270 FV 6.2 DETONATION
RussRuss
Member Posts: 28 ✭
I had to pull my motor after I lost compression in a cylinder. Turns out I was having detonation in about half my cylinders (worse in back 4) and I could see the start of it in the other 4. From some googling around, the prime culprits are bad gas (unlikely since I get from same marina as my buds and no one else had issues), clogged injectors (unlikely since all 8 show at lease some detonation), timing off (controlled by computer on this MPI). The one other problem I had was a bad IAC, so for a little while I had to open the throttle to start and the motor it would rev up at start. I don't know if that is related to the detonation. I am getting repaired now but the last thing I want to do is have it happen again. A Mercruiser mechanic said it is not real uncommon on large single engine cruisers where you are pushing the motor hard.
I also had a mechancic scan for codes to see if some sensor was maybe misbehaving but nothing came up (I also had no alarms). I would be willing to pay for a scanner (~$500) if it would help me monitor something that would tell me I still having detonation issues. I just need to find out if it is starting to detonate again before a wreck the motor. You really can not hear it like in a car, so I am thinking I will get a small borescope and keep peeking in back cylinder to look for the start.
Any advice from you smart guys would really be appreciated!
I also had a mechancic scan for codes to see if some sensor was maybe misbehaving but nothing came up (I also had no alarms). I would be willing to pay for a scanner (~$500) if it would help me monitor something that would tell me I still having detonation issues. I just need to find out if it is starting to detonate again before a wreck the motor. You really can not hear it like in a car, so I am thinking I will get a small borescope and keep peeking in back cylinder to look for the start.
Any advice from you smart guys would really be appreciated!
Comments
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but in order to determine your specific issues, can you tell us how the engine is used? Are you idling around, or, are you WOT often?
one manner to address the situation is to retard the timing a few degrees. allowing the charge to not be touched off until closer to TDC will eliminate a lot of predetonation.
another is to make certain your fuel pump is providing not only the pressure, but the volume required at WOT. Failure to do so will cause a lean condition that isn't present at lower RPM's or load. If it is providing the volume AND pressure, this restricts it from future concern.
it only takes a few POPS to wreak havoc on an engine, depending on the RPM, the load, and the location of the piston face/stroke it can be catastrophic in one single case.
predetonation is defined by the firing of a cylinder before intended- the things that can make that happen on an otherwise healthy engine:
- ignition too far advanced
- hot spot in cylinder
- octane too low or fuel too unstable allowing heat/compression the ability to ignite
- running out of fuel; ie low fuel pressure/volume, or bad distribution of fuel to individual cylinders
- cross firing
- using improper spark plug (though many recommend fancy metals, I strongly oppose; copper is just fine)
Too much fuel brings it's own list of issues, but those are generally much more suitable than running lean- running lean will smoke an engine quick like, while running rich will just clog her up.
your PCM will record the number of knocks. It may be interesting to have those harvested and scrutinized- to determine where and how the predets happened. If series of small, or great big ones, if low in the range or high, ect.
Boat Name: King Kong
"Boat + Water = Fun"
- new crab style cap/rotor in spring
- I have not yet winterized, so I assume I can check fuel/water separator for water?
- I had one overheat issue when first launched, but it was only for a few minutes and at idle (I had two hoses switched like a DA and pumped all my antifreeze into river.
- plugs were dark from flooding when I had IAC issues, no damage etc.
- I had one "event" when troublshooting IAC. My bud was at helm with throttle open to clear flooding and when it started it really reved up for a few seconds. Could this one time event cause the damage?
- I had motor scanned and will go back and see if he could see when detonation was detected. He only mentioned overheat in computer.
- I checked fuel pressure, but not at WOT. I did not check fuel flow, not sure how to do that. Any suggestions?
- Has anyone had there injectors cleaned/flow checked. I dont think I have an injector problem but this would eliminate the question.
- I use boat pretty normal, some idling, lots of just on plane, a shorts bursts of WOT here and there for fun.
- I think boat is propped right and original - I am just under 5000 at WOT
Thanks again and keep advice coming!! I cant afford another rebuild - I need the cash for fuel and beer :-)
one of the shortfalls of the way Merc has performed MPI is it lacks an oxygen sensor. W/o that, it can only operate off of pre scripted tables residing on the PCM- which makes the burden heavier on the other sensors that handle fuel trim. There are tables that provide for reference of engine temp, RPM, and MAF/MAP, and may advance timing based on those. (or retard timing, too). Merc plays it safe and only allows a small default timing advance before the engine closes loop (starts to rely on input from sensors for fuel trim and spark)... that advance should only be 8~10*.
