Diesel Engine Backfires?

[S&S]

If I had to guess (and I'm guessing it's not continuous, just occasional), I'd say sticking intake valve.

[Chief Engineer]

Does the "pop" originate out of the air intake?

When was the last time you change your fuel filters?

BTW on most marine diesels the "air filter" is actually a silencer....Yanmars are especially noisy with the silencer off.

Originally Posted by Ocean Girl

Every once a couple minute or so it seems my engine pops or backfires. Never heard a diesel backfire or make that noise before. It only seems to happen above 1600 RPMs not at low idle. It is a newish (70 hours) Volvo MD20. My Air filter is off, could that be the source? - no abnormal temps or pressures.
Erika

[SkiprJohn]

If the timing is off a bit then do not run above the 1600 rpm to create the problem. I burned out a pre ignitor once by making an engine run when the timing was off. It required pulling the head. Not a fun thing to do.

Yes, check the valve clearances first but by all means don't keep the engine running when it is at backfiring rpms. Its trying to tell you something.

Regards,

JohnL

[Stillraining]

Yes it could be a valve clearance issue..but with only 70 hours on it I would not think so..But have your mechanic check all valve clearances ( lash ) when he is in there doing the timing.

As said by another change your filters as well...If you wanted to start there as the cheapest step first befor your trip and see how it does that would be a good idea and progress from there... but I doubt that is all it is. IMHO..
It would be nice for you if it was though so I hope I am all wrong on my Guessing...My shoulders are broad I can handel being wrong..

Let us know what you find out..and I truly hope its a simple fix.

Take care.

[SkiprJohn]

I forgot to say I hope it is really something simple and easy to fix. Good luck on your troubleshooting.
What I burned out on my head was two pre-ignition chambers that needed to be removed and new ones be pressed into the head and 3 glow plugs all because my engine was out of time and I sprayed a fuel/oil mixture into the air intake. Boy was that stupid. A very costly and time consuming lesson.
That engine was severely out of time though so I don't think you'll have hurt yours just by having a few pops now and then.
regards,

[Ocean Girl]

Thanks guys so much, I really wasn't taking it that seriously, it was just a pop every couple of minutes. But you all have scared me enough to get my mechanic on it and goad me into taking a refresher diesel class, I should be working on this engine! Oh well, one thing at a time.
Thanks again you guys!
Cheers,
Erika

[SkiprJohn]

Please let us know what it turns out to be. We're all willing to learn.

[Stillraining]

Originally Posted by Ocean Girl

But you all have scared me enough to get my mechanic on it and goad me into taking a refresher diesel class, I should be working on this engine! Erika

Erica:

I look a diesel engines like playing chess..there are some rules..there is a system..and there's lots of variables your opponent throws at you.

Making it somewhat pragmatic on steps to take diagnosing problums but there are many directions of "Attack" you can take and still not win.

Starting out just trading pawns (replacing stuff) is rarely the way to go...except in the case of fuel filters..thats as cheap of fix as it gets..and always a good start.

Once you have played the game enough times..you start to know your opponents moves better... if he does this you do that...and can react in a more determined and pointed direction.

Sometimes you still get stalemated or placed in check yourself. and have to rethink what you learned last game...sometimes you forget a move..This is just how it is..always learning always getting suprized and seeing a move you have never seen before from your opponent even in the same situation he played last game .

Im never done learning the moves..but Im winning more often then I use to...

Check mate?

[Ocean Girl]

Thanks everyone! My move went smoothly I kept the RPMs low and never heard that unusual sound.
An interesting note to all this - Before leaving the slip I decided to check the fuel level at the tank. I have a fuel gauge and it read 1/2 tank but I had a nagging feeling not to trust it, so I crawled into my bilge and started tapping on the tank and the tank sounded empty! I put a couple gallons in the tank for the trip but could that be the culprit for the odd noise?
Love my new marina
Thanks for all the help.
Erika

[Stillraining]

Atta Girl...I would have done the same...just hard to advise someone else to take the same corse

My Answer...I would no.....diesels are not like gas in that you can starve them out and have then pick right back up again..and refire with an electric ignition and inso doing create a back fire..what happens is air is compressabal.. fluid is not..your injector pump meters a high pressure discharge to each injector line in a closly and critically timed arangment..this influx up pressure is what opens,fires, triggers or pops your injector ( use the term that you like ) this does not mean the line becomes empty of fuel or even void of pressure it just drops below the required pressure to open the injector...

