Distributor is oscillating

[gregkjones]

Hello everyone,

I have a 1998 Sea Ray330 with 454s. Recently after a trip to Catalina, when I backed off the throttles to slow for the trip into the harbor my right engine was surging between 1000 and 1200 RPMs. The next week I went to start it up and it still did it but only after advancing the throttle to around 1000RMP. When I add more power it gets rough.

I looked at the distributor and it was oscillating. I bought a new cap & rotor when I looked at the old cap and could see that some of the posts were being gouged by the rotor. After installing the new cap and rotor, low and behold, it was still oscillating! I thought it was just the old cap itself since it was a little lose when I had checked it out, but the whole distributor (not just the cap) is moving around about an 8th of an inch as the rotor turns. It is tight too, not loose so I am guessing something pretty ugly may be going on in there. Any ideas why this could happen. I checked the plugs, they look ok; look like its running a little rich (or perhaps there is some blowby) but nothing too bad. I ohm checked all the wires, they are good too. Hoping it isnt something big time like a bent cam/gear/bearings or something. Maybe bad bearings in the distributor shaft etc??? Please help.

 

[OldSkool]

Most likely the distributor bearings are going bad. Not an uncommon event for an older GM motor. Replace the distributor and all should be good.

 

[gregkjones]

Thank you OldSkool. I'm gonna replace and will post results.

 

[gregkjones]

replaced distributor. no more issue with that and the tach stopped jumping around too but the overall problem of engine roughness and lack of full pwr continued.

mechanic went back and revved it up and finally got a MAP code. Not mass airflow but the manifold air pressure sensor. i've been told that they would have to pull the intake plenum to replace this part. i am thinking there may be a little info lost in translation between the mechanic and the person i spoke with on this. the part is cheap however, the labor to pull the plenum and the gaskets adds up to around $700. i can pull this plenum myself to do this and get new gaskets but have not checked out the boat yet to see if i can just pull the manifold air pressure sensor without pulling the plenum before i do this because i could swear it just sits right on top of the plenum (the part you always need to avoid stepping on and breaking when moving around on the engine working on it). Does this sound typical or is there perhaps something else that would require an intake plenum to be pulled?? I cant see mercury putting a sensor in a place that requires so much effort to pull out. Do not want to do it for nothing especially since this plenum is very delicate bakolite material and if it breaks, i'm looking at another $375 plus if on the off chance I drop so much as a washer in there during the surgery, this job just gets more and more out of hand (and I've seen easy jobs go bad like this despite great care). What do you think??