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Member
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Discussion Starter · #1 ·
I have read a lot of threads about overheating issues, but those are all (unless I missed one) mainly the car heating up at idle, due to lack of airflow, etc. This overheating at speed has happened before in similar situation, but I have done trips at speed without it occurring.

Cruising at about 70 MPH, for the last 20 minutes out of about 40 minutes into my drive to the Springfield Rt66 meet... Weather at the time was mid 70s
Engine began struggling and an audible know started. Slowed to 55 and got to 1st exit, parked at a gas station.

after an hour or so to cool, and adding some water, I limped home and the knock dissipated... it sounds normal again (thankfully)
Idled very rough after that, if at all (stalled a few times). Checked all possibilities for vacuum leaks, but there were none.
Some relevant info about my 70GT:
  • stock radiator
  • 5 blade fan
  • shroud in place
  • weber 32/36 dgev
  • separated intake
  • sprint manifold
  • 4 speed
  • mechanical fuel pump
  • plastic fuel filter about 12" from carb
  • Pertronix ignition, Flamethrower 3 ohm coil (resistance wire replaced with straight wire run)
No coolant smell, or leaks whatsoever (except when it overheated of course).

I will be replacing the 5 blade with a 7, put in a new cap, having the radiator re-cored, new temp sender installed and putting in a new thermostat, but I was hoping anyone else that has had this happen might help me understand the cause, and add some tips/tricks that they used and found to help prevent it.

I am an absolute novice when it comes from understanding what cause and effect of various systems of a car might have on other systems

Also, hoping OGTS gets aluminum radiators back in stock soon.

Thanks! Mark
 

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Super Moderator
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14,289 Posts
I agree it sounds like a stuck thermostat.
 
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Opeler
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3,151 Posts
OPGTS lists aluminum radiator for automatic transmission. Might have slightly reduced capacity compared to manual transmission model. Telephone OPGTS,
and ask about the difference.
Aluminum Radiator, Opel GT w/Auto Trans. Part: 11023AT Price: $238.99.

New aluminum radiator for Opel GT's equipped with an automatic transmission. Recommended for Summer driving and for performance-built Opel GTs. Installs in original location, and outperforms original-style radiators. Works with original mounts, and adds a convenient drain plug! (Note: Designed for automatic transmission Opel GT's - Manual transmission GT owners see our part #11023A). Requires a 70% coolant/distilled water mix for operation.
 

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Member
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165 Posts
Discussion Starter · #5 ·
Advice on which thermostat to use, 160, 180... other? I live in Chicagoland, and the car is licensed (Enhanced Antique) for use April to Oct only.
 

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Just Some Dude in Jersey
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15,879 Posts
Just as an FYI: You can turn on your heater (and roll down the windows to let the heat out) if you have an emergency overheating situation. If your 'stat was stuck dead closed, your heater flow would still work and might help you get home. As said, your symptoms sound like a stuck thermostat. I'm a bit worried about your knock sound and subsequent rough running.......
 

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Just as an FYI: You can turn on your heater (and roll down the windows to let the heat out) if you have an emergency overheating situation. If your 'stat was stuck dead closed, your heater flow would still work and might help you get home. As said, your symptoms sound like a stuck thermostat. I'm a bit worried about your knock sound and subsequent rough running.......
And it was 20 min out of a 40 min run. The knock sounds worriesome though.
 

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Registered
1970 Opel Gt - Purchased July 1972 - Chartreuse - restored - 3000 miles as of 02-16, 2021 -
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901 Posts
My GT was running hot about 3/4 to the right on the temp gauge, 50% being the middle. Radiator was original and had been serviced twice over a period of 3000 miles- still ran hot - then replaced the old original radiator with the new aluminum one from OGTS, but first flushed the system with the heater open, using a chemical flush component and replaced the water pump just for good measure while the radiator was out of the way. I also changed the thermostat to a 160. The GT now runs around 1/4 to 1/3 to the left of center which is so much cooler than ever before regardless of the outside temp. I recently completed a 500 mile round trip and never got close to hot, temp stayed very consistent as mentioned above - I however mostly stayed at or below 60 miles per hour for the majority of the trip - however did run it at 70 miles an hour for about an hour - temp stayed the same, give or take just a little. So I think that the old radiator was just shot/clogged. I had changed the thermostate to a 160 prior to doing anything else with not change/help- but agree that is the best place to start the trouble shooting process. Good luck......
 

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Pedal Smasher
1973 Opel GT
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2,881 Posts
Flush the coolant. If it comes out kinda brown, I’d run water through it till the water is clear. Replace the thermostat with a new 180 degree one. If a new stat and coolant flush doesn’t fix it, you have a bigger problem.
 

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ILLUMINATI
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12 Posts
I have read a lot of threads about overheating issues, but those are all (unless I missed one) mainly the car heating up at idle, due to lack of airflow, etc. This overheating at speed has happened before in similar situation, but I have done trips at speed without it occurring.

Cruising at about 70 MPH, for the last 20 minutes out of about 40 minutes into my drive to the Springfield Rt66 meet... Weather at the time was mid 70s
Engine began struggling and an audible know started. Slowed to 55 and got to 1st exit, parked at a gas station.

after an hour or so to cool, and adding some water, I limped home and the knock dissipated... it sounds normal again (thankfully)
Idled very rough after that, if at all (stalled a few times). Checked all possibilities for vacuum leaks, but there were none.
Some relevant info about my 70GT:
  • stock radiator
  • 5 blade fan
  • shroud in place
  • weber 32/36 dgev
  • separated intake
  • sprint manifold
  • 4 speed
  • mechanical fuel pump
  • plastic fuel filter about 12" from carb
  • Pertronix ignition, Flamethrower 3 ohm coil (resistance wire replaced with straight wire run)
No coolant smell, or leaks whatsoever (except when it overheated of course).

