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New Rebuild Overheats...Any Ideas?

R

roberob89

NAXJA Forum User
Location
hampton, va
Alright, I've searched the database and though I've found numerous topics regarding overheating issues, there were variances in each that make me think "this could not be my problem". Besides, there are too many possibilities for me to afford trying each one. So here's my situation...

I have a 94 XJ 4.0HO that I just rebuilt. Specs on new motor are as follows:

- bored .040 over
- block decked .006
- melling high volume oil pump
- crower cam 256HDP
- head rebuilt w/ chevy valve train (shaved - can't recall how much)
- stock pushrods (rocker-arms shimmed for correct pre-load)
- new thermostat 180*
- new radiator hoses, lower has spring inside
- again, just rebuilt, currently has roughly 20 miles on it.

Pre-build stats - everything stock, no-overheating

Using stock injectors. Could this be causing a lean condition enough to make it run hot? Runs hot while running or idling (it's dead summer and I'm running the heater to cool it HELP!)

Any suggestions? Where do I start first?

Sorry if this has already been posted. Thanks for any and all interest. Man, if I don't get this up and running, my wife is going to kill me, surely you can relate!
 
Mine ran hot for 500 miles. New motors run hot until they break in a bit. How long has it been?



roberob89 said:
Alright, I've searched the database and though I've found numerous topics regarding overheating issues, there were variances in each that make me think "this could not be my problem". Besides, there are too many possibilities for me to afford trying each one. So here's my situation...

I have a 94 XJ 4.0HO that I just rebuilt. Specs on new motor are as follows:

- bored .040 over
- block decked .006
- melling high volume oil pump
- crower cam 256HDP
- head rebuilt w/ chevy valve train (shaved - can't recall how much)
- stock pushrods (rocker-arms shimmed for correct pre-load)
- new thermostat 180*
- new radiator hoses, lower has spring inside
- again, just rebuilt, currently has roughly 20 miles on it.

Pre-build stats - everything stock, no-overheating

Using stock injectors. Could this be causing a lean condition enough to make it run hot? Runs hot while running or idling (it's dead summer and I'm running the heater to cool it HELP!)

Any suggestions? Where do I start first?

Sorry if this has already been posted. Thanks for any and all interest. Man, if I don't get this up and running, my wife is going to kill me, surely you can relate!
Click to expand...
 
Only about 20 give or take a few. It's only been 2 days since I completed the rebuild. And due to the heating issue, I've been driving it minimally.

Will it harm it drive it like this? I would hate to have busted my ass and worked so hard for nothing. Could it really be this simple? Any other feedback/experience with this?
 
It's running hot around 230-240.

It gets up to around 210 real fast and if I have to stop at all, it rises to 230-240 and I turn the heater on to cool it down.
 
As long as it isn't boiling over and you have burped it, run it. The only thing I would even think about is monitoring the plugs and tailpipe to make sure it isn't running lean. Given the new cam, it will take a while for the computer to learn and compensate. Just watch it and drive it, ideally around town and not for extended periods on the freeway until you hit a hundred miles or so and the temps work their way down. It is very common for an engine to run hot during breakin. '
 
I didn't touch the cooling system. Prior to rebuild it ran a little cool, so I don't know.

Well the general consensus thus far seems to be just to give it some time. Everything else seems fine, purrs like a kitten. Suppose I'll just wait it out. If anyone else can concur this, I'll rest easier.

Thanks. Much appreciated.
 
Have you tried to "burp" the cooling system? OF all the things you mention, you did not mention doing this, and if it is still the stock closed system, then there is a chance it has an air bubble trapping the coolant. Since it heats up really fast, that seems to me to be either because the coolant is NOT flowing well or, possibly the head gasket is leaking slightly in to the water jacket. I know it is a fresh rebuild, but that doesn't mean there isnt still a chance the head gasket could have a slight internal leak.

Try burping it, pressure test if you have the tool to do so. Why chance ruining your hard work waiting on it to break in?
 
It doesn't have the closed system, it has the newer style with the coolant recovery bottle and all. I thought it would burp itself with this set up...I ran it parked on an incline with the radiator cap off for a bit any way
 
roberob89 said:
It's running hot around 230-240.

It gets up to around 210 real fast and if I have to stop at all, it rises to 230-240 and I turn the heater on to cool it down.
Click to expand...
A good many years ago the engine I built for my autocross/hillclimb car ran very hot. Rejetting the carb wasn't able to help it. My mistake was listening to advice from someone who said he knew what he was doing. The cam was too much for the engine and, as you are suspecting for your situation, I had a lean-burn condition. This was supposed to be a sort-of daily driver as well as a weekend warrior vehicle, so putting on a bigger carb and dumping huge loads of fuel through it wasn't a very attractive option, so I sent the cam back to the manufacturer and had it reground to milder specs. After that all was well.

You might try playing with an adjustable MAP sensor to see if making it run richer helps. If so, then you could think about injectors. Given that you've bored it, increased compression slightly, and are running a bigger camshaft, it would not surprise me if you need a bit more fuel.

Have you put it on a sniffer to see what the air-fuel ratio is?
 
Sniffer?? Where would be the best place to get that done at?? Are there any codes I might be able to pull off the computer to tell if it is running lean?
 
Eagle said:
You might try playing with an adjustable MAP sensor to see if making it run richer helps. If so, then you could think about injectors.
Click to expand...
If you're using the stock PCM (I assume you are), you can't trick it into running richer with an adjustable MAP sensor or with larger injectors. The PCM uses the signal from the O2 sensor to adjust the mix. The only times that the aforementioned items would make a difference is during warmup when the engine is cold and at WOT.
Maybe you have a bad temp sender on the rear of your head. It's been known to happen and cause incorrect higher readings. Just a thought.

K
 
the temp sender on the rear of the head is brand new...could be bad out the box?? And i don't think it's a flow problem because when i turn on the heat it immediately goes down...I'm stuck!! Thanks everyone for all the help...ROB
 
roberob89 said:
the temp sender on the rear of the head is brand new...could be bad out the box?? And i don't think it's a flow problem because when i turn on the heat it immediately goes down...I'm stuck!! Thanks everyone for all the help...ROB
Click to expand...
If the cooling system is in good working order, I believe the stock cooling system has more than enough capacity to cool your motor even if it's running hotter than your old motor. Temp goes down when you put on the heat, so I think we can assume that the pump is working and that you don't have air in the system. So maybe there's a slight restriction in your rad or you have a defective or crappy t stat (even though it's new). Or did you get over zealous with the rtv when you put in the t stat and cause a partial blockage there? Just some more thoughts...

K
 
Since the coolant temp. rises very quickly I suspect that you either have air trapped in the cooling system or your t'stat is stuck partially closed. As an easy experiment, try running the engine without the t'stat and see if it still gets hot.
 
After the t-stat test - 2 things usually cause o-heating in traffic; bad fan clutch or worn out radiator. At 10 years old you are aproaching both of those problems.
Also does the aux fan come on? you might check that also.
 
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