Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

I am getting my boat ready for the season and here is what is going on. So far, I have only ran the boat out of water in my driveway.

Engine SN: OD524716 Thunderbolt IV Ignition

The engine starts fine. I let the engine get up to operating temp and let it idle. It will idle fine for around 5-10 minutes, sputter for a minute and then shut off. I hooked up an axilery gas tank and it did the exact same thing. I have been able to recreate this problem almost 10 times. When the engine stalls, it can be tricky to start. ESP, if I leave the muffs on and the water running. If I turn the water off and let it sit for a couple of minutes, it starts right up. If I leave the water on and let it sit for a couple of minutes, it can be tricky to start.

When the boat is idling in the driveway the temp gauge only shows ~135*. If I idle the boat higher the temp will rise to about 145. I can also get the engine temp to rise to 145 if the slow down the flow of water to the muffs using the value on the hose. I have not found the perfect flow of water to get the engine to idle at the proper engine temp. The engine has not stalled when the engine temp reads ~145* due to increasing the idle or cutting back the water to the muffs.

On the water, while running the engine, the engine temp gauge usually shows ~155.

So, is running the engine at a cold engine temp for an extended period of time causing the engine to die at idle? I have a well and the water coming out of the tap is cold. The combination of cold water and pressure from the hose is keeping the thermostat open and the engine running pretty cool.

I am a crazy for even thinking this?

I know with Fuel Injection computer controlled engines there are controls with in the PCM which modify spark and fueling based on engine temp to encourage the engine to warm up faster, idle higher, etc. Is the engine reaching some sort of weird glitch in the Matrix where it expects to be running warmer but it's not so it gets fussy and putters out?

Pretty much everything else looks good.

I changed the plugs, engine oil, impeller, water/separator fuel filter, fuel filter, the battery gauge shows good voltage, and the engine will rev strong.

I believe the thermostat is working properly. I cut the water to the muffs and let the engine warm up to 180*. When I turned the water back on, the temp gauge went down to 155* pretty fast. As I keep the water running, it slowly creeped back down to 135*-140*.

Now that I think of it, I am going to test the thermostat in a pot of hot water...

Any other suggestions?

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

The thermostat tested fine.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

Not unusual for engines to run much colder (just on thermostat opening temp) when on the muffs, there's no load whatsoever on the engine.. Just one thing, never shut the water off to the muff, or even slow it down. It will damage the impeller.

The TB-IV does not have any temperature inputs, it's just a spark advance controller based purely on engine speed, which it derives internally.

From your serial number it appears you have a 2 bbl Mercarb carburetor, and that's probably the second place I would look. The first place is to check that when it has stalled and will not re-start, see if there is spark. The spark should be strong and blue, and jump a minimum of 1/2"....

The other quick check is to make sure the choke is opening fully.

Chris.......

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

Thanks for the advise Chris.

Yes, I have the 2 bbl Mercarb. I did check the spark to one of the spark plugs when I was having problems with starting and there was spark. I don't know how to check the spark in other locations. The carb has an electronic choke and the only thing I know it is doing is warming up the fuel/air. I don't know how to check to see if the choke is open.

The boat is a 1991 Thompson with VERY low usage. This is my second season with the boat so I am still learning. When I changed the thermostat and fuel filter last year, I broke the factory paint off the bolts. Almost everything is original. It's over due for an overhaul.

The boat is picky about gas but other then that, it ran excellent last summer. I am going to put it in the water today and see how it performs.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

when the engine is cold and turned off, move the throttle lever to forward gear full throttle to give it a good pump of fuel, then return the lever to neutral. Take the flame arrestor off the carb. The top plate of the carb should be fully closed preventing you from looking down the carb.

if it is fully closed, start the engine. As the engine warms up, that plat should slowly begin to open and will open all the way until it is about 90* vertical.

if any of that is not happening then your choke is not working correctly and needs adjustments.

Engine must be cold/cool, and you must pump the throttle 1-2 times before starting the engine as this lets the choke plate close.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

sti1471 said:

when the engine is cold and turned off, move the throttle lever to forward gear full throttle to give it a good pump of fuel, then return the lever to neutral.

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Does the throttle lever need to be in gear for this? I can pull the lever toward me to move the throttle without being in gear. This is what I do to start the engine when it is cold. Give it 1 pump by moving the level all the way forward. Then return the throttle to neutral. Then bump the throttle forward to where it would idle ~1500. Under normal conditions, the engine would start 99% of the time.

sti1471 said:

Take the flame arrestor off the carb. The top plate of the carb should be fully closed preventing you from looking down the carb.

if it is fully closed, start the engine. As the engine warms up, that plat should slowly begin to open and will open all the way until it is about 90* vertical.

if any of that is not happening then your choke is not working correctly and needs adjustments.

