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1970 Datsun 240Z — temperature climbs in traffic but drops once moving

RotorCommuter84Community StarterAug 3, 20261970 Datsun 240ZCleveland
1970 Datsun 240Z, about 96,500 miles. Coolant temperature rises while sitting with the A/C on, then drops once the car is moving. Coolant level looks correct. Fans run, but I do not know whether they are reaching full commanded speed. I am in Cleveland. I am trying not to assume the first explanation I see online.
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11 replies

ShopDriverCommunity StarterCommunity StarterAug 3, 2026permalink
Intermittent stuff lies if you only catch it once. A pressure test beats staring at a dry driveway if the 1970 Datsun 240Z is losing coolant with no puddle. If the cheap test changes it, stop and reassess. On “temperature climbs in traffic but drops once,” that was the fork that mattered for me. Not gospel, but it saved me a headache. If it repeats twice more I'll book time.
TowCommuterCommunity StarterCommunity StarterAug 3, 2026permalink
Fair point. Counterpoint: Write temp, speed, and in-gear vs coast when the 1970 Datsun 240Z does it — then decide on parts. Don't erase codes until someone who can help has seen them. This is from a 1970 Datsun 240Z in Cleveland with 96,500 miles. Verbal 'we'll take care of it' is not a plan. This is the version I'd tell a friend.
TowCommuterCommunity StarterCommunity StarterAug 3, 2026permalink
I wouldn't throw parts yet. Move a coil or plug on the 1970 Datsun 240Z if it's cylinder-specific and see whether the code follows. Save the big job until that test fails. I'd want that in writing, not as a promise. Your commute and mine probably aren't the same, but the test still applies. If this is nothing, great. I want that in data.
CleanBuyer879Community StarterCommunity StarterAug 4, 2026permalink
I hear you, though I'd still want a second check. A pressure test beats staring at a dry driveway if the 1970 Datsun 240Z is losing coolant with no puddle. If the cheap test changes it, stop and reassess. On “temperature climbs in traffic but drops once,” that was the fork that mattered for me. This is from a 1970 Datsun 240Z in Cleveland with 96,500 miles. Keep sale price, fees, and rate as three different conversations. Parking it somewhere I can hear it better next time.
ShiftHunter834Community StarterCommunity StarterAug 3, 2026permalink
I'd want a measurement, not a hunch. A pressure test beats staring at a dry driveway if the 1970 Datsun 240Z is losing coolant with no puddle. If the cheap test changes it, stop and reassess. Run the total, not the monthly, before you commit. I'd rather be boring and correct. No warning lights, which is why I'm documenting it.
ProjectOwner137Community StarterCommunity StarterAug 3, 2026permalink
Ask what changed right before it started. Move a coil or plug on the 1970 Datsun 240Z if it's cylinder-specific and see whether the code follows. Don't erase codes until someone who can help has seen them. If it doesn't change the out-the-door or the repair estimate, it's noise. Make the next action something you can finish this week. No warning lights, which is why I'm documenting it.
RotorCommuter84Community StarterCommunity StarterAug 3, 2026permalink
I went back under the 1970 Datsun 240Z and it only happens after a heat soak, not on a cold start. I already checked the easy fluid levels.
ShiftHunter834Community StarterCommunity StarterAug 4, 2026permalink
Intermittent stuff lies if you only catch it once. Move a coil or plug on the 1970 Datsun 240Z if it's cylinder-specific and see whether the code follows. If the cheap test changes it, stop and reassess. On “temperature climbs in traffic but drops once,” that was the fork that mattered for me. I'd want a dollar figure or a measured spec, not 'should be fine.' If your situation is the opposite, ignore me. If this is nothing, great. I want that in data.
BatteryDetailerCommunity StarterCommunity StarterAug 4, 2026permalink
I wouldn't throw parts yet. If a shop can't duplicate it on the 1970 Datsun 240Z, bring a video plus freeze-frame data, not just a story. That's the branch I'd chase this week. I'd want that in writing, not as a promise. Your commute and mine probably aren't the same, but the test still applies. Only after sitting overnight so far.
RotorCommuter84Community StarterCommunity StarterAug 4, 2026permalink
OP update: scanned the 1970 Datsun 240Z again and still only have the one pending code, no new lights. This is me being the wet blanket on purpose.
ShopDriverCommunity StarterCommunity StarterAug 4, 2026permalink
If it were in my driveway, two 1970 Datsun 240Zs can feel identical and have different causes, so measure before you copy someone else's repair. That's the branch I'd chase this week. Take it or leave it — that's what I'd do. Not trying to scare you — just skip the expensive first guess. This is me being the wet blanket on purpose.
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