Leakdown on a never fired rebuild - need some coaching please

SoCal_Lynch · 11-18-2020, 05:52 AM

Good morning. Have a '78 ROW large port 3.0L engine I am looking at that was supposedly professionally rebuild 10+ years ago then stored since. Asking price on the engine is attractive, seller is a good guy but engine condition is all word of mouth since it has changed hands a couple times. It has been stored inside in with the plugs in and fuel injection system installed.

The seller offered to do a leakdown prior to my purchasing it this weekend. He said the engine rotates freely but is stiff with plugs in. When the leakdown test was started the PSI never got to 90 and there was 50%+ leakdown via the crankcase. Apparently you could clearly hear it hissing.

I would expect an engine that hasn't been fired and run-in to not have the rings sealed yet. I am hoping to get some coaching on this leakdown behavior and how concerned I should be about the ability to service it and then carefully fire it up and use it?

Any thoughts or experiences to share here? I don't want an attractive price to lure me into an engine that needs to be re-rebuild, at the same time I don't want to be unfair to seller and let a lack of knowledge have me miss out on a good engine.

Thank you in advance for any responses.

dkirk · dkirk's Garage

Leakdown tests are usually done on a recently-run, warm engine. This assures rings and cylinder walls are properly lubricated. Personally, I wouldn't be concerned with the leakdown readings you're getting from this engine. A cylinder borescope inspection would yield better information on internal condition.

Black 993 · 11-22-2020, 01:23 PM

The leakdown you saw past the rings (i.e. to the case) is actually pretty good evidence the backstory on the engine is true, since if it's really never run, the rings haven't seated and you'd get significant leakage there. I would roll it over a few times and take a look with a boreoscope. If you see brand new pistons and cylinders, it means the engine was rebuilt and you might want to roll the dice if it's a good price.

JAM_Parts · 11-23-2020, 03:50 AM

Remove plugs and put some oil in the cylinders, I am sure that the compression would go up, not sure what it means. If it is on an engine stand so much the better you can drain the oil out. Be prepared to remove and clean the pugs a few times on starting the engine.

SoCal_Lynch · 11-23-2020, 10:08 AM

Thank you for the responses. The seller agree to bring the engine to a shop and get it properly fired up. I will follow up once that has happened. I appreciate the responses, kind of a big purchase to get wrong.

Emo993 · 11-23-2020, 11:50 AM

I had the same thing this year with a 2.2 long block I purchased, leak down was also 50 psi on all cylinders. Finally with PMO and Ignition ran it on test stand for 20 min plus to break-in cams and rocker and all but one cylinder came up to only 4 percent leakage. I'm not worried about the sixth, needs to be in car and have some load applied to set the rings. Best, Mark

E Sully · E Sully's Garage

When I did a top end on my 3.2, the leak down was about 10 percent before I ran it, very minimal after break in with 180 psi compression test. It had nikasil cylinders and new rings. New valves and guides were also installed.
I would want more information. I would want to know what this professional rebuild consisted of. Were new pistons and cylinders installed, was the top end done, was the case split for new bearings? Did they change any head studs.
Also, are the cylinders nikasil or alusil. If they are Alusil and all they did was new rings, this is not a rebuild, and can cause issues if they used old cylinders and just put in new rings. Re ringing alusil can be very problematic.
Without some documentation it is just a used engine. Hopefully the shop can give you some good information after they run in the engine.