GM Cooling System Woes

2007 Chevy Express 3500, 6.0.

This weekend at SOFR I had some overheating issues. On the way down I got stuck in traffic for about 20 minutes and when I got to where I was going and stopped with the engine running to look at a map the dash lit up. The temp gauge was pegged