OK now in vision air getting into that line..the air is basically a spring ( remember it is compressible ) well that spring does not aloe the proper transfer of pressures to fire the injector ( this is to simple of explanation but Im trying to give you a visual ) So you question was.... could low fuel tank level cause backfiring?...No ..not on a continual every couple minute basis like you described ocuring...your engine would stumble and eventually die as more and more injectors received air contamination.

Hope that helps

[Ocean Girl]

Stillraining, Thanks! It does help a lot! I was just wondering If I should apply the same thought pattern to my diesel as I did to my VWs that is - Push, suck, bang, blow. (do I have it right? its been a while since I have worked on any engine!). Anyways, it was a good trouble shooting technique for finding out where the problem was in my VWs engine when they broke down ( everyday ). My guess is the "pop" noise is during the Bang part of the cycle.
Erika

[Stillraining]

That is corect...but it is firing off time for some reason...you never really did clarify once and for all if this was out the exhaust or back through the intake manifold..if the latter it leans more toward valve isues if the former more toward fuel delivery isues...and what I took you as meaning from the get go.

Since you only have 70 hours you can sort of rule out sticking or burnt valves ..and like I said SORT_OF or in other words were going to look elsewhere first then come back to that if we are stalemated.

However every manufacturer I know of wants a re-checking of valve lash ( the clearance between the end of the valve stem and the bottom tip of the rocker arm in a totaly valve closed position after a reasonable break-in time..This is stated as TDC or Top Dead Center...and checking that in a particular rotation of the four stroke firing order...

As the valves seat farther into their valve seat due to wear and reduce this clearance it can create a no-sealing condition where the valve is not totally closing....This is not called valve sticking this is a valve clearance issue or another part of the "Timing issue" ...but could lead to a sticking valve or worse a burnt one as exhaust gasses rush by when they are not supposed to or where they are supposed to in the case of intake valves.

Yours should be done if it hasn't been already.

Hence my first place to check after the Fuel filter change and while in there doing the pump timming...As you have to pull the valve cover for the later two so It makes no sence not to check both.

Again I hope this helps a little.

Scott

[Event_Horizon]

Does the engine only backfire after it has reached operating temperature?

Clogged air filter can lead to uncombusted hydrocarbons migrating through your exhaust and causing backfire, or preignition on the next compression stroke.

With so few hours on your engine I'm inclined to believe that there isnt anything mechanically wrong with it...though it wouldnt be unheard of.

[Stillraining]

Originally Posted by Event_Horizon

Does the engine only backfire after it has reached operating temperature?

Clogged air filter can lead to uncombusted hydrocarbons migrating through your exhaust and causing backfire, or preignition on the next compression stroke.

With so few hours on your engine I'm inclined to believe that there isnt anything mechanically wrong with it...though it wouldnt be unheard of.

She says she is running without the air filter housing...also she says there is no black smoke...you have to have a pretty dirty filter to cause what you are suggesting..which creates an air starved fuel rich and hence, fairly constant black smoke out the exhaust in order for that to happen..in my experience.

[yottiejohn]

Hi

Diesels are very simple
Just give them a CLEAN supply of fuel
at the right time
and compress = they work

Just an off the wall, instead of chasing "possibles"
the almost empty tank made me think of possible fuel contamination

like the one post said "fluid" is NOT compressable
and can cause a lot of damage to your injector tips

Check the fuel to make sure it is clean and no water
an half empty tank can condense an awful lot of water into your fuel
droplets of water getting into the system
would cause minor bangs or pops

Good luck

Regards

John