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1988 Mercruiser 120 2.5L stalling after warming up

johnnyseko

johnnyseko

New member
*Reposting in the correct forum...

Just joined to try to get some more insight into my unresolved problem - and hopefully post positive results after repair.

My 1988 Cobia Monte Carlo is giving me a headache -

Starts fine, idles fine and I can idle out of the no wake zone no issues. Run it up to @3000rpm, running great and then within 5 minutes will sputter and stall. Will restart, but will not go above idle without stalling again. Then while idling back to the dock will run for approx 5min before stalling again. Then have to allow to cool for a few minutes and it will start and idle, repeating the process until I get back to the dock (This cycle went on for about 6 stalls/restarts until I got to the dock). Verified this issue over multiple days while trying to repair.

This problem suddenly appeared after running fine for the last season and beginning of this one. I thought for sure a fuel issue as it occurred same day I had just topped it off at the marina and we had gotten a ton of rain just prior. Here is the current work I've done so far and it still isn't fixed -

-New Fuel lines
-Cleaned and straightened breather line (there was a low spot collecting fuel I eliminated)
-New anti siphon valve
-Emptied gas tank and refilled with fresh fuel from marina
-Added new water separator housing and filter
-Changed fuel filter at fuel pump
-Cleaned filter at carb inlet
-Cleaned air filter/flame arrestor and checked butterfly for proper movement
-New condenser
-New spark plugs
-New ignition coil

Also, in 2021 I rebuilt the carb. New points, rotor and condenser. New fuel pump. Adjusted the timing. Was running fine since that work was done. Also new spark plug wires 2022.

Not really sure where to go from here (besides a real mechanic). Seems like an electrical issue when it gets hot? I did double check the temps with an IR temp gun and nothing is out of the ordinary and all engine gauges are good when running.

Thanks for your time.

**I've since just brought it to the mechanic. We'll see what he says. I ran it over to the marina @ 1000rpm and it took just under an hour to get there. Didn't have an issue at all and it killed me not to try to get it up on plane...
 
Take gas cap off when it happens to see if you have a plugged fuel tank vent, or can run it off a remote 6 gal OB tank.

Do you have spark when this happens? Do you have fuel being supplied to the carb when this happens?
 
Thanks, vent was cleared and working not obstructed. I wasn’t able to check the fuel pressure or spark. Unfortunately I ran out of DIY time and brought to the mechanic. I’m waiting to hear back from him. I’ll post results.
 
I finally got the boat back repaired after 2 weeks. I was never able to speak directly with the mechanic but he had a heck of a time getting it fixed. (my hobbs logged 19hrs!) The manager of the marina told me that the mechanic thinks 99% it was the points. That said, he also cleaned the carb and changed the fuel/water separator (which was brand new when I brought it in). I know he ran it on a remote tank as well and the engine still stalled. He had to be towed back in a few times. I wish I had more details but that was all I got.

So this brings up a follow up question - How often should I change the points? They were less than 2yrs old. Should I just plan on replacing the points/rotor/condenser every spring? It's certainly easy enough.
 
When I worked in the industry,
I would accumulate at least 2, 1 gallon zip lock bags of points, condensers and rotors every spring.
 
Points should be replaced annually if the boat has been run a lot, but it's not necessary if it is only used a few times.

Do you turn the key to ON and leave it? If the points are closed, that burns them fairly quickly. Also, make sure the engine block ground is great, clean & tighten the battery cable clamps and make sure the resistance from the distributor to the block is very close to ) Ohms. The resistance form the block to the battery POST (not the cable clamp) is as close to 0 Ohms, too. This is important.

Are the battery cables original? If so, you might think about replacing them, especially if the ends near the battery are more stiff than the rest of the cable. I replaced mine since the boat is 39 years old and I don't know the boat's history.

I would also ask- how did you gap the points and did you measure the dwell?

Did you check the compression?

You need to connect a timing light to make sure the spark is still there- if you can, connect a spark intensity gauge like the one in the photo.

41inMkG0iFL._AC_UL232_SR232,232_.jpg


You also need to find out whether it's a spark or fuel problem- when I was trained by Mastercraft, the instructor told us that he had always used a plastic bottle with some gasoline in it to add gas when the engine faltered and I have used one many times to identify a problem when others had been stumped. You can loosen the nut on the flame arrester- if it backfires, it's not going to blow the arrester off, but it does provide a small gap for the gas to be sucked in. If the problem is corrected, look at the fuel system.

