4.3 engine stops running. Fuel pump?

[watrski4ever]

I have a 1997 4.3 V6, carbureted engine with a Thunderbolt V ignition system that has been dying after a few minutes of running perfectly. So far it has always started back up and runs perfect again until it dies again. I initially suspected ignition malfunctions because of the way it stopped, without spitting or sputtering or bucking, the things I would think would indicate fuel...but I was wrong. After checking spark as it died with an in-line spark tester, I could see it spark until it stopped.
So now I'm looking at fuel issues. Does anyone know if this engine is equipped with a low oil pressure cut-out? The oil pressure is good, but if the sender is malfunctioning, it may be sending a signal to the electric fuel pump. Sound plausible?

 

[Bondo]

Does anyone know if this engine is equipped with a low oil pressure cut-out? The oil pressure is good, but if the sender is malfunctioning, it may be sending a signal to the electric fuel pump. Sound plausible?

Ayuh,.... You should have one, down low, port side of the block, just above the oil pan,....

It's a simple pass-through switch to the fuel pump, 'n yes, they do go bad,.....

 

[alldodge]

I have a 1997 4.3 V6, carbureted engine with a Thunderbolt V ignition system that has been dying after a few minutes of running perfectly. So far it has always started back up and runs perfect again until it dies again. I initially suspected ignition malfunctions because of the way it stopped, without spitting or sputtering or bucking, the things I would think would indicate fuel...but I was wrong. After checking spark as it died with an in-line spark tester, I could see it spark until it stopped.
So now I'm looking at fuel issues. Does anyone know if this engine is equipped with a low oil pressure cut-out? The oil pressure is good, but if the sender is malfunctioning, it may be sending a signal to the electric fuel pump. Sound plausible?

Is this a carb and does it take some cranking to get it fired back up, or is it instant?

Edit: how about a serial number?

 

[achris]

Sounds very much like the oil pressure switch for the fuel pump. Most likely corrosion on the terminals, or a broken/corroded wire. The reason it starts back up and runs another few minutes is because there is a 'bypass' on the starter motor so that the pump is powered while cranking, before enough oil pressure is reached to power the pump through the switch, Hence you can start the engine, but when you release the key, power is no longer going to the pump (failed oil switch circuit), until it runs dry and stalls, when you hit the key again and give it another carb-full of fuel....

Location of that switch is at the back of the engine block, below the exhaust manifold, port side. Look for a pressure switch with a purple/yellow wire and a purple wire. If you have a test light, turn the key ON and probe each terminal. The purple should have power, the purple/yellow not. Start the engine, now both should have power. If not, find out why ...

Chris............

 

[watrski4ever]

1997 V6, 2 bbl carburetor, (engine SN OKO282235) MCM 4.3 LX Gen+. Starts right up typically. Thanks for all the help. I will do some more testing tomorrow.

 

[alldodge]

1997 V6, 2 bbl carburetor, (engine SN OKO282235) MCM 4.3 LX Gen+. Starts right up typically. Thanks for all the help. I will do some more testing tomorrow.

Your serial number is off by something but for a 1997 LX I'm seeing a TB-IV ignition. For it to start right back up with next to no cranking, there wouldn't be enough time to fill the carb back up. Now if it took a bit of cranking then the fuel pump would be on during cranking, and then off when key is released. This would cause me to agree with Bondo as the oil pressure switch. If it immediately starts back up with next to no cranking, then it might be something else

 

[achris]

Your serial number is off by something

Quite right. I think it should be 0K028235

but for a 1997 LX I'm seeing a TB-IV ignition

Not quite right... V6s changed from TB-IV (w/Gen II block) to TB-V (w/Gen + block) in mid 96....

Also, it could start up again if there were sufficient fuel still in the carb to get it to fire. The pump needs very little time to fill the carb, as the lines are already full, and it would only have to pump about 150cc.

Chris.....

 

[alldodge]

Not quite right... V6s changed form TB-IV (Gen II block) to TB-V (Gen + block) in mid 96....

I'm sure your right but looking at the Merc site I see this


That sure looks like a TB-IV to me

Also, it could start up again if there were sufficient fuel still in the carb to get it to fire. The pump needs very little time to fill the carb, as the lines are already full, and it would only have to pump about 150cc.

But by your leave sir

 

[Fun Times]

The sensor under the cap doesn't designate the ignition system to be a TB 4 or 5. You can even find that sensor on some EFI's.

Two ways to tell when you run the serial number is either look at the top line where it say "Service Literature" for MERCRUISER TB5, W/PS, SGLPCEX, WORO, GEN+ 4.3LX ALPHA.......Or go to wiring harness and electrical components and look at the design of item 34. Once you're familiar with the designs of the module shown in the parts catalog, it becomes easy.

 

[achris]

Same distributor used on TB-IV as TB-V... The only difference between the 2 systems is the module. The dissys are EXACTLY the same....

