I can only think of two possibilities...lip seal in the water pump is sucking in air, or...there's a pinhole in one of the cylinder walls that's allowing exhaust gas to pass, but not water to pass the other way. While glaze busting the cylinders, there WAS an odd little rust spot in #5 that I attributed to water getting in there during head removal (don't you LOVE they way they hid those drain plugs behind the new style motor mounts????), but it went away with a single pass of the hone, and nothing looked unusual. However, this engine, while in great condition overall (.0002-3 taper in all cyls...barely a ring mark to be seen) the original owner (the know-it-all father-in-law) didn't seem to believe that you needed to change antifreeze regularly. Thus, the jackets were FULL of rust flakes, necessitating me taking a pressure washer nozzle down into the block to clean it up. After the pressure washer treatment, the walls of the water jackets were grey iron with ocassional dark rust spots here and there. However, if by doing this, I knocked all the rust loose, the problem should've gotten worse, and it didn't. Actually, it improved...slightly...but it is still there.
Prior to this, this engine would NEVER get more than 5° above thermostat in any condition except pulling a grade when 110° out with the AC on. Now, it gets up to 210° just putting around town at 45 MPH.
Anyone heard of any bad block castings on this vintage LA engine? Also, anyone had a water pump do exactly what this may be doing? No bearing noise, and the weep hole is clean, but it's a two year old "rebuild" (the original blew the seal right out the nose on a trip to Vegas on the hottest day of the year).
The car, an '86 M-body 5th, is a cream puff, and I've grown to love it, even with its various faults. Strengths: comfy "rich Corinthean leather" seating all around; low noise level; excellent handling; excellent AC; straightforward, honest design, no engineering "gotchas," and it has Lee Iacocca in the trunk under the spare cover. (Ricardo Montalban is a no-show.) Weaknesses: Choppy ride at times; A-904 TF's lockup converter locks up at 36 MPH in 3rd gear...no matter WHAT the load...no part throttle unlock...have to downshift to second to knock it off; the usual headliner replacement (normal in any car this old); jiggly front sheet metal on rough roads; rather ungainly styling; and the standard MoPar feature...HARD to work on in places, something I think that's been a MoPar standard since the '60s. You need every wiggle and U-joint extension you may have in your rollaround, but if you're smart, everything comes apart fine.
Hate to trash this car for a porous cylinder wall. My wife loves it, too (it was her mom's when new) and it hasn't even hit 100K yet, and the paint and interior are still quite nice and draw compliments wherever we go.
Any opinions?
Caveat: If you have an old engine with low mileage where the original owner ran long oil change intervals with mediocre oil, and you see crusty sludge on the rockers (or wherever,) do yourself a favor....strip the top end, clean it out BEFORE switching to synthetic. I didn't, thinking the syn would sort of erode the carbonized oil and slowly send it to the filter. I got away with it, but after my clean up after dear Dad-In-Law, I'd never chance it again. The amount of crap loosened up and laying in the pan was a bit scary. Oddly enough, the pickup screen was clean, but every oil change, that filter (Fram Double Guard) was HEAVY. I changed out the oil pump just to feel safe, and later disassembly of the original proved me right...there was a LOT of abrasive wear in it.