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MG Midget and Sprite Technical - Radiator cap anomaly

Last week I took my midget to the Lake of the Ozarks for "50 Years of Spridgets," and on the way down I experienced some hot running, which is unusual for my car. It was a pretty warm day, and the extended cruise saw my temp gauge climbing up into the hot end. Coming back wasn't so bad, since the weather was cooler.

Anyway, today I took off my radiator cap while tinkering with the car, and notice something odd. The return valve that's built into the cap had apparently busted its spring, and was hanging freely from the bottom of the cap. (See attached photo.) Normally the spring holds it up against the main pressure seal.

My guess is that my cooling system was therefore unable to build pressure normally, and was boiling and showing hot on the gauge.

I put a new cap in today, took the car for a nice run, and the gauge stayed firmly in the middle of the range, as it used to.

I'd like to think I've solved this one, but what do you guys think? I've never had a radiator cap go bad before, but it sure seems that this might have caused the symptoms I saw on the trip.

Ciao,

-:G:-


Gryf Ketcherside

Gryf, the cap indeed was the cause of the problems. I've not seen a cap do that before either!
Clive Reddin 76 Midget

Gryf

That was one freakin HOT day, I had never experianced vapor lock in my 43 years until that day...considering how many cars had fuel pump problems that day, Im guessing they where suffering vapor lock as well and not pump problems...caause that what I was almost certian was my issue until david lieb and I talked it over...that was a day where minor problems on reg. temp days where exasterbated by the heat....no doulbt my out of adjustment acceleratere cable and slightly sticking choke contributed heavly to my vapor lock, that on a typical day would not have....

My car normally runs 165-175...that day I was pushing ever bit of 195-200 and my oil pressure was putting in a valourous fight to hold ground at 40psi at 3500 rpm


making my point the long way around the block....its probable been broken for a while...but on that day...any weakiness was made well known..

I have only changed the cap on my car once as I didnt know its age...but my truck I change every year when i change thermostat...and its always worn out...the seal is shot, and the spring has no tention.

BTW..did you get the photo yet in the mail

prop
Prop

>>> BTW..did you get the photo yet in the mail <<<

Yes!! Thanks for reminding me, and thanks for sending it up!

>>> That was one freakin HOT day, I had never experianced vapor lock in my 43 years until that day... <<<

As for vapor lock, I had a bit of that, too. I noticed that one of the 1500 owners at the Lake had removed the steel fuel pipe that runs across the back of the engine - just a couple of inches from the head, and right above the clutch housing; a real hot spot, if you ask me - and replaced it with a long rubber fuel hose that ran up behind the battery. I took a similar route this afternoon, by fitting a rubber hose as well, but I snuggled it down between the battery and the heater box. That way it's separated from the engine by the heater, and from the tranny by the heater shelf.

So - hopefully I've corrected both the overheating and the vapor lock. We'll see!

Cheers,

-:G:-
Gryf Ketcherside

thats a great Idea....Im thinking about using the core from my heater box and mounting it just behind the front grill to the far driver side so it will get fresh air and hopefully an extra 10 degrees of extra cooling....going to use the heater valve to access it....Im thinking about using copper tubing instead of rubber so as to help disapate the heat better...but not sure if that idea will backfire and take advantage of heat soak instead...will see

btw I found a company here in missouri just outside of joplin that makes shelby cobras and they have a nice little "functional" heat vent they use on there cars...about 55 dollars for the 2 for both sides

prop
Prop

Gryf,

Not sure that a lack of pressure will cause a higher operating water temperature? I would think more to do with the ambient temperature on the day?

The purpose of a sealed and pressurised system is to prevent evaporation and provide additional headroom if the water temperature goes up near normal boiling point on a hot day such as you experienced.

We had record high here in the UK of 100°F a few years ago. It was near Heathrow, and we were on the M25 in the same area at the time. The 1380 normally runs around 170°F, but that day was getting close to 200°F. Made me very pleased to have a 10lb/sq in radiator cap that would hold off boiling until ~250°F, assuming a 50/50% antifreeze mixture, if really needed.

Also satisfying to cruise past a worryingly high number of 'moderns' on the hard shoulder!!

