Cracked exhaust header = Overheating?

I've been fighting an overheating issue for a couple of years now. The entire cooling system is new (last 6 months). I will only overheat if the outside air temp is over 93F. The hotter it is outside the faster it will overheat, but yesterday it was 99F and it took about 5 minutes of highway driving to get it up to the red. I can keep it out of the red if I keep the A/C off and keep moving. I also have a few other symptoms but I don't know if they are related.
-Very poor gas mileage (10-12mpg)
-Strong gas/unburnt exhaust smell
-Black ring around tailpipe with a day of cleaning it off

Power on the truck is good, no problems getting up and moving even on 31s and stock gearing.

I do have a cracked exhaust header that I plan on replacing in the near future. I can see a crack at the collector and the last time I seafoamed it, smoke poured from that location.

Is a cracked header enough to overheat the engine bay and cause the engine to overheat?

The only other possibility I can think of is that I have a blown head-gasket that only rears its head when the engine is totally heat soaked.

The engine only has about 60k on it (junkyard pull), but the swap was done before I owned the truck so I don't know any of the history of the engine.


I do have a temp gauge and a cooler on the tranny so I know its not that.

I take it that this is the '91 in your profile. The cracked manifold adds heat to the engine compartment, but it's hard to say how much. It also can make the o2 sensor richen up your mixture. You are burning almost twice as much gas as you should be. That could pay for a new exhaust manifold pretty quick. You are ruining your motor by running it so rich too. Maybe it would even fix your overheating condition which risks warping your head.

Hallo. I have a cracked exhaust manifold too, but I don't think the engine heats up by that. There is imo always an overpressure, so the O2 sensor don't rich the mixture.
There is an extra heat and exhaust gasses in the motorbay. Therefore my hoodvents do a very good job.

'92 XJ

I searched for you. http://www.naxja.org/forum/showthread.php?t=989330&highlight=exhaust+manifold+crack

your air/fuel is off,
a cracked manifold doesnt help, if its been cracked for a year, its about time to change it, youve gotten your money out of it
if you have black exhaust tracks, you could have also fouled your spark plugs, o2 sensor(s), and cat. might want to check em, maybe clean em off/test them.
could also be bad fuel system, causing extra load on the motor SLASH just not running right.

IF I HAD A 91.... w/ a cracked exhaust manifold....
i'd pickup a 99+ intake, and do it all at the same time, then replace the stuff i mentioned in my last post.