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Why did engine blow up

  1. Jan 12, 2016 #1
    hi all. im new to this forum and have a question i hope someone can shed some light on.
    im trying to determine, if possible, step by step why my motor blew up. here goes.

    its a 20 y.o. 4cyl mazda diesel

    i had the motor rebuilt 2 years ago by a professional.
    it had a 1 year warranty so well and truly out of warranty.
    it had done 27000klms without any problems

    about november i suddenly started to hear a very small tappet noise. it was very mild but i thought i should let guy listen even though out of warranty.
    he listened and agreed it sounded like tappets. he was too busy at the moment and said bring it back next week. ok to drive. while listening he revved the motor to 3500rpm. i will never go over 3000rpm.

    as i drove away the noise was worse. i reckon because he revved it so hard.

    i drove probably about 30klms. while coasting down a hill there was a sound that sounded like a mild backfire. i thought at this time to get it towed back to his workshop and not drive until he checks it.

    he didnt have time to work on it for a week. but in that week he sometimes had to start it to move in his workshop.
    after 3-4 days of this, one day he started it as normal, and at revvs that he claims to be about 2000rpm, motor suddenly blew up.
    a conrod had pushed out the side with such force that it even pushed the starter motor out of the way.

    once he stripped the motor down, he was shocked to realize that 1 conrod was gone. he could not find it at all and believed that the bits of metal in sump about 2cm in size were the remnants of the conrod.
    2 conrods were ok.
    1 conrod was slightly damaged
    there was nothing wrong with the head at all. no damaged valves. meaning original sound was not a tappet sound.
    2 pistons were visibly undamaged. 2 were damaged
    crank was ok
    the worst affected area was no3 cylinder.
    no3 piston was intact except the gudgeon had been torn from the bottom. the gudgeon sheered in half.
    he couldnt tell if the conrod from no3 was the missing one or not.

    because the motor had been so badly damged, its hard to even replace the pieces back to original place, to determine what was the horse and what was the cart.
    but i was able to determine from the salvageable pieces that for some time no3 gudgeon has been rubbing on the bore. it is only a slight mark a few thou deep. quite hard to see at first. this is defiantly 'a' horse, mayby 'the' horse.

    what im not sure of is what does this tell me?

    for a gudgeon to rub on the bore, that means the circlip must come out. if the circlip was installed incorrectly, it couldnt have lasted 27000klms before gudgeon started rubbing.

    what i think happened is the circlip came out for whatever reason, allowing the gudgeon to rub on the bore. this was the tappet sound i heard for about 1 week of driving.
    but how does this cause the motor to blow up?
    how does this completely shatter a conrod?

    while coasting down the hill and i heard what i thought was like a backfire, i now realise it wasnt a backfire but something cracking.
    how does coasting change the way that the parts are treated.

    well, i hope i have explained all the details for you guys to understand what im on about. ask away if theres something you dont get or i explained wrong.
    hope to get some insight on this.
    please, im a normal joe blo that is not uni educated. please try to keep answers in normal every day lingo. thanks

  2. jcsd
  3. Jan 12, 2016 #2


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    Sounds like the failure initiated at the gudgeon pin.
    Maybe the little end seized due to a blocked oil way in the con rod?
  4. Jan 12, 2016 #3


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    A common cause for this type of failure is that the big end bolts come loose on a con rod . There is then rapidly increasing slop and rattle until that big end finally comes apart .
  5. Jan 13, 2016 #4
    do you mean the gudeon pin sheered first, which then pushed the circlip out to allow it to rub on the bore?
    the hole in the piston where gudeon goes has no visible wear at all.

    every big end bearing was in perfect condition. for a bolt to work loose that means any big end slop should be visible on the bearing. and this wouldnt explain how the gudeon pin was able to rub on the bore for a while.
    the big end would have had to develop the slop at the same time the gudgeon started to rub otherwise i should hear 2 separate noises.
  6. Jan 17, 2016 #5
    i guess noone knows?
  7. Jan 18, 2016 #6