The fact that you need to cool down tells me that you may have a vapor lock issue- run the blower and if the problem ends, improve the air circulation volume going into the engine compartment. Did you check the fuel pump diaphragm?
 
Thanks guys! I've put a few hours (@4) on it since getting it back with no issues - except yesterday. I ran it for about an hour, 1/2 time @idle (no wake) and 1/2 time at @3000rpm. Well, there were races on the lake yesterday and I had to cross the course to get home. When the police told me it was ok to go across I got it just under 4000rpm to get through quickly - just before I got to the other side pylons it stuttered and stalled..... same issue as before! Right in front of all the anchored boats watching the races...UGH. I was ready to throw my anchor but gave it a minute and was able to start the engine after a couple of tries. It stayed running and I put it in gear and idled all the way home (@30min) without it stalling again.

I'm leaning toward vapor lock being the issue? It was pretty hot here yesterday. I'll try the blower trick, then I've got to figure how to increase air circulation. This may run into next season as I have to pull the boat in a couple of weeks.

That's a good list, jimn.

I'm going to take a closer look at the ground and battery cables as well as resistance.

I did gap the points and measure dwell angle with a gap tool and multimeter. Compression checked ok too. Did the timing with the light and checked for spark.

The boat stutters and stalls so quickly I don't think I'd be able to add gas to stop the stall in time. I did install a new fuel pump 2yrs ago. I haven't checked fuel pressure (need adapter) but I did install a new clear tube that is supposed to show when the diaphragm goes bad (it runs from the pump up to the carb and should never have fuel in it, which would indicate a diaphragm leak)
 
1. Lake test free from ANY activity so if an issue ocurrs you can freely float and trouble shoot. Never use the boat in a hazardous situation until you are 100% confident it WONT stall.....ugh!

2. Once you can lake test all alone and far from shore, have basic test equipment with you, if it stalls, immediately check for spark with spark checker plugged into one spark plug wire. If you have spark then that is not your issue.

3. If spark then remove flame arrestor and pump fhrottle to see if gas squirts into carb. If yes then its not fuel. If NO then its a fuel issue.

4. If spark and fuel are ok and it wont start then check choke to see if it closed.

5. If nothing is obvious then check all wiring connections to confirm they are clean and tight at coil and distributor. Make sure they are not brittle or cracked insulation.

The fact that you were running at high rpm and then it "shuttered and stalled" could be either fuel or ignition related.

Pleasse describe what SHUTTERED means to you? Like, " it seemed to run out of gas" or it started to vibrate like a miss fire
 
I wasn't lake testing it. I received it back from the mechanic a couple of weeks ago claiming problem fixed. I have since taken it out a few times without issue. I thought the problem had been fixed.

I have to go through the wiring. The spark plug wires are all new as of last year and in good shape.

Shuttered - I'd say an immediate drop in RPM, down to @1000rpm and stalls from there - yes like it ran out of gas.
 
If you really feel its a running out of gas issue,
Pay very close attention to gas tank level.

It ran great for several outings, what gas level did you start with and what was it when the issue ocurred? How many if any times havs you refueled and what level was it at when refueled?

What did the mechanic find as the root cause of the problem?
 
johnnyseko said:
Thanks guys! I've put a few hours (@4) on it since getting it back with no issues - except yesterday. I ran it for about an hour, 1/2 time @idle (no wake) and 1/2 time at @3000rpm. Well, there were races on the lake yesterday and I had to cross the course to get home. When the police told me it was ok to go across I got it just under 4000rpm to get through quickly - just before I got to the other side pylons it stuttered and stalled..... same issue as before! Right in front of all the anchored boats watching the races...UGH. I was ready to throw my anchor but gave it a minute and was able to start the engine after a couple of tries. It stayed running and I put it in gear and idled all the way home (@30min) without it stalling again.

I'm leaning toward vapor lock being the issue? It was pretty hot here yesterday. I'll try the blower trick, then I've got to figure how to increase air circulation. This may run into next season as I have to pull the boat in a couple of weeks.

That's a good list, jimn.

I'm going to take a closer look at the ground and battery cables as well as resistance.

I did gap the points and measure dwell angle with a gap tool and multimeter. Compression checked ok too. Did the timing with the light and checked for spark.