Richard
Richard Wale

Richard,

Not quite. The higher the pressure that water is at, the higher its boiling point. The whole point of a pressurised cooling system is to allow the coolant to run at a higher temperature before boiling. It's controlled by the pressure at which the filler cap spring allows the pressure of the system to unseal it. If the seal isn't being made efficiently in the first place (for example, in Gryf's case, the spring isn't doing its job) the system won't run at design pressure, so will boil at too low a pressure - hence Gryf's problem.

That's why a radiator cap of this type is a service item, and should be at least inspected regularly, and replaced every few years.
Steve Clark

Gryf,

Yeah, that weak radiator cap lowered the boiling point of the cooling system and allowed you to run hotter. I was most worried about cooling that day, too, but I had just put on a new cap and all was fine.

It was me that ran the fuel line around the battery. My father-in-law had a '75 that had constant vapor lock problems. He even added an electric pump to boost the mechanical fuel pump, but getting rid of the steel line and using good rubber fuel hose routed as far away from the heat of the engine solved the problem completely. Adding an in-line filter helps, too. I should replace the last bit of steel line that runs over the transmission bell housing, but will wait until I take the system apart to fix the fuel sender unit to make that fix.

I'm getting the itch for another caravan run somewhere.

Lee in St. Louis
L Fox

>>> It was me that ran the fuel line around the battery. <<<

So it was! Now I remember. I couldn't run mine behind the battery, since the clamp bar on mine pulls the battery down and back, and it's pressed against the bulkhead. That's really the main reason I routed the hose between the heater box and the battery.

By the way, I have two filters in my fuel line. One's just before the pump, and the other's at the carb. I got the clear type so I could see at a glance if I have fuel in the line. The first filter lets me know if fuel is reaching the pump, and the second one shows if fuel's getting to the carb.

-:G:-
Gryf Ketcherside

Gryf,
You might contemplate inserting a wooden spacer between your battery and the bulkhead. This could give you room for the fuel line as well as moving the posts a little further away so as to delay the "my battery acid ate a hole through the bulkhead" effect.

Since you have the clear fuel filters, perhaps you have observed them and can answer the age-old question of "how is the fuel getting to the carbs when there is just air in the upper thrid of the filter?"
David "easily puzzled" Lieb
David Lieb

>>>>>>>>>>>the age-old question of "how is the fuel getting to the carbs when there is just air in the upper thrid of the filter?"<<<<<<<<<<

WHew, Thank god Im not the only one. I always thought it was just me.....sort of filed that question in the ol folder next to "where does my renagade socks ultimatle go to once there put in the dryer, and why is it when you look at a full moon, its all ways the same image....with the face off to the side....you never see the backside of the moon during a full moon, and why is 60 channels always really bad, with nothing good to watch, but on a rare occasion....all 60 channels have something really good to watch,,,but there is other things that need attention...so you cant watch tv....anyway

prop
Prop

Oh heck

true poetic philosophy there Prop

so true

:-)
Bill

>>> Since you have the clear fuel filters, perhaps you have observed them and can answer the age-old question of "how is the fuel getting to the carbs when there is just air in the upper third of the filter?" <<<

I put that one in the same category as "How do bumblebees fly?" FYI, a few years ago when I had my condenser problems, I initially diagnosed a fueling problem because the filter at the carb was near empty. Once I got the ignition fixed and the car was running great, the filter was STILL near empty. Go figure. The pick-up tube in the filters must be located such that they draw fuel even when the level is low.

-:G:-
Gryf Ketcherside

I dunno bout your filters, but I have a cheapo with a tube out one end, a tube out the other end, and a filter element in between. No "pickup tube" anywhere. It defies gravity as well as logic, but it works.
David "no, I can't tickle myself, either" Lieb
David Lieb

Maybe the funneling effect in the filter combined with the pressure of the fuel pump created a worm hole within the fuel system...And we cant see the fuel leave the filter cause time and light has been bent...LOL.

OKAY.....Im off to bed early tonight...lol.
prop
Prop

This thread was discussed between 06/07/2008 and 08/07/2008

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