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Thread: Over-revved the Engine - Poof! (Update: 2017 engine swapped into 2015 car)

  1. #1
    Still Plays With Cars Loren's Avatar
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    Over-revved the Engine - Poof! (Update: 2017 engine swapped into 2015 car)

    Soooo... the Mirage came home on a flatbed today.

    I haven't started the search yet, but has anybody got any info about what happens when you over-rev a 3A92 Mirage engine?

    No, I didn't do it, but it was my car, and I just as well could have. Anyone who has autocrossed for any length of time (or likely even driven "spiritedly" on the street) has managed to miss a 2-3 shift and hit first gear. Typically, the astute driver recognizes this before the clutch is even fully engaged, pushes the clutch back in, and the engine carries on with just a minor over-rev.

    That's more or less what happened today, except my co-driver must have let the clutch out a little more than that. This being at an autocross event, he was at the top of 2nd gear. 6200+ rpm, reached for 3rd and grabbed 1st. Kudos to my transmission for smoothly allowing this to happen! I'm not too mad at my co-driver. As mentioned, I could have just as well done it myself. I don't do it often, and I haven't done it in this car, but I have done it.

    I don't know what's broken. The engine turns over freely. Too freely. So freely that I first thought that the starter wasn't engaging... but, that didn't make any sense. Popped the hood and looked. You can see the accessories on the front of the engine turning. So, the engine is cranking. But, there is zero compression. No resistance. But, no strange noises, either. (that part is good) Removed the oil cap. You can see the timing chain turning, so it's not a broken timing chain.

    My only guess right now is that it somehow slipped the timing chain, and now the valves are out of synch. Thus no compression.

    I've never bothered to do a great deal of research on this engine, didn't think I needed to. I'm not even sure if it's a non-interference engine. And I don't know what the chain tensioner setup looks like. Is it possible that the timing chain could have slipped a few teeth (possibly by design) without breaking anything? I don't know.

    If you know anything, hit me with your knowledge!

    I'll be trying to figure this out over the next few days. If it seems like I can fix it without a lot of money in parts, I'll likely do that. If it seems likely that something is hard broken, I know salvage engines can be had for very cheap.

    I'm the 2nd owner, and the car has 40k miles. I guess I should be covered under the 5yr/60k warranty. But, I'm hesitant to even bother trying to make a claim. It would be nothing but hassle if I took it in, just to have them tell me that "the ecu shows that this engine was revved to 8972 rpm, this is clearly abuse and your warranty is void".

    So... I'm probably just going to fix it myself.


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    Administrator Daox's Avatar
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    Bummer!

    I'm not sure if its an interference engine or not. I would guess it is, and my other guess would be that the valves floated and hit the pistons bending them enough to kill your compression. Its possible timing jumped, but I think that is less likely. Popping off the valve cover should allow you to check that fairly easily though.
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    Loren (10-21-2018)

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    Member daleWV's Avatar
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    So sorry to hear that this happened Loren. I'm strongly inclined to agree with Daox's diagnosis. I hope you will keep us updated on what you find!

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    Loren (10-21-2018)

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    Moderator Eggman's Avatar
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    Sorry about your loss Loren. Here's hoping you and your Mirage get back on the road soon.

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    Still Plays With Cars Loren's Avatar
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    Stunning. This car has been sold in the US since 2014 and globally since 2012. The engine family has been in production since 2004!

    I can't find a single clear reference anywhere as to whether or not this is an interference engine!

    I guess that points to reliability. Nobody's breaking timing chains, so it doesn't matter if it's interference or not... unless you money-shift it.

    I did find a list of engines that states interference or not. This engine isn't on it, but all but one Mitsubishi engine on the list IS interference. And, I'd imagine that to get a high compression ratio, things need to be rather tight in the combustion chamber. So, I'd be surprised if it wasn't an interference engine.
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    Senior Member Cobrajet's Avatar
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    Sorry to hear about this. I have bounced my Mirage off of the rev-limiter a couple times pulling out into traffic (when I shouldn't have). Never heard of this kind of transmission-induced over-rev situation before.

    From what I understand about overhead cam-vs-overhead valve engines, valve float is supposed to be something that solid lifter OHC engines DON'T do. This is why they have much higher rev limits than pushrod motors. Motors with bent valves are usually pretty clattery.

    When you say 'no compression' I assume you really mean ZERO? Could this have something to do with the MIVEC system? Stuck or broken actuator, perhaps, which is throwing the valve timing off?

    Are there any CELs?

  9. #7
    Still Plays With Cars Loren's Avatar
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    Somebody else PM'ed me about this.

    There seems to be a fundamental misunderstanding about throttle-induced over-revving vs. mechanical over-revving. I'm used to being on sports car and autocross forums where people know what I'm talking about. I don't fault you guys for not knowing.

