broken pistons

[efret]

I got an 86 MR2 and I'm in the proccess of rebuilding the motor with alot of help from my dad. We have it all torn apart and all 4 pistons are broken and need replaced. What i was wondering was that caused by using low octane fuel or is it a flaw in the design and it's not worth buying new ones?

-Thanks

[Timon]

Considering all the other people here, and so far you're the only one with 4 broken pistons (no offense intended) - I'd doubt it is a flaw in the design.

[PHOENIX]

I'm gonna guess and say you had a fuel problem, most likely some detonation to ruin all the piston heads. Be sure to check out your fuel system and Air intake system to make sure they are running properly.

[toy-driver]

Pictures?

[efret]

Well i just got the car in this condition.

Sorry but no camera.

Thanks for the help.

[tjmr2]

The 4AGE is a high compression engine and needs high octane fuel to keep from detonating. The broken pistons were probably caused by either low octane fuel, or timing that was too far advanced, or a combination of both.

Use good fuel and set the timing to factory, you will have nothing to worry about but smiling till your face hurts.

[toy-driver]

Really? Mine has had 87 octane ran through it most of it's 225,000miles

[tjmr2]

And you are at sea level, bring it up to 5500 feet elevation to start then try and go over a 10,000 foot pass, I'll bet it pings.

[te51levin]

Originally Posted by tjmr2

The 4AGE is a high compression engine

9.4:1 is not high compression in my book. In fact it's rather low, particularly for a relatively modern engine with EFI and pentroof combustion chambers. In most parts of the world they (bigport 4AGEs) will happily run forever on 87-octane swill, if they are properly tuned and maintained. Sadly, most of them are not.

If he had detonation issues bad enough to bust all four pistons, either they were comically cheap (non-OEM) pistons, or there was a serious tuning or neglect issue, like massive carbon deposits, grossly incorrect ignition timing, chronically lean fuel mixture under load (dirty filter or clogged injectors), etc.

[jus7o]

I am high jacking this thread and inserting my own noobesh question. I am in the process of rebuilding my big port motor two pistons are bad and wanted to know if the .50 over sized AE92 hi comp pistons would be a good alternative fit into my stock 86 big port set up? I think they would and am continuing a little searching now but wanted the mr2.com to help confirm for me!!

Thanks

[te51levin]

They won't work because they use 20mm wrist pins, while your '86 has 18mm pins. It'd be easier to find a later short block that has 20mm pins.

[toyotaspeed90]

Originally Posted by te51levin

They won't work because they use 20mm wrist pins, while your '86 has 18mm pins. It'd be easier to find a later short block that has 20mm pins.

well.... he should make sure of what he has..... not sure how many aw11's I've seen with the original untouched motors.....

[te51levin]

I've seen plenty, but you raise a valid point. Something to watch out for.

[Jackstand Queen]

4 broken pistons is a highly improbable occurance. I can think of only Two scenarios that could cause it: a single, brief, incident of super lean fuel mixture. With just the right amount of fuel/air mixture, gasoline has the explosive power of Tri Nitro Toluene (TNT).

Therefore, you should trouble shoot every device having to do with the fuel delivery: every sensor, switch, pump, filter, injector, solenoid, and even the ECU . . . . . Trouble shoot all these items to the Nth degree!

Otherwise, the possibility of the exact same event happening again is a too probable.

The other scenario is that the previous owner added too much Nitrous Oxide or NOS for the motor too handle.

[toyotaspeed90]

Now that I think about it.... it would depend on what the original poster meant by "broken"....

I've seen many 4age's grenade a rod and start taking bits everywhere -- which ultimately hit other pistons and generally destroy the skirts.

[jus7o]

if its broke dont fix it!