Engine overheating! (V8 Jeep not YZ)

I have a 79' Jeep with a 304 V8 that keeps overheating. This new motor has only about 5000 miles on it, but the 304 it replaced had the same exact problem.

I have installed:

New radiator ( 3 core, about 1 year old, still looks very clean inside )

New Hi flow water pump ( Flow Kooler )

Several new thermostats ( currently running a 160 degree w/ 8 small hole drilled in it )

New hoses with springs in top and bottom.

I have even run with out any thermostat and it will still get hot running down the interstate. :cry:

As long as its below 60 outside it runs cool, gage about 1/4 off the cold mark, regardless off the driving speed.

It can be 110 in the shade and it will stay cool as long as I don't get on the interstate.

It only overheats after about 20 mins running down the interstate at about 70-80 mph and 2500 rpms or higher.

I can dog all over it in the city on a hot day w/ no problems.

Do any of my fellow YZers know what I should do.

* Timing also in specs and A/F mixture on carb is around 14:1

Try a jeep forum there out there too :cry:

Well you know I'm in Thumper talk right know and just taught you guys would know as much about any engine as any other forum.

Dose it blow water out is the temp gauge correct

I have a over flow tank, every thing appears normal. I changed the sending unit a few months ago and the gauge its but 2 yrs old.

I didn't see any mention of what coolant you're using

Are you using any antifreeze in your rad?

This will raise boiling temp.

I checked a container of Prestone I have and a 50/50 mix will

raise the boiling temp to 264 F.

A 70/30 mix will be 275 F.

I had a 1972 Jeep Wagoneer with the same problem and CJ as well. They all want to run hot due to the block design, small radiator, and small engine compartment. I ran a 4 core radiator, 160 t-stat, temp. controled clutch fan with stock steel fan, a automatic t-stat controlled electric fan on the front of the radiator (for lower speeds) and a external trans cooler to help keep some heat out of the radiator (automatic). This kept it pretty cool most of the time. Hope this helps.


Do you have an auto trans? Does it have a lock up torque converter? I had a similar problem on an S-10. It would run at about 180 around town (not surprising since I had a 180 degree thermostat installed), but would peg the temp gage when driving on the freeway. I replaced the radiator, the water pump, and even ran without a thermostat. Same thing. I talked to a bud who's a pro mechanic. He said to check the towque converter, that if the lock up TC wasn't locking up, it would generate a boatload of BTUs a highway speeds (when it was supposed to be locked up and not generating much heat at all). Sure enough, that was the problem. It was an easy fix, as the electrical connection had become disconnected.

To check the TC Lock up, cruise down the interstate on a long level section. Hold the speed and engine RPM constant. Tap the brake pedal enough to break contact ont he break light switch (which also powers the TC). You should see a 200-300 momentary RPM increase as the TC unlocks from the crank. No rpm change means that the TTTC wasn' locked up.

This looks like a classic radiator problem. I know you said it's not very old, but that's what it looks like to me. I would take the radiator to a radiator shop and have them remove one of the tanks and flow check for blocked tubes. In areas with hard water (almost everywhere), this can happen very quickly under the right conditions.

What you're seeing is that the cooling system is capable of disipating heat up to a certain rate. When heat is created faster than that, it overheats. A partially blocked radiator can't unload BTU's as fast as a clear one can, and the engine running at freeway speeds simply produces heat faster that it can be gotten rid of.

"Gray" - (Former Chevrolet and ASE Master Technician)

To: Vanisland - I use the green kind, cheap stuff from wal-mart, Super tech or something, but the problem was still present whe I used the more expensive Prestone brand. I run around 60-70% mix, probly closer to 60% now.

To:Birdie426 - I feel bad you wrote such a long detailed reply since....I have a 3 speed manual. But Thanks for the help.

To:grayracer513 - I was told the same thing about the radiator by some clown who didn't know sh1t about anything... But hearing it from you changes my mind. I do have "hard water" where I live and fill my radiator. I will plan on doing what you said, Do you recommend using disstilled water after the radiator shop work? When I was in the navy I learned disstilled water is alot less corrosive but don't want to go spend the $ on it if I shouldn't use it. I'm trying to save up for a $400+ dollar aluminum radiator before summer hits.

If you have bad engine grounds the radiator could be grounding the engine through the coolant and causing electrolasis in the radiator. A new radiator could be clogged in as little as 30 days with a weak ground. Check all engine and chassis gropunds. If you have an A/c condensor in front of your radiator make sure it is not clogged or blocked or anything in between them. Have you confirmed a working gauge with a known good one or a pyrometer? Weak grounds can also cause inaccurate gauge readings.

Do you have the wrong water pump, maybe?

You didn't mention if the cooling system had been pressure tested. It should hold around 18 psi, which raises the boiling point well beyond 220.

Davidyz450: I never heard of such a thing. I would be surprised if that was happening to me. I have a fiberglass body and during the restore I had to make take extra steps to ground everything to the frame and / or engine. I look it the top of the radiator and it looks very clean. I dunno???

As far as the gauge...I was told by another Jeep guy on the Jeep forum I just joined ( as suggested in the 1st reply on the thread ) that he has the exact same jeep and problem and he found that the gauge was wrong and he ignores his.

YZ on Nos: I have tried diff water pumps. This lates pump is made just for the 304 by Flow Kooler $110 ailuminum and made in Japan. I'm 100% sure its right.

Yankee500: I haven't had it pressure tested.. I think a bullet in my head would be cheaper. Jk I will look in to doing that as well. Thanks

It sounds like like you need to confirm the over heating with an accurrate gauge. As for the grounds you can do a voltage drop test. Take a volt meter while the engine is running and connect the negative lead to the negative battery post and the positive lead to the engine block or manifold. Put the meter on volts DC and it should read less than .05 volts. Due the same to somewhere on the chassis and radiator core support. They should also read less than .o5 volts. If not clean grounds and/or add more ground staps. But don't ignore gauge. Verify whether there is a problem.

I am a regular at jeepaholics. I have a cj-7 with a ford 5.0. I worked at a radiator shop in Palm Springs for 3 summers as a kid.

There are two reasons radiators overheat, Lack of air flow and lack of water flow. If you had an air flow problem your symptoms would be opposite of what yours are. I had that problem until I added a better electric fan.

Either your radiator is partially blocked or it is not big enough to cool the jeep. If it is not pushing a substantial amount of coolant past the bottle and onto the ground, it is probably not really overheating. It should displace about a quart of coolant into the overflow just when it warms up.

A 180 degree thermostat should be right for your AMC.

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