Exhaust "Explosion" or "Afterfire" in my 2003 6.2 MX MPI Mercuiser'

[w317chester]

Exhaust "Explosion" or "Afterfire" in my 2003 6.2 MX MPI Mercuiser'

So I just finished installing my fresh completely remanufactured engine into my boat. The engine is a 6.2 MX MPI MIE Mercruiser (serial number M317417). I attempted to start the engine for the first time last night and I turned the starter over multiple times and after attempting to start the boat about the 7th time a extremely loud explosion went off in the engine (or exhaust possibly). The "explosion" was very loud. Much louder than a typical engine backfire. It was as loud as a shotgun. I don't think it caused any damage but can't be sure without actually looking into the engine. Just wondering what could have possibly caused this and want to make sure it doesn't happen again. Here are some notes/thoughts/things i've checked:

1. I'm 98% sure I set the ignition timing correctly. I set the distributor rotor pointing when the engine was at TDC for the number 1 piston just as it says to do in Mercruiser manual #31 for my engine. I checked TDC by lining up the 0* mark on the harmonic balancer with the mark on the timing chain cover. I also verified TDC by using a piston stop tool threaded in the spark plug hole. This confirmed that the harmonic balancer mark was correct. Is it possible that my engine remanufacturer put the harmonic balancer on 180* out so that the timing mark is showing 0* (TDC) when this is in fact not TDC between the compression and power stroke (like it should be) but rather TDC between exhaust and intake stroke? I'd think this would be very unlikely but maybe possible. I had the engine (longblock) remanufactured by a very reputable, professional company so I would like to think they got everything correct on their end. I've done the install myself however.
2. I've checked the spark plug wires to ensure they are all on the distributor and plugs in the correct firing order. All new spark plugs also gapped at 0.060" and plug wires are in good shape. Not completely sure that the distributor cap is even getting ignition to it so I need to check that tonight (not that this would have caused an "explosion" though).
3. All brand new Bosch fuel injectors have been installed as well.
4. Fuel rail is getting fuel pressure to it. Haven't measured the PSI but fuel is also getting to the cyclinders (or at least the number 1 cylinder as I smelt gas on the spark plug when i removed it).
5. Is it possible that after having tried repeatedly starting the engine that there was a lot of unburnt fuel in cylinders that got dumped into the exhaust causing the fuel to ignite in the exhaust and thus the "explosion"? However, I don't know how the fuel would have got ignited in the exhaust since the engine wasn't warm and thus no heat in the exhaust.
6. I did see a little bit of smoke coming from the front of the engine from in between the thermostat housing and the throttle body when I was turning the engine over but I have a feeling this may be a separate, unrelated wiring issue. This was before the explosion occured. Not sure but just wanted to mention this.
7. This may be a dumb question also but is it possible i have the starter wired backwards causing the engine to spin backwards when I try to start the engine? Not sure if a DC motor can spin backwards if current is flowing in the opposite direction.

Any help here would be very much appreciated as i've worked my butt off on this engine and not sure the next step to take. I'd almost be willing to buy someone a visa gift card on here for whoever provides/leads me to the correct solution or answer lol.

 

[Scott Danforth]

sounds like you did not have spark, however had fuel. the cylinders and exhaust loaded, then you got spark and a good exhaust pop.

I blew the mufflers off my street rod trying to start with the ignition kill switch on. you may want to check your exhaust shutters and rubber exhaust hoses, however I am sure you should be fine.

the bit of smoke from the front of the engine may be concerning.

 

[w317chester]

Thanks Scott, that's a little bit reassuring. I've spent way too much money on this engine and can't afford any major damage.

I'll check the spark plugs to see if they're sparking when I get home tonight. I just wonder why the engine wouldn't have started if I was getting spark and fuel, and the timing was set correctly?

 

[Scott Danforth]

you may want to double check your timing.

 

[MikDee]

Right now, it sounds like your Dist. is 180 degrees off!

Wait a second! You said you were on TDC of the crank, then set the Dizzy for #1,,, BUT, were you on the Compression stroke, or the Exhaust stroke? TDC on the crank comes up twice with each engine cycle on a 4-cycle engine. If accidently on the exhaust stroke, you would probably get an explosion!

Go back, and crank the engine by hand, or with the ignition off or, (coil wire off) until YOU start to FEEL compression (with your finger, or a hose, or a comp gauge) on the #1 cylinder, then line up the crank timing marks to TDC again, then remove, & reset your Dizzy then to fire on Cyl #1.

That piston tool will only tell you if you are on TDC, but Not if you are on Compression, or Exhaust!

 

[w317chester]

you may want to double check your timing.

Thanks Scott, Is it safe to expel my theory about the harmonic balancer being put on 180* backwards and thus me seing a "false" TDC when I line up the mark on the harmnic balancer with the mark on the timing cover and also using the piston stop tool? I think the harmonic balancer is keyed so it can only go on there one way anyways. Not sure though.

 

[w317chester]

Right now, it sounds like your Dist. is 180 degrees off!