The boat stutters and stalls so quickly I don't think I'd be able to add gas to stop the stall in time. I did install a new fuel pump 2yrs ago. I haven't checked fuel pressure (need adapter) but I did install a new clear tube that is supposed to show when the diaphragm goes bad (it runs from the pump up to the carb and should never have fuel in it, which would indicate a diaphragm leak)
Click to expand...
How much fuel was in the tank when it stalled and how accurate is the fuel gauge? The gauge may show 1/4 tank, but if it's a 20-25 gallon tank like the one in my 4 Winns, the total depth is only about 5"-6", so 1/4 tank means the gas is only 1-1/2" deep and when it sloshes because of deceleration and waves, it can easily cause a disruption in fuel flow.
 
I'm pretty certain I've eliminated any fuel issue besides the fuel pump, which hasn't shown any signs of abnormal behavior. I always keep the tank over half and when I did drain the tank I checked the operation of the fuel gauge with a multimeter. The mechanic said he was 99% sure it was the points.

Yesterday I removed the carburetor and disassembled it. Bowl was just clean fuel. I checked the float drop and cleaned everything inside and reassembled. Engine started right up and needed no adjustments. While I warmed the engine I rechecked the dwell and timing and both are spot on.

I'm not sure where I saw it but someone suggested to check all wires by shaking them while the engine is running so I grabbed the harness and shook....engine died. I restarted the engine and did it again. Engine quit again. I unplugged the harness and it was dry and nasty (I'll see if I can upload pics). After cleaning and using some dielectric grease on the connections I started the engine again. I shook the harness, engine kept running. I went though the other wires and engine kept running. I decided to take the boat out. After 30 min engine stalled from 3000rpm to @1000 but didn't quit. I idled home and ran out of time for more repairs.

So now I think I'm zeroing in on a wiring / grounding issue? These are some old wires and I'll have to go through and possibly replace whatever I can. Hopefully I'll get back on it later this week with a multimeter in hand.
 

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Pull that connector apart again,
All the male pins have slots. CAREFULLY spread them a very small amount then plug back in.
Also if there was no hose clamp on the connector install one. This was done for these many years ago to keep good contact berween pins and sockets for that connector. Hose clamp is to be fairly tight.
Every boat I work on has this including mine....mine is a 1987....
 
johnnyseko said:
I'm pretty certain I've eliminated any fuel issue besides the fuel pump, which hasn't shown any signs of abnormal behavior. I always keep the tank over half and when I did drain the tank I checked the operation of the fuel gauge with a multimeter. The mechanic said he was 99% sure it was the points.

Yesterday I removed the carburetor and disassembled it. Bowl was just clean fuel. I checked the float drop and cleaned everything inside and reassembled. Engine started right up and needed no adjustments. While I warmed the engine I rechecked the dwell and timing and both are spot on.

I'm not sure where I saw it but someone suggested to check all wires by shaking them while the engine is running so I grabbed the harness and shook....engine died. I restarted the engine and did it again. Engine quit again. I unplugged the harness and it was dry and nasty (I'll see if I can upload pics). After cleaning and using some dielectric grease on the connections I started the engine again. I shook the harness, engine kept running. I went though the other wires and engine kept running. I decided to take the boat out. After 30 min engine stalled from 3000rpm to @1000 but didn't quit. I idled home and ran out of time for more repairs.

So now I think I'm zeroing in on a wiring / grounding issue? These are some old wires and I'll have to go through and possibly replace whatever I can. Hopefully I'll get back on it later this week with a multimeter in hand.
Click to expand...
The connections do look like crap- that would definitely cause problems. Was the plug partially dislodged? The corrosion/dirt would replicate grounding problems and voltage loss- if the pins that correspond to the purple ignition wire have high resistance, the voltage will drop low enough that it won't allow the coil to saturate properly and that means the spark won't be as intense as it needs to be. The coil's secondary output needs to be very high- HEI is in the 50KV range, IIRC, a points ignition is in the 35-40KV range.

Sounds like you nailed it.
 
jimn said:
The connections do look like crap- that would definitely cause problems. Was the plug partially dislodged? The corrosion/dirt would replicate grounding problems and voltage loss- if the pins that correspond to the purple ignition wire have high resistance, the voltage will drop low enough that it won't allow the coil to saturate properly and that means the spark won't be as intense as it needs to be. The coil's secondary output needs to be very high- HEI is in the 50KV range, IIRC, a points ignition is in the 35-40KV range.

Sounds like you nailed it.
Click to expand...

Not quite,

Point ignition factory coil output is typically 10,000 to 20,000 volts MAX.

