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Thread: How robust is the 3a92? What are its failure modes?

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    How robust is the 3a92? What are its failure modes?

    Our little engine is proving to be far more robust than I would have guessed. The only engine failures I have read about were due to mechanical over-revs due to improper downshifting in race like situations. I have read about our engines being tuned to rev to over 7.5K with moderate boost pressure with the stock compression ratio of 10.5:1 and (separately) handling the abuse of autocross without complaint.

    I am curious if the 3a92 has a weak point. Not sure if I am wording this correctly but I am curious where our engines are likely to fail in stock form?



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    Senior Member Cobrajet's Avatar
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    Quote Originally Posted by ahausheer View Post
    Our little engine is proving to be far more robust than I would have guessed. The only engine failures I have read about were due to mechanical over-revs due to improper downshifting in race like situations. I have read about our engines being tuned to rev to over 7.5K with moderate boost pressure with the stock compression ratio of 10.5:1 and (separately) handling the abuse of autocross without complaint.

    I am curious if the 3a92 has a weak point. Not sure if I am wording this correctly but I am curious where our engines are likely to fail in stock form?
    If it has a weak point I think we have yet to find it. These engines are tough because they are very simple.

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    We know these engines last long if not over-reved, if not run with non fully synthetic oil (non PAO and ester-based ), not run with low coolant, and maintained correctly.
    How long these really last, I don't know. It is easy for example to replace the rod bearings on an old engine before they start knocking, but the tolerances are slim and must be fully met.

    Similar 3-cylinder engines of other makes, eg. Suzuki, Daihatsu/Toyota, have shown they can last very long, much longer than most other 4-cylinder engines. Important in that sort of engine is the fluids, particularly proper PAO-based plus ester oil, and that it gets changed before it becomes black. If not, the oil return holes in the groove for the lowest piston ring will clog up. Think Forrester, Yaris... Knowing how that can happen, we can easily prevent it.
    Last edited by foama; 10-02-2020 at 08:40 AM.

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    Still Plays With Cars Loren's Avatar
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    It's an interference engine (valves can contact pistons if they get out of synch). And it has REALLY weak valve springs.

    I know from experience that a botched shift (hitting 1st when reaching for 3rd when at 6000 rpm in 2nd) that zings the revs past 10,000 rpm will instantly overpower the valve springs causing the valves to "float", and bend all of the exhaust valves when they contact the pistons.

    In normal use, this shouldn't be a problem. I'm not sure I'd rev this engine past around 7k without fitting stiffer valve springs, though.

    It's a well-designed and robust engine, otherwise. Seems that it can take plenty of abuse. Just don't over-rev it.
    Simplify and add lightness.

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    Quote Originally Posted by ahausheer View Post
    I am curious if the 3a92 has a weak point.
    It absolutely does. In addition to thin valve stems, there have been several documented cases of lost compression due to a failed head gasket.

    Bent Valves, Need help picking a new engine.

    3A92 Images (pics of disassembled Mirage 1.2L engine)

    Bit of rod-knock?

    Engine swap this weekend. Pics

    3A92 1.2L 3-cylinder Mirage / Space Star engine info (specs, specifications)

    Some threads are dedicated to a failure, some threads have a failure buried in some post within.

    Engine failure under normally aspirated conditions doesn't inspire confidence in plans to boost.

    Taken care of and operated within rational boundaries the 3A92 can last a long time. Worked hard and thrashed, it'll crumble.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 manual: 49.6 mpg (US) ... 21.1 km/L ... 4.7 L/100 km ... 59.5 mpg (Imp)


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    I've yet to see a failure. 2 of the documented failures were just plain abuse. The first link was a guy that swapped cams and didn't install them right or did I read that wrong? The second one was a blown head gasket likely cause by running it out of coolant with a damaged radiator. Loren over revved the hell out of his. Rocketship had a blown head gasket.

    Overseas Foama keeps getting carbon buildup on his valves. He has a 1.0L

    There have been a bunch of engines replaced under warranty due to knocking. This is really the only real issue I've heard of. Mine is a little clattery and annoying. It'll likely last another 200,000km or so and not change much. I believe the earlier 2014-2015's were the ones mainly affected and mostly ones that see cold weather. Cold starts and overfuelling in the winter would wash out the cylinders and thin out the oil. There has been recalls to reflash the ecm to fix that years ago. I've yet to replace an engine or have a need to open one up. I'm not sure how long the timing chain and tensioner or guides are gonna last? Probably no point in changing them in a 450,000km car. Might as well drop in a different engine if your gonna go through that much trouble.
    Mirage videos:

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)


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    Senior Member Dirk Diggler's Avatar
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    Found out after the fact our 2017 G4 had its engine replaced and we couldnt find out why because our dealer went out of business for being shady. As to the toughness, for 10k miles the engine was severely under cooled until I spotted the coolant reservoir was very low. Turns out it was a bent tube when the original engine was replaced. As of now, its stable after multiple roadtrips. To me this means these engines are well designed to be tough and sip gas and thats it.7

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    I’m fixing to find out the limits myself pretty quick 😂

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    Quote Originally Posted by Fummins View Post

    Overseas Foama keeps getting carbon buildup on his valves. He has a 1.0L
    On long fast highway trips a lot of oil in the blowby gets into the intake manifold via the pcv valve. The longlife oil in these parts is 100% PAO and ester based, and thus causes gummy goo when it gets onto the hot valve stems. That goo hardens within 72 hours or so if the car wasn't restarted sooner. If it hardens, the valves remain a bit open and compression is gone. No compression means no start.
    Added a catch can and that problem is now cured!
    Last edited by foama; 10-15-2020 at 05:35 PM.

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    Senior Member strawboss's Avatar
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    In fact the reason I bought the Mirage was because of the engine.
    I bought a diesel powered Smart Fortwo in 2008 because the FE of my pickup in the city was terrible and the Smart was the only vehicle that would fit in my garage with my boat, ( other than a motorcycle).
    I was fascinated by the little engine and fooled around with it a bit, deleting the egr among other things.
    But a couple of things about it irritated me and after 6 years traded it in on the Mirage.
    I knew the basic architecture of the Smart engine and looked for a car with the same engine.....voila, the Mirage.
    The selling point for me was/is the timing chain. I never did like timing belts, the worst one I owned was a Ford Escort that powered the water pump with it's timing belt.When the pump kakked it would strip the teeth on the belt and that engine was an interference type....
    This engine doesn't seem to have many faults,thankfully.


        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES Plus 1.2 automatic: 37.9 mpg (US) ... 16.1 km/L ... 6.2 L/100 km ... 45.6 mpg (Imp)


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