Overheating on the highway

[bighause]

My 89 4.0 renix MJ is overheating on the highway but stays cool at low speeds.

 

I'm out of ideas... In the past few months I have replaced the radiator, water pump, thermostat and housing, heater valve, all hoses, gauge temp sensor, coolant, and added a factory electric fan. Pretty much everything is new except for the heater core and the engine. I flushed the heater core very well during the conversion but I noticed my coolant looks very rusty after about 1 month of use.

 

I am running a bottle of coolant flush through the system right now, what else should I look at? I know its not the gauge because it is getting hot enough to boil over.

 

What would cause it to overheat just cruising on the highway but not low speed crawling in 100 degree temps? Nothing is blocking the front of the radiator either.

[HOrnbrod]

Still have the original cat. converter? Might be clogged. Also get a pressure test done on the cap and radiator.

[bighause]

I am running the original cat, is there a way to find out if it is clogged? It seems to have plenty of power (could be the 4.10 gears :D )

 

I'm concerned that after less than a month the new coolant is now brown instead of green. If the coolant channels in the engine are corroded up how do you fix this?

[CWLONGSHOT]

Buy some radiatior flush and get a bit intimate with your MJs cooling system.... :brows: ;)

 

 

Then fill it back up with a good 50/50 mix.

 

CW

[Jerry]

System properly 'burped'?

http://www.comancheclub.com/forums/viewtopic.php?f=2&t=18625&hilit=how+to+burp

[Comanche County]

I had the same problem on an XJ recently. It had a new radiator when I bought, it was getting hot on the highway from day one, so I replaced everything including the cat and put in a new spring in the lower radiator hose which was collapsing. It ran a little better but still warm on the highway. Turns out the new radiator was already clogged. I put in another radiator, flushed it at the same time and it ran perfect.

 

If you can hear knocking in the exhaust, especially when you shut it off, it could be that the material inside the cat has broken free and is not allowing proper air flow.

[HOrnbrod]

My old stock engine used to overheat on the highway, but was fine around town. Replaced virtually the entire colling system, nothing helped. While installing a dual exhaust system, we cut out the cat. It was almost completely blocked by a gummy tar looking goo. There were no rattles in the cat. In my case, the only way you could tell the cat was clogged was by looking into the inlet. Finished up the exhaust system (w/o cat) and it never overheated again on the highway.

[Oizarod115]

you need to be absolutely sure the junky bottle isn't losing pressure, ANY pressure loss on that truck will cause overheating quickly, bad cap and or bottle, i had a new one from MOPAR that wouldn't seal in under a year

[CWLONGSHOT]

All good and viable suggestions. (Esp. leaking pressure bottle and collapsing lower hose! :bowdown: Something not too many even consider!!)

Just don't forget,

I flushed the heater core very well during the conversion but I noticed my coolant looks very rusty after about 1 month of use.

. . . . Me thinks he still has some foreign material in the coolant passages, possibly even partial blockages. I say he needs to start with a clean system. Cheapest way to get there is to thoroughly flush the system. May even take a couple times.

 

Mine used to get hot on the freeway at higher crusing speeds. Mine is more from margional cooling and a WARN 8000 smack in the middle of my grill! :shake:

 

Just my 2cents, CW

[mvusse]

My 89 4.0 renix MJ is overheating on the highway but stays cool at low speeds.

 

I'm out of ideas... In the past few months I have replaced the radiator, water pump, thermostat and housing, heater valve, all hoses, gauge temp sensor, coolant, and added a factory electric fan. Pretty much everything is new except for the heater core and the engine. I flushed the heater core very well during the conversion but I noticed my coolant looks very rusty after about 1 month of use.

 

I am running a bottle of coolant flush through the system right now, what else should I look at? I know its not the gauge because it is getting hot enough to boil over.

 

What would cause it to overheat just cruising on the highway but not low speed crawling in 100 degree temps? Nothing is blocking the front of the radiator either.

 

The rusty color tells me there is iron in the water, which means you probably used tap water. The iron turns to rust and the rusty sludge will clog your new radiator in no time, causing overheating issues when driving at high speeds. That's why you're supposed to use distilled water, or in case of emergency and no other option, rain water.

 

Overheating issues on the highway is usually a coolant flow problem. Overheating in stop and go traffic is an air flow problem.

[CWLONGSHOT]

My 89 4.0 renix MJ is overheating on the highway but stays cool at low speeds.

