1998 Mercruiser 5.7 TBI Backfiring Under Load & Loss of Power

[98BaronMag]

Hello all,

I've been pouring over dozens of old threads the last few weeks trying to sort out some issues I'm having, but was mostly just trying small, inexpensive fixes. After this weekend, I decided to find a shop to work on my boat. Unfortunately, no one in my area particularly wants to work on a sterndrive, especially one that's going to take some hours troubleshooting. It seems I'm on my own here, so I'd like to dig into the project myself. I thought starting a thread for my specific issues would be the best way to find the most pertinent information. I'm an alright shade tree mechanic, but have no experience in marine scenarios. Hoping you guys can help guide me in the right direction.


My boat is a 1998 Lund Baron Magnum with a Mercruiser 5.7L TBI Alpha 1 sterndrive. Since I bought the boat last year it has always seemed low on power. When I initially bought it, top speed at WOT was about 40mph at 3600 rpm trimmed up as much as possible without prop blowout. This was on a 19p Mercury Black Max prop. I thought perhaps I was overpropped and that the aluminum prop was blowing out prematurely. I switched to a 17p Mercury Mirage Plus. This brought my WOT rpms up to about 4000 - still about 40mph. I became more concerned about this lack of power towards the end of last season, when the motor began to backfire occasionally at cruising speed and developed a 'knocking' on acceleration (starting in the 1500-2000 rpm range) I cannot discern if the knock is coming from the engine or the drive but it is loudest when accelerating, once at cruise speed it is not as noticeable. With those issues in mind, I took my boat in last year and asked the shop to go through it completely to try and figure out where this was coming from. They changed all the fluids and spark plugs (which they said were the originals as they had motor paint on them!) and told me plug wires and distributor were fine and they found no other issues. This year, when I took it out for the first time, the backfiring had worsened, making 5 or 10 pops in rapid succession instead of just one or two as it had been last year. I decided to take matters into my own hands and do some digging. I replaced the fuel/water separator and checked the distributor cap. The pins inside the cap as well as the rotor tip were severely corroded - not a bare spot of metal to be seen. Distributor cap also had original motor paint on it. Satisfied I had found the issue, I ordered a new cap and rotor and installed them. This weekend, I had the boat out for testing. The knock, backfire, and low power conditions all persist, but only under load. In neutral the engine runs fine, perhaps a bit of a rough idle, but especially over 1000rpms it sounds smooth and normal. In addition, it starts much harder than before in certain scenarios since replacing the cap/rotor and fuel/water separator. If the motor has been off for 10 minutes or more, it will crank and sounds like its firing but takes 20ish seconds and a bit of throttle to get it going. If you shut it off it will immediately restart without issue. I'm thinking perhaps the hard start condition has come from some air in the fuel system since replacing the fuel/water separator, but I read in other threads that the TBI has no bleeder valve and is supposed to be self-bleeding.


I'm looking for some starting points to dig into these problems. As of now, I'm planning to loosen the fuel line fitting at the throttle body and try to pump any air out of the line there. For the other problems, I'm much less sure of a starting point. I'm thinking about checking timing, compression, fuel pressure, exhaust flappers, replacing plug wires, and checking my work on the distributor and fuel filter, but this is a very shotgun approach and I'm sure to waste a lot of time and potentially money doing it this way. I'm hoping you kind folks can give me a sense of direction on where to start for the highest probability of sorting this out.


S/N on the motor is 0L049032 if that helps anyone identify exactly what I've got.

Thanks,

-Marcus

 

[ESGWheel]

Nice and through explanation and agree, this is the place to help you.

To get started there is still a bit of ‘shot gunning’ required as its clear there was a lack of maintenance on this engine / boat For the Ign System, only the wires remain to be replaced, so do it. As for checking the compression, good idea and post those results. There are more maintenance items but not necessarily related to your issue. However, worthwhile doing (example impeller and sterndrive oil change, heck perhaps even the engine oil?). My point is getting to a baseline where you know the routine maintenance items are addressed so not getting misled by these items during troubleshooting. Its akin to thinking you need to do a front-end alignment on a car but the real issue is low tire pressure on the right front tire. Make sense?

For backfiring: there are some basic causes which are outlined in this short description link which is good reading for background / understanding.

 

[98BaronMag]

Absolutely, I'm ready and prepared to do some serious troubleshooting. I know the engine and drive oils have been changed as I had a shop perform that last year. Engine oil reads over full which I know has been said can be a cause of poor performance, but I've always checked it on the trailer and I'm not sure if that has a meaningful impact on reading. Will plan to do the spark plug wires shortly. Is there any advantage to paying for 'marine' replacement or are just good quality heavy duty wires enough?