Engine must be cold/cool, and you must pump the throttle 1-2 times before starting the engine as this lets the choke plate close.

Click to expand...

I don't think the choke plate is closing. At least I have not noticed it ever closed in all the testing I have been doing. I will give it another shot.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

My test on the water was unsuccessful.

The boat started fine and I idled out a couple hundred feet to where the water was deep enough. I pushed the throttle all the way down and the boat got up to plane as normal. It was running just over 4200rpms like normal and almost to full speed. A minute or two later, the engine started to pop. The boat would lose power and try to regain it and then pop again. I slowed down and back off the rpms and the engine stalled. It was tricky to start. It idled okay but if I put any load on it the engine would pop is stall. I trolled it back to the launch at just over 1000rpms and the engine stalled.

My cousin was ready with his boat so he pulled me back to the launch.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

Since it does it on the main and the side tank I'd check for fuel at the carburetor itself. Next time you run it get some carb cleaner or a tspn of gas. After she quits shoot a small amount of either fuel in the top of the carb. Bet it starts. However briefly, it will die out pretty quickly though.
If it does you have fuel starvation happening. With the age and unknown previous care, I'd check the carb over. It may have a poor float chamber fill rate from a sticking float or clogged needle valve. Did the fuel in the old filters smell OK?

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

gmredline said:

I believe the thermostat is working properly. I cut the water to the muffs and let the engine warm up to 180*. When I turned the water back on, the temp gauge went down to 155* pretty fast. As I keep the water running, it slowly creeped back down to 135*-140*.

Click to expand...

OK it sounds like your carb needs gone through and the idle screws readjusted...

But you are turning the water off while its running??? If so you better put another water pump impeller in the drive,, it only takes 5 seconds to wipe out an impeller with no water

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

gmredline said:

My test on the water was unsuccessful.

The boat started fine and I idled out a couple hundred feet to where the water was deep enough. I pushed the throttle all the way down and the boat got up to plane as normal. It was running just over 4200rpms like normal and almost to full speed. A minute or two later, the engine started to pop. The boat would lose power and try to regain it and then pop again. I slowed down and back off the rpms and the engine stalled. It was tricky to start. It idled okay but if I put any load on it the engine would pop is stall. I trolled it back to the launch at just over 1000rpms and the engine stalled.

My cousin was ready with his boat so he pulled me back to the launch.

Click to expand...

You didnt mention what your water temp gauge showed during this test. Because you turned off your water to your muffs and probably burned out your impeller, you may have overheated the engine during this run. (after restoring water to the engine/muffs, the temp most likely went down because of the water flow from the hose, not because of flow from the impeller. There is an off-chance you got lucky, but experience says this is probably not the case.). best of luck.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

The engine temp was right at 150*. I am done messing with it for the season. I am planning some upgrades to my engine for the winter. Maybe a full 5.7L swap or vortec heads, intake manifold and carb. I got some research to do and then I will probably start a new thread.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

Does this spring belong to my fuel system?


I found it laying on top of my intake manifold.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

gmredline said:

Does this spring belong to my fuel system?

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Looks kinda like an accelerator pump return spring, or maybe carb internal filter relief spring?

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

gmredline said:

Does this spring belong to my fuel system?

I found it laying on top of my intake manifold.

Click to expand...

It actually looks like the spring that goes in the fuel pump and sits on top of the fuel filter holding it in place. If you are missing that from your fuel filter, then your fuel filter is not being held in place, and it letting a lot of un-filtered fuel pass by into the carb. It can plug your carb rather quick causing the issues you are having.

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

It very well might be. I did not find the spring until after I changed my fuel filter. I didn't want to tie the two together because the spring could have been there before. I thought I was safe because I watched these youtube vids of rebuilding a merc carb and did not see the spring anywhere in the vids.

MerCarb Marine Carburetor Rebuild - Part 1 - YouTube
Mercarb Marine Rebuild Parts 2 - Assembly - YouTube

Re: Can an Engine run too cool? 5.0L Mercruiser stalling at idle on Muffs

Don't tie two springs together. If you already have a spring in the pump, above the filter, then that's what should be there.

There is also a spring behind the fuel filter in the carb fuel inlet. That spring in your picture looks very much like it... (#9 in this picture)