1999 4.8 Broken rod - piston hitting valve - HELP...

[easygoin43]

Thanks in Advance for any help in finding my cause:
I have a 1999 silverado 4.8 2 wheel dr. 171,000 miles ran great till one day knocked parked it. I just bought this as indicated from the original owner and never any problems with truck. He went and bought a new one- drove this in and out of driveway 5 times and I drove to put on trailer to get home. Knocked loud tinny sounding knock and ran like crap.
Diagnosis: 0 comp in one cyl. - leak test, air into cyl. exhaust open air in exhaust, same for intake, closed both and hit the air heard clunk- piston went down. Took off oil pan and splash shield rod had broke hitting splash shield only (no block or head gasket damage). piston stayed up in cyl. Took head off yeaterday top of piston and valve appear same as others very little carbon- HOWEVER- it appears that a valve may have been hitting and spinning the piston around in cyl. swirl marks in top of this piston in circular pattern.
Questions:Why broke rod?, did timing go to allow piston to hit valve?, is it possible for ther eto be enough pressure to push the piston up to valves while running?, the cylinder is fine, and I will be able to replace piston,rod,rings,gaskets and be able to go - BUT I want to fix why it happened.. Sorry for long post- tried to give all info. ANY HELP GREATLY APPRECIATED.. Chris

[CJ0717]

Have you looked at the valve stem keepers yet. Hard to believe but once in a great while a valve stem key may get thrown then the second key goes (could have been the slight ticking noise you were hearing (valve stem keys rattling around under the covers. The valve then drops down and smacks the piston. Then the show is over.

Where was the rod broken? Are the rod bearings wiped out? Sludge can build up in an engine. Starves the rod mains and creates a wiped out bearing. This then causes alot of stress on the rod and it breaks.

Regardless of all the damage, if your into rebuilding engines it might be salvageable. If not then a used motor from the scrap yard around $500-$1000 or better yet if you can afford it a new crate engine. Probably around $4500 and up.

I used to work for GM Powertrain and saw quite a few casualties in my time. The root cause can be very deceiving.

[MT-2500]

Originally Posted by easygoin43

Thanks in Advance for any help in finding my cause:
I have a 1999 silverado 4.8 2 wheel dr. 171,000 miles ran great till one day knocked parked it. I just bought this as indicated from the original owner and never any problems with truck. He went and bought a new one- drove this in and out of driveway 5 times and I drove to put on trailer to get home. Knocked loud tinny sounding knock and ran like crap.
Diagnosis: 0 comp in one cyl. - leak test, air into cyl. exhaust open air in exhaust, same for intake, closed both and hit the air heard clunk- piston went down. Took off oil pan and splash shield rod had broke hitting splash shield only (no block or head gasket damage). piston stayed up in cyl. Took head off yeaterday top of piston and valve appear same as others very little carbon- HOWEVER- it appears that a valve may have been hitting and spinning the piston around in cyl. swirl marks in top of this piston in circular pattern.
Questions:Why broke rod?, did timing go to allow piston to hit valve?, is it possible for ther eto be enough pressure to push the piston up to valves while running?, the cylinder is fine, and I will be able to replace piston,rod,rings,gaskets and be able to go - BUT I want to fix why it happened.. Sorry for long post- tried to give all info. ANY HELP GREATLY APPRECIATED.. Chris

Pistons do not go up unless piston or rod breaks or rod bearings shell out.
But a timing chain break or valve can drop hitting piston.
Even if you figure out what failed.
I would replace it with a new or re-man or used engine.
Actual cheaper than a complete rebuild and better warranty.

Gm parts places have some good new/rebuilt engine with good warranty.

[j cAT]

I agree with the others on this ...considering the mileage and unknown useage/maintainence installing a good used engine would be better..with lower mileage..

[CJ0717]

Since it did run up to the moment the rod broke, for sure the valve must have dropped. See if you can find the valve stem keys loose in the valve train area.

