5.0L V8 Merc runs fine...shut down - won't start again??

[Viddi]

1992 5.0l V8 Alpha 1. New dissy cap, coil, sensor, plug wires and ECU

I have been having intemitent starting problems since I bought this thing, and I am almost ready to drill a hole in the hull!

General problem: She starts fine, runs fine (somtimes). Sometimes she'll stall out when I bring her down to idle speeds. When this happens it'll turn over strong, but won't start. If I leave it until the next day, she'll fire right up again.

The same happened when I ran for about 20 minutes. Dropped passengers off at shore...I killed the engine, but when I tried to start her up again...same thing.

Another interesting scenario - I had the Gimbal bearing, Ujoint bellow, and a whole new drive installed. Yesterday I was out for the break in and did about 3 1/2hrs of low speed trolling at about 1000rpm and she ran GREAT!! When I decided to head back in, I still had to keep it under 3500rpm (as per break in instructions), but I went a little faster 2300-2800 RPM and we decided to stop and cast for a bit. When I idled down, she stalled out, and wouldn't start again We towed her back in, and she started up fine again this morning

Sorry for the long story, but any insight or suggestions would be greatly appreciated!!!

PS. I originally thought it was a result of water in the fuel system. Earlier in the season there was water in the fuel water separater, but I think I have that sorted out now...

 

[Bifflefan]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

you didnt mention if you have checked the spark and fuel when its doing it thing.
Thats where id start and go from there.
you need to determine what kind of problem your having.

 

[jtybt]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

Shift adjustment or lower shift cable sticking.

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

you didnt mention if you have checked the spark and fuel when its doing it thing.
Thats where id start and go from there.
you need to determine what kind of problem your having.

Fuel - Yes, it is getting fuel.
Spark - I haven't been able to check yet when it is acting up...

What would happen only at higher speed/RPM that would keep it from sparking?

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

Shift adjustment or lower shift cable sticking.

Hmmmm, I think the shift cable is good. I had to adjust it when I installed the new drive, and it was smooth.

BTW I have the old style "two lever system" with throttle and shifting on different levers...I don't know if that has any bearing on what your train of though was...

 

[jtybt]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

If the lower shift cable is smooth, then it needs adjustment.

independent throttle/shift has no bearing on the issue.

 

[victorc1]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

i had the same problem with my mercruser 3.0l 1996 and it was the filter in the carburator follow the gas line in to carburator and there is a little filter in there, mine even had rust particles in it. i cleaned it out with carburator cleaner but that wasnt enough i took the carb apart and it had rust in the jets so i rebuilt it to.
p.s. check the fuel pump dont do like i did i started from the carb and i got stuck again because the fuel pump was rotten on bottom and wasnt pumping enough gas.

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

If the lower shift cable is smooth, then it needs adjustment.

independent throttle/shift has no bearing on the issue.

By smooth, I mean that when I took the cable off the shift bracket to adjust it, it was able to slide in and out easily. The instructions I was reading to adjust the shift cables said that if it doesn't slide smoothly, it should be replaced.

I am still unsure as to how the shift cable sticking would have any bearing on my problem. I didn't actually shift when it stalls, I just throttled down. Why would it work again a few hours later?

Could you please elaborate a little more...Thanks!

(BTW, my boat does not have a shift interrupter...the previous owner bypassed it.)

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

i had the same problem with my mercruser 3.0l 1996 and it was the filter in the carburator follow the gas line in to carburator and there is a little filter in there, mine even had rust particles in it. i cleaned it out with carburator cleaner but that wasnt enough i took the carb apart and it had rust in the jets so i rebuilt it to.
p.s. check the fuel pump dont do like i did i started from the carb and i got stuck again because the fuel pump was rotten on bottom and wasnt pumping enough gas.

Thanks for the feedback Victorc1. Was your problem exactly like mine as described?

Are you talking about the small canister like filter that is inline b4 the carb? I had that off and used some hose to pump fuel through it, but it seemed fine?

How tough/long does it take to rebuild a carb, or aprox how much does a shop charge to do it? I just want to get back on the water...I have missed most of the season messing around with little things

 

[jtybt]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

I'm curious how you shift out of gear without a shift interrupt switch.

I must have mis-read your post. I thought it was happening when you were shifting.

How does it idle? What is idle RPM? . I'll re-read again.




Are you carbed? Did you check the choke plate when it wouldn't start?

I'm doing a little too much guessing here. You have to give me as much info about your engine to see where the source of your problem lies.

Could be a bad neutral safety switch in the throttle lever. Any other indication there's an electrical problem?

You see...I can't tell if it's an electrical or fuel problem.

First thing is determine if fuel or electrical. Check for spark and look down carb and pump throttle to see if the accelerator pump has fuel to pump. You have to do these thing right after she won't start.

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

I'm curious how you shift out of gear without a shift interrupt switch.

I must have mis-read your post. I thought it was happening when you were shifting.

My apologies, I think I am confusing you more than I am helping you. Upon a little more investigation, yes, I do have a shift interrupter switch. I was referring to the neutral safety switch that has been disabled.

I am fairly confidant that it is not an electrical problem. I have gone through the starting flow chart, and replaced most of the electrical components.

You are right though, I need to check after the problem hits. The trouble is when it happens I have to get towed back to my bouy, b/c the launch is too far away so I can't pull it out. By that point it is usually just a matter of making sure we get back to shore safely. By the next morning she starts up fine again!

I was just hoping that somebody recognized the symptoms and a light may go off...

Thanks for your help so far. For what it's worth it i carbed with a TB4 ign.

I may be able to head down to the cabin tommorow and I'll try to replicate the problem and check things out when it happens...unfortunately no internet at the cabin

 

[jtybt]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

So it's very important to check for fuel as soon as the engine quits to see if there's any gas at all getting to the carb. Pump the throttle as someone watches for the accelerator pump squirting gas. Then pull a plug wire connected to an old spark plug touching the block and crank it (3-4 revolutions) to see if there's spark getting to the plugs. Two easy and important tests.

You also have to make sure the choke is fully open.

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

I haven't made it down to the cabin yet, but will be going later today.

I was however doing a little more reading and came across a few threads on vapor lock. Does anybody think this may be a possibility?

How long does it usually take before you can restart when it is vapor locked?

 

[jtybt]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

If there's not enuff air getting to the engine room or you have a small confined space for the engine, heat build up can vapor lock the fuel line. I have heard of ice wrapped in a wet towel around the metal line to the carb has worked to alleviate the problem. I can't attest as to the effectiveness.

What can happen, an air pocket/bubble can be trapped between the carb float needle and fuel pump in a high spot in the fuel line. The mechanical fuel pump on carbed GM engines is rated at 3-7 PSI. The pump may not be able to force open a stuck float valve.

This ain't the only way a vapor lock can happen.

I've had it happen on my bike before but not on a car or marine engine. This is only what I've heard.

 

[Viddi]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

Thanks for all the help everybody... I have spent many many hours learning on this site and it is an invaluable resource!

I just wanted to post for all the people who may read this in the future with similar symptoms - The carb was the culprit. It has been rebuilt and is running great now! I guess some of the gunk was getting sucked up and stuck. Then when it sat for a bit it would settle back and start again. Frustrating to say the least!

 

[batchawanabay]

Re: 5.0L V8 Merc runs fine...shut down - won't start again??

Congrats. on solving your problem. Wish someone would respond to my previous post. I have a near identical set of symptons to your situation but engine is 4.3 EFI. I have been suspecting some kind of fuel pressure problem on mine or vapour lock but can't believe heat is the problem because situation occurs even when engine cover is removed while underway and stopped.