HEI/Electronic are typically 20,000 to 40,000 volts
 
kghost said:
Not quite,

Point ignition factory coil output is typically 10,000 to 20,000 volts MAX.

HEI/Electronic are typically 20,000 to 40,000 volts
Click to expand...
I haven't worked on many points ignition engines in the last 20 years and haven't looked into the specs since before that- the HEI I was referring to are the kind on GM injected engines, which were in the 50KV range, as of the time I went to Mastercraft training in the late-'90s. 40K vs 50K- not sure it feels very different when one hand is on the top of the cap and the other is on the exhaust manifold, though. :) Reminded me of the scene in 'Down Periscope' when the guy was checking the wiring with a pencil.....
 
jimn said:
I haven't worked on many points ignition engines in the last 20 years and haven't looked into the specs since before that- the HEI I was referring to are the kind on GM injected engines, which were in the 50KV range, as of the time I went to Mastercraft training in the late-'90s. 40K vs 50K- not sure it feels very different when one hand is on the top of the cap and the other is on the exhaust manifold, though. :) Reminded me of the scene in 'Down Periscope' when the guy was checking the wiring with a pencil.....
Click to expand...
Considering we are NOT working with Fuel Injection the 50k coil is not relevant.

The numbers I posted come from Merc service manuals for non EFI engines.

A point ignition with at best 9-10 vdc to coil when running is hard pressed to produce greater than 20kv
 
Thanks a ton for your input!

Friday I looked through all of the wiring. I pulled the harness again, pried apart the male connectors slightly (great idea!), cleaned again and greased. I made sure the clamp holding it together was tight when I reassembled. I also checked the ground voltage at every point I could find. It seems the engine block fluctuated. I removed, cleaned and greased every connection I could get at. The battery terminals didn't look too bad but I'll probably replace in the spring anyway. I also replaced some wiring connections with crimp connections and shrink tube. There was one quick connect bullet purple wire at the alternator that was loose (those are in the manual that come with it but I'm not a fan). Also a jump from the harness the previous owner had done that I had to replace (Pretty sure it was the temp sender).

I took it out and ran for just under an hour. No issues.

I just took it out yesterday again (after a long rainy weekend). Had to drop it at the marina for winterization. Before dropping it off I ran it for just under 2 hours. Various RPMs but tried to keep it over 3000RPM, NO ISSUES!!

So I have to imagine that it is fixed. I'll have to wait until spring to get it back and see how she does.

Thanks again!
John
 
johnnyseko said:
Thanks a ton for your input!

Friday I looked through all of the wiring. I pulled the harness again, pried apart the male connectors slightly (great idea!), cleaned again and greased. I made sure the clamp holding it together was tight when I reassembled. I also checked the ground voltage at every point I could find. It seems the engine block fluctuated. I removed, cleaned and greased every connection I could get at. The battery terminals didn't look too bad but I'll probably replace in the spring anyway. I also replaced some wiring connections with crimp connections and shrink tube. There was one quick connect bullet purple wire at the alternator that was loose (those are in the manual that come with it but I'm not a fan). Also a jump from the harness the previous owner had done that I had to replace (Pretty sure it was the temp sender).

I took it out and ran for just under an hour. No issues.

I just took it out yesterday again (after a long rainy weekend). Had to drop it at the marina for winterization. Before dropping it off I ran it for just under 2 hours. Various RPMs but tried to keep it over 3000RPM, NO ISSUES!!

So I have to imagine that it is fixed. I'll have to wait until spring to get it back and see how she does.

Thanks again!
John
Click to expand...
That loose purple wire could easily have been the cause- that's for the field coil in the alternator and if it loses 12VDC, the alternator will no longer output voltage.
 
The two bullit connectors at the Alt should have a hard plastic shrink tubing completely covering the connections.

When you replace the Alt with a new OEM it comes with the heavy duty shrink tubing.

You may want to consider using high quality shrink tubing in the spring and if you use standard sfuff then use two to three at each connection. Install the two or three on the wire (s) and pull one at a time and shrink, then pull another over shrunk and shrink it....you get the point.
 
kghost said:
The two bullit connectors at the Alt should have a hard plastic shrink tubing completely covering the connections.

When you replace the Alt with a new OEM it comes with the heavy duty shrink tubing.