Here's what I saw when I put that serial number into the same Merc parts catalog as you...

 

[watrski4ever]

Quite right. I think it should be 0K028235



Not quite right... V6s changed from TB-IV (w/Gen II block) to TB-V (w/Gen + block) in mid 96....

Also, it could start up again if there were sufficient fuel still in the carb to get it to fire. The pump needs very little time to fill the carb, as the lines are already full, and it would only have to pump about 150cc.

Chris.....

You are correct, I mistakenly added an extra 2. 0K028235 is the correct SN. I assumed a TB-V beause my distributor is equipped with a knock module which, according to my shop manual, didn't come on the TB-IV. Whether that's correct or not; I don't know.

 

[Bondo]

Ayuh,.... The knock sensor is a T-Bolt V feature,....

 

[watrski4ever]

SUCCESS!! The problem was indeed the oil pressure switch. Now, does anyone have a part number for this thing? I did a search on ebay just using "Mercruiser oil pressure switch" as my keywords and didn't find one that looked like mine.

 

[achris]

Glad you found the problem. Here's your number, 87-864252A01, and a link to an ebay sale -> http://www.ebay.com/itm/OEM-MerCruis...f0aa1c&vxp=mtr

 

[watrski4ever]

Thanks and thanks again for all your help!!:joyous:

 

[achris]

You are correct, I mistakenly added an extra 2. 0K028235 is the correct SN. I assumed a TB-V beause my distributor is equipped with a knock module which, according to my shop manual, didn't come on the TB-IV. Whether that's correct or not; I don't know.

You're absolutely right. Knock sensors are only found on TB-V. But here's the hook. A system without a knock sensor could still be TB-V. The 5.0l Alpha engines don't have knock sensors, but are definitely still TB-V.... The only way to tell positively is to look up the number on the module...

Chris......

 

[DougIndianLake]

Sounds very much like the oil pressure switch for the fuel pump. Most likely corrosion on the terminals, or a broken/corroded wire. The reason it starts back up and runs another few minutes is because there is a 'bypass' on the starter motor so that the pump is powered while cranking, before enough oil pressure is reached to power the pump through the switch, Hence you can start the engine, but when you release the key, power is no longer going to the pump (failed oil switch circuit), until it runs dry and stalls, when you hit the key again and give it another carb-full of fuel....

Location of that switch is at the back of the engine block, below the exhaust manifold, port side. Look for a pressure switch with a purple/yellow wire and a purple wire. If you have a test light, turn the key ON and probe each terminal. The purple should have power, the purple/yellow not. Start the engine, now both should have power. If not, find out why ...

Chris............

Do you have any idea why someone would have bypassed the oil pressure switch?

I got here b/c I was have the exact same issue as watrski4ever. When I went to look at the oil pressure switch I saw that it was rigged so that the purple wire and the purple/yellow wire were taken off the leads and joined together. I bought the boat this way and it's been running great for 2.5 seasons. Now it started stalling. I simply connected the wires to the oil pressure switch as they should be connected and my problem was solved; however, for the life of me, I can't think of why it was done in the first place, why it ran great for years, and why putting it back would fix this issue.

Doug

 

[achris]

Do you have any idea why someone would have bypassed the oil pressure switch?

I got here b/c I was have the exact same issue as watrski4ever. When I went to look at the oil pressure switch I saw that it was rigged so that the purple wire and the purple/yellow wire were taken off the leads and joined together. I bought the boat this way and it's been running great for 2.5 seasons. Now it started stalling. I simply connected the wires to the oil pressure switch as they should be connected and my problem was solved; however, for the life of me, I can't think of why it was done in the first place, why it ran great for years, and why putting it back would fix this issue.

Doug

2 things.

1. Often the bypassing is done to solve a bad connection problem, or a failed oil pressure switch. The other reason it might be do is so the pump fires up and runs as soon as the key is on the ON position, effectively priming the carburetor. This is particularly 'nice' for the dribbly Webers of the 90s. Not great if you leave the key on though.

2. On our forums we prefer people to start they own threads rather than hijack someone else's.

Cheers,

Chris....

 

[DougIndianLake]

2 things.

1. Often the bypassing is done to solve a bad connection problem, or a failed oil pressure switch. The other reason it might be do is so the pump fires up and runs as soon as the key is on the ON position, effectively priming the carburetor. This is particularly 'nice' for the dribbly Webers of the 90s. Not great if you leave the key on though.

2. On our forums we prefer people to start they own threads rather than hijack someone else's.

Cheers,

Chris....

Thanks for the reply Chris. And sorry about hijacking the thread. I am not a frequent user of forums and I thought that keeping it within the topic of oil pressure switches would be more efficient than starting a yet another thread on oil pressure switches. I will continue this conversation on a new thread as you suggest.