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MG Car Repair/1980 MGB LE Running Hot



You've helped me before so hopefully you can give me some guidance on a methodical way to investigate my problem.

I purchased a US spec. 1980 MGB LE about a month ago.  The car was located in Colorado at an altitude of about 8000 ft above sea level.  It has all of the original emission control equipment intact.  The car ran a little rough at idle but no smoke, good oil pressure and good power and response during the test drive.  I attributed the rough idle to a vacuum leak or damper oil etc.

I drove the car back home (about 400 miles# to my home which is at about 750 feet above sea level.  I mention the altitudes in case mixture might play a role in my issue.

As I drove the car home, it was running very hot.  The temp needle was at or near the red on the gauge the entire time.  I ensured that the coolant was topped off through the plug in the thermostat housing.  Coolant seemed to be flowing through the thermostat as hot water was present in the upper radiator hose.

It was a very hot day #around 100F# so since I knew MGBs don't handle heat well I just chalked it up to the extreme conditions and the nature of the beast.  The car did not boil over but was obviously running very hot.

After I got home, I drove the car a couple of times and again the temp was very high even though the ambient temp was only in the 95F range#.  

Shortly after getting home, the water pump failed.  The car didn't overhead or anything, I just noticed coolant dripping from the weephole.  It was one of the budget type with a stamped impeller.  I replaced it with the more robust type with the cast impeller and installed a 160F thermostat instead of the 180F standard unit that was in the car.  I was hoping this combination would increase coolant flow and allow the car to run cooler.  There was no difference.

I thought the timing might be incorrect so I checked it with a timing light with the vacuum advance disconnected, the line plugged and at about 1400 rpm.  It was spot on at 10 BTDC.

Then I remembered my aviation training and that a lean engine would run hot.  I assumed that the car had been adjusted lean for the high altitude from which it came.  I followed John Twist's method and found it was indeed too lean.  I richened the mixture appropriately and hoped that would keep it cool.  Again no change.

Car has the original Stromberg carb.  I added damper oil to the carb #it had leaked out# but didn't help the idle issue.  I thought I would rebuild the carb and dip it to ensure that all of the passageways are clear.  I doubt this was done correctly before.  I don't know if this could contribute to the hot engine but I hope to help the rough idle with correct carb operation.  Previous owner had removed the water choke and installed a manual conversion.  I don't know if I will leave that in place or not.

I adjusted the valve lash on the engine.  Both intake and exhaust at .015 with the engine warm.  There was a great deal of variance in the valve lash when I started.

I talked to the previous owner and received a list of work he had done to the car and in retrospect it looks like he was trying to solve the same heat problem.  He claimed that all late US spec engines were leaned out to the extreme for emissions and that I should not be concerned.  He said that he thought the gauge is probably wrong.  I own another 1980 MGB and even if you ignore the gauge, it doesn't "seem" as hot when driving or working on it.  

His list of updated items included:

Radiator flush and refurbishment from a shop

Removal of engine driven fan and installation of electric fan with thermostat #I checked these out and the are all working as well as the two factory fans.  All were wired to an adjustable thermostat which I dialed down to ensure they are running nearly constantly.#  The original thermostat for the fans was bypassed in lieu of the new adjustable unit.

Rebuilt head

Rebuilt Carb

Installed Mallory dual point ignition and distributor

Valve adjustment.

I've owned 11 MGs in various running conditions so I'm not a newbie by any means.  I know that I can't take the word of the previous owner for any of the work he says he did, especially since I found the bad valve adjustment.  

My question for you is where to start and what steps to take to address this issue.  Here's what I have in mind.  Please correct me as necessary:

1. Use an infrared temp gun to see if the engine is actually running hot or maybe it is a gauge or sensor malfunction. #Should I see 160 or 180F with my 160F thermostat?  Where should I aim the gun for the best reading?#

2. Do a compression test on cylinders #Looking for a head issue#

3. Do a cooling system pressure test #Even though the system does not boil#

4. Do a complete engine flush and fill to ensure no debris is in the water jacket.

5. Remove the radiator and have it shop tested for flow

6. Stop fighting it and just get the bonnet hot enough to cook burgers on it for my friends and make a nice planter out of the rest of the car.

Howard, I'd like to approach the problem methodically and logically so any input you can provide on how to get this issue resolved would be greatly appreciated!  Would you suggest solving the rough idle issue first?  I've done the carb cleaner test and can find no vacuum leaks so that is pushing the idea of a carb teardown since I suspect the little o-ring is letting all of my damper oil leak out anyway.  

I could always re-install the water pump driven fan and see if that helps but it seems to me that 4 fans on the radiator is a bit much.


Hi Dave,

It sounds like you have done a lot so I will just tell you what I did each time I got a car with "over heating" complaint.

My first question to my customers was "Is it loosing coolant?" If not I agree with your using a temp gun to measure the heat as it is showing hot on the gauge. The thermostat housing is where to test the temp. If it is hot then I would first put the test dye in the radiator(available at any auto parts store) and run the test for CO in the coolant.

If the temp gun said it is not hot then I would look at the gauge voltage stabilizer to see that it is putting out 10v. (as I remember the 80 B has an electrical temp gauge not the old bulb type) If that is ok check the charge rate to see that it is not too high (13.8v to 14.5v at 2000 RPM)

What fuel are you using? High test or regular? Using high octane fuel in a low compression engine will cause one to over heat. If you are using high octane fuel and you are not sure of the compression ratio, you can run the fuel level down to almost empty and put in low octane fuel (only a couple of gallons) but don't alter the timing that you already set. Note if you get any "Ping" on acceleration from a low RPM. If you do then fill it up with high test but if you don't get the tell tail "Pinging" then you do have a low compression engine and you must run regular gas.

Does your Stromberg have the adjustable metering needle with the adjustment as a 3mm Allen down in the top? Even though leaded gas is gone and plug readings went with that gas, you can still get a rough reading with unleaded gas.

Make a steady throttle run down a road that you can pull off of the side with safety. At cruise speed depress the clutch and shut the engine off at the same time. Be careful not to turn the key back to lock the steering. Quickly pull off the road and pull all the spark plugs and look at them. If the center has any signs of black powder the mixture is rich (unlikely in your case) What you want to see is a clean center and a slight flat black on the edge of the threaded portion of the plug.

Surging is a symptom of a lean mixture when using light steady throttle settings on the road. You can still be lean even if you have a good quick throttle response due to the oil in the top pot.

Somewhere in that you should be able to locate the cause of the high readings. let me know.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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