 

I'm out of ideas... In the past few months I have replaced the radiator, water pump, thermostat and housing, heater valve, all hoses, gauge temp sensor, coolant, and added a factory electric fan. Pretty much everything is new except for the heater core and the engine. I flushed the heater core very well during the conversion but I noticed my coolant looks very rusty after about 1 month of use.

 

I am running a bottle of coolant flush through the system right now, what else should I look at? I know its not the gauge because it is getting hot enough to boil over.

 

What would cause it to overheat just cruising on the highway but not low speed crawling in 100 degree temps? Nothing is blocking the front of the radiator either.

 

The rusty color tells me there is iron in the water, which means you probably used tap water. The iron turns to rust and the rusty sludge will clog your new radiator in no time, causing overheating issues when driving at high speeds. That's why you're supposed to use distilled water, or in case of emergency and no other option, rain water.

 

Overheating issues on the highway is usually a coolant flow problem. Overheating in stop and go traffic is an air flow problem.

 

:agree: :agree: :agree:

[90eliminator]

What is the best way to flush the system. My MJ also has brown coolant. I have a all new parts to upgrade to a open system. Can I just put a pressure washer into the inlet and fire away till the system come out with clean water....??? might try it tomorrow night, so if thats a bad idea...let me know..thanks

[bighause]

I forgot to mention that when I was replacing parts, I converted to an open cooling system in the process. I used pre-diluted coolant when refilling the system (I know better than to use tap water), the previous owner definitely neglected the truck and I'm sure if he ever added coolant he used tap water.

 

I am still flushing the system since it says to run it for a total of 3-4 hours even if it takes several days. I plan to drain and refill the system tonight and see what happens. I checked for the lower hose collapsing under acceleration and high revs but see no signs of this.

 

Thanks for the replies, keep the ideas coming

[mvusse]

After you flushed the system, did you drain the entire system, or just the radiator?

[bighause]

I removed the lower radiator hose from the water pump and drained the radiator with the truck facing down a hill to get as much coolant out as possible out. Is there a better way to drain the system? It didn't seem like enough out, but I did spill a lot in the process.

 

I drove the truck to work today and it is better, but it's still overheating, it just takes longer to do it. I guess I'll pull the new radiator out see if it is clogged. Hopefully the coolant won't turn brown again in a month. The radiator is a single core just like the factory one I replaced, I'm wondering if I need a 2 core to keep up with the 100 degree temps :???:

 

What else can I do to flush the engine other than keep adding coolant flush?

[mvusse]

Power flush it with a water hose and a flush kit. Then evacuate the system with compressed air before filling with coolant.

 

Always worked for me.

[cheapmj]

mine has been overhating for two months and replaced everthing but the radiator cap i think 1 row radiators is not just not enoungh to cool it

:fs1:

[mvusse]

mine has been overhating for two months and replaced everthing but the radiator cap i think 1 row radiators is not just not enoungh to cool it

:fs1:

 

That should be the first thing to replace. It's the cheapest part, and if it doesn't work right your coolant will boil at just over 210 degrees, well within specs, instead of 260

[CWLONGSHOT]

mine has been overheating for two months and replaced everything but the radiator cap i think 1 row radiators is not just not enough to cool it

:fs1:

 

DING DING DING. . . Not when you add bigger tires, winches and other heavy parts to the truck, combine this with 20+ years of use/abuse!! These trucks had marginal cooling from the factory. Making the motor work harder equals more heat generated and this equals overheating.

 

CW

[bighause]

With my current 4.10 gears and 33's the motor should be under less strain than the stock 3.08 gears and 28's. I don't have a winch or heavy items bolted to the truck (yet).

 

No doubt a 2 or 3 core radiator would be better, but it seems like with everything replaced in the cooling system but the engine block, it would be up to the task since it only had a 1 core radiator for the last 21 years.

 

The truck failed emission testing recently, could that have something to do with the overheating? Next thing to replace if the radiator isn't clogged would be the 21yr old cat I guess. :wall:

[ratrapp]

mine overheats on the interstate only with the a/c on.well,it doesn't overheat but climbs farther over 210 than i like to see.it has a new waterpump,radiator converted to open cooling,mopar 195 t-stat.i can run 55-60 mph with the a/c on all day long but once it hits 65-75 mph with the a/c on for more than 5 miles it slowly climbs up.i even replaced the headgasket and head and it didn't make a difference.without the a/c on it doesn't get any warmer than normal.i'm thinking partially clogged cat as it's about 11 years old and does seem a little low on power.