[easygoin43]

I would like to comment on the responses- AND THANKS ALOT for looking at this- it all helps me to try and diagnose... Keep them coming....
First I know the original owner and he takes truck to dealer for any work and has alll records - there were very min. trip to dealer nothing major and relgiously kept up on oil changes. I know tihs is tru because last nite I got the piston out. The cylinder is in perfect shape same as others- the valves too are very clean no carbon/sludge this engine looks like new still on the inside. Took off both valves in this cyl. the keys/springs were perfect and tight. There is one spot on piston where the intake valve hit and put a 1/32" gouge in top but the valve has no marks where it was hitting. I will attempt to get some pics downloaded.
The rod bearings were in excellent shape too- normal wear and no additional play at crank. I know I am very lucky to have this happen and not ruin a block or crank...
I have checked all injectors with OHM meter and they all read fine.
I need to see if there is a way to check timing? Can you do like the old days with timing light and mark on harmonic balancer?
I cannot afford to replace the engine and since the condition seems ok I am going to proceed with a slight hone and replace piston/rings/rod/bearing/headbolts/gaskets/spark plug on that cyl. I have priced it out at $262.00 plus tax. Also- think I will also change intake valve to be safe.
BUT-- I still want to TRY to find the cause before I fire her up.
My 3 theorys are:
1. Bad injector flooded cyl. caused stress on rod trying to compress fuel and snapped. This plug was soaked and not carbon like others upon removal first time. Appears to be lighter in color and not firing.
2. Timing off - she ran rough in this condition but just not sure if it was running rougher then having only 7 cyl. going - so timing off - hit valve first time and broke rod- then piston was forced up in top of cyl. by pressure in block and the valve kept tapping top of piston to cause it to spin in cyl. and swirrl pattern on top of piston.
3. Stress on rod and it just went. (ever hear of this?)

I am saving my $$ to get parts and will have in a week or so -before I fire it- I will check spark on that coil with all plugs out. Like to check timing but how?

Please let me know your thoughts- Thanks Again.
Chris in Michigan

[CJ0717]

Love to see someone that wants to rebuild these engines. I think that is the right move.
If the valve keys were in that throws out the valve smacking the piston causing the piston rod to break.
The injector if stuck open could cause a hydrolock condition and stress the rod to where it braeks. (only if it was a defective rod to start with. Hydrolocked cylinders won't normally break a rod.
Is this a zero clearance engine? if so if the timing chain jumps a few teeth (from being old and streatched, then that would cause the piston hit an opened valve. Doing a rebuild I would normally replace the timing chain/belt anyway.
You should be able to see if the timing is on. Rotate the engine where #1 is at Top Dead Center on the compression stroke and see if the timing marks line up i.e. cam gear/Crank Gear The CAM gear timing marks are what you need to focus on. If they are off a few teeth that would do it.

Good Luck
CJ

[easygoin43]

Also I have documentation from owner on exactly what happened when she went:

It broke at startup. I was on vacation (at home) between Christmas and New Years, so the truck sat a few days in cold temperatures. The night of Dec. 28th I was going to take my truck to the store. When I started it, I immediately heard the loud rod-knock noise and the check engine light came on...so it was never driven.

When I popped the hood, I did smell gas...so maybe you're on to something with your guess #3, but I think #1 is more likely. I saw air bubbles (probably the gas I smelled) coming from a seal/gasket by the front left piston where I thought the noise was coming from. When I showed a neighbor where I saw the air bubbles, he said that was where the exhaust exits the piston. The neighbor isn't a mechanic, so I have no idea if he knows what he was talking about.

Before the issue, the only time the check engine light would come on is if I was running really low on gas...when that happened the check engine light would blink repeatedly until I got gas. Once I got gas, the check engine light would stay on (not blinking) for 20 miles or so. I figured that once the light comes on, it doesn't turn off until the engine error codes clean themselves up.

The truck was driving great before the issue, especially after having new U-joints installed in the summer. I was never concerned about the truck's reliability...I took it up to Manistee in October and November for deer hunting.

Please let me know your thoughts again- can"t add pics from work - will add tonight...

Thanks to everyone
Chris

[CJ0717]

Yep the way you describe it now sounds like a hydrolocked cylinder. Must've had an injector stuck open.
I've heard of this potential but never experienced it in a regular V engine. Saw it happen on a rotary engine (typical when a quick startup and shutdown) Ends up you gotta pull the spark plug and rotate the engine to free it up. Mazda's poor design.
Anyway's good luck on the rebuild. Sounds like you've done these before so it should be a fun project. It always is for me. Something about hearing it rev back to life after rebuilding it.

Ciao
CJ

[j cAT]

Originally Posted by easygoin43

Also I have documentation from owner on exactly what happened when she went:

It broke at startup. I was on vacation (at home) between Christmas and New Years, so the truck sat a few days in cold temperatures. The night of Dec. 28th I was going to take my truck to the store. When I started it, I immediately heard the loud rod-knock noise and the check engine light came on...so it was never driven.

