misses, backfires on top speed.

[Aloysius]

Re: misses, backfires on top speed.

Get down to basics, and forget "codes" in that primitive computer.

What causes backfiring in a carbureted or TBI engine?
1) Ignition
2) Ignition
3) Valve problems

Since 90% of backfiring is ignition related, that's where to start. The tach is acting up, so you know you gotta ignition/pulse count problem. Find the cause of the tach problem. Running the trim pump causes a voltage fluctuation which affects something ELSE in the engine management system.

 

[Bondo]

Re: misses, backfires on top speed.

What causes backfiring in a carbureted or TBI engine?
1) A Lean condition...
2) Ignition
3) Valve problems

Ayuh,... I fixed that for ya...

 

[Aloysius]

Re: misses, backfires on top speed.

Well, yea, I agree bondo. To be so lean to backfire at full throttle/high rpms would include lean surge and stumble. However, since fuel distribution on a v8 is pretty inconsistent between cylinders, he could have one or 2 running critically lean. This would show up on the plugs.

He says it happens hot or cold. If it was that lean, it probably wouldn't run at all cold.

But I agree that overall, lean mixtures can cause backfire.

 

[monterey98]

Re: misses, backfires on top speed.

i appreciate you all came with more input, let me keep you up to date.

my plugs is all wet, fuel pressure is high all the time, i forgot to see the pressure when running top speed but sure is high, when i pull the plugs show wet/back(ish)/oily color.

is even worse now, i took the boat for a spin last saturday, 3 people backfires when top speed (red line) and then i got 4 more friends, guess what? cannot pass 30mph, backfires and misses like crazy, it happens all the time now, forget about the trim stuff.

the only thing i need to do is a leak down test, to make sure the engine is 100% cause my compression test show 180psi all the way.

so its getting cold over here, i dont know what else to do.

 

[dingdongs]

Re: misses, backfires on top speed.

have you tried running it in the pitch black to see if your leads are breaking down etc on your driveway.you may see some arcing

 

[monterey98]

Re: misses, backfires on top speed.

running on pitch black?
can you translate? lol

aloysius says "Find the cause of the tach problem". if i knew how to find. lol
thats why im here bothering. ha ha

...?

 

[Haffiman]

Re: misses, backfires on top speed.

Even if your compression test reads out, I suggest you remove the valve covers and check the high of the valve stems when closed and unloaded, back off rocker arms nuts.
If one or more valves are higher than the rest, tulip valve and off with heads.

 

[nimrod69]

Re: misses, backfires on top speed.

I had the exact same problem with my 2000 5.7 at high speed. Turned out to be a riser gasket leaking, and water was entering a cylinder a high speed and causing misfire. If you pull your risers, you should be able to see signs of water getting in.

 

[Mr.LG]

Re: misses, backfires on top speed.

hi there guys.

the mechanic found a purple wire not conected from the alternator.
is this the source of my whole problem?

Hey Monterey, a purple wire not connected to the alternator MIGHT be the cause of your problem. That purple wire on your alternator is the excitation wire. This has nothing to do with the tachometer.

If this wire is not connected, the Alternator might not be working properly, and hence you might be having voltage drops in your coil, leading to the conditions your are describing.

By what you have described your problem is related to your ignition system, not fuel system.

Have you connected this correctly to your alternator already?
Is the problem still happening?

If so, lets start looking at your ignition system, step by step.
1st, ensure your coil is receiving the proper voltage to produce good sparks.

 

[ChrisCraftJohnny]

Re: misses, backfires on top speed.

Have you checked the gear switch interupter? You know this is a kill switch right? When you switch from forward to reverse this switch "kills" (grounds) the ignition for a slpit second so as to relieve the force of engagement of the outdrive gears from the engine torque. This way the outdrive will be able to disengage the gear it's in. If the outdrive was under direct engine load you would not be able to switch gears.

Read the ADULTS ONLY on this topic. Don S explains it very well.

If you have any fault in the switch interrupt system it can cause all kinds of ignition woes.

CCJohnny

 

[monterey98]

Re: misses, backfires on top speed.

Haffiman
Im hoping that i dont have a major engine problem, thats why i have to eliminate everything else before go and open the engine or perfom a leak down test.

nimrod69
thats a good tip, thanks, but as i said, thats will be a $1k job including parts and labor, i guess i will have to change it next march/april. could be the solution of my problems right?

Mr.LG
My purple wire was hanging and the mechanic installed it back, still doing the same thing.

cc johnny
the switch you mention has 2 purple wires? i did test this way: i was slow in forward gear and unplug it, the engine died, i had to plug it back to make run, i was looking for more information about this switch but couldnt find it, dont even know what is it or what shape.

thanks guys. keep it coming

 

[Mr.LG]

Re: misses, backfires on top speed.

Ok. But lets continue to to look into your ignition system.
First the electrical part. What about the voltage on the coil?
Besides me, I saw 2 or 3 earlier posts asking you to confirm that.

Get yourself a multimeter and do this:
.Get your boat on the water. Start it and go to 1300 RPMs on idle. What is the voltage reading?
. Do the same thing under current load (for example while using the trim). What is the voltage reading now?
. Now increase the RPMs to the point where it was failing under load (was it 4000?) and repeat those 2 steps.

If voltage readings at the coil are within the normal operating range. Your electrical system should be ok.

During those tests did you engine run smoothly? Temperature and oil pressure were normal? If so, can you get to max. RPM smoothly (on idle)?

If all of that checks, then both your electrical and your ECM should be ok (there is more to chek, but for now this should be enough). Next step would be to check your coil and spark cables.
.Check your spark. What colour is it? Is it constant or some times failing?
.Get your multimeter and measure coil resistance (this is described in the service manual. I think for your engine is number #24)
.Also with the multimeter measure resistance of your spark wires.

All of that checks? Post your results... then we can go further...

 

[nimrod69]

Re: misses, backfires on top speed.

You shouldnt have to replace the manifolds and risers, just the gaskets. Cost is $5.00 and 20 minutes of your time.