Mercruiser 454 - Intermittent Break Up at High RPMs
 

Hi Everyone,

I have a 1997 Baja with a Mercruiser 7.4L 454 and have been trying to chase down an Intermittent issue since last season.

Out of the hole, the boat gets right on plane without any issues and runs strong (sounds great too). If you stay below 3,800 RPMs the boat will run fine.

When you go over 3,800 RPMs, sometimes the motor will break up (sounds almost like a misfire but not quite and looses power but will not completely shut down). Soon as you back off the throttle to under 3,800 RPMs the problem goes away. Again, this doesn't happen all the time, it seems intermittent.

As an example- got in the boat warmed it up today, then ran it WOT @ 4,800 RPM for several minutes without a problem. Then idled for a few minutes and then ran at about 4,200 RPM for about 30 seconds then the problem surfaced so I backed off to 3,000 RPMs then back up to 4,000 RPM and the problem was right there. Decided to run back at 3,200 RPMs for about 20 minutes. Then did another WOT run at 4,800 RPM (just over 60MPH) for several minutes again without a problem.

Here is what I have replaced/checked so far:
1. Replaced Fuel Separator
2. Removed In-Line Fuel Filter
3. Replaced Fuel Strainer in VST Tank
4. Checked Gas tank vent and it was clear
5. Check anti-siphon valve and fuel pickup tube in tank (tube was clean and no anti-siphon valve was present)
6. Replaced Fuel Hose from tank to mechanical fuel pump
7. Replaced Electric Fuel Pump in VST tank
8. Replaced Float/Needle Valve in VST tank
9. Cleaned Intake Filter/Flame arrestor

My original thought was a fuel delivery issue but I have just about exhausted this option. Before I move on to the ignition system, I thought I would check in here to see if anyone else had this problem?


This intermittent problem is really getting the best of me! Any thoughts on what you guys think the issue be?

Thanks!
John

Replace the Ign Coil.

Watch the tach carefully when it's breaking up. If it jiggles around or looks very erratic you may have a bad distributor, play in the shaft will cause breakup at higher RPMs like you describe.

check plug wires or coil going bad.

+1

For grins try unplugging the vent line from the VST tank. I just had a similar problem and found the tank was shooting gas into the manifold intermittently, flooding out the #8 cylinder. Other than hard starting when it was hot it only caused a skip at 2800-3200 RPM. Drove me nuts trying to find it. I also replaced the float valve. If you suspect the ignition try putting a timing light on the wires and watch the strobe while running it. In my case I found it by shorting out the cylinders one at a time. The cure for now is it's venting back to the fuel tank and has been running great since. Good luck

I know this post is a little old, but John, did you ever figure out the problem. had my boat out this weekend, 88baja, 454, and had the same problem, with a skip/misfire/hesitation at high RPMs. coil and distributor sound like a good start, just wondering what you did to fix it.