 

[ESGWheel]

Was this the same shop that said the cap and rotor were good? Point being suspect everything they did. Overfilled oil (when boat is level on the trailer or in the water) is not good. Does it look clean (a nice golden and see thru color?). Does the dipstick smell like gas? Is the oil back and opaque? Is it milky in color? All these are signs (except the first one) of an issue. How much does it read overfilled, a lot or just a hair? An overfilled crankcase can cause foaming of the oil and cause bearing failure due to not being properly lubed, not your symptoms you are seeing. Also overfilling can come from a lot of blowby of the rings > gas getting past rings and adding to the oil, so your oil ‘condition’ can tell a lot. What is it like?

Get marine wires. They run close to the exhaust manifold and the lengths are different. Also, in general do not cheap out on replacement parts. It’s a boat and you spend a very limited time in it, no reason to risk getting stranded due to bad parts. In general OEM or a quality brand like Sierra are recommended.

And if you are new to boating there is a ton of other stuff to consider, if interested I’ll PM you with some good info.

 

[Fun Times]

From the description it also sounds like you really need to know if the engine internals are in good condition (or not) before just putting a bunch of new outside engine parts in place... such as performing an compression test, leak down test, vacuum test, borescope inspection to each cylinder with plugs removed, fuel pressure test (< Takes a special Merc fitting to test (91-806901), ignition timing thoroughly inspected such as initial timing and advanced is in proper working range and item number 9 (Module) is working correctly, https://www.mercruiserparts.com/bam/subassemblydetail/36099463/28169633

https://www.mercruiserparts.com/5-7l-efi-gm-350-v-8-1998-0l010035-thru-0l339999

Make sure the spark plugs are NGK, BPR6EFS or AC-Delco equivalent.

 

[98BaronMag]

Actually, the engine and drive oil were changed twice within a month...once at the shop who 'checked' the distributor and again at the shop I had winterize it. I didn't ask them to change oils with the winterization and didn't know they had until I saw the bill. Guess I should have asked more questions... Oil looks fine to me - nice golden color. I'll admit I've never checked it right after running to notice any foaming, but I've never seen any milkiness or dark/black color. If memory serves it reads quite high on the dipstick - maybe 1/4 to 1/2 inch over full. When I investigate things further tomorrow, I'll also pull the spark plugs and see what that tells us about how it's running. I'd appreciate greatly any extra info you have @ESGWheel. @Fun Times Yes, a compression test is definitely coming up soon and I'll post results of that once it's done. Planning to test timing as well. Now, I don't have a house

 

[98BaronMag]

I apologize for the abrupt ending there...had some connection issues with the website. The post was supposed to continue as follows: Now, I live in an apartment and don't have access to a spigot for running it on muffs. I am hoping to do compression/timing/fuel pressure tests running it at my storage location in a big tub of water. Definitely have to keep a close eye on temperature so the water doesn't warm up too much. Is that a viable plan or just begging for trouble?

 

[Bondo]

or just begging for trouble?

Ayuh,..... Welcome Aboard,..... Unless it's an Olympic sized swimming pool, you can't do that, the impeller has to be underwater,....

 

[Scott Danforth]

I apologize for the abrupt ending there......... just begging for trouble?

impellers are NOT self priming. so when you fire up the boat, the whole outdrive must be under water to have the raw water pump flooded.

this is why you must use a hose and muffs with enough water pressure and flow to operate the motor when on a trailer.

When I initially bought it, top speed at WOT was about 40mph at 3600 rpm trimmed up as much as possible without prop blowout. This was on a 19p Mercury Black Max prop. I thought perhaps I was overpropped and that the aluminum prop was blowing out prematurely. I switched to a 17p Mercury Mirage Plus. This brought my WOT rpms up to about 4000 - still about 40mph. I became more concerned about this lack of power towards the end of last season, when the motor began to backfire occasionally at cruising speed and developed a 'knocking' on acceleration (starting in the 1500-2000 rpm range

4000 RPM is way too low. the knocking you are hearing is most likely pre-detonation of the motor eating itself. The low WOT is probably from a litany of issues.