You may want to consider using high quality shrink tubing in the spring and if you use standard sfuff then use two to three at each connection. Install the two or three on the wire (s) and pull one at a time and shrink, then pull another over shrunk and shrink it....you get the point.
Click to expand...
The clear or black plastic shouldn't be hard if it was OEM, it should be semi-soft vinyl and the bullet connectors are used for serviceability by factory-authorized service techs- warranty work doesn't always pay full shop rate, so any time that is required for making complete wire repair connections is wasted as long as the seal works well. It doesn't seem so bad when someone repairs the connections for their own application, but when someone has a lot of boats to work on in a day/week/month, the little extra time really adds up.
 
jimn said:
The clear or black plastic shouldn't be hard if it was OEM, it should be semi-soft vinyl and the bullet connectors are used for serviceability by factory-authorized service techs- warranty work doesn't always pay full shop rate, so any time that is required for making complete wire repair connections is wasted as long as the seal works well. It doesn't seem so bad when someone repairs the connections for their own application, but when someone has a lot of boats to work on in a day/week/month, the little extra time really adds up.
Click to expand...
Its soft when new.
After many years the oem shrink tubing becomes quite hard and possibly brittle
 
The shrink butt connectors I'm using seem ok for now. The tubing seems pretty thick compared to the standard shrink tubing you'd get if you just got the tubing separate from the connector.

The one I replaced (purple wire) was pulling out of the old shrink tube and loose. No idea if/when it had been replaced but it was definitely dry & brittle.

I've got some time to think about upgrades and further repairs over the winter. I'm considering getting a new harness but right now not sure it'll be worth it. That only covers about half the wiring anyway. I definitely have to clean and tidy the wiring up. I got this boat in 2020 (my first boat) with no idea how much time is on the engine. It was like a time capsule though - stored in a boat house on a lift most of the time. The guy I got it from inherited it and it became obvious hadn't driven it much, if at all, when we took it out for a test drive.

Right now I've got the mechanic at the marina just doing the standard preventive maintenance and the lower unit I'm unable to service. The bellows was replaced back in 2020 as soon as we got it delivered it was leaking.

I'm open to any recommendations or advice from owners of older boats. The last mechanic I called to have a look at it said they don't get involved with old boats....seems the norm for most marinas here on the lake.
 
1988 is not old. It is Alpha one gen 1.

Hundreds of thousands of these still operational.

My boat is a 1987......
Most of the boats I work on are 1980's and some in the '90's.

The Alpha one gen 1 generation of product is one of Mercury' s best designs.

Almost 30 years and still going strong.
 
kghost said:
1988 is not old. It is Alpha one gen 1.

Hundreds of thousands of these still operational.

My boat is a 1987......
Most of the boats I work on are 1980's and some in the '90's.

The Alpha one gen 1 generation of product is one of Mercury' s best designs.

Almost 30 years and still going strong.
Click to expand...

Great to know!!!
 
kghost said:
Its soft when new.
After many years the oem shrink tubing becomes quite hard and possibly brittle
Click to expand...
I have seen it become harder, but I bet it becomes brittle in high temperatures. Still, the previous owner of my '84 4 Winns redefined 'overheat' and none of the insulators are hard. Having worked on literally a couple thousand boats, I haven't needed to replace more than a few terminals because of this, but the unsealed butt splices, spade/quick disconnects and bullet type? Far more than should have been necessary. Yeah, sealed or Heat N Shrink terminals cost a lot more but c'mon! Even in boats that sold new for more than a million dollars use crappy PVC-insulated terminals that sell for a couple of cents apiece and that's inexcusable!
 
kghost said:
1988 is not old. It is Alpha one gen 1.

Hundreds of thousands of these still operational.

My boat is a 1987......
Most of the boats I work on are 1980's and some in the '90's.

The Alpha one gen 1 generation of product is one of Mercury' s best designs.

Almost 30 years and still going strong.
Click to expand...
35 years is old, for a boat unless it has been well-maintained (not saying anything about the OP's boat). You know that many parts are becoming more difficult to find and I know servicing dealers who don't work on anything past about '87 but they need to work quickly if they want to stay on top of the boats coming in for service since it's not always easy to have enough techs who are trained in (and who know) older boats. Hell, it's hard to find techs who know much about the electrical side of this kind of work and then, Mercury came up with DTS and that confuses a lot of experienced techs.

The first time I went to service training, the topics on that day were electrical and at the end of the day, the test required us to connect a Bosch relay to a light to the NC terminal and a bilge pump to the NO, then switch between them. The group who were attempting this ahead of me looked at the relay as if it was some mystical jewel, almost like Dan Aykroyd & Steve Martin on Saturday Night Live in their 'What the hell is that?' sketches.
 
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