1996 Mercruiser 5.7LX Alpha Multiple issues!!

Hello, this is my first post to this site but I am in desperate need of some input! I have a 1996 Mercruiser 5.7LX Alpha 1 with the Thunderbolt V Ignition with roughly 250 hours, all freshwater on Lake Michigan. All season I have been fighting poor running issues. First it has a high speed backfire. Its not continuous but when running out to my fishing spots it has a random loud backfire through the carb. Engine continues to run fine just seems to skip a beat and boom then gets right back up and runs fine. Seems to do this about every 15-20 minutes on average sometimes it wont do it sometimes very often. I usually run about 3000-3200 rpm which is just enough to get it up on plane but does it at other rpms as well. I seem to only be able to ever hit about 4100 tops as well. Also from a dead stop when you hit it it bogs considerably before getting up on plane and will occasionally have the same backfire then as well. Lately it has been blowing smoke (blue/white) upon startup with visible oil residue in water but seems to go away after warmup. When it is smoking it runs very rough and vibrates and wants to stall. This past weekend it has developed a vibration at between 1000-2000 rpms. Also has some smoke and oil dripping out breather tubes as well.

Now for the frustrating part, it has been to three different "reputable" mercury marinas around my area, not shadetree mechanics. I am fairly proficient with mechanical engines (no expert) but just have not had the time to work on it myself this summer. But I keep getting the feeling that everyone is clueless on whats going on. This summer alone the boat has had a complete tune up (plugs, wires, cap, rotor), carb rebuilt twice, timing set and checked multiple times (10 deg BTDC), ignition module replaced, manifolds replaced, flappers replaced, impeller replaced, intake gasket replaced, thermostat replaced, anti siphon valve replaced, fuel filter replaced three times, all new fuel lines, fuel tank checked and was spotless and vent works fine, compression test was good (four months ago). And marinas thought blowby was normal for age of engine. Fuel is all fresh. Temp usually runs about 175-180 at speed

I am at a loss at what to do. Valve float causing backfire? I am going to check compression again myself. The backfire just has me stumped. I know engine may possibly be getting tired but has never been ran hard just cruising right on plane out to fishing spots and back. Any thoughts would be great or if you need any other info just ask!

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

250 hours is NOT an old engine. My previous V6 had 1200 hours when I sold it and it's still going strong (and should for another 1200 hours). The bit that concerns me is that 4100 rpm is the max revs. That engine should be pulling 4400-4800 at WOT. I would change prop, down at least 2". You may find that the extra load (generated by too big a prop) is causing hot spots on the piston crowns. That might account for the backfiring. My current V6 will bang on the rev limiter with the 17" prop I'm running. With the quality of fuel nowadays you really should be running up as close (or slightly above) the top end of the recommended WOT revs.

The other thing worth checking with a TB V is that all the electrical connections are good and clean. That's all the way from the battery post terminals to the connectors under the dash to the engine harnesses.

Chris.........

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

Sounds like an exceptionally high float level. I was going to suggest a non operating power valve but it appears it is a rich backfire. I have seen something as simple as the power piston hanging up in the down position leaving the power valve wide open. This could also be caused by a decreased vacuum in the airhorn of the carb. I've asked this question before......Who rebuilt the carb?

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

If this engine has the Quadrajet carb I am thinking secondary air valve could be opening up to soon . This is the the big butterfly valve that that is right behind the primary section of the carburator . When you have the throttle wide open , the quadrajet has a huge secondary butterfly . Now if the if the butterfly valve that is sitting on top of the carb is opneing to soon , you will get a lean condition and hence , occasional bacfiring . There is a way to put more tension on this valve to slow it's opening to maybe help your problem . With your engine at 4100 RPM may agravate this condition

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

pikeman8 said:

Hello, this is my first post to this site but I am in desperate need of some input! I have a 1996 Mercruiser 5.7LX Alpha 1 with the Thunderbolt V Ignition with roughly 250 hours

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A 96 with 250 hours-that is very low, has it been repowered to your knowlege? The thunderbolt ignition system has the ability to manipulate timing due to the knock sensor (wich has a torque spec) too tight and it will sense more than it should too loose and it won't recognize the advanced timing. When the engine was timed was the proper timing procedure followed?- if not you will be chasing your tail, the systems will be fighting one another.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

The vibration, smoke, and oil residue, would seem to indicate something mechanical. Why 3 marinas couldn't find it? A vacuum gauge may help isolate. I would make sure damper hasn't spun and still points to #1 TDC. While this may have started as a backfire, it might have morphed into something bigger. I'd do the compression again and possibly a leak down test. What do the plugs look like?

