Melted #2 Piston! Theories wanted.

[DSMTC03]

Hi. I just recently changed the headgasket on my 95 GST and had been driving it for about a week of normal driving. It had a rough idle since reassembly but the timing marks were lined up correctly so I figured I would fix the idle problem this weekend. Well I didn't make it to this weekend because, on my way home from work, I accelerated pretty hard up until about 100mph and let off the gas only to see smoke shooting out behind me. I immediately pulled over and shut down the car. I brought the car home and pulled the spark plugs only to find that #2 had the electrode melted off. A compression test showed only 30lbs of compression in the #2 cylinder whereas the others were about 145 across the board. Upon removing the head I found the #2 piston to have a "flat spot" on the side of it where it melted. Obviously there was a very lean condition in that cylinder. My question is what would cause only one cylinder to lean out this much. I am thinking maybe an injector failure? Is there any way to test the injector flow rate? All of my gaskets on my intake manifold, head, etc looked good with no tears. All of the other plugs looked great. If anybody has any ideas what to look for just let me know. Thanks in advance.

 

[mavisky]

If it looked something like this then I'd guess you ran lean and detonated.

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[DSMTC03]

Yeah mine doesn't look nearly as bad as that one does. What was the culprit? Injector?

 

[1gnasty]

yep that is definitely detonation want ecu are u ruining

 

[mavisky]

Yeah mine doesn't look nearly as bad as that one does. What was the culprit? Injector?

Too much boost, too little fuel, long pulls, probably a little bit of each.

 

[Defiant]

It's an epidemic:
http://www.dsmtuners.com/forums/showthread.php?t=148744

 

[DSMTC03]

I updated my profile. Does anyone have any theories as to why I melted just this one piston?

 

[DSMTC03]

Nobody has any ideas as to how only one cylinder could lean out this much?

 

[eclipsegs-t92]

i see in your profile that you dont have any fuel mods to support your turbo mods hence the lean condition. i dont see that you have an aftermarket boost gauge installed. DONT rely on the stock boost gauge at all your boost may be spiking like mad too. you said as soon as you saw smoke you pulled over and shut it off right? the reason #2 cylinder was the first to go was because it was the weakest, your motor is only as strong as its weakest part.

 

[Pitman99505]

How long had you been running the t-28 with no supporting fuel mods or boost gauge? I agree, I do not think it was injector failure, it was pushing so much dense air with that T-28 and no supporting fuel mods. It is okay to run a T-28, but you cannot run the boost very high with out leaning it too much and causing detonation which seems to be what happened. If in fact you do not have a boost gauge, with your exhaust so free flowing with a 3" dp, test pipe, and cat back you were more than likely experiencing boost creep. So you really have no idea how much boost you were running. After you replace your pistons i would highly reccomend getting some sort of logger along with supporting fuel mods for that T-28. Sorry that happened to you, Hope it all works out for you in the end.

 

[Defiant]

Nobody has any ideas as to how only one cylinder could lean out this much?

It takes about six revolutions for that to happen once a piston gets that close. You can rest assured that they were all that close, but that one went first. Aluminum goes plastic a bit over 400°, and combustion temperatures top 4000°. You'll notice the starting point was that small riser around the valve cutout. It got heated up locally just enough (so much for running without piston squirters) to get soft, then a ping or knock at just the right time in just the right place, and bang-o.

Do not bump your threads.

 

[DSMTC03]

I do have a boost gauge. Sorry I didn't have it in my profile. I was only running 15lbs of boost until I got injectors, 190 walbro and an SAFC. I never noticed boost creep. The T-28 was installed while the headgasket was done because my T-25 was on its way out. Its just a standard T-28, not a killer or anything like that. All of my other cylinders have great compression and the plugs are even brown except for the #2 cylinder. I am just curious if a slightly clogged injector could cause a cylinder to lean out to the point of meltdown. I am also curious if that could possibly been related to my idle problem. I will be installing 550cc injectors regardless. I just want to be sure that will fix my problem. Thanks for the replies and keep the ideas coming. I only want to have to put this motor together once.

 

[Defiant]

I am just curious if a slightly clogged injector could cause a cylinder to lean out to the point of meltdown.

It could, it's just not likely. Fuel systems generally don't have much opportunity to pick up dirt to start with, nor will it get past the fuel filter.

I only want to have to put this motor together once.

That's the best one this week.

 

[DSMTC03]

It could, it's just not likely. Fuel systems generally don't have much opportunity to pick up dirt to start with, nor will it get past the fuel filter.
That's the best one this week.

The car was apart for 3 weeks while I was waiting on parts and the injectors sat in the open air under the hood. I got a better look at the intake gasket today and I can see where a little amount of "soot" residue appears to have leaked past the gasket in a few spots. I think I may have found my problem. I have a feeling this residue was left by the oily/smoky air in my intake leaking past the gasket while my car was dying. I will be having a talk to my mechanic on just how "tight" the intake manifold bolts really were. I assume that an air leak between the intake manifold and the head would cause a VERY lean condition, correct? This would probably also cause my rough idle correct? I probably should have just done all the work myself in the first place. Thanks for the help so far guys.