    Ranger Mike

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    Having blown up many gasoline IC engines over the decades, I can offer a guess.
    The cylinder is in pretty good shape so detonation as a cause could be ruled out. This leaves mechanical failure due to lack of lubrication. The piston/wristpin/connecting rod design is one that has free floating pin in con rod small end as opposed to the shrink fit design that does not require a circlip on each side of the wrist pin. This feature is important as the diesel engine is a hot running beast to begin with and lubrication is sued to cool the piston bottom and lubricate the wrist pin. It sounds like oil was not doing its job ,causing the tapping sound. Eventually the wrist pin seized in the piston bosses and pulled the piston apart. The con rod swung around and took out the side of the engine block on the way out. The circlip ring groove disintegrated in the piston destruct mode, giving the wrist pin a moment to hit the cylinder wall. ( long term interaction of broken circlip means a lot more damage to the cylinder wall then you described.
    In summary it sounds like you had one wristpin to piston - con rod clearance that was out of specification, things got hot and ..BOOM
  8. Jan 18, 2016 #7
    Hi Adrian I'm a car guy and I'm sure you have an old RF with the overhead cam.
    I guarantee that if you remove the timing plate and look at the belt you will find it toasted.
    Secondly and more importantly inspect the 2 pully notches and the third cam shaft mazda marked bolt that has to line up with the timing mark.
    Third have a look at the tensioner pully if it is in tact.
    My guess is the tensioner pully failed because it wasn't replaced on the rebuild and if kms were over 100000 on old motor its a disaster waiting to happen.
    When the chain wiggles the cam out of alinement because it is loose the piston and 2valves per cylinder begin to make contact.
    When you drove the chain slacked when coasting and when you throotled up it moved again.
    After returning he throotled up and the valves and pistons impacted destroying the motor.
    To know forsure if he truly completed the rebuild take a good look at the water pump gasket should be nearly new if not you got a bad rebuild if kms were high.
  9. Jan 18, 2016 #8
    U should take the cover plate off and snap a good picture I'm really curious because those cars can go long way on those motors design. looked it up and the bolt for timing is m8 1.25. Backfire is the misfire because timing cylinder out of sequence and belt is slipping and tapping is valve rod to piston contact. Recommended to change belt every 60000m or 100000km.
  10. Jan 19, 2016 #9
    ranger mike.

    good thoughts.
    only problem with this theory is that the piston that had the bottom torn off it was 'almost' seized in the tdc position. the top of this piston, and every piston, was in perfect condition, as were the valves. im not 100% sure if this motor has clearance or not. it may do.
    what you're essentially suggesting was the the motor continued to run for a short time after the piston was pulled in half. this couldnt be the case though as apart from sound, the motor performed/went the same right up to the time of death. if it continued to run for a short time with 1 piston torn in half, it would have went much differently to what it did. which is why i think the gudgeon was rubbing on bore before time of death.

    dave bj
    i have had a timing belt tension fail on this motor in the past. 15 years ago. so am well aware of the potential problems and symptoms assoicated with timing failure. im 100% sure that timing was not an issue.
    the problem is purely from tdc down.
  11. Jan 20, 2016 #10
    Every diesel engine I know is an interference valve design, so if the top end all looks good, that pretty much eliminates a timing belt/gear problem.

    What is the engine model on this? Car, Pickup?

    It does sound like there was a problem with the wrist pin.. perhaps the wrist pin keeper was never even installed.. I had a chainsaw missing a wrist pin keeper that ran for YEARS, and the wrist pin had just rubbed a little on the cylinder.. Since the engine is usually oriented so that gravity doesn't move the wrist pin, it might not move on it's own for a long time.
    When it did finally move, it starts to create shavings, that hurts all sorts of stuff, and adds a lot of friction.. the other 3 cylinders will keep going even if it tries to seize, which can bend and break the con-rod really quickly.

    Without any pictures to look at, I can't say much more... Pistons were damaged.. where? I think you could find important clues in the 1 damaged but not destroyed cylinder components... Perhaps the crank was ground and the wrong size bearings were installed.. causing low oil pressure, and no oil flow to the small end bearing? Even if its just on 1 bearing, the garbage in the oil that can create could plug/restrict the oil filter and cause a chain reaction on the other bearings.

    Lastly, was there any antifreeze in the oil? main bearings DO NOT TOLERATE antifreeze, and it will ruin them in short order.. that can cause something that starts with a tick and ends with a boom.. Check the old head gasket!
  12. Feb 4, 2016 #11
    An engine does not actually blow up. There are many types and causes of engine failure. The most catastrophic and visual is a broken connecting rod punching a hole in the block or crankcase. A connecting rod is the part that connects a piston, which is moving up and down very quickly, to the rotating crankshaft. If the rod is broken, the crank keeps turning and can push the broken rod right through the side of the cast iron or aluminum block or the crankcase, which holds the oil. In either case, the engine will stop immediately and the hot oil escape through the hole in block or crankcase and onto the hot exhaust causing a lot of blue smoke or even a fire.
  13. Feb 4, 2016 #12

    Ranger Mike

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    excuse me???????..engines will continue to run until oil is lost and things weld up / seize / or part ways. it is not an INSTANT stop. If it was, then how would oil get sucked into the exhaust and turn to smoke? The most dramatic failure is a crankshaft breaking. Big boom on that one...pistons coking apart make a lot of smoke as the oil is pumped into the cylinder and 1200 degree valves light it off in spectacular fashion. The messiest failure is the con rod popping thru the oil pan and oiling the track..very messy...About the only instant failure is a distributor or magneto breaking the drive shaft but since the oil pump still works, the engine spins down..a broken cam shaft or timing chain can make for a severe failure but the engine will not IMMEDIATELY stop..
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