Overheating after over revving

[awakened]

Here is my situation. Car is a 5spd 86 GT. Stock motor. Recently installed gt40 tubular intake, 70mm bbk TB, shorty headers, catted h-pipe with flowmasters. I also installed the following to upgrade the cooling system. SVE aluminum radiator, new high flow water pump, 180 degree thermostat, SVE silicone hose set. Car ran great and with good temp. Probably had about 100 miles of occasional driving when I had a brief over revving event. Felt like throttle stuck while shifting from 2nd to 3rd. As soon as I let off the gas and hit the clutch the rpms spiked. Stabbbed the throttle quickly and revs came down. No noticeable problems with the motor after that event except that the car now over heats. Still has the stock temp gauge so I don't know the actual temp but the needle creeps out of the "normal" range and moves to the "H" on the gauge and I need to shut it down. It will overheat like this idling or around town or highway driving. Car does not smoke and continues to run great while the temp climbs. Oil shows no sign of water. When I shut it down the lower radiator hose and driver side of the radiator are just warm and not as hot as they should be. Upper radiator hose and passenger side of the radiator are extremely hot. This suggests I'm not getting good circulation.

I pulled the thermostat housing thinking maybe I blew some crap loose in the cooling system and it blocked the thermostat but everything looked fine. Coolant is very clean. The new SVE lower radiator hose doesn't fit great (a little oval in one spot) and the hose feels thin. I thought this might be collapsing with the high flow pump but I swapped in a standard hose that fits better and keeps its shape better but that doesn't change anything. Car continues to overheat. The hose
wouldn't explain the sudden change to overheating anyway.

When I start the car with the radiator cap off, the coolant stays still for a while then as the car warms up the coolant starts to flow out of the opening a little until the thermostat opens and the coolant really starts to spill out fast.

It appears that the coolant is flowing correctly (thermostat opening, pump working, tubing not clogged) but the fact that the lower radiator hose is only warm while the car is overheating suggests it's not flowing good.

Any idea what could be going on? Could the over rev have done something? Next I will check the temp of the motor with an infra red gun to make sure that the gauge is not giving me a bad reading. Any other suggestions would be appreciated.

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[dynodon64]

To start, you didn't over rev the engine. The computer has a 6200 rpm rev limiter. You can hold the pedal to the floor and it will just rev limit. It won't hurt the engine when you hit it.

As for the over heating problem, you need to verify it is over heating. If the lower hose isn't hot, then the radiator is working. Is the water over heated when the thermostat opens and it pushes out of the radiator cap?

If the water is super heated and the engine is over heating, then you may have blown a head gasket. It can be hard to test though.

[awakened]

I didn't know there was a rev limiter in the computer. The revs did seem to stutter when they spike d so that makes sense. The water hasn't over heated enough to push open the radiator cap. I've been cautious to turn it off when the needle goes too high so I haven't let it go that far. I've wanted to pick up an infra red thermometer anyway so I will check the actual temp tomorrow when the gauge pegs. Hopefully there is just something wrong with the gauge. If there is a problem with the head gasket wouldn't I get water in the exhaust (smoke) or in the oil?


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[dynodon64]

A head gasket can blow in many ways. Sometimes it will get water in the oil, oil in the water, water out the exhaust. It all depends on where it blows. Alot of times it will blow between to cylinders. Can be seen with a compression test. That's why it can be hard to trouble shoot.

Again verify that it is first over heating.

[Walking-Tall]

Sounds like the car's not overheating, and a cooling system doing it's job well. I'd be checking the temperature gauge's sending unit, the "coolant temperature sending unit", to the right of the distributor.

~ Haynes manual procedure:
- Disconnect the wire at the sending unit
- Turn the key to On but not Start
- Ground the wire with a 10Ω resistor, and the gauge should read in the Hot vicinity
- Ground the wire with a 73Ω resistor, and the gauge should read in the Cold vicinity
- If the gauge indicated as above, replace the sending unit
- If the gauge did not indicate as above, the circuit may be open or the gauge may be faulty

[Broncojunkie]

High-flow water pump, high rpm, and silicone hose could be sucking and collapsing radiator hose.

[Mgino757]

Make sure the spring is in the lower hose. Without it, it is very possible to collapse the hose.

[awakened]

The original hose didn't have the spring. The new silicone hose didn't come with a spring. I agree that the spring should be in there but don't understand why the SVE hose set does not come with one. Especially since they are softer than the original style hose. I will put a spring in just as a precaution but it overheats at idle and I can see/feel the tube is not collapsed at idle at least.

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[dynodon64]

Is it really over heating? Has it ever pushed water out the overflow hose? If you haven't let it get that hot yet, then it hasn't over heated. You need to verify it is over heating and not just seeing it on a gauge.

[grabbergreen84]

Does it smell hot?

[awakened]

Bought an infra red thermometer last night but didn't have a chance to check it. I will take a look this morning and let you know what the temp is. Also, can I pick up those specific resistors at an auto parts store or are they a specialty item that I need to get from an electrical supply place? I would like to verify the functionality of the gauge.

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[dynodon64]

If you have a volts ohm meter, just get a 100 ohm potentiometer and use it instead of the resistors. This way you can test the whole range of the gauge.Just use the outside two pins. You can use the ohm meter to set the pot at any ohms reading then hook it to the wire from the sending unit and the other one to ground.