I will be replacing the 5 blade with a 7, put in a new cap, having the radiator re-cored, new temp sender installed and putting in a new thermostat, but I was hoping anyone else that has had this happen might help me understand the cause, and add some tips/tricks that they used and found to help prevent it.

I am an absolute novice when it comes from understanding what cause and effect of various systems of a car might have on other systems

Also, hoping OGTS gets aluminum radiators back in stock soon.

Thanks! Mark
Check for crudded radiator (even if not plugged up corrosion on inside of collant passage surfaces will reduce heat transfer) failing water pump, thermostat.

Sometimes a (out of the car) radiator flush and maintenance can improve radiator capacity. In the past saw too many stock radiators with thick oxide layers.
 

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Opeler
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6 Posts
Decades ago one of my GTs overheated at highway speeds after the distributor shifted on its own from correct timing to severely retarded timing. Problem corrected by putting the timing back to specs and wrenching that hold-down bolt just a little tighter. A significant loss of power accompanied my overheating. I doubt this is the cause of your overheating troubles.
 

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Member
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Discussion Starter · #13 · (Edited)
Thanks for all the tips and ideas... I have a new 180 degree thermostat installed, a new cap, and proper coolant mix level re-established.
Automotive lighting Gas Auto part Circle Rim

The thermostat I removed was stuck open about 1/8" so I am hoping that was the cause, as previously suggested..

1st couple of runs after, the gauge never got completely past the "E" on temp gauge. Strangely, I did have adjust my idle screws to re-achieve a steady idle which I am not sure I understand. I will also be checking my timing as @jgrosh suggested, I think that too was affected.

Luckily, motor seems strong still so hoping I dodged a bullet.

Once driving season winds down I will either do the flush, or preferably install a new aluminum radiator.
 

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Registered
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108 Posts
I have read a lot of threads about overheating issues, but those are all (unless I missed one) mainly the car heating up at idle, due to lack of airflow, etc. This overheating at speed has happened before in similar situation, but I have done trips at speed without it occurring.

Cruising at about 70 MPH, for the last 20 minutes out of about 40 minutes into my drive to the Springfield Rt66 meet... Weather at the time was mid 70s
Engine began struggling and an audible know started. Slowed to 55 and got to 1st exit, parked at a gas station.

after an hour or so to cool, and adding some water, I limped home and the knock dissipated... it sounds normal again (thankfully)
Idled very rough after that, if at all (stalled a few times). Checked all possibilities for vacuum leaks, but there were none.
Some relevant info about my 70GT:
  • stock radiator
  • 5 blade fan
  • shroud in place
  • weber 32/36 dgev
  • separated intake
  • sprint manifold
  • 4 speed
  • mechanical fuel pump
  • plastic fuel filter about 12" from carb
  • Pertronix ignition, Flamethrower 3 ohm coil (resistance wire replaced with straight wire run)
No coolant smell, or leaks whatsoever (except when it overheated of course).

I will be replacing the 5 blade with a 7, put in a new cap, having the radiator re-cored, new temp sender installed and putting in a new thermostat, but I was hoping anyone else that has had this happen might help me understand the cause, and add some tips/tricks that they used and found to help prevent it.

I am an absolute novice when it comes from understanding what cause and effect of various systems of a car might have on other systems

Also, hoping OGTS gets aluminum radiators back in stock soon.

Thanks! Mark
Mark,
This will probably get argued about, but a similar situation happened to me on a 1971 GT, Stock everything. First, bugs had gotten into my radiator on the first trip after I just cleaned out the front of the radiator. GT's are a bug trap. Second, I was running the Petronix 2 ignition and flame thrower coil. When the car was getting hot, past the half way point on the temp gauge, the car started running like crap and I had a knock (pre ignition knock). When I got home, the car would not start. Once it cooled down, the car started but ran like crap. After MUCH tinkering with the carb with no luck, I opted to change out the Pertronix unit and wa-la the problem went away! The Pertronix 1 is absolute junk, don't us it. The Pertonix 2 does NOT like heat and it will shut off or fail when it gets hot. OPEL GT distributor location, right by the heat source (radiator). I have switched to the Pertronix 3 and I have had NO problems with the ignition failing period. Yes the Pertronix 3 is more expensive and some say is the exact same as the 2. Heard this MANY times, however I have switched ALL of my oldies except my 46 Dodge WC (waiting on Pertronix to offer one) to the Pertronix 3 and I have had -0- problems. Prior to the switch, I have had random problems with ignition failure. The "knock" you heard may have just been pre-ignition knock. If it went away, it was most likely NOT a bearing. Bearings do not fix themselves. they are either good or bad.
 

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Had a Very similar situation happen to me a couple years ago. Mine was fatal for the engine. Was told it was running too lean. Maybe something to check.
 

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Mid-West Opeler
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2,412 Posts
Thanks for all the tips and ideas... I have a new 180 degree thermostat installed, a new cap, and proper coolant mix level re-established.
View attachment 438159
The thermostat I removed was stuck open about 1/8" so I am hoping that was the cause, as previously suggested..

1st couple of runs after, the gauge never got completely past the "E" on temp gauge. Strangely, I did have adjust my idle screws to re-achieve a steady idle which I am not sure I understand. I will also be checking my timing as @jgrosh suggested, I think that too was affected.

Luckily, motor seems strong still so hoping I dodged a bullet.

Once driving season winds down I will either do the flush, or preferably install a new aluminum radiator.
This Saturday 02/10/2021, there is a car show at 501 County Farm rd 11am 3pm Sheriffs Office Wheaton
would be great to see you
 
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