Merc 5.7L Thunderbolt 5 Ign, RPM bogging down at 3K occasionally??

[Eddie P]

Ahoy Ladies and Gents, I have a question regarding a tune up of my 1999 Stingray 220DS. We have a Mercruiser 5.7L 250HP engine with a carburetor equipped Thunderbolt 5 Ignition system.

I'm the third owner on the boat, we've had it for 4 years. Last "tune up" of the ignition system was 4 yrs ago with the previous owner (I have the invoice showing a tune up and tune up kit at least from his previous mechanic). I've been replacing the plugs every other year and have had the ignition system inspected by boat mechanics every year.

Each year I do the typical stuff - new fuel-water separator, oil, oil filter, outdrive gear oil (every other), etc.. and every other year or so I change the spark plugs and impeller on condition as it's metal in the Alpha 1 outdrive itself. So I'm pretty proactive as far as maintenance goes, but I admit I am not a crack boat mechanic on the ignition system. I haven't changed the wires or messed with the ignition, for example (but was going to this season). At the beginning of the season my tune up kit didn't ship out on time so I just had my mobile boat repair guy change the plugs themselves. The day after we did the work, my tuneup kit showed up. Great. Anyhow I have it and it's ready to go when we are.

So this season was an epically fantastic boat season. Put about 30 hours on the boat over about 30 days of on-water operation (We cruise to remote beaches and swim, kids are too young for water skiing and boarding yet thus the low hours). Great performance from the boat, no complaints at all until the very end of the season.

The time before last, I noticed the engine occasionally hesitate on throttle up/acceleration while coming off step, at about 3000 RPM on the way to 3500K. The first time it did it just once, very briefly, and I continued to accelerate to 3200-3500 RPM where the engine was running smooth with no repeat through the day. That was one event out of at least four or five throttle ups.

Yesterday was a little bit different. After running all morning just fine in all RPM ranges, we were on our way back to the marina when my hat flew off my head (about 3200K RPM) necessitating throttle down and a spin around to pull said hat from the water (no dramatic engine ops, just a smooth throttle down while turning back and decelerating). After fetching the hat, we turned around and throttled up again to steam home. The engine would accelerate in smooth ops to about 3000 RPM where a stuttering and stagnating of RPM with associated power stagnation would occur due to the rough engine. No backfire, no engine quitting, just a rough operation and no more power increase available from 3K. I throttled down smoothly to about 2000-2500RPM where normal operations would resume. Another try at power up, same thing on this occasion. So I trolled along for a bit and thought about it. I suspected ignition or fuel. Not fearing any drama as we were now in the home port area, I exercised the throttle range again. This time, no problems. Accelerating smoothly, running fine all the way to 3500K. Smoothly decelerate back down to idle... run at idle for a minute, try accelerating again, and the behavior is back. So I throttle down again and just idle in to park. Our boating season is over due to the change in weather anyhow (good timing, pardon the pun). But of course this issue looms in my head as I want to make sure I'm on the right track.

My question for you gurus is: Does this sound like a classic case of an engine tune up being required? Ignition? New spark plug wire harness changeover time?

I've posted and chatted a bit elsewhere, some points to consider by others were possibly water intrusion into the fuel. Check the water separator as well as carb filter.

I have a new tune up kit and a new wire harness as well as plugs. I'll buy a carb filter as well and of course my typical winterization process changes the oil, oil filter, outdrive oil, etc.. Am I on the right track? Do you guys have any other observations as to what this could be? Again, this is a new intermittent issue. Seems like the engine runs great at all PRM ranges, at least right now, except on power up and there is some sort of range where it begins to run rough and isn't happy. The history of the engine is consistent butter smooth operations. When it does accelerate through the rough range, it runs great. But this was just a snap shot in one day, I'm not sure if it would have gotten worse (I suspect it would have) if I'd have just kept running down the prim rose channel into the sunset.

Thanks in advance. Please post away and pour your knowledge onto the deck like a load of King Salmon. I will be in awe of it all, and it will be appreciated!!

