95 Mercruiser 5.0lx stalls/stumbles after getting warm and sitting

[Badfish95]

I installed the new fuel pump this evening and took it to the lake just before sunset. It ran better than it has in a long time! It even seems to start up quicker. The overflow tube that runs to the carburetor was completely blocked. After letting some carb cleaner sit in the tube, I was able to blow it out. I'm pretty sure the fuel pump was my problem all along. We'll see this weekend if it continues to behave after sitting for a bit.

 

[Badfish95]

Unfortunately, the boat is still acting up.... It definitely seems heat related. The other night when it ran fine I didn't have the back seat in so the whole engine compartment was getting fresh cool evening air. I reinstalled the seat and the stumbling and hesitation is back. I have no idea what it could be at this point.

 

[alldodge]

What kind of boat is this?
Having the boat start to stumble when the seats are put back in it gets me thinking your air vents are clogged up

You installed a new fuel pump, but I would sure like to know what the fuel pressure is

 

[Badfish95]

The boat is a 1995 Crownline 202br with a Mercruiser 5.0lx, 4bbl quadrajet. The air vents were replaced when I restored the deck three years ago. When the seats go in it causes the temperatures in the engine bay to warm up. This in turn causes me to lose spark intermittently after sitting for awhile. When I first take it out it runs perfect. If I sit for awhile in the cove and then throttle it back up it hesitates, stumbles and sometimes even stalls out.

I have ruled out fuel as the culprit. New fuel lines, fuel pump, fuel filters and it ran the same way off of a portable tank with fresh Marina gas. I also checked the tank vent and it is free and clear.

I called a local Marine mechanic who I trust and went over everything I have tried. He explained that he has seen this a handful of times with Mercruiser TB IV systems. He stated that the ecm's are not made anymore and having chased these problems with multiple customers he now recommends swapping to a Mallory Marine Distributor. He is going to look up the part number and get back to me.

This isn't the first time I have heard this advice either. I spent $400 on the last ECM alone...

 

[Fun Times]

Before buying and switching the distributor/ignition assembly as that TB4 module isn't that weak to heat, plus the fact that you tried another one... 'You may want to rig up some sort of engine compartment outdoor air temperature wireless weather station so you can see what the temp is actually reaching in there as Mercruiser does not recommend the temp at the flame arrestor to go over 176 degrees. With today's technology this is just a quick example to consider using just for an idea starting point, http://www.ebay.com/itm/wireless-we...-humidity-rcc-dcf-with-2-sensor-/390654221049

And here's some points/concerns from Mercruiser about heat, http://www.boatfix.com/merc/bullet/95/95_12.PDF

Next time you're out and it starts acting up while underway, try lifting the engine hatch up a bit to get more air to see if it will start to run better all of a sudden at that same engine/boat speed.

Also be sure none of the reinstalled engine compartment cover/s aren't resting on any part of the engine restricting air flow and especially not touching the main wiring plug running between the dash and engine as it could be rubbing and causing intermittent electrical shorts.

 

[Badfish95]

Thanks for having a look Fun Times. I see your points and read the service bulletin from Mercruiser. The problem is, I reinstalled he ventilation hoses exactly as they came out. 3" vinyl vent hoses that were dry rotted were removed and 3" vinyl vent hoses reinstalled in the same locations. The engine compartment should actually breathe better than stock since I removed two pieces of vinyl wrapped plywood that fit in front of the battery on one side and trim pump on the other. The storage compartments are relatively free of equipment too. We have 3 life jackets that ride in each side of the engine. Also, I did not re-carpet the area behind the seat or the dividers. It is just painted plywood. This should be better for releasing heat instead of trapping it. I have a probe that plugs into my voltmeter and will take a temperature reading the next time I take it out, but I HIGHLY doubt this is my issue. In fact, when the boat acted up the other night, I put two bumpers between the hatch and the rear seat to give it a hood scoop effect. It did the same thing...

Here's the best photo of the engine compartment I have:



I am desperate to salvage this year's boating season, so..... I went ahead and ordered an MSD 83606 marine ready to run distributor and an MSD Blaster 2 coil from Summit Racing. This should bypass the TB 4 ignition module(which is obsolete) and rule out ignition problems. My mechanic said that he has run into this multiple times before and the new ignition system is his fix...

 

[NHGuy]

Please. please let us know what you get for results.
BTW if you still have the bog you may need to increase the secondary air door spring tension. It is supposed to be 3/4 turn from loose.
Many folks set them too loose thinking that if allowed to open easily it will accelerate well. But it causes the bog because it causes an over fuel condition. The engine can only use so much fuel before you get a rich condition.

 

[Badfish95]

It's FIXED! The new distributor and coil did the trick.
The MSD 83606 marine ready to run distributor arrived yesterday from Summit. Ordered on Tuesday night and arrived Thursday afternoon.

The installation is really straight forward. It comes with a three wire harness. Red to positive side of coil, orange to negative and black to ground. I used waterproof butt connectors that heat shrink to the wires for a good seal. Since I have an alpha drive, I also needed to connect the shift interupter to the negative side of the coil. The original grey tach wire also goes to negative on coil. The original keyed 12v purple wire attaches to positive side.

Spun the engine to TDC on cylinder one and dropped the new distributor in with the rotor facing the number one terminal on the cap. Installed the cap and transferred the wires over from original merc cap.
Once it fired up I set the timing to 8* BTDC. I then tightened the hold down clamp and Headed to the lake.

I ran it up to WOT. It must be propped right since with just myself in the boat it maxes out at 4,600 rpm. I then dropped anchor and let it sit for 45 min with the seat in and sun deck down. The engine room temp never got above 130* per my voltmeter thermometer. I might take my digital smoker thermometer out next time just to be sure.






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It ran great all afternoon and into the evening. Glad to be back on the water!

 

[Fun Times]

Good to hear it all worked out for you and thanks for the quick how to on installing the new distributor assembly from MSD.:thumb:

 

[Badfish95]

My son and I spent all Saturday on the Lake and the boat ran great! Boating is a lot more enjoyable when you have confidence in your equipment.

I thought I would follow up with how I set my MSD timing curve up. The kit comes with bushings and springs that are used to set the mechanical advance curve in the distrubutor. Previously this was all done by the Mercruiser Electronic Control Module. The timing curves for these modules can be found online and mine looks like this:



MSD has 6 different spring options and 5 different bushing options. The springs control the curve and the bushings control the total advance(does not account for initial or base timing). The 305 in my boat has a base time of 8* and 22* of additional advance for a total advance of 30*. The MSD comes with the blue bushing installed and is supposed to be 21* of advance. The next bushing up is a green one and is 23* of advance. I decided to play it safe and stick with the blue bushing. Next you need to compare your engines stock timing curve with the spring options supplied by MSD. I chose option D. A is the slowest advance curve and F is the fastest. I chose D since the MSD curve was within the min and max set forth by Mercruiser. The advance is all in with 29* total advance at around 3,000 rpm, but still within the min and max.