Wait a second! You said you were on TDC of the crank, then set the Dizzy for #1,,, BUT, were you on the Compression stroke, or the Exhaust stroke? TDC on the crank comes up twice with each engine cycle on a 4-cycle engine. If accidently on the exhaust stroke, you would probably get an explosion!

Go back, and crank the engine by hand, or with the ignition off or, (coil wire off) until YOU start to FEEL compression (with your finger, or a hose, or a comp gauge) on the #1 cylinder, then line up the crank timing marks to TDC again, then remove, & reset your Dizzy then to fire on Cyl #1.

That piston tool will only tell you if you are on TDC, but Not if you are on Compression, or Exhaust!

Thanks for the input MikDee! I'll check that TDC was on the compession stroke and not the exhaust stroke. And perhaps my understanding of the crankshaft rotation is a bit off according to my previous question to Scott. Just so my understanding is correct.....When the crankshaft rotates a full 360* does the piston move up and down once (completing 2 of the 4 strokes) or does the piston move up and down 4 times (completing all 4 piston strokes in on full rotation of the crankshaft) I know this is a dumb question and think I know the answer but just want to clarify. If it only completes 2 of the 4 strokes per 1 full crankshaft rotation then it's very possible my timing is 180* off!

 

[MikDee]

Okay, On the Chevy V8 blocks, #1, and #6, are at TDC on the crank at the same time, that's a given, it never changes. The Camshaft turns at 1/2 the speed of the Crank so it determines which one is ready to fire, and which one is on exhaust. So, one revolution of the Crank, it could be ready to fire on #1, OR, #6? Without having access to checking the valves, the only way to tell is Check, or Feel for compression on #1. A complete cycle of the engine is actually 2 revolutions. So, you must reset your Crank to #1 (compression stroke), pull out your Dizzy, then reset it to #1 again, and reinstall. I hope this is clear enough? It can be quite confusing.

 

[w317chester]

Okay, On the Chevy V8 blocks, #1, and #6, are at TDC on the crank at the same time, that's a given, it never changes. The Camshaft turns at 1/2 the speed of the Crank so it determines which one is ready to fire, and which one is on exhaust. So, one revolution of the Crank, it could be ready to fire on #1, OR, #6? Without having access to checking the valves, the only way to tell is Check, or Feel for compression on #1. A complete cycle of the engine is actually 2 revolutions. So, you must reset your Crank to #1 (compression stroke), pull out your Dizzy, then reset it to #1 again, and reinstall. I hope this is clear enough? It can be quite confusing.

Thanks for the explaining MikDee. It's very clear. I think it's very likely I have my ignition set out one complete revolution of the crankshaft like you're saying. I'll check this weekend. If you're right I owe ya a gift card!

 

[Villain lll]

Thanks Scott, Is it safe to expel my theory about the harmonic balancer being put on 180* backwards and thus me seing a "false" TDC when I line up the mark on the harmnic balancer with the mark on the timing cover and also using the piston stop tool? I think the harmonic balancer is keyed so it can only go on there one way anyways. Not sure though.

You are correct. With the alignment key, the balancer can only go on one way. No way to make a mistake. Usually a "backfire" or a explosion up thru the intake is either a timing issue or a bad valve issue. An "Afterburn" or a "pop" in the exhaust is usually caused by a lean carb set up. I agree with the others to go back and validate your stroke cycle.

 

[achris]

Pull the dissy, turn the engine one revolution, drop the dissy back in... Problem solved.

 

[MikDee]

Pull the dissy, turn the engine one revolution, drop the dissy back in... Problem solved.

I see you had to jump in here Chris,,, Basically, Just what I said! But, I also gave him reasons why!

 

[achris]

I see you had to jump in here Chris,,, Basically, Just what I said! But, I also gave him reasons why!

And over complicated the issue....

 

[MikDee]

And over complicated the issue....

You Think? Too much information? Does anyone else think so?...

 

[w317chester]

You Think? Too much information? Does anyone else think so?

MikDee, I think your advice was spot on and very helpful and informative. Plus I asked for more detail anyways. You answered my questions very clearly. And yes I had my ignition set one full revolution out on the crankshaft. Changed it when I got home and she crunk right up. PM me your address. Gonna live up to my promise and send you a gift card. Everyone was very helpful to me on here however. Thanks!

 

[MikDee]

MikDee, I think your advice was spot on and very helpful and informative. Plus I asked for more detail anyways. You answered my questions very clearly. And yes I had my ignition set one full revolution out on the crankshaft. Changed it when I got home and she crunk right up. PM me your address. Gonna live up to my promise and send you a gift card. Everyone was very helpful to me on here however. Thanks!

Chester, I thank you for the kind words, and the offer,,, but, I cannot accept, because there are many more people on here that are quite knowledgeable, some more then me, some less. I just happened to read between the lines, and get to the root of the problem. Most on here could probably figure it out as well, either sooner or later, and it wouldn't be fair to them to accept this gratuity. Although I'm glad to be of help, it's a relief, like a detective solving a crime. I envy you with a new 6.2 motor, plus I root for you to now go out there, and enjoy it! It does my heart good to make a fellow boater happy, with a healthy engine! Save your money for your boating, and family enjoyment! Thanks again, Mike D.