who's good at online diagnostics?

Alrighty... so my father bought a 1999 Tahoe a little over a year ago to have a cheap 4wd for them when the weather gets bad. has ~160k miles on the truck, engine was swapped for a junkyard engine about 30k miles ago. Since he's had it he has changed the radiator, waterpump, intake reseal, thermostat, front and rear brakes and a pitman arm. Considering he paid less than $3k for the truck, he's still coming out ahead.

Now, here's the problem. The intake was resealed about last July at a shop, where at the same time they installed a new waterpump. Sadly, they did not flush the cooling system at all. It was still gunked to hell and back. Around the middle of december, it started overheating at odd times randomly. Never consistent enough to do any real diagnosis. Starter took a dump shortly after and he parked it during the worst snow storms we've had in a couple years. :floyd:

About 6 weeks ago, I helped him drag it into the garage, put in a new starter nd a new t-stat at the same time. It was back to running and having heat again. The t-stat had opened so far that the blade of the valve was caught under the lower brace and it couldn't shut. It was an odd sight, something I've never seen before. As soon as I pried it past the catching spot, it slammed shut.

Worked for a few days then it went back to randomly overheating again. It always happens while moving, under load or not doesn't seem to matter. Every time it starts to over heat he loses heat from the heater. He can usually pull over, stop for a couple minutes and when he restarts, it goes back down and away he goes. The last few times, when he shuts it off, about 30 seconds after turning the engine off you can hear the hot coolant rushing into the overflow tank on the firewall. Once it purges, you can restart and it is back to normal.

We can find no signs of coolant in the oil, or oil in the coolant, it is not smoking (doesn't even have the standard SBC worn valve guide start up smoke), and its not using coolant.

Thoughts? Ideas? I considered that when the shop resealed the intake, that they blocked off the rear passages, or they installed an incorrect waterpump.... but it ran fine for several months after the work was done.

sounds like blockage

Do a system flush with cleaner.

Bad cap? A lot of people don't replace those because they are $20+. I wouldn't be surprised if the cap is losing pressure causing the coolant to spike in temp and overheat.

New cap, he bought it at the same time as the radiator. He flushed the engine side several times, until it was nothing but clear water running through it.

I still wonder if it might not be a blockage though. I may have him get one of those flush kits with the good cleaner in it and try it.

Take all the hoses off, pop out any and all drain plugs, get the garden hose out, go crazy.

Ranger50 wrote: Bad cap? A lot of people don't replace those because they are $20+. I wouldn't be surprised if the cap is losing pressure causing the coolant to spike in temp and overheat.

+1 on a new radiator cap. Bad radiator caps can cause cooling systems to do absolutely weird things, and I'd classify those symptoms as "weird."

FWIW, I change my radiator caps every 75,000 miles or so just to prevent future headaches.

My guess is blockage or a bubble. LT1s had to be burped, not sure about the Vortec.

How cold outside. A bad antifreeze mix will freeze and block the system for a while. Then thaw and return to normal.

Did you dremel the housing for the t-stat to prevent future 'hang ups' just for giggles I'd clearance everything and put in a new 195 t stat.

sachilles wrote: How cold outside. A bad antifreeze mix will freeze and block the system for a while. Then thaw and return to normal.

I did have that happen one time. Ran the engine until the heat got high,shut it off for a few minutes. Repeated until clear/melted.

My son's Sable had a similar problem. It was running cold, so we swapped the T-stat. After the swap it wanted to run hot. Tried two other new T-stats with the same problem. Stuck the original back in. No more overheating. Some of the aftermarket stuff is junk. Try an OEM T-stat.

The one aftermarket T-stat I tested didn't open until 240 degrees.

Pour some CLR in the radiator and run it for 30 minutes. Drain and fill with water to see if it still overheats. Don't use after market T-stats.

sergio wrote: Pour some CLR in the radiator and run it for 30 minutes. Drain and fill with water to see if it still overheats. Don't use after market T-stats.

Never put CLR in an aluminum radiator (they say its ok for under 2hrs but I'm not trusting it without a total flush in a situation that I have nothing to lose. It is fine for copper.

I flush with vinegar... And let it sit overnight after running it up hot. This made me wonder if you have put in any of that Lucas 20 degree cooler additive. It really does work. It's a surface tension breaker. I have it in my Ford because a few friends of mine that race recommended it.

Aftermarket t-stats are fine in these trucks but I would boil it and make sure it's true. It will most likely go over about 10 degrees which is fine.

Did this thing have Dexcool in it ever?

tuna55 wrote: Did this thing have Dexcool in it ever?

Yes, it was a dexcool car originally. It had been "flushed out" before my father purchased the 'hoe. It's since been flushed a couple more times with water.

A friend was driving in the middle of the night

Truck started overheating, it had been running hot. No water to be found.

Had a six pack of Coke,(the drinkable kind) so he poured some into the radiator.

A few miles down the road , the temperature was back to normal.

Bobzilla wrote:

tuna55 wrote: Did this thing have Dexcool in it ever?

Yes, it was a dexcool car originally. It had been "flushed out" before my father purchased the 'hoe. It's since been flushed a couple more times with water.

After witnessing the beginning of a Dexcool clog on a S10, I can't imagine how bad it could be if it got bad enough to hurt something. I'd say it needs to be REALLY flushed REALLY well.

tuna55 wrote:

Bobzilla wrote:

tuna55 wrote: Did this thing have Dexcool in it ever?

Yes, it was a dexcool car originally. It had been "flushed out" before my father purchased the 'hoe. It's since been flushed a couple more times with water.

After witnessing the beginning of a Dexcool clog on a S10, I can't imagine how bad it could be if it got bad enough to hurt something. I'd say it needs to be REALLY flushed REALLY well.

Bob I spent some time on BITOG today and there are some super duper chemical flushes which some people swear by, but nothing fantastic and well accepted when dealing with the red sludge of death that is Dexcool. If someone flushed it before you saw it, then you will never know how bad it was - it could be a mess where you can't see it. I'd investigate some of the better flushing products and get that trash out of there forever.

Well, the radiator was so clogged before he replaced it that the water coming in the driver's side took forever to get to the passenger side.

Test the continuity between the pin on the sensor and the block. Sensors sometimes go bad, and corrosion in the threads of the sensor will also make funky readings. Increased resistance shows up as high temps.

Of course, there are other smoking guns like the gunky rad and stuck thermostat, but its worth a look. My dad's 88 pickup with a TBI 350 had some funky readings. It would heat up slowly and normally, but the gauge would keep climbing steadily... then out of nowhere it would drop back to normal, continue to climb again, then drop to normal.

Since it started in the winter we assumed it was possibly icing so we checked the mix, but it was spot-on 50/50. Replaced the Tstat... same issue. Turns out it was bad continuity in the sensor. It would go back and forth between proper resistance and infinite resistance. The gauge is designed to rise slowly, but fall quickly. So, it would lose continuity and slowly climb. Then you would hit a bump and it would be working and the gauge would snap back down.

Replaced the sensor and all was good.

suggestions on best coolant flush out there?

I use vinegar on aluminum radiators.... Mixed 50/50 with distilled mineral free water.