    It's not at all uncommon in motorsports or performance driving, in fact, it's common enough to have it's own name! They call it a "money shift", because you botched the shift... and, obviously, it does damage to your engine and costs you money.

    So, y'all are used to the rev limiter. And it does it's job. Trust me, in the autocross environment, I've visited the rev limiter a LOT in this car and every other car. Why? Because autocross courses are typically right at the top of 2nd gear. In the Mirage, they'll be right about 50-55 mph. Maybe you might get up to 57 if you grabbed 3rd, but then you'd have to immediately downshift back to 2nd for the next turn. More often than not, it's not worth taking the time (and disruption in focus, and potential for mis-shifting) to shift to 3rd at an autocross, so we stay in 2nd... and sometimes ride the rev limiter for 3 seconds. It's not big deal. The rev limiter does what it's supposed to do, it protects the engine in those situations.

    But, that's not what we're talking about here. The other way to over-rev an engine is mechanically. You're FORCING the engine to turn faster than it should, you're not "asking" it with the throttle (where the ECU can say "no, I'm not going to allow that"), you're "making" it with another set of circumstances (and physics takes over). No rev limiter can save you from this. The most common way for this to happen is exactly what happened to my car. Accelerating at the top of 2nd gear, reaching for 3rd, and hitting 1st. In the Mirage, 2nd gear tops out at about 54 mph. So, popping it into 1st gear and releasing the clutch immediately has you in 1st gear at 54 mph. If we assume (sorry, I can't test this at present!) that 30 mph in 1st gear is 6,000 rpm... then 54 mph in 1st is somewhere around 10,800 rpm! Even if you don't fully release the clutch (and you'd be amazed just how quickly you realize this mistake and push the clutch back in!), you can still zing the engine to 8-9k or more.

    People do this all the time in motorsports, and usually get lucky. But, some engines are less forgiving than others. I remember the early 90's Celica engines had a problem with this because they already had a really high rev limit (8k, as I recall), and they were pretty sporty cars. So, people would be out having fun (even on the street), letting that little engine rev... as you do. And then at 8,000 rpm in 2nd, they hit first gear and probably 12-14,000 rpm!

    Perhaps the Mirage engine is one of those unforgiving engines? Or perhaps my co-driver's reflexes weren't on-point (and he's not really used to my car yet), and he let the clutch all the way out for long enough to break things? I wasn't driving. I wasn't even in the car. I wasn't even WATCHING at the time. So, I can't say for sure exactly what happened. It happened right at the finish, and a lot of people HEARD it happen... probably not a good sign.

    But, that's where we are. The engine has been spun up to some unknown, but really high, rpm.

    No CEL because it's not starting. But, I did read codes... nothing but a pending O2 fault. (pretty typical state for my car with the header and no pre-cat)

    No weird clattery noises, which is why I was first thinking it wasn't bent valves, but a slipped timing chain. But, I suppose a valve doesn't have to be very bent to not seal.

    No compression... I haven't measured it, but the engine spins freely, almost as if the starter was not engaging.

    I doubt that the MIVEC system has enough range to completely eliminate compression. But, if it does... it's clearly broken! And, if the case, it could surely be fixed. But, used engines are so cheap that it just doesn't make sense to get that deep into it. I'm leaning a lot toward just putting in a $500 used engine. I looked last night. 2017 engines are available for $300.19 (yes, 19 cents) to $1600. And expanding the search to include 2015 engines, they are $250-1800. There's even at least one fairly close to me for $650.
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    Senior Member timw4mail's Avatar
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    Quote Originally Posted by Loren View Post
    But, used engines are so cheap that it just doesn't make sense to get that deep into it. I'm leaning a lot toward just putting in a $500 used engine. I looked last night. 2017 engines are available for $300.19 (yes, 19 cents) to $1600. And expanding the search to include 2015 engines, they are $250-1800. There's even at least one fairly close to me for $650.
    Wow...that is really cheap!

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  11. #9
    Still Plays With Cars Loren's Avatar
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    Yeah. Reliable car. Cheap car. People buy them and crash them... but, there's little market for used engines. So, they sell cheap.

    I looked at this before I ever started autocrossing the car. I know I'm taking this car far beyond what it was designed to do and I'm likely to break stuff! I looked into prices of used transmissions and engines before I even bought the car.
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    Senior Member Cobrajet's Avatar
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    Quote Originally Posted by Cobrajet View Post
    Never heard of this kind of transmission-induced over-rev situation before.
    Sorry, Loren. I guess this part of my post was unclear. I absolutely knew that your co-driver had 'grabbed the wrong gear' and sent the engine over-revving past it's electronic limitations. I just meant that I had never heard of it happening on a Mirage before and causing an engine failure.

    I suppose not too many people here autocross them!

    You are right about simply replacing the engine rather than spending a bunch of time tearing it apart trying to find the exact mode of failure. These little engines are pretty cheap used, and after 40k of hard use a fresh motor in your car would be the way to go.



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