Runaway Engine - Whats broke? - Ford Powerstroke Diesel Forum
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Old 01-24-2008, 02:11 PM
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Unhappy Runaway Engine - Whats broke?

long story short, we were trying to get the injectors to prime by using gas through the air intake. Unfortunately, the guys trying it didn't mist it and they didn't know how to stop a runaway engine. Once gas hit the motor it started running for a few seconds then just ran away for about 5 or 6 seconds and then shut-off.

We removed the valve covers and found some broken rocker arms and push rods. Is it likely that valves or pistons are damaged too? I haven't pulled it all the way apart and we can't do a compression test w/o repairing the push rods and rocker arms, a $600 job just to find out IF the pistons and valves are ok.

Thoughts?
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Old 01-24-2008, 02:27 PM
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What truck/engine?
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Old 01-24-2008, 02:32 PM
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What truck/engine?

Probably

99-03 7.3L
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Old 01-24-2008, 02:38 PM
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Why can't you do a compression test? The rocker arms and push rods don't hold the valves closed, the valve springs do. You can test for compression, because whatever valves that are associated with the broken rocker arms and push rods are now closed, and can't be opened. All the pushrods and rockers do are open the valve, the valve spring closes it.

I'd be willing to say that yes you have some piston damage, but maybe not.
Diesel pistons are tougher than gas pistons. There is always the chance that you could have broken a piston top edge and that will be floating around scarring your cylinder walls if you attempt to fire it without checking compression.

Just do a compression test and see where that gets you, if all are good then you can start thinking about re-assembling the valve train. How about the CAMSHAFT?

If all is good then I would replace ALL the pushrods, and ALL the rocker arms.
Keep us informed!
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Old 01-24-2008, 02:47 PM
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Its a 95 7.3L.

So, with all of the rocker arms and push-rods removed, I should be able to do a compression test? I was told that I have to crank the engine for a compression test. Is that true? Or can I manually move the engine?

Is there any way to inspect the camshaft with just the valve covers removed? I'd like to see if the lobes have any scarring or damage.

Won't cranking or turning the engine if a piece of piston is broken score the cylinder wall?

I don't want to cause anymore damage and I don't want to completely tear it down if I don't have to.

On a brighter note, I found a reman'd engine for $2650. If the valves, piston(s), and cam are toast, a new engine will probably be cheaper.
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Old 01-24-2008, 02:52 PM
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You could have major damage, or you may get lucky. Most of the time a run away is bad. You could probably do a cylinder leak down test. If it ran high RPM for 6 minutes, I would not expect much from the internals.
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Old 01-24-2008, 02:55 PM
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well......honestly i wouldnt bother fixing the valvetrain problems untill you know. the best way to check visually would be with a borescope.

You can do a compression test as it sits.....crank it five revolutions or so with the starter and see what the compression is.....you will need the 7.3 adapter that screws into the glow plug hole....

Honestly id say there is about a 60% chance that there is pistion damage. Its tough to say why your motor shut down too.....it could have shut down cause it lost compression....it could have shut down cause it ran out of gas to run on..... Every runaway ive seen has actually melted pistons and i saw one that completely incinerated a whole valve too....

a compression test will tell you a ton....i would also run a cylender leakdown test (just take all the rockers off for that)

IF the engine ends up being fine, not only do i think you should replace all of the damaged rockers and pushrods and inspect the rest good...but if it was my motor i would replace every one of your valve springs...
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Old 01-24-2008, 03:55 PM
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Quote:
Originally Posted by Marc View Post
...but if it was my motor i would replace every one of your valve springs...
AHHHHH!!!!!!
Always the soundest of advice!

I didn't even think of that! I've never seen a runaway, from all the stuff you described it doesn't sound good
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Old 01-24-2008, 05:12 PM
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Thank you fellas. I appreciate the help. I'll go with the compression test first. We have a compression test tool with the 7.3 glowplug adapter. At the least, I plan on replacing all of the rocker arms, push-rods, and valve springs.

It only ran away for 5 or 6 seconds, but it pegged the rpm gauge.
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Old 01-24-2008, 05:26 PM
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Send a message via Yahoo to bigcountrysg
The only runaway diesel engines I have dealt with was in the Marines. They ran away from diesel injection timing being off. But they ran away running on diesel fuel. Alot of difference there and what your dealing with. Good luck to you.
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