I’m not sure what to do anymore, any input from anyone knowledgeable of these trucks is greatly appreciated. I’ve got a 1998 Mazda B4000 with a 4.0 OHV engine. For the past 2 years since I’ve aquired it, I’ve noticed a sight coolant leak from the rear of the driver side head. Never mixed/ burned coolant in the oil or vise-Versa. Ive just added coolant to the resivour ever so often, putting off a head job until the problem worsened. Well, it finally did and recently, I’ve replaced the heads (with brand new ones) the head gasket, bolts, new valve guides, (heads assembled by machine shop), no new lifters or push rods as they were in good shape, pistons looked good, no scoring of such, lower intake manifold gasket, upper intake manifold gasket, new water pump, thermostat, radiator, fan clutch, tensioner pulley, belt, rad cap, new egr, cleaned map sensor, cleaned injectors, etc. The truck runs fantastic, does not burn oil/coolant no smoke, has good vac, just as before. After everything was put back together, I’ve got about 300 miles on the job. I’ve got this weird popping/knocking noise that is driving me nuts once the truck warms up. It’s only noticeable at idle once the truck is warm. It does NOT over heat, I’ve even gone past trusting the temp gauge on the dash and hooked a live scan tool to the OBD2 port to read the temp. It’s witthin normal operating range. Since the head job was completed, I’ve had this horrible problem. The noise is coming from the thermostat housing/ hot spots on heads where I believe the noises to be steam formation as they are not consistent and only appear when the engine is warm. I’ve burped/purged the cooling system with a funnel while it warms up with the front end raised, heater on and I’ve even read about the TSB these motors had in the late 90’s about needing a coolant bypass to help with flow! So I made one myself since they are not available anymore! A couple of brass fittings and a t, some heater hose and a 1 1/4” nipple for the lower heater hose worked just fine with no signs of leakage. Temp sensor was installed to the rear of the brass t on the intake just as the factory bypass would mimic. I’ve tried everything I could think of, everything is flowing fine as I’ve inspected. If my hand is placed on the upper coolant hose I can feel the coolant rapidly pulsating and can feel all the knocks/noises through the hose. The coolant system is not being interrupted by exhaust gasses and I did not see any bulbbes after the durations of time needed to purge the system. I’m using the proper 50/50 mix antifreeze. I live in south Texas, it hardly ever gets below 40’ down here but I’m using straight 50/50 with no added distilled water that could possibly lower the boiling temp of the coolant. I’m stumped. If anyone has any off the wall suggestion as to fix this problem it would be greatly appreciated. That’s my whole story. Sorry for the long post.

RonD, that is very possible. Maybe it’s something I overlooked. I’d hate to tear it all apart again but that ultimately might be the solution. I don’t feel comfortable driving it this way. I’ll put the infrared on it tonight to see any noticeable differences.