[awakened]

Took the car out for about 15 minutes and right when I got home the needle was on H. Kept the car running and used the IR thermometer which is accurate to +/- 3°. Reading fluctuated a little but was 235 - 245°F at the spot in the picture just above the t-stat. Temp was reading 155 - 170°F right below the temp sensor. I assume the internal temperature is a little higher than on the surfaces that I am measuring. Idled for about 5 minutes like this before I shut it off. Coolant was not over flowing.



Ignore second picture. I couldn't delete it.

[awakened]

I'm going to check the timing next. Its a long shot but the distributor looks a little off based on a couple marks that were on the shaft and block that I use for reference when pulling the distributor.

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[Walking-Tall]

Bought an infra red thermometer last night but didn't have a chance to check it. I will take a look this morning and let you know what the temp is. Also, can I pick up those specific resistors at an auto parts store or are they a specialty item that I need to get from an electrical supply place? I would like to verify the functionality of the gauge.

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Through that Haynes procedure with the gauge sender wire grounded with the two resistors, we can surmise that the temperature sending unit is a variable resistor, with temperature variances, that connects with ground. So, when the engine is cold, a simple connection between the sender's wire stud and ground with a multi-meter set to measure resistance, should result in 73Ω, and with the engine hot, show a 10Ω resistance. Also, so, the gauge's (and therefore the sending unit's) "window" or range from Cold to Hot is 63Ω. So if the sending unit doesn't show you a cold resistance of 73Ω, and then decrease as the engine warms up, I'd say the sending unit is not doing it's job, and needs replaced. Interesting information would be to map what temperature window the factory gauges represent, based on actual measured coolant temperatures and what the gauge shows us.

This brings up another point. I think I can see Teflon tape on the coolant temperature sending unit threads in your one picture. There's only one wire that goes to the temperature sending unit. The temperature sending unit body, the threads, therefore needs to be what makes good solid connection with ground, the intake manifold. A quick look at a wiring diagram shows this as well. Whether or not the Teflon tape is adding resistance or basically hindering the sending unit from ground in your case might be an issue. Whenever I have sealed up an unruly sending unit with Teflon tape, I've made sure to only apply it to the top portion of the threads, so that it seals up any leakage, and the bottom portion of the threads make good contact with the intake manifold.

[JACook]

I don't like to see Teflon tape anywhere on an automobile. Teflon-based pipe joint compounds generally
work much better, without the undesired side-effects. If you have to use Teflon tape to get a sender to
seal, a separate ground wire needs to be added from the sender body to a clean ground on the engine.

[dynodon64]

Change the thermostat. You say that the lower hose is only warm, but the top gets really hot when the thermostat opens. Your test show 240 degrees. Sounds like the thermostat may be sticking. I see you mention a new 180 degree, but it may still be sticking. Your not letting it run long enough to see if it really is over heating. Until it pegs out the gauge or pushes it out of the cap, it's not over heated. The cool lower hose says it cooling properly. If it was over heating, the lower hose will be as hot as the top.

[Mgino757]

I doubt you messed this up, but did you maybe install the thermostat in backwards? Springs go into the intake manifold.

[awakened]

Didn't check the timing yet (runs good so can't be to far off) but I let it idle in the driveway for about 15 minutes with the gauge on H. It never boiled over but I was getting temps of about 225 -235°F on the intake right where I indicated earlier near the thermostat. I'm sure if I was driving it at an elevated RPM it would have boiled over but I don't want to force it. Best case is it runs really hot but not enough to boil over. Still not ideal.
The thermostat is in correctly. When I pulled it to check to see if it was clear of gunk i was also curious to see if i screwed up the orientation in a late night assembly session, but it was good. I will wait till morning and get a new thermostat and also remove the Teflon from the sender and check the resistance when cold and hot. I may run it briefly without a thermostat to see if the system is flowing good. If I get noticeably cooler temps with no thermostat that should confirm that the thermostat is the problem. Running briefly without a thermostat shouldn't cause any other problems right?

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[grabbergreen84]

Should be fine, but definitely not advisable for the long term, as it can keep the engine from warming up - and I think there might even be a scenario where running without a 'stat can lead to an overheat!

[Walking-Tall]

The thermostat really establishes the minimum coolant temperature, and coolant remaining near to the thermostat's opening temperature or not is up to a good working cooling system. Running briefly without a thermostat won't be a problem with your eyes on things for shutdown if it starts really sweating. At worst, it will mean a much longer warming up time period, because cooler coolant will be flowing freely without that closed thermostat (which is just a fixed, calibrated restriction to free flow once it opens. Some race vehicles simply use a disc with a certain size hole in it in place of a thermostat to maintain a certain operating coolant temperature) for quick warm-up. The coolant might get to a higher engine temperature afterward if something else in the cooling system isn't up to par.

[awakened]

Ran it without the thermostat this morning for about 20 minutes let it idle for another 10. The temp never got above here.

Temp with the IR gun was 155 - 170°F right above thermostat. Looks like it was a bad (brand new) thermostat. I bought another and that will go in this afternoon. Also, I checked the resistance of the temp sender and it was 350ohm after sitting all night and 17 when the temp gauge was at the reading in the picture above.

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[PaceFever79]

Cool. I'm curious what brand was the failed stat?

I've had several fail before their time.

[awakened]

Mr gasket. 180°

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[PaceFever79]

Mr gasket. 180°

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That was one of the brands that failed me. More than once! Don't mean to flame Mr Gasket but do some google on it and you'll find plenty of complaints on them sticking.

Next time try a 180F Stant SuperStat