This year, when I took it out for the first time, the backfiring had worsened, making 5 or 10 pops in rapid succession instead of just one or two as it had been last year. ............... I replaced the fuel/water separator and checked the distributor cap. ..................The knock, backfire, and low power conditions all persist, but only under load................ I'm thinking about checking timing, compression, fuel pressure, exhaust flappers, replacing plug wires, and checking my work on the distributor and fuel filter,

this new to you boat should have had the plugs, wires, cap, rotor, bellows, gimbal bearings, raw water pump, drive alignment, fuel filter, oil filter, all changed prior to ever even getting the boat in the water. Additionally you should have re-packed the trailer wheel bearings.

your motor is suffering a lean sneeze where the flame front in the cylinder is lazy as you do not have a proper air fuel ratio for it to burn and make power. so the motor will backfire thru the intake under any load.

additionally, the rattling you hear from the motor is most likely pre-detonating like crazy due to lean condition. this is will kill your motor in a very short time if you do not correct the issue.

how old is the fuel. if the fuel is older than 1 year. pump the tank out and get new fuel. Make sure the new fuel octane level meets the requirements of the motor.

replace the fuel filter. pour the contents of the current fuel filter into a glass jar or plastic bag and inspect for water and other contaminates. if you see a line of separation, you have water contamination. go back to the tank and check for water in the fuel tank.

measure the fuel pressure. make sure it is to spec. that is 30 psi

inspect the spray coming from the two TBI jets. they should be perfect cones. if you see any large drops or dribbles, pull the TBI unit and send in to get the injectors cleaned

pull the drive and inspect the bellows, gimbal, u-joints and alignment.

pull the exhaust elbows and inspect the flappers. if they are missing, find the pieces. they will plug up the exhaust cavity and limit your motors ability to breath.

 

[alldodge]

I'm planning to loosen the fuel line fitting at the throttle body and try to pump any air out of the line there

I wouldn't do that, it's not needed. The TB self bleeds

Without the special fitting the Fun Times mentioned you could remove item 3 on the TB and a fuel pressure gauge can be attached.

With it hard to start and backfire leads me to think it's either fuel pressure and/or injector issues

 

[98BaronMag]

Well, thanks everyone for the warnings about my large tub of water idea. Looks like I'll have to ask a friend to borrow their driveway to run it on muffs. Going to check some basic things today - Pull plugs and inspect, check exhaust flappers, check fuel pickup line and check valve...etc. I'll update you all this afternoon with my findings. Considering pulling the drive like @Scott Danforth mentioned because the 'knock' really sounds to me like it could be u-joints.

 

[98BaronMag]

Alright, went through a few things today. Here's my findings. Pulled elbows and inspected risers and exhaust flappers. Risers look good to me, excellent honestly, for a boat that's likely never had them replaced. Very little rust/corrosion. Flappers are in place and whole although they do make a definite 'tink' noise when pushed back and released. Perhaps this is where my knocking noise is coming from? Next, I pulled the spark plugs. All the plugs look similar, although they have varying levels of oil on the threads. Completely dry at the electrodes. They were replaced probably less than 50 hours ago, so I'm not sure if this is concerning. Advise if I should replace plugs again based on the pictures to rule out any issues. Finally, fuel system. I gained access to the intake tube and removed it from the tank. Screen at the bottom was clean, but check valve was difficult to suck air through and had a buzz/vibration as I pulled air through it. Manually moved the ball a couple times with a small screwdriver and pulled air through it again - no buzzing noise. Still slightly tough to pull air through, but perhaps that's normal. Thinking I'll replace it just in case in order to rule it out. I can't imagine it's very expensive. I'll follow this post with photos and descriptions of what I did/saw.

 

[98BaronMag]

First, Port side elbow, riser, and exhaust flappers. I noticed nothing out of the ordinary, except maybe some carbon deposits.

 

[98BaronMag]

Starboard side, same deal. Did notice where there must have been a small water leak...unsure if this is an issue or just needed more tightening on the hose clamp

 

[98BaronMag]

Spark plugs. Delco #MR43LTS Aligned in the photo how they came out of the engine. Closest plugs near the belt. A few electrodes shown close-up for detail.

 

[cyclops222]

Are these the newest plugs you put in ? about how many hours at cruise or WOT ?

 

[98BaronMag]

They were put in last year by a shop. I'd say maybe 10 hours idle, 10 hours cruise, couple hours WOT, knowing that WOT is only hitting 4k rpm with whatever issue I'm having.

 

[alldodge]

I don't see anything of concern, put same plugs back in

 

[Scott Danforth]

Still slightly tough to pull air through, but perhaps that's normal. Thinking I'll replace it just in case in order to rule it out.

the anti-siphon valve would be hard to pull air thru. what you are seeing is normal.

did you check compression? all cylinders between 140 and 150 psi?
did you check fuel pressure? 30psi?

 

[cyclops222]

Plug tips look GREAT !! BUTT
ALL that black oil on all the left side row of new plugs thread area. Is totally BAD for so little hours on new plugs. Almost like steam cleaning is done at higher rpms.
THe right side ones look much better. Were the plugs fully compressed when newly installed ? What make & grade of motor oil is in the engine ?