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MG Midget and Sprite Technical - Over-Pressurized cooling system

Just when you think you've got your car together and are looking for good stretch with clean hands and messed up hair....something totally awesome happens and you get to take your spridget apart again!

OK hear's the skinny. I just installed a rear oil seal when doing a transmission swap. I have a 1275 with a timing cover to PCV to manifold crankcase vent. Fresh air coming from the canister to the valve cover elbow. After driving the car up to temperature the cooling system is over-pressurized and once the overflow tank is full blows coolant out directly related to the engine speed. No change with a closed valve cover cap, vented or no cap at all. I did have some coolant loss before the engine out that I had thought was a crappy rusty lower crossover pipe.

My hypothesis of cyl head problems were confirmed with cyls 2 & 3 test 115 and 110 psi while 1 and 4 both are at 145 psi.

Anything else I should check before I take the head off for inspection? Anyone else had this problem that ended well? Thanks. CJ
CJCharvet

Doesn't sound liek a venting issue at all. Sounds like a head gasket allowing combustion gases into the coolant.
David "But I have been wrong before..." Lieb
David Lieb

Yeah, I was just ruling out the most recent changes as contributing factors. Thanks David.
CJCharvet

CJ. I agree with David. Either a head gasket failure or a cracked cylinder head. Have the cylinder head checked for cracks when you remove it and before installing a new head gasket.
Les Bengtson

Id say try re-torqueing the head 1st you might get lucky, My guess is the same as davids.

Just curious, did this happen on the 1st start up after the engine install??

If so, its a fairly common problem (at least for me), because you have air pockets thur out the cooling system, and will certianly give big time pressure differances, Also when you do the compression check, allow the connection to stay on the holes for 5 minutes and see how fast the pressure drops for each clyinder. My toyota truck is notorious for air pockets and over pressuring when I drain the coolent and replace it


Take the rad cap off and pull the thermostate and let the engine run for about 15-20 minutes this will allow the water pump to push the water thur really fast, you might keep a rag to cover the rad hole with to keep it from spilling out and keep adding fluid as nessary to the rad. then replace stat and leave rad cap off (still) and run engine another 20 minutes adding coolent as needed then let the engine cool an put rad cap back on and run agian for 20 minutes and see what happens from there, most likely it will be cured if its just air in the system


just be warned if this is the case for over pressurizing its going to blow massive amounts of steam, and I mean big time steam and even gushing gysors of coolent out the rad hole ...It looks bad but its just working the air out, Id also take the heater core out of play by looping the hose to the heater tap on the head...you can always add it back in after the problem is solved

Another hint ...if you have to replace the coolent agian, fill up the system thur the theremostat hole, the coolent seems to flow better backwards to take out the air....Just something I stumbled across sometime back.


Another hint...fill a 5 gallon bucket with water if the engine starts to over heat (on the gauge) pour the water over the a good size portion on the front of the rad while trying not to let the water splash on the engine, this will cool it down in a hurry

Prop
Prop***The End in 2012

Thanks everyone, this is by far the most helpfull BBS I have ever been a part of. The heads coming off tonight. Thanks for the tips Prop, It usually takes me about 20 min to get all of the air out of the system and I thought that may be the problem at first. Never tried removing the t-stat though. CJ
CJCharvet

Thanks again everyone. Cheap aftermarket head gasket failure. Live and learn right? Cyl head was fine with no cracks and straight.
CJCharvet

Thats great news CJ, glade to hear the good tidings of others on durry seasons of the year.

Now thats just sad, Good news is a poor guy with a busted midget, and the great part of his trouble is a blown head gasket....WOW, come on spring time.

prop
Prop***The End in 2012

It's 75 degrees here on the eastcoast of Florida so I did get to drive it today. Just got rained on pretty good though.
CJCharvet

This thread was discussed between 27/01/2009 and 29/01/2009

MG Midget and Sprite Technical index

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