When I popped the hood, I did smell gas...so maybe you're on to something with your guess #3, but I think #1 is more likely. I saw air bubbles (probably the gas I smelled) coming from a seal/gasket by the front left piston where I thought the noise was coming from. When I showed a neighbor where I saw the air bubbles, he said that was where the exhaust exits the piston. The neighbor isn't a mechanic, so I have no idea if he knows what he was talking about.

Before the issue, the only time the check engine light would come on is if I was running really low on gas...when that happened the check engine light would blink repeatedly until I got gas. Once I got gas, the check engine light would stay on (not blinking) for 20 miles or so. I figured that once the light comes on, it doesn't turn off until the engine error codes clean themselves up.

The truck was driving great before the issue, especially after having new U-joints installed in the summer. I was never concerned about the truck's reliability...I took it up to Manistee in October and November for deer hunting.

Please let me know your thoughts again- can"t add pics from work - will add tonight...

Thanks to everyone
Chris

with that description I agree it was a problem with the cylinder filling up with something ...then with the cylinder full of liquid it blew..

normally this would be coolant...not fuel ...If fuel were leaking, would have evaporated a bit and leaked down into the crankcase by the rings..after a few days of sitting ..did you have coolant issues...had to add coolant often,,?

[easygoin43]

Pictures added........
No coolant issues. I inspected the head gasket, block, and head but do not see any signs of blown head gasket.
Again- thank you all for the help- As of right now I am thinking about a bad fuel injector. But still does not tell me why this plug was burning lean and the top ring was stuck into the piston in one area? See pictures..

Thanks
chris

[j cAT]

[quote=easygoin43;6100112]Pictures added........
No coolant issues. I inspected the head gasket, block, and head but do not see any signs of blown head gasket.
Again- thank you all for the help- As of right now I am thinking about a bad fuel injector. But still does not tell me why this plug was burning lean and the top ring was stuck into the piston in one area? See pictures..

Thanks
chris


quote:

looking at this picture , the top compression ring looks rusted ...how does this compare to the other pistons ..

[easygoin43]

I have to agree after looking at picture close your right. Last weekend I proceeded to clean everything- this is what I found. Head looks fine, but I have noticed that this specific cylinder is different on the intake side. the port on head for intake is cleaner then the rest. the only sign of gasket issue is on intake gasket on the intake side of it near bottom, and the intake port is smooth and has a powder like on the walls - not glossy and carbon like all others. ALSO- found tonight- coolant in the back of the intake plentium. I am thinking that since no coolant ports in the intake that it may be coming in from throttle body area - there is a coolant line in the throttle body? does this make sense?
Why the rust on this piston? Remember the truck sat for sometime out in MI. winter then it popped on startup.
What do you think about this- coolant being sucked into intake near throttle body and then since this cyl is close to that it was coming thru intake valve and collecting in cyl.? But what about the stuck fuel injector theory? Hydro lock of some kind? Thanks a million for all the input- I have ordered rings and rod. hope to get back together soon. thoughts?

[CJ0717]

Man oh man,, knowing that the piston rings appear rusted is not a good sign. Exposure to gas won't do that. Having coolant or moist air would. Possible for a cracked cylinder wall. If your rebuilding this engine absolutely have the block and specifically that cylinder magnafluxed for cracks. Sometimes cracks are so thin that only magna-flux will expose them. The intake valves are normally loaded up with crud and the exhaust would normally be just blackened but clean. With the intakes being the same as the exhaust would suggest a steam clean or some sort while running. This condition may have been there for a while prior to the rod breaking. I still think a hydro-lock condition broke the rod. Great pics though. Amazing how the valve spun the piston around. Not much clearance in this engine from the depth of the eyebrow cut in the piston.

Food for thought.

[CJ0717]

Looking back a past posts. Ran great. Parked it. Really cold day. Sat for a few days. Started it, loud knock ran terrible and shut down to diagnose.
Think about a crack in the block. Hot engine, cold temps outside. As the engine cools down a cylinder with the intake valve in a slightly open position will cool down first. Hot coolant and cold cylinder cavity. Coolant and vapor bleeds into cylinder. Any H20 would then freeze. Go to start a couple days later and you have either liquid in cylinder that is ready to close the intake valve and go into the compression stroke. Unfortunately there is either alot of coolant or a chunk of ice in the cylinder. Turn it over and bang there goes the rod.

Again, food for thought. Only a thorough investigation of the block and heads will uncover the truth. The culprit is still there.

Oh, by the way have you looked at the injector for the #6 cyl. Was it stuck open?
I never really diagnosed a bad injector so I wouldn't even know how to tell if it is stuck open or not.

CJ