Overheated Engine - How Hot To Ruin Rings?

[Captain Obvious]

Off topic alert. I'm asking here because the level of experience is much higher here than where I should be asking. I just bought a cheap disposable Hyundai. It was overheated significantly by the previous owner and because of that, it needs engine work. No surprises here. I knew about all that before buying it and I got it cheap enough that even if I have to replace the engine completely, I think I did pretty well.

 

Here's where I am... I know the head gasket has been compromised and I'm assuming that the head has warped as a result of the heat and will need to be planed flat. But before I pull it apart, I'm trying to figure out what else is wrong. I don't want to dump a bunch of money into the head only to find that the engine should just be completely yanked instead.

 

The engine has 120K miles and the compression test numbers don't look too good. Wet looks great, but dry is below the service limit. So it sounds like rings or worn cylinders, but at 120K miles, I would be disappointed if it was just normal wear.

 

So here's the question... Just how hot do you have to get an engine before rings lose their temper? Is it conceivable that the engine overheating episodes ruined the rings? is it conceivable that the pistons and bores are OK, and I could get out of this with just tossing new rings on the pistons?

[Chickenman]

A lot of variables, but as when you take the head of you can examine the cylinder bores and get a better idea. If it got hot enough to damage the head gasket it probably got hot enough to take the tension out of the rings at a minimum.

 

If you see any cylinder wall scoring or a polished or Blue look to the cylinder walls, then you may want to have a hard think about things. There will likely be piston damage as well if the bores are scored. Engines often experience combustion detonation when they are over heated. That can break rings and while it may still run, the downward spiral to a nasty demise has begun.

 

Modern engines operate with very tight piston to wall clearances and are not tolerant to overheating. Thin wall casting iron cylinder blocks distort like crazy if over heated. Aluminium blocks are even worse. If an aluminium block engine over heats, it's pretty much toast. Major work will be needed, but often the engine is scrap.

 

About the only thing that will help the situation is if the engine was run with a good Synthetic oil. Synthetic oil doesn't lose it's shear strength at high temperatures like Dino oils. Whatever, you'll get a better idea of the situation when you pull the cylinder head ( s ) off. I'd lean towards a good used engine if you see cylinder wall damage. . The vehicle is probably not worth the price of rebuilding the engine.

Edited October 14, 2015 by Chickenman

[djwarner]

Oil control rings are separated with a spring ring. The spring will lose temper with overheating before the piston rings. This will lead to excessive loss of oil.

 

If the cylinder walls look reasonable, you may still need a ring job. 

[Zed Head]

Overheating usually leads to compounding of interacting problems.  Excess heat causes excess piston expansion leading to localized heat in the bores and more expansion.  The aluminum pistons usually bind up in the bores before things get too too hot.  Motorcycles seize all the time from over-heating.  I've never seen any discussion of rings losing temper due to overheating.  I have seen discussion of crankshaft bearings losing lubrication due to oil-thinning though.  Seems like certain engines have certain failure points.

 

As far as block problems due to being aluminum, you really need to know if the bores are sleeved or not.  The old Chevy Vegas were known for ruining blocks due to over-heating, but they were still saved by sleeving the bores and rebuilding.

 

I'd find a Hyundai forum to get the details on that engine.  But like dj says, the evidence is probably on the cylinder walls.

[black gold man]

I would find a junkyard that has guarantee policy and get a used motor.

[gundee]

Lots of unknowns there.

I know of two cases where the person drove it till it quit running.

After it cooled off and added water there was not enough ring

compression left for it to even start.

The yards around me have so many engines they don't even have

the room to put any more inside.

The engines outlive the cars now. Exchanging for a nice low mileage

one would be a good idea I think.

Machine shop costs and parts really add up fast when repairing.

Tough call.

[grannyknot]

I would find a junkyard that has guarantee policy and get a used motor.

This^

How much can a used Hyundai engine be?

[rcb280z]

Agree with locating a "good" used engine. Too many variables with the one in question. Probably be less money and for sure less time on a used one.

[Zed Head]

I think that CO just bought this as a project.  No fun to just replace a whole engine.  More fun to make bad parts good again.

[Captain Obvious]

Thanks for the input guys.

 

So about just how hot it got? Of course, I wasn't there, but here's what I know. I was told that it was misfiring badly when they shut it off, but it runs and sounds fine now. I also know that when I went to pull the plug wires off to run the compression test, the plastic extensions that reach down between the cams to the plugs were a little melted onto the spark plugs and the plastic extensions snapped off leaving some melty portions stuck to the plugs. I don't know what kind of plastic they used for the wire extensions to reach the plugs, but whatever it is, the engine had exceeded the melting temp for that plastic.

 

Runs fine now. Plug wire extensions had melted, and in general smells a little burney plastic when you open the hood. That's about all I know at this time.

 

One of my pushbacks on a replacement engine is the ability to do a compression test on a replacement engine that has already been pulled out of the donor vehicle? Sitting on the floor in the corner of the shop? If I bring a starter along, I might be able to bolt it directly to the block even if there isn't a transaxle attached? Anyone got any bright ideas? If I'm going to drop one in, I certainly want it to be proven to me that it's better than what I've currently got.

 

I guess I was thinking of pulling the head first anyway and having a look at the bores. I've been into other import motors with twice that mileage and I could still see the remnants of the original hone crosshatch. So I guess my path forward is, pull the head, look at and measure the bores, and if they look and measure good, I may cheap out and just throw a set of rings in it and see what happens. But if the bores measure worn or if there's any scoring, metal transfer, or discoloration, I'm going to bail on the job and look for a replacement drop-in instead.

[Captain Obvious]

I think that CO just bought this as a project.  No fun to just replace a whole engine.  More fun to make bad parts good again.

 

There's actually a lot to that!    This project is also a learning experience for a kid. The intent is that he will be involved in the repair, whatever direction it takes, and in the end, he'll be driving this car. I think there would be more experience in fixing the broken engine than just dropping a replacement cold chunk of metal.

 

Not to mention that I've been known to frequently throw a completely unreasonable amount of time at fixing unfixable parts...

[rcb280z]

I was assuming you were wanting it on the road soon but......Then maybe tear it apart and rebuild it. If it got that hot I would rebuild the whole thing. Great learning experience actually for all involved. Have fun! Ah to be a kid again and tearing into my first engine. Where have the years gone?