200 range, you may be getting excessive blow-by past the rings when the cylinders fire. If you have a high level of carbon buildup in the cylinders, that would increase the compression ratio to a point where it starts pushing a decent volume past the rings when the cylinder fires. If the rings in any cylinder behave slightly differently at extreme pressures from any of the others, and in a repeatable manner, then you may pick up an engine quiver. -Note, this is purely speculation, but it's my best guess. I'm curently working on my '87 245 and a friend's '86 245, both of which have significant carbon deposits, high compression test results (195-205 on all cylinders), good ignition systems, no vacuum leaks, and both have a similar quivering at idle speed. The idle is constant (doesn't drift more than a couple RPM), but something is definately causing a regular, repeating, shake (even with new engine mounts).

FYI, the Haynes manual indicates that the engines (B230F) should have compression tested around 170 at the factory before it left the assembly line. I may end up pulling the head some Saturday and cleaning the cylinders. Hopefully that will solve the problem. I'll keep you guys posted.

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 234K

before you pull the head, unless you need the practice, go to your friendly gm dealer and get a can of top engine cleaner and try that. good luck, chuck.

Quote: "before you pull the head...go to your friendly gm dealer and get a can of top engine cleaner and try that."

Thanks, I may yet try that route.

I've not had the need to clean a lot of carbon out of an engine yet, so I've been asking the shop guys around here what they use. Sea Foam and Techron seem to come well recommended, with Sea Foam producing great results. I'm almost tempted to pull the head, just so I can document what I find inside.

Thanks Chuck, and God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 234K

How can you tell a smooth idle if it's shaking? Has anything been done to plugs, cap & wires recently?


Even if the 95,000 is true, I'd suspect air leaks at the injector seals, and maybe even the intake manifold gasket on an '81.

I use Propane to test for vacuum leaks (less messy than carb cleaner or other sprays). Take the nozzle tip off a propane torch and replace it with some snug fitting rubber hose about 2 feet long. Practice with the valve to get a moderate gas flow (not a roaring blast).

With the engine at a warm idle, open the gas valve and poke the end of the hose around each injector for a couple of seconds. If the seals leak, you should hear an RPM change when the propane gets sucked in and burned.

Do the same around any other suspected areas, like hidden vacuum hose ends and the intake manifold gasket itself.

To block the breeze from the fan, lay a piece of cardboard from the fan shroud to the engine.


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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.