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I have a 5.7 TBI merc cruiser that was replaced 3 years ago.

I have a 5.7...

I have a 5.7 TBI merc cruiser that was replaced 3 years ago. these problems started after the new motor was installed. First, I am not able to get more than 4000 rpms from this motor when it should be turning at least 5000. With that in mind, after driving for a while or pulling a skier, the motor will back fire. It sounds as if it is running lean when it back fires. The new motor was a long block with all the othe items switched over to it. I have replaced plugs, wires, mapping sensor(s) rotor, cap, fuel filter. The dealer that did the work says the he thinks its the computer. Other persons I have talked to, say it sounds as if its a fuel issue. The fuel pump, and computer are the only two things that have not been replaced.

Mechanic's Assistant: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else important you think the Marine Mechanic should know?

Not that I can think of. Thank you,

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Answered in 1 minute by:
3/7/2018
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,826
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
Verified

Hello my Name is ***** ***** am I speaking with? I read what you wrote. Is it backfiring through the exhaust or is it popping back through the TBI?

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Customer reply replied 4 months ago
My name is***** is back firing through the exhaust. The motor while on the trailer will tach to 5000, and runs very smooth. Once there is a load on it, that's when the problems start.

Hi Stan. Okay backfiring through the exhaust can mean low compression, incorrect timing, incorrect firing order, excessively rich fuel condition. Are you able to check the compression, firing order, timing, and fuel pressures? Is that something you can do?

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Customer reply replied 4 months ago
I have checked the firing order it is correct. I have checked the compression and that is correct. I understand able to check the timing due to the computer not allowing me too. The dealer stated that the timing was set when they replaced the motor. That is why he believes that it's the computer. Not allowing the it to advance or retard as needed.
Customer reply replied 4 months ago
This is the original fuel pump and distributer.

All you need is a timing light to check the timing. Do you have a timing light?

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Customer reply replied 4 months ago
I do. Also what is your opinion regarding the computer being bad?

On the computer, anything is possible. Has a scan tool been hooked up to the engine to see if the computer will communicate with the scan tool?

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Customer reply replied 4 months ago
Also, if the timing was off, wouldn't it back fire and run ruff all the time? Even when it is sitting on the trailer.
Customer reply replied 4 months ago
in fact that's how they set the timing. They over rode the computer to set the timing.

Also, if the timing was off, wouldn't it back fire and run ruff all the time?

No

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Customer reply replied 4 months ago
now what do I do? Some people say timing, computer or fuel. Can a fuel pump get lower pressure after it gets hot. Maybe that's the problem?

Well, what are your actual compression numbers? Was fuel pressure checked? If yes, what is the number there?

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Customer reply replied 4 months ago
I did not check the fuel pressure. I do not remember the exact compression numbers, but when I did check them, I compared them to what the manual stated and they were within range.
Customer reply replied 4 months ago
I did not check the fuel pressure. I do not remember the exact compression numbers, but when I did check them, I compared them to what the manual stated and they were within range. Do you think that I should check them again?

Yes.

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Customer reply replied 4 months ago
you believe that it is either compression, timing or fuel? One of those 3 things or a combination of?

Low compression, incorrect timing, incorrect firing order, and high fuel pressure will all cause backfiring out of the exhaust.

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Customer reply replied 4 months ago
What about the fact that the motor has never been able to tach over 4000rpms in the water, and it should be able to tach 5000.

Well.... High fuel pressure would not cause a lack of RPMS. but low compression, incorrect firing order, incorrect timing would.

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Customer reply replied 4 months ago
Okay, these are the same answers that I have received before. Thank you. I will go back and recheck again. Maybe I will find the problem. Thank you.
Customer reply replied 4 months ago
One last question. If it was back firing through the TBI, what would cause that. The same things?

No that would be completely different. Which one is it doing, out of the exhaust or out of the TBI?

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Customer reply replied 4 months ago
I believe that it is through the exhaust, but I have a covered motor. The first time it happened, I though I hit something with the prop. It made a metal sound.

We've being going round and round for the last hour assuming it's backfiring out of the exhaust. Do you want to go double check and get back to me?

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Customer reply replied 4 months ago
I just spoke to my son in law, and he said that he thought it back fired through the TBI. It's hard to say. Everything is covered up. This boat is not going back in the water for another 2 months. I am just trying to get a head start. That Wil be too long to get back to you I'm sure.

So what do you want to do?

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Customer reply replied 4 months ago
I am just going to go back and recheck everything again and if I can't locate the problem. I am going to take it to another dealer and have them look at it. I just hate throwing money into it with no results. Sorry I took so much of you time.
Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,339
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
Verified

Hello, My name is Tan.

Has the engine run correctly since it was installed?

Your symptoms could be fuel starvation if the engine runs well up to a certain rpm then backfires. Have you tried running the engine on a different fuel tank?

Do you have the engine serial number? 5000 is awfully high.

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Customer reply replied 4 months ago
Hello, The first time taking the boat to the water, I ran it very easy, still in the break in period. The motor ran fine, but I did not tach it over 3500 rpms the entire weekend and no back fire.. The next trip, I began to increase the rpms and noticed that it would not tach over 4000. During the weekend, I was pulling a skier and we had a back fire and loss of power. After waiting for a few minutes, we started and went about our ride. No problems. The motor will only backfire when there is a load or after it has been running for a longer period of time. It does not backfire all the time, just once in awhile.
I only have one gas tank, so no just the same tank. As for the rpms, everyone that I have spoken to as well as the manual, state that 5000 is not a problem. The original motor which was a 5.7, ran at 5000 with no problems.

Ok, you could still have a fuel restriction from the boat side fuel system. This can be debris in the fuel tank blocking the pickup tube or the anti-siphon valve sticking.

You can try running the engine on an auxiliary tank or we can plumb in a vacuum gauge to check for a restriction.

To plumb in a vacuum gauge you will need a brass tee the same size as the fuel supply line to the engine, a vacuum gauge. Once plumbed into the fuel supply line run the engine watching the vacuum gauge. It should never exceed 2" of vacuum. If it does we have an issue with the fuel supply.

Here is a diagram showing where to plumb the vacuum gauge.

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Could I get the engine serial number, please? I'm coming up with 4400-4800 for EFI the MPI's go up to 5000.

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Customer reply replied 4 months ago
That has crossed my mind also. Do you think that it is more of a fuel issue rather than a timing issue? The serial # 0L012665.

To me, it is more likely fuel related as the engine starts and runs good till you load it. Backfire through the throttle body is lean fuel condition.

The spec on your engine is 4400-4800 rpms.

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Customer reply replied 4 months ago
Do you think that the fuel pump could be failing after it has been running for a while?

No, if the fuel pump quits the engine dies. These electric fuel pumps run or don't run, nothing that would cause your symptoms.

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Customer reply replied 4 months ago
Okay, I will look for an obstruction in the tank and/or lines. Thank you for the diagram. Thank you for your time and wisdom.

Your welcome.

I hope I’ve provided the information you were seeking. If you are happy with my service, your 5-star rating at this time is most welcome. If you need more help, please let me know so l can continue to assist you.

Thank you..........Tan

Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11,339
Experience: 40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
Verified
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