View Full Version : 94 Prostar LT1 Not firing on all cylinders

07-11-2011, 05:43 PM
During ski show practice a few weeks ago, I noticed that our boat was not sounding right. In low RPMS (under 20), it was chugging and seeming to make quite an effort to plane. My husband heard it a few days later and said the boat was not firing on all 8 cylinders so he decided to replace the spark plugs. One plug wire was corroded but he cleaned it and said all was well.

Ski show day, it seemed to work ok, but then we took the boat on vacation and under 20 or 21 RPM, the engine seemed to chug (my husband said it sounds like it's farting in the lake), then after we were planing and running faster, the engine seemed to smooth out and everything was fine. You could hear the engine change and watch the RPM and speed suddenly increase.

Same problem yesterday trying to pull 4 ballet skiers out of the water...I was one of the skiers and it was quite a drag. None of us got up.

Two different mechanics suggested an Air Idle Valve was the problem: one was a mercruiser mechanic and wasn't 100% sure. The other one (inboard mechanic) was just backing up what the first guy said. A local chevy mechanic said a map sensor, vacuum line or fuel pressure regulator.

Has anyone else had a similar problem with the LT1 or does anyone know these parts for our boat so maybe we can try replacing them?


07-11-2011, 06:23 PM
A down and dirty way of locating a misfiring cylinder is with a timing light. Just hook up the inductive lead to each plug wire at a time. The light should blink rhythmically and not skip a beat. If it does then you have a misfire on that cylinder. It could be the plug, wire or coil pack and would require further investigation...

If you have never replaced the plug wires on your LT-1 yet, I just ordered a set for mine from www.inboardsonline.com You will have to call them with at 1-877-327-7175 and order over the phone using these numbers: Parts S556010A & S556010B (need both). They are two sets of plug wires made up for the left and right banks.

I don't have them in my hand yet so can't speak for the quality or the fit just yet, but for around a $100 I am willing to give them a look.

07-12-2011, 05:11 PM
I had the Same Problem once turned out to be a spark plug in the rear cylinder was bent over. New plugs boat ran great. Now change them all the time. I suggest you check both Plugs and wires.

07-13-2011, 12:05 PM
Thanks for the information. We did replace the spark plugs a few weeks ago but not the wires. Chances are the wires have never been replaced either. I'll let my husband know about the timing light. Seems like an easy thing to start with.

Any ideas about the Map sensor or fuel regulator part numbers?