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overheating 74 mgb

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mykec74mgb Avatar
mykec74mgb mike culver
rochester ny, USA   USA
I bought this 74 mgb with I believe an early motor in it last year. The car sat in a barn for 4 maybe 5 yrs. I got the car running - rough but OK, fixed the clutch & brake systems and drove it around my neighborhood. This spring I rebuilt the carburetors and replaced the master brake cylinder and rebuilt both front calipers and rear wheel cylinders. The brakes do not drag or bind.
Now here is the problem - I started the car up to finally drive it got down the street and it starts to overheat ( I replaced the temp sending unit this spring).
Got home and found the thermostat was stuck closed. Replaced it with a 160 deg put the car cooling system back together and fired it up. The car in my driveway warmed up to the "N" position on the gauge so I took off down the street. I got maybe 500 ft away and the gauge started heading towards the hot position then it started to cool back to the "N" for a couple of minutes so I drove it into traffic and the it went back and pegged the hot position. It was getting late and after I let the car cool down I drove (nursed) her back to my driveway
The coolant looked good when I changed the thermostat. after the thermostat change and ran the car in the driveway I checked the radiator and it was getting hot - some fluid did shoot out the overflow from the neck of the radiator after it started to overheat.
Does one bleed or burp the cooling system - adding fluid as the motor is running?
could this the problem?
would I be able to tell if the water pump is pushing water through the system or enough water by looking at the motion of the water via the radiator cap opening?
lastly is there a way to figure out what water pump I presently have with out taking off the motor?

any suggestions would be greatly appreciated.


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michaelodonnell123 michael odonnell
north bergen, NJ, USA   USA
Did you conifirm that the thermostat was working before you put it in? You can put it in a pot of water on the stove and let it boil to make sure it opens up properly.

Michael '74 MGB
good photos

mykec74mgb Avatar
mykec74mgb mike culver
rochester ny, USA   USA
Yes I did, I put both the old and the new to confirm that the old did not open and the new one did.

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kuz1 Avatar
kuz1 Keith Kuzma
Prattville, Alabama, USA   USA
have you confirmed the temp gauge is accurate? After I changed my voltage stabilizer 180 became N ,prior 180 was H

tomkatb Avatar
tomkatb Larry Baygents
Dayton, Ohio, USA   USA
1963 MG MGB
It sounds as if all the minor stuff is OK. The system is simple. The fact that the gage seemed to read hot and it started to overflow means the gage, a known issue, likely works. A meat thermometer and a bit of putty can be used to verify engine temp. Do not let the wife see you with it or it can't go back in the drawer.

You can sort of test for flow by wearing a glove and squeezing the radiator hoses. Do it cold and hot so you see a difference. Watch that fan blade they can hurt.

My next step would be to try an Prestone engine flush. It helped mine. I think maybe a DPO might have used stop leak or something and may have clogged up the radiator/block. If the flush does not help I would have the radiator tested and repaired as needed. They can do wonders for $100 or less. When I changed my clutch I had mine professionally flushed and flow tested. They said it was fine so apparently the Prestone helped.



77 Spit

AVIMAX Ryan Nadler
Toronto, Ontario, Canada   CAN
Even though it sounds like it may not be the case, check that the voltage stabilizer is not just dangling loose and is attached to the firewall and therefore grounded. It only takes a second to confirm and in my case I was chasing a suspected overheating issue for years only to find the voltage stabilizer was not grounded and this was the source of my gauge reading hot. The voltage stabilizer should be located on the firewall inside the car near the steering column. I changed everything, radiator, water pump, thermostat, temp sending unit, gauge all with no results until I came across something on the internet about the voltage stabilizer. I checked the schematic and sure enough it was supposed to be grounded through its housing. Wish I had known to check that first. But at least I know everything in my cooling system is new!

74 Bracken B Avatar
74 Bracken B Dennis Glass
Vacaville, CA, USA   USA
1974 MG MGB "Loon"
Mike, this may sound silly, but what is the condition of the water pump? The reason I ask is that I also have a 74' that I purchased 2 months ago and had the exact same symptoms! I took it a step further and grabbed the fan and wiggled it from side to side and it had too much play in it for me. The DPO (and I do mean DUMB!), when he installed the water pump, he used silicone gasket sealer and just about covered the entire hole for the coolant return. I went down to our local Kragen auto supply, picked up a 160 deg thermostat and water pump (yes they had one on the shelf) for $48.00, put it all back together and it works great! The needle is on the right portion of the "N" now. During the real hot days the needle climbs a bit but not a bunch. I'm going to install an electric fan next and that should solve it all. Check the pump though, it might be a part of the problem. I hope this helps.

P.S. If your a fan of silicone gasket sealer, (which I am not) use it sparingly as my DPO was sloppy and nearly cost me an engine.

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mgb922 Avatar
mgb922 Tim Dowell
Solomons Maryland, USA   USA
1979 MG MGB
1979 MG MGB
I've seen coolant systems do all sorts of crazy things. I live in Arizona for a while and used tape water to add to the antifreeze. The minerals in the water clogged the heater core and part of the radiator. I've also seen the impeller of the water pump come loose of the shaft just enough to work and low RMP but start slipping at higher speeds. So really you should check everything mentioned. As Dennis said could cost you an engine.

Sprite Lou Avatar
Sprite Lou Platinum Member Lou A.
Springfield, Oregon, USA   USA
1970 MG MGB GT "Higgins - RIP"
1973 MG MGB GT "Eliza - FIRST PLACE - MG2013"
1976 MG MGB "Daphne"
kuz1 Wrote:
Quote: have you confirmed the temp gauge is accurate? After I changed my voltage stabilizer 180 became N ,prior 180 was H

x2 on the volt stabilizer - just waiting on a rebuilt (solid state components) from Limey (Eric Marshall) - can hardly wait to see if this helps my temp reading "issue".

If you send him 2 cores, he'll send you a rebuilt one for F-R-E-E !!!

'76 MGB RD MKIII "Daphne"
'73 MGB GT MKIII "Eliza" - First Place - MG2013
RIP Peter Cummins
RIP John D. Weimer
RIP Mick McGuire

"Bits-4-Brits" fuse block & relay kits --- X1 (for now)
VTO LeMans Wheels from Hap --- X1
"Black-Label Factory OD" --- X2
"Custom Side Cover" --- X2
"Hiltonized" SU HS4s --- X2
"Schlemmerized" Dizzy X1 (for now)
"MGCCARS" Alternator X2
"DBWood" custom, one-of-a-kind heater valve --- X2
"Ingramized" Bonnet & Hatch Struts --- X2
"DuBois-ed" SU Fuel Pump --- X3
"Peter-Built" Shocks --- X1
"Classic Auto LEDs" --- X2
WAY More parts from Lord Roop than even HE remembers!

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