FWIW: This should have a 750 cfm weber with 3 step rods. The air door is weighted, and there is no adjustment. Should be T5 w/knock sensor and module.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

Thanks for all of the quick input. A few items I may have left out in my first post----This engine has never been repowered it does in fact have 250 hours on it. All cruising no extended idleing or high speed operation. Last year the motor required a valve to be replaced on the #5 exhaust valve so at that time both heads were rebuilt at the dealership/marina. I was very upset at the time and they offered no real explanation of why other than the idea of bad gas. I think they just seen the low hours and assumed it was just docked and never ran. But although I cannot always judge quality of gas when you buy it I have never let tons of gas sit for extended periods w/o stabil. After the top end motor ran great at first.

Motor has ran up to 4600+ rpms until this year and boat has original prop. It is a recent problem so although a lower pitch prop may get the rpms back it would be a mask of an underlying problem...

The motor does have a weber 4bbl carb. First time it was off the boat was this year was initially rebuilt by the mercruiser dealer/marina. The second marina/dealer swore it was poor float level or related carb issues so it was removed again but no adjustments were made claiming it was right at spec. And only associated gaskets were replaced (not a complete rebuild again).

Motor seems to advance well when the timing light is on it. Timing was checked by myself by grounding the purple/white wire on the ignition module per manual at 750 rpms on my hand held tach. Grounding the wire retards the timing about 2-3 degrees at 750 rpms. I like the idea of thinking the harmonic balancer has shifted. I will look into it.

The smoke and vibration is a recent development since the last marina visit. I am afraid that it may be the underlying problem surfacing further since NOBODY could figure out the backfire issue.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

A minor note: Idle rpm is 650, not 750. Pur/white wire needs to be grounded before starting engine.

A backfire is typically a lean condition. Running hard with a lean condition can cause other engine damage. You may have tuliped a valve, etc.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

Next time you get the smoke and vibration, shut it off immediately and peer down the carb and see it it's dumping. It still seems to me it's dumping fuel. If it is you'll see it.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

OK, my 2 cents. The oil on start-up sounds like valve seals or something internal. Do a cylinder leak test to get a good idea of any internal issues.

The temp should be 150 ish not 175. So see what the thermostat is - it should be a 145 degree.

As for the 4100 RPM - way low, should be in the 4800 range. Get the oil and backfire issues fixed and see what rpm is afterwards.

It sounds like the 3 marina mechanic shops replaced a lot of stuff that had little/nothing to do with the problem (oh well it moves idle inventory in the parts department).

For kicks, I would see if there is some 'slop' in the distributor gear.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

tpenfield said:

OK, my 2 cents. The oil on start-up sounds like valve seals or something internal. Do a cylinder leak test to get a good idea of any internal issues.

The temp should be 150 ish not 175. So see what the thermostat is - it should be a 145 degree.

As for the 4100 RPM - way low, should be in the 4800 range. Get the oil and backfire issues fixed and see what rpm is afterwards.

It sounds like the 3 marina mechanic shops replaced a lot of stuff that had little/nothing to do with the problem (oh well it moves idle inventory in the parts department).

For kicks, I would see if there is some 'slop' in the distributor gear.

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All good points but keep in mind if he's dumping fuel while sitting he is probably "washing" cylinders on startup. Would certainly explain the smoke and residue in the exhaust. Lot of shotty rebuilds lately.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

I will start with a compression test and see what that shows because I have a gauge. If nothing there I will try getting a leak down tester. I don't personally have one but know somebody who does.

Wouldn't a rich condition and dumping fuel result in black smoke? I have never observed black smoke. I have looked into the carb (from an angle) to see if the accelerator pump was working properly as well as the choke opening properly and all appeared fine. At this time I did not see excess fuel pouring into the manifold.

Would the knock sensor cause backfire? I tried as reading a previous post to tap the block while running and there was definitely a noticeable change in rpms.

