misses, backfires on top speed.

[monterey98]

HELLO GUYS

Im new here, but got a lot of help in the past with other problems.

my situation now is this, i bought a 98 monterey 24ft with a 5.7 efi v8 that was running great, after riding pulling tubes, washing the engine and treating with wd40 and all that good stuff, noticed that from 3 weeks ago the tachometer dont stop to go crazy, jumps/pegs, do all kind of crazy stuff, and when i play with the trim up to get top speed, i hear popping/backfiring noise coming from the engine, dont know if is from exhaust or TBI.

but yesterday i got a new cap and rotor and new plugs and wires also, when i water test it today, noticed no change, but also notice a different noise besides missing poppin and backfire, noise that looks like timing chain.

also brand new fuel filter, i got no water in my fuel.

million dollar question is, what you guys think could be?
timing chain loose can make missfire/backfire?

help me out guys.

thanks

 

[monterey98]

Re: misses, backfires on top speed.

...

 

[TilliamWe]

Re: misses, backfires on top speed.

Is the hull dirty?

 

[monterey98]

Re: misses, backfires on top speed.

lol
you kidding me?
stop joking man, i need serious input.

 

[dubs283]

Re: misses, backfires on top speed.

we need serious info - like, what make/model engine and drive??

start with disconnecting the sender for the tach, see if the problem goes away

did you check/set timing??

what voltage is the alternator charging at??

 

[monterey98]

Re: misses, backfires on top speed.

ok here we go.
mercruiser 5.7 v8 efi digital ignition. outdrive alpha 1.

i didnt disconnect the tach sender cause i was thinking that makes no sense, its just a gauge that reads whats happening with the engine, i think the tach goes crazy cause is receiving that from the engine, am i right?

alternator reads 14 as i recall.

i will check the timing tomorrow, i will borrow a timing light to see if is changing or if its in factory settings. i got a "mechanic" friend that will help.

what else could be ?

i almost forgot to mention, i noticed a vibration, i think either the puller for the power steering pump or the alternator, happend when top speeding, i also have almost 500 hours on that engine, every now and then i have to top off the engine oil to bring up the level. All this tell you all something?

 

[TilliamWe]

Re: misses, backfires on top speed.

Well, Monterey, if you actually knew what the sound was, you might realize my question was serious. Ever heard of "lugging" a marine engine? Well, when you operate a boat with a fouled bottom, you over heat the valves and tulip them. They make a sound that sounds very similiar to a backfire, when it's actually detonation. This doesn't usually cause the tach to jump around, but that could be a totally unrelated problem. So, I'll ask again, have you been leaving the boat in the water all this time? Is the hull dirty?
The fact that you say you are washing the engine down and spraying with WD40, implies that it's in the water, and has been for 3 weeks. Well, around here, that's more than enough time to foul a hull, begin lugging the engine, and deforming the valves.
Of course, your issue could be that you don't have marine stuff on your engine and you are getting stuff wet that isn't supposed to get wet, when you "wash the engine down."
Which is really uneccessary, by-the-way. Good luck.

 

[monterey98]

Re: misses, backfires on top speed.

well, i washed the engine once and i dry using my air compressor, just because i like my things clean, my boat stays on the trailer all the time.

whats been causing the detonation then? and why? what about the tachometer, why is he jumping crazy? need a new gauge?

my hull is not dirty, is yellow ish cause of the salt water, good polish should go away.


what else?

 

[Bondo]

Re: misses, backfires on top speed.

i didnt disconnect the tach sender cause i was thinking that makes no sense, its just a gauge that reads whats happening with the engine, i think the tach goes crazy cause is receiving that from the engine, am i right?

Nope,.... A Bad tach can short out the igntion....

 

[TheKatsass]

Re: misses, backfires on top speed.

I dont know how those V8s operate as a boat motor, but it sounds like detonation, bad timing(some crank sensors you can adjust), bad gas, too much load. Does it recreate the symptoms revving out of gear? Do you run 87 or 91? Is it full EFI or TBI? Is it a mechanical or electric pump?

 

[dubs283]

Re: misses, backfires on top speed.

some crank sensors you can adjust

not on mercruisers

on the mefi style ecms, timing is adjustable, but a special procedure is required to place the ignition into service mode

if this is a mefi style - being the digital ignition i belive it to be - the ecm gets the crank signal from the ignition sensor in the distributor, there is no crank sensor mounted on the motor

 

[monterey98]

Re: misses, backfires on top speed.

well, i will have to disconnect the tachometer and test it, i didnt yet.

i run cheapest gas.

gas is fresh and clean (no water)
my 5.7 its EFI.
electric pump.

when im flushing on the trailer seems normal, tach dont get crazy and no popping/backfire/detonation, i guesss only happens when under load right?

 

[dubs283]

Re: misses, backfires on top speed.

i run cheapest gas

try using non-oxy(no alcohol) premuim gas - 92 octane

cheap gas is just that - cheap, and can cause a lot of problems

 

[TilliamWe]

Re: misses, backfires on top speed.

well, i will have to disconnect the tachometer and test it, i didnt yet.

i run cheapest gas.

gas is fresh and clean (no water)
my 5.7 its EFI.
electric pump.

when im flushing on the trailer seems normal, tach dont get crazy and no popping/backfire/detonation, i guesss only happens when under load right?

Correct, under load only.

Humor me, get the bottom really clean, go out, and see if the popping noises subside or stop. 3 weeks in saltwater, I'll bet you a nickel your hull is fouled more than you think. And it doesn't take much to affect performance. Especially if you were already at the very bottom end of your WOT RPM range.

 

[scoflaw]

Re: misses, backfires on top speed.

The OP says his boat lives on the trailer. Hows it gonna bottom foul?

 

[monterey98]

Re: misses, backfires on top speed.

well, i took it to a real mechanic, so he can test whatever needs to be tested, i will update you guys early next week.

this things only happens in the water, my hull doesnt seem dirty, is white as it can be, out here in florida is salt water all over the place, and i will appreciate all your sugestions, keep them coming

 

[TilliamWe]

Re: misses, backfires on top speed.

...

this things only happens in the water, my hull doesnt seem dirty, is white as it can be, out here in florida is salt water all over the place, and i will appreciate all your sugestions, keep them coming

Plenty of fresh water there too! Of course it tends to turn the bottom brown.

 

[dingdongs]

Re: misses, backfires on top speed.

if you can get it running in pitch black have a look as it sounds as if ignition is earthing out and may see some arching in the dark.timing chain is sounding bad as ignition is off and on rapidly.might be a good idea to make sure all terminals are cleaned too as it sounds as if it only occured after throwing water about engine bay.

 

[monterey98]

Re: misses, backfires on top speed.

ok, so wash or no wash? not even when is dirty oily and all bad stuff, cant i wash just the engine bay as the guy said? or leave dirty forever?
can i apply wd40 every time i use?

 

[scoflaw]

Re: misses, backfires on top speed.

I would'nt be power hosin it too often. How dirty is this thing? Bilge is one thing, stay away from the electrical stuff for sure. Probably got water in some place it don't belong, now you've probably got a bad connection from corrosion, maybe, maybe not.