1996 SD 5 speed
Last week my water pump went out, changed it with new Airtex. After refilling with same antifreeze that came out of it, I found oil in in it when I went to bleed air. Assuming it's the head gasket I put a bottle of blue devil in it (per instructions) been driving around town for a week 150 miles, temps have been normal at 194. Today it began over heating, attempted to bleed air a dozen times already, changed reservoir cap with new one, checked all the hose clamps and tightened them a little more. Now here is my problem, normal idle is at 194.5 UG but now it fluctuates 194.5/201.5 @700rpm. Now when I take a drive down the road it immediately cools to operating temp 186.5 / 194.5 with engine under load until I reach 5th gear and rpm drops, temp immediately starts to climb to about 220 (takes only about 5 seconds) dropping it back into 4th it cools back down just as quick @3000 rpm and runs cool until I put it in neutral to coast to a stop and it quickly climbs again. Now I plan to do the heads here soon but, before I break it down I thought I would ask if any one can confirm that this is symptomatic of a blown head. New fan clutch, thermos, resv cap and hoses in last 3,000 miles. Just don't want to do the head job and be in the same "boat" when I finish and would like to get another to weeks out of the blue devil if I could any input is appreciated. FYI right side was lifted when bleeding.

 

Why wouldn't you spend the few extra bucks for new coolant? Doesnt seem worth the savings.
Head gasket repair in a bottle snake oil products never work. Usually they clog coolant passages and make the problem worse. Maybe someone else here has had some good luck with them but I've only seen problems.

Has the radiator been replaced or rebuilt? These are famous for having deposits at the bottom reducing the effective cooling area. If you are still running the original radiator it might need to be rodded out or replaced. Did you double check to make sure the belt was installed correctly? There is a way to install it wrong and the pump runs backwards.

 

have you pressure tested your coolant system

 

Coolant was only a week old before pump went out didn't foresee a problem reusing it, I found oil in it only after refilling it and opening air bleeder plug. As far as the blue devil goes, it was only a temporary fix, didn't plan on letting it go over a month (just buying time) but, with all the great reviews I would have thought it would have lasted more than a week. Radiator is original but, it hasn't been a problem (it been cooling fine) , flushed it 2 weeks ago (assuming that's probably what finished the water pump off), belt is on correctly, as I said it hasn't given me any problems for 150 miles in town driving since the blue devil.
No haven't pressure tested system but, it's not building pressure as it should hence the cap change. I'm quessing it's just the head gasket. About to order set from Lucky8 and will post back with pictures and my scathing review of Blue devil. Thanks guys! Either of you have a fail safe procedure for removing the manifold bolts?

 

Pull and pray. Hahaha

 

Yes, get the motor to operating temp and they'll unbolt like new although they'll squeal like an old guy getting a prostrate exam. Well some squeal for joy.

Anyways, not just head gaskets can be the cause of all this excitement. You have a front cover gasket that can fail around the coolant ports your water pump uses to access the block by. Also the intake gasket/pan can let oil enter the coolant path without you knowing it. There is also an oil cooler in one side tank of the radiator. The tanks can develop pin holes and leak. It's internal.

Have you checked your oil? That is far more important than oil in the coolant.