JDW

99 4.6 232k-overheating at highway speeds and under load when towing. Normal temps at street speeds under 55 mph. I am having a vacuum/EGR issue that I can't figure out yet. Will this cause engine temp to rise or is this a fan problem? Thermostat was replaced to no avail.

Good old Bill

Your description is a text book radiator plugged up. Will try to explain, your engine is a heat generating machine and at highway speed and especially towing it is producing its most power and thus generating its most heat build up. The radiator is the thing to remove this heat, if its functioning at less than its designed efficiency, the temp continues to rise. (Overheating). T-stats, water pumps, fan clutches all have their job to do, overheating only at highway speeds is not their job. Their malfunction will affect temperatures at slower driving conditions.

JDW

Thanks-I'll flush and back-flush it this weekend and see what I get. I did run some leak stuff in this winter to try to fix the heater core (did not work). Maybe this set up in the radiator?

Good old Bill

Unfortunately you have become another victom, what has happened is, the stop leak looks for its leak to seal, when the additive and coolant hits the top tank of the radiator it starts to gell due to the air in the top tank, and plugs up the coolant passages of the radiator itself. You might have to have a radiator shop rod out the core. In years past when radiator caps were on the top tank we could see the solid mess obstructing the tubes. Now a days the top tank has to be removed for the visual inspection.

JDW

Thanks Good Old Bill-turns out i have a couple holes in the radiator that keeps the system from being pressurized. I need to try to locate a radiator this weekend. Now it's on to fixing my EGR problem? Took everything apart last night and still can't hardly go down the road without shuttering at all RPM's and speed. Codes went away, but not the problems.

Good old Bill

Others more qualified than me on the subject, back in my day EGR issues were predominately at idle or stopping. EGR not closing would cause stalling. Another type of issue was the temperature vacuum switch allowing the EGR to come on prematurely, (Cold engine).

JDW

thanks again-much bigger issues than EGR right now. Turns out the rear end was cracked causing the shuttering, then finally let go and imploded. Took off the rear diff cover and the diff gear housing was split in 2 and the gears sheared off. Looking for a rear axle as I type. also looking for a new-used truck.