I will try a compression check here when I have some time and let you know the results. I have been prolonging it because I am dreading what it will reveal. What is the target for this motor?

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

pikeman8 said:

Would the knock sensor cause backfire? I tried as reading a previous post to tap the block while running and there was definitely a noticeable change in rpms.

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It changes timing so although i have never experienced this first hand it seems it could have the potential to...right?. your comment about it being at random speeds and erratic leads me to believe an electrical problem. i was just thinking of the electrical components in your system.

any thoughts?

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

pikeman8 said:

I will start with a compression test and see what that shows because I have a gauge. If nothing there I will try getting a leak down tester. I don't personally have one but know somebody who does.

Wouldn't a rich condition and dumping fuel result in black smoke? I have never observed black smoke. I have looked into the carb (from an angle) to see if the accelerator pump was working properly as well as the choke opening properly and all appeared fine. At this time I did not see excess fuel pouring into the manifold.

Would the knock sensor cause backfire? I tried as reading a previous post to tap the block while running and there was definitely a noticeable change in rpms.

I will try a compression check here when I have some time and let you know the results. I have been prolonging it because I am dreading what it will reveal. What is the target for this motor?

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Regarding a compression test . . . I recently did a compression test on my 7.4L Mercruiser. It came in at 134 +/- 7, which is pretty consistent. Then I did a leak test. 4 of the 8 cylinders showed 50% leak while the other 4 showed about 10% leak. The point being a leak test is more revealing, so if you can get hold of a leak tester, go for it.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

175 at the temp gauge is normal for this engine which comes with 160 stat on a raw water cooled engine.

Compression and leakdown.

Have any pics of plugs after the blue/white smoke? Any water in the cyl?

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

Okay been VERY busy with other things lately but got out and did compression and leakdown on the engine today with. What do you think. Results as follows:

Compression:
#1 - 169
#2 - 166
#3 - 158
#4 - 166
#5 - 170
#6 - 165
#7 - 160
#8 - 160

Leakdown:
#1 - 21%
#2 - 18%
#3 - 20%
#4 - 20%
#5 - 21%
#6 - 22%
#7 - 20%
#8 - 18%

Leakage during each cylinder was observed through the oil filler cap and dipstick. That explains the blowby it seems but with the numbers all that close it does not seem to be a problem at this time.

Checked damper to #1 cylinder and it has not spun. Its right on.

Checked connections on dash and module for loose ignition conections. all seemed clean and tight.

Spark plugs all were evenly colored between the 8 but were darker than normal. Tea colored. What is the correct plug for this engine? Plugs in it were Champion RV91MC this does not match what is listed in manual. Checked spark and seemed weak. Tested coil per manual and failed basically every test. I know this is not a foolproof method but I feel it may be work a try to get another coil. Also coil has significant sticky sludge inside the high tensile fitting. Is this normal from factory or is this a sign coil may be leaking? Thanks again for help!!!!

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

My fathers merc powered boat had a problem like you are discribing that ended when we changed the coil. It too was leaking a dark brown goo.
I hope it fixes it for you as well, good job!

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

pikeman8 said:

...Plugs in it were Champion RV91MC this does not match what is listed in manual.

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Bin the Chumpions! I would use NGK.
If it's not a Gen+ then BR6FS, gapped to 35 thou... (0.9mm)
Gen+ use BPR6EFS, gapped at 45 thou (1.1mm)....
(We'd need the serial number to determine the engine generation.)

pikeman8 said:

Checked spark and seemed weak. Tested coil per manual and failed basically every test. I know this is not a foolproof method but I feel it may be work a try to get another coil. Also coil has significant sticky sludge inside the high tensile fitting. Is this normal from factory or is this a sign coil may be leaking? Thanks again for help!!!!

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Dielectric grease is put around the coil tower on assembly, but if the coil failed all the tests, then I would be putting a new coil in. Use a high quality automotive coil (non ballast). Merc want way too much for a genuine.

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

Serial #F795528

Re: 1996 Mercruiser 5.7LX Alpha Multiple issues!!

You have the non Gen+ engine, with Thunderbolt IV ignition, so the right plugs are NGK BR6FS.

Set of plugs and a new coil and you should be a happy man..

Chris......