EvstaG

89 4.0 is getting very hot under engine load. Gauge climbs steadily under sustained engine load, I pull off the road when the gauge hits the red. Speed doesn't seem to be a factor, it overheats just as bad on the highway as it does climbing some twisties up a mountain. The Jeep has just been returned to the road after sitting for 12 years, so I don't have a baseline of the cooling system being in good working order. Here's what's been done so far:

-new rad
-new waterpump
-new fan clutch
-new heater core
-new t-stats (tried 195 & 180, no difference)
-A/C condenser removed
-bypassed heater control valve (was leaking)
-Hoses are in good shape and not collapsing, no blockage in hoses
-Volvo 22psi cap
-New Volvo 240 coolant bottle, went with it after I tried 3 brand new Dorman Jeep bottles (fun fact, even the Volvo bottle blows apart before the 22psi cap releases)
-Coolant has been citrus flushed twice and coolant has been cycled at least 5 times in the last three months
-chemical block test performed once by me once by a professional mechanic, no evidence of combustion gasses in coolant
-leakdown test performed, no abnormal readings
-System is properly bled every time coolant is replaced, true 50/50 always put in

The only thing left in my eyes is the head gasket/cracked head, that is somehow only allowing combustion gasses into the cooling system after the system gets hot (if engine is started cold it will idle all day at t-stat opening temperature) and under cylinder pressures (meaning that the 15psi or so cooling system pressure isn't enough to push coolant back into combustion chamber if engine turned off hot).

I'm having a tough time rationalizing the head/head gasket failure as the overheating is the ONLY symptom. No coolant loss, no white smoke at any point, no coolant/oil mixing, etc.

Just looking for any insight anyone might have, I'm not usually one to get stumped by something as straight forward as a cooling system.

jeep94Norway

what do you use to bleed the cooling system?

try to stay under 1500 rpm when you drive and see if you stil overheat. if you dont overheat with low rpms I bet the head gasket is bad/moving with high rpms and let exhaust go in the cooling system. many turbo builders experience this

and why are you using a 22 psi cap? this means your cap dosent release pressure before 1,5 bar. this means you are 0,3 bar over stock radiator cap?.

Edit: by a vacuum gauge/tester and see what readings you get.

EvstaG

For bleeding the system I get the jeep on a downhill, let it run with the coolant bottle cap off/coolant sensor in the back of the head unplugged, heat on full.

Low RPM's don't really seem to help (but I see why that would make a big difference on turbo engine), honestly if anything it seems like the high RPM's might help a bit by getting the water pump spinning faster, I can't say for sure. But who know's what's going on in there if the pump is cavitating or something if the head/head gasket is letting gas into the system.

jeep94Norway

my experience is that the 242 amc engine is difficult to bleed properly without the right tools. I only use the cooling system refiller tool. this gives a indication off leaks too. worth a try? 75$

EvstaG

Might be worth a try, but I've bled at least a couple other Renix Jeeps with this method and had no issues, and I've driven at least a couple thousand miles with the Jeep behaving this way, when I've improperly bled the system on previous Jeeps I've noticed temperature rises almost immediately.

jeep94Norway

aim not saying that you are wrong, or not bled the cooling system good enough. my point is that the cooling system refiller can indicate a leek somewhere.

and another thing you can try is to install a vacuum gauge, and do it the good old fasion way

EvstaG

Yeah, I'll have to buy an adapter for my vacuum gauge and see if there are any leaks present, although there definitely isn't any evidence of one currently.

I'm using a 22psi cap because it's designed for the Volvo coolant pressure bottle that I'm using in place of the Jeep bottle, is made in Germany, and seals better than any of the 89 Cherokee replacement caps I've bought. Additionally, in a properly working system, the 22psi cap should *theoretically* give you some extra cooling ability.

I've also been using the 10psi Volvo cap to keep that as the certain "failure" mechanism in the system, as I mentioned the coolant bottle seems to let go and explode before the higher pressure cap releases.

freegdr

Aux fan working ? Replace the hoses only thing you didnt do.

cruiser54

I'm suspicious of the radiator. Do you have an IR temp monitor?

Compare readings between the top of the rad and bottom of it and note the difference.

EvstaG

Aux fan does work, but it's 5 speed and no A/C, my other 89' didnt even come factory with an Aux fan, so in theory this one really shouldn't need it either. And I did replace the hoses actually, I noticed the lower hose that came on the rig didn't have the spring inside so I replaced them, no difference.

My thoughts have also been that it seems like a radiator issue; however, being that it's such a simple device, is new, isn't leaking, and seems to allow good flow, I had sort of discounted that idea. I can get my hands on an IR temp gun, any idea what sort of readings I should expect when the Jeep gets up to operating temps?

I also should consider that ones the temps start climbing, if it is a head/HG issue there could be some gasses getting into the radiator, which would put a big dent in the cooling capability of the system. I'll see about getting some temp readings and report back though.

jeep94Norway

with stock thermostat you should read something between 60-70 degree celcius. on the lower hose. if the engine just idling for 15 min you should read something around 75-80 I think. these temperature is when the car is only using the fan, and not when the car is moving.

I dont understand why you dont take vacuum test on the cooling system. that would be the first thing I would do. then mount a vacuum gauge on the intake manifold and see what the engine is saying. a vacuum gauge give you all the information you need. its basically "computer"

tech

I also think it could be a radiator , what radiator did you use ? the core thickness is very important and how many rows does it have ? I would all so add in Aux fan , with the fan you have now you are only cooling haft the radiator the Aux fan would cool the other haft .

EvstaG

I am going to do a vacuum test on the cooling system, I just don't have the tools quite yet, and I'm here to listen to everyone's ideas, but I'll definitely be looking for leaks with the vacuum. FWIW, the engine pulls normal manifold vacuum at idle.

Honestly, the radiator is just a stock replacement from the local auto parts store, IIRC the brand was Spectra, but I bought it a couple years ago for a different Jeep and it never got installed so I'd have to confirm the specs on that one.

This Jeep does have an Aux fan, and it does turn on when the coolant temps climb. But the aux fan and the clutch fan have minimal bearing on total airflow through the radiator when you're driving 70mph down the freeway where it still overheats.

tech

Most stock Radiators only have a core that is one inch thick the OEM radiator for heavy duty use core that is 1 1/4 inch thick with a wider tube for the coolant to run through . the radiator I used is the Champion two row all aluminum radiator that has a core that is 2 inch thick with tubes that are about 3/4 of a inch wide . Some radiators that will fit the Cherokee 4.0 are really for the 4 cylinder Cherokee and not really large enough for the 4.0 engine . It sounds like thats what you have , the harder you push your XJ the hotter it gets until you just let it run with no load on it , then it will cool down .

EvstaG

Alright, so I got the system pressure/vacuum tested and bled properly, no change. Swapped with a junkyard OEM radiator that looked reasonable, no change. Currently swapping the head with a 2686 I know is in great shape.
Here's what she looks like under the head.

1. How important is it that all of those driver's side coolant passages are blocked? The same ones on the head are clogged but soft enough to be poked through with a screwdriver; however, the passages on the block side are extremely hard, a hammer and small punch didn't make much difference.

2. The hole between the front (cyl 1) two head bolt holes should enter the water pump area, correct? Currently I can go a little more than an inch with a screwdriver and it stops.

Just trying to make sure coolant is going to be flowing everywhere it should be when I get this replacement head back on.