350 mag idle vibration

[67stingray]

I just had an engine out service to replace a leaking oil pan. Years of neglect also requred new rear coupling, engine mounts, starter, and valve covers. Ignition coil, rotor cap, spark plugs and other bits went into the service. Before getting the boat back from the marina, they told me the engine had a vibration at idle speed and sligtly above they could not figure out. They maintained when the engine was re installed they double and triple checked everything and its not an alignment or engine mount problem. They told me they though it was injector related and claimed they were only able to get 6 injectors out and feared of breaking 2 that were stuck so left them alone. Although they claimed that after cleaning the 6 the vibration got better, when i picked up the boat its obvious.

The engine physically vibrates in place at idle and slightly above, either in neutral or while its in gear. At higher rpms it smoothes out and runs great. I just filled the tank with fresh fuel with no change. Any ideas before I start throwing parts at it, specifically injectors as the marina recommended? Could this be from bad new engine mounts or misalignment of the engine when it was reinstalled?

 

[Scott Danforth]

Check the firing order

 

[67stingray]

Check the firing order

Ok will do that in next few days. I am wondering if they screwed up the engine alignment when they replaced the rear coupler and motor mounts. Not sure if that would manifest only at low/idle rpms like i am getting or the whole power band.

 

[kenny nunez]

If the engine had not been started for a period of time before the service it could be a compression problem. A weak cylinder will smooth out with increased rpm.

 

[67stingray]

If the engine had not been started for a period of time before the service it could be a compression problem. A weak cylinder will smooth out with increased rpm.

Compression test was done by marina, said all cylinders were within spec. But the engine was run all season before service happened in mid July. Fuel pressure to rails was good, spark plugs were supposedly changed, including new dist. Cap and new coil. Plug wires were changed within the last 2-3 years.

 

[Scott Danforth]

After you check firing order, do a balance test at idle. Pull a plug wire , observe RPM. Replace wire, move to next. When you get to the cylinder that isn't firing the RPM will not drop. If you pull a plug wire and the RPMs go up, you found a miswire