those sensors which are players, in no particular order, are Mass Air Flow or Manifold Absolute Pressure, engine temperature, air intake temperature, Throttle Position Sensor, Camshaft Position Sensor, and Crank Position Sensor. If one of these sensors are providing inadequate information (outside parameters) you would hope it provides a fault code, but it doesn't always. So, what you will encounter are knocks, improper performance and economy, hard starts, ect... and because the sensor is providing information that is within specs (so not allowing a code to be set) but bad information for proper performance.
on a cold engine, I say again, the engine is running solely off of a script- and ignores the sensors (except tpi and iac) to run. this is called open loop operation. if an engine is running improperly before achieving at least 100* temperature, it can likely be determined the sensors (provided above) aren't the issue- not always, now, but most often.
there are two ways to advance timing. either the computer does it dynamically when all sensors check out and it closes loop, or, at the crank. it's advanced at the crank by physically keying/indexing the timing chains cog. since you're running a MPI computer controlled set-up, you'll not want the advance at the crank, but installed 'straight up' 0* (or, whatever Merc recommends).
another reason an engine will predetonate (we're talking an unhealthy engine as opposed to a healthy one in my first post) is a sloppy timing chain. If the chain slips the computer loses proper mapping of the entire process, and your running will not be precise. If it fires a few degrees sooner than the computer thinks it's firing (due to indexed timing chain) it will absolutely cause a knock. If the engine closes loop and the computer thinks it's safe to advance timing, it compounds the advance- which can cause a catastrophic predetonation. Having a properly indexed and tight timing chain is imperative. Fortunately, these items follow a wash basin type graph on failure rates, meaning there is a higher percentage of failures early, then a long low rate of failures following, and then a spike near the end of their life. If you make it through the beginning, you'll likely make it to the end without worries- your rings will likely start getting tired before the chain gives up.... don't use a regular ol' automotive timing chainset- use either a gear driven set (no chain, gear to gear from crank to camshaft) or a double roller (no morse style chain). These are louder, which makes them undesirable in cars, but they are more precise and they last a lot longer.
make sure your temperature sensors are right....... the number one killer and cause of predetonation is when an engine is hotter than it thinks it is, an advances timing in what it thinks is a safe environment to do so. That can be caused by a thermostat or a temperature probe (both items under $15), and on a closed cooling system, add a radiator cap to that list...... three items under $15 that will kill your engine if they aren't right. Don't trust the thermostat. Instead of taking it out of the box and installing it assuming it is right, drop it into a pot on the stove instead, along with a thermometer. The thermostat should start opening ten to fifteen degrees below its rating, and be wide open at or around its rating. If you observe when that particular (hopefully properly functioning) thermostat opens, you'll have some variance of trust that it will continue to operate in a similar manner. So, if it opens at 165* fully, and you install it, you should have faith that while in your engine doing its thing, it's wide open at 165* as well. reference your dash gauge at idle after fully warming up- if it coincides, you're good. If it doesn't, it may be time for a new temperature sensor. If you have closed loop cooling, pressure test your cap. Any parts store can do this for you.
fuel pressure, as discussed prior, is also important. the engine running short of fuel at higher engine speeds is a serial killer. Many people only measure pressure at idle, which is a quick check, but you'll want to measure at speed and under load, as well, to make certain the engines fuel pump is providing not only pressure (which has a direct and imperative relation to volume injected at the injector), but also volume. The easiest way to do this is to get a rail pressure gauge, install it, and go for a short run at wide open throttle. The gauge will demonstrate the highest pressure achieved. You WANT there to be an excess of fuel volume, and pressure to remain constant at idle and WOT UNDER LOAD. You DON'T want to see it drop at any point. Reference the merc manual for expected fuel pressure at the rail. EVERYTHING about proper fuel delivery is based on that pressure. Not enough pressure means not enough fuel, which means leaning out the engine, which means predetonation, which means damage is immanent. Too much pressure means the characteristics of the injectors spray change, likely from a fine mist to a pencil stream, which is ingested as drops instead of a steam (atomized), which is not only more difficult to ignite (limiting predetonation) but sometimes doesn't clear until the exhaust stroke, which causes that terrible rich smell. and wastes fuel.
you made a comment that the far cylinders were the ones demonstrative of predetonation damage. Those cylinders being robbed of fuel, it sounds, which means I would personally focus all of my attention on fuel pumps, fuel pressure regulators, and obstructions in the rail. The rails sometimes capture debris from the pump (read up on paint flaking due to ethanol laden fuel), or the interior of fuel lines (invisible from the exterior) deteriorate and flake off, and are captured in the rails. Bottom line: clean your rails. Swap fuel lines with FUEL INJECTION fuel lines. Inspect the interior of your pump for sluffing.
You've got a novel and some direction to take- hopefully all bases are covered. I'll be around to assist if you need it, and basically because I love this sh&t. :-)
x
Boat Name: King Kong
"Boat + Water = Fun"
PC BYC, Holland, MI
Paul
Boat Name: King Kong
"Boat + Water = Fun"
Everybody's gotta have a dream, right?