Blowing core plugs

Help!
I have an 89 4.0 with a brand new rebuilt motor that is repeatedly
pushing core plugs out of the block. The block was built by a
machine shop(not me or I'd have a better idea what is wrong).
I've had the head off twice and I've checked the cylinders for cracks,
nada. Could there be aninternal crack in the head that I can't see??
Supposedly it was all magnafluxed before He put it all together, BUT.....
Any suggestions??
TY
:eye:

It could be that the holes for the plugs are just a bit oversize, and pushing out the old ones wrecked the interference fit. In that case, the best fix is to "stake" them in place, or drill/tap three or four small holes (equally spaced) in the boss around the thing and use screws/washers to hold the thing down.

I don't know of any other fix, short of soldering or brazing the thing in place - which makes it a HUGE pain in the arse to replace later. Always do something with future replacement in mind...

If problem is because of to high pressure:

Alternative 1:
Cylider head cracks can be a PITA to detect. Remove cap from coolant bottle and start engine. Look for exhaust bubbels in coolant bottle, and smell for exhaust fumes in bottle. (engine should be hot)

Have to ask: Do your car have the original cooling system, or is it modified to open system (cap on radiator)

Alternative 2:
Pressure release valve. Could be stuck closed. Try changing filler cap.

Alternative 3: (rare but not impossible in combination with loose fitting plug)
Water not circulating properly-partially faulty water pump (i.e. missing "wings" on pump). Causing water to boil locally in block, causing pressure build up.

Alternative 4: (rare but not impossiblee in combination with loose fitting plug)
Thermostate opening too abruptly and on too high temperature, causing pressure to fall quickly resulting in water boiling locally inside block, again leading to sudden pressure rise.

Alternative 5: (my money on this one)
Wrong size plug/worn out plug.


I believe this covers all viable scenarios... (except coolant freezing and pushing out the plugs, but unless you work in a ice-factory and park inside that's not really plausible i guess) :laugh:

ok, so to answer a few questions,
A. new water pump
B. New brass, tight fitting plugs
C. now there's water in the oil
D. new t-stat
E. stock closed system
F. water still in recovery bottle

Thanks all for your help

Water in the oil? must be cracked head or block, or head-gasket... ouch..

Hard to say why there is water in the oil. Did you change oil since you pulled and re-installed the head? Did you use a new gasket?

Back to the plugs. What does the electrode look like? Is it smashed?
When the fellow rebuilt the motor, did he mill the head or deck the block?
If so, he may have removed too much material.

If you believe you are having clearance problems, you can remove the head, pull a cylinder up to TDC, put some playdoh on the piston, and lay the head on the block (no rockers btw, they just mess up the process).
You should be able to determine if the plug is too close to the piston at this point. Also, if your plug is hitting, what about the valves?

I did change the oil after the head was off the last time, there was just a hint of water in the oil on the dipstick, Very minimal.
Now it's definitely milkshake-y.
I don't think the machinist decked the block.
I will pull a spark plug in the morning to see what they look like.
Yes it was a new head gasket.

I'm about to put original head back in to replace the one in it,
it's the only thing I can think of to rule out a cracked head.
thanks again

Zuki-Ron said:

Back to the plugs. What does the electrode look like? Is it smashed?
When the fellow rebuilt the motor, did he mill the head or deck the block?
If so, he may have removed too much material.

Click to expand...

.... am I way off here... or are you? We're talking coolant plugs in the block side right? not spark plugs?

Yes coolant plugs

Sounds like bad head gasket or a cracked head, but could be that #11 head bolt that needs sealant upon installation.

wow just tried to find head bolts and the stealership wants 250 bucks for a complete set, napa schucks and autozone seemed to strike out. any help?

Well it turns it WAS a cracked head like we all thought, BUT,,,,
It was cracked not on the cylinder side but on the top, right next to the hump for one of the valve springs. And down through the water jacket and exhaust port.
Thanks for all your help and advice.