Cheers-

 

[Bondo]

Re: Merc 5.7L Thunderbolt 5 Ign, RPM bogging down at 3K occasionally??

I've posted and chatted a bit elsewhere, some points to consider by others were possibly water intrusion into the fuel. Check the water separator as well as carb filter.

Ayuh,.... I'd dump the filter into a bowl, 'n look for water 1st....

 

[Pete104]

Re: Merc 5.7L Thunderbolt 5 Ign, RPM bogging down at 3K occasionally??

If you got water past the filter into carb, get it asap!
Are you sure you didn't weeds or underwater growth wrapped up in the prop? Like the hat flew into the lilly pads!

 

[Eddie P]

Re: Merc 5.7L Thunderbolt 5 Ign, RPM bogging down at 3K occasionally??

Thanks gents for the quick replies. No chance of any weeds in the prop or shaft. We only boat on Lake Tahoe - 70 foot water visibility and no weeds in the lake, just sand, giant granite boulders and the occasional giant trout. Prop is ship shape and turns freely BTW, after pulling from the water.

I'll get a good look into the carb and dump the fuel filter into a bowl to check for water!!

 

[Eddie P]

Re: Merc 5.7L Thunderbolt 5 Ign, RPM bogging down at 3K occasionally??

PROBLEM SOLVED!!!!

I just wanted to update anyone who might come across this thread in the future with the same issues haunting their Mercruiser 5.7L.

Last fall we changed the oil, filter, fuel filter, etc. We didn't find any water in the fuel and everything looked normal and it ran fine on the trailer so we tucked her away for the winter.

This spring I did a full tune up in hopes of hosing the problem in a wide brush attack and it needed a tune up anyway as it was time. Spark plugs, wire harness, ignition, etc. The old plugs, when pulled, were fouled a bit. It still started and ran, but they looked like they were being run in a very rich environment. New plugs went in to replace.

Hoping it was the tune up that would make a difference! Off we went to run the boat. First couple days on the lake were perfect, very nicely running boat, good response on throttle up, etc. I suppose over those two days we put on about 2 hours of run time.

After sitting for a week dry and covered, out we went again yesterday. (nice warm day) Start up was fine but then she began to run wierd - some smoky exhaust and choppy, sorta rough idle. I took her out to see if I could clear the condition - and wouldn't you know it... the exact same thing happened as the initial problem only a little bit worse. (engine losing power, would die if I kept the throttle forward). After it died on me during trouble shooting a second time, we called off any more investigation on the water and successfully started up again. Back into port we trolled and let the kids play on the boat launch beach for the day (not a bad fall back plan, everyone but me being happy).

I called my marine mechanic while the kids played and he suggested I call his colleague, another independent marine mechanic who does more in depth work. We figured we would start with a compression check and then test run her on the trailer. He took the boat that day (yesterday) and I gave him all the details, etc. He right away suspected the choke. So he said he would run it in the morning and call me up.

Findings- Engine block was just fine, new (!!) plugs were dirty but not fouled yet. On running, the choke was going to full choke on, during acceleration! And sometimes just going full on when starting and staying there. Well we are at 6200 feet, and that's not good at all. Thus, we had a smoking gun.

The Choke was R&R'd and I had my boat back from him less tha 24 hours after him taking it on.

I took it out today and extensively tested the boat operation on the lake by myself under all the typical conditions I had trouble with on those two times before.

No problems! Ran her pretty fast too to clear out those plugs that suffered some extra rich mixture under that whacked out choke.

Also, the boat ran a whole lot better in general as I suspect the choke has been acting up in a less severe way for some time and running the engine rich. I noticed my fuel consumption going up last season hour for hour and at times I had some smelly exhaust after starting and would have to clean a mild exhaust line up near the stern of the boat after pulling her off the water. When the engine was really going through it's fits the exhaust scum line was worse. Now it is back to normal - non existent!

I'm a pretty happy camper as you guys can guess. We have a happy boat back and feel like we have been cleared for a really great boating season after a scare.

Hopefully this writeup will be of some help to someone searching these threads for some ideas.