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After engine rebuild 120 PSI compression; Need to be broke in more?
Topic Started: May 17 2013, 12:18 AM (1,747 Views)
YostFMX
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Just finished my XFi rebuild, honed, new base model pistons and rings, rebuilt the head, stainless lapped in valves, cleaned the lifters/rebuilt... It runs good, broke in the way my book says, warm up, run it from 30 up to 50 MPH 10 times to seat the rings. I checked the compression after that and they are 120 - 130 PSI. Do I need to break it in more before checking?
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scubaman
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All are 120-130? Did you check the bore for wear prior to rebuild? Did you put in std piston and rings?
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snowfish
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Basic GearHead

I'm guessing you forgot to hold WOT went testing compression. :smackface
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metromizer


Don't use synthetic oil during first 500 miles, or your rings may never seat. Also, new rings will seat faster if the engine is under load... don't let it idle for extended periods or 'baby it', until the rings seat.

Try removing the oil cap while the engine is idling, place the palm of your hand 1/8" from sealing the opening and 'feel' the vapors escaping out of the oil fill hole. As the rings seat, this 'blow-by' that is trying to pressurize the crank case will decrease significantly.
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YostFMX
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WOT? What is that?

Standard pistons and rings. And the top part of the cylinder measured (that the ring didnt ever hit) the same as the rest. It had 145 PSI compression before the rebuild.
Edited by YostFMX, May 17 2013, 10:18 AM.
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Aaron_37
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YostFMX
May 17 2013, 10:14 AM
WOT? What is that?

Standard pistons and rings. And the top part of the cylinder measured (that the ring didnt ever hit) the same as the rest. It had 145 PSI compression before the rebuild.
Wide open throttle. You need the air flow for proper compression testing.
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Deleted User
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I check compression with wide open throttle, the coil wire disconnected, and the fuel injector fuse removed from the box under the hood. Crank the engine until the pressure no longer increases. This takes a few revolutions of the engine. Position the gauge where you can see it while cranking the engine.

I have a compression checker from Harbor Freight that lives in a little red plastic box. It reads low. I bought another one. It reads more like what I expect.

The Harbor Freight compression checker will read 120 PSI, while the other one reads in the 180 PSI range.

The HF compression checker is going in the recycle bin.
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YostFMX
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What is the coil wire? I did all of that but didnt take off "the coil wire" as I dont know what that is... tested 175 PSI on all cylinders. The first cylinder when the compession gauge was hooked up makes a kind of popping sound, not sure where its coming from? What does that mean?

I'd like to test again when this coil wire thing off, where is it and what does it connect to?

Edit: Oh the connector out the left side of the FI?
Edited by YostFMX, May 17 2013, 01:00 PM.
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Coche Blanco
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Run it for a few thousand miles before you worry about whatever the numbers say. As long as they're evenish you're fine.
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YostFMX
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Coche Blanco
May 17 2013, 12:57 PM
Run it for a few thousand miles before you worry about whatever the numbers say. As long as they're evenish you're fine.
Makes me feel a lot better :)
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Old Man


YostFMX
May 17 2013, 12:55 PM

I'd like to test again when this coil wire thing off, where is it and what does it connect to?

coil wire goes from the coil that is attached to the firewall to the middle of the distributor.
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Old Man


YostFMX
May 17 2013, 12:18 AM
rebuilt the head, stainless lapped in valves,
did you install new valve stem seals? did you install new valve stem guides? did you check the clearance on the valve stems to the valve stem guides? did you have the valve seats ground, and the valves ground or did you just lap the new valves to the old seats?? if you just lapped without regrinding did you insure that the valve seat seating area was not too wide? .051 to .059 (1.3 to 1.5 mm) All of the above is important for a "head rebuild"
the lapping without grinding and/or the seats being too wide will eventually affect your compression if not already affecting it.
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Memphis metro


Sell it while its running and before it blows up.
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Aaron_37
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Coche Blanco
May 17 2013, 12:57 PM
Run it for a few thousand miles before you worry about whatever the numbers say. As long as they're evenish you're fine.
I too have found that my rings take a while to seat in properly regardless of what I do for break-in.
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YostFMX
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New Valve stem seals. I put grinding/lapping compound on the valves and ground them in till it was smooth, pretty sure thats all good to go.
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