Third gen 88 5.7 over heats on freeway/highway


Asked by Jun 17, 2013 at 10:51 PM

Question type: Maintenance & Repair

1988 Iroc z 59,000 original miles tuned port, all factory, no mods, new rad, new cap, new
water pump, new thermostat 180 degree , new electric fan motors, new fan relays,
airdamn intact, zero holes or damage etc, 50/50 coolant ratio, no water in oil, no water
out exhaust , new belt and hoses, on the freeway or highway it runs 240, in bumper to
bumper traffic cools off to 220, run around short roads all day 220, put it on the freeway
watch it climb, pull over or coast off and on, back to 220, not pushing water back into
overflow tank either, fans kick on and as they should. great around town or sitting in
traffic, cant make a long run. Added mechanical guage to check accuracy of stock gauge
and it was close (within 5 or so degrees as the best you can tell from the generic factory
marks).  I am now very frustrated and broke any suggestions?

14 Answers

invest in a thermal gun, and point it at various parts of the radiator to find the hot spots there...possibly coolant flush, may fix the situation if the radiator is plugged~

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Radiator and coolant was replaced yesterday with new, same result.

1 people found this helpful.

I have a thermal gun that I use for tire temps on my racecar. I have already used it on the car, checking hoses and radiator temp. Whole radiator is basically close to same temps (hotter on the inlet hose/side), but really no distinct hot spots.

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Have the intake manifold gaskets been changed? Sounds to me like the coolant is not circulating through your motor correctly. Heats up with highway load but circulates enough under low loads?? Sounds like you've changed everything else. Or correct pump - rotation?

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if the coolant isn't thick and chunky would pass the waterpump, but if under attack due to an acidic coolant mix might disintegrate and stop pumpin' properly~


I would double check the water pump for correct rotation first, then check that the cooling fans are set up correctly ie pullers or pushers.

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The correct thermostat for your car is a 195°F one. Have you checked your ignition and valve timing? If either are retarded (late) that will cause it to heat up at cruising speeds. Do you have a valve tap? If so, your knock sensor may be "hearing" it and retarding the ignition timing severely. The knock sensor may be bad or the knock sensor module may be bad. Again, severely retarding the ignition timing. Are you absolutely sure this car only has 59,000 miles on it? At high mileage, say 159,000 I'd suspect the timing chain has stretched and/ or jumped. Again causing the valve timing to become retarded. Another key symptom of this is late shifting of the transmission. Do you have a serpentine belt? Is it correctly routed? Some water pumps rotate opposite the direction of the engine. Usually the symptoms you are describing are caused by poor coolant circulation, ignition or valve timing, or overloading the vehicle. BTW, in my opinion 220°F is too high as well. Unless you're stuck in traffic, on a 90°+ day, with the air conditioning on. HTH. -Jim

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I am wondering if your head gasket is damaged. Probably overheating has damaged it. A possibility - who knows?


I agree with js08016, sounds like a poor coolant circulation issue. To me, i would look at the low temp thermostat. 195 is where it needs to be. These cars already have an issue of running warm as it is, without proper cool off time in the radiator, the coolant will continue to just get hotter. 180 opens too early and closes too late to properly control the coolant temp. Also was the intake gasket recently replaced? FYI: I owned an 89 GTA with the 5.7 tpi and it ran around 215-220 during the summer with the ac on. 100+ degree weather can be rough.

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Sounds like a typical F-Body that has hit a parking stop one to many times. Is you air dam under the bumper intact, and is it solid? Not flopping around. See where his license plate is, it says pastuby? Since the F-Bodys don't have a grille this front air dam is the only way it gets air flow after 30 mph, if it's only slightly damaged it would explain why it only overheats on the highway

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Click the pic to zoom in

2 people found this helpful.

i have the same problem with my 1988 3rd cen camaro all water related parts brand new but still heats under load only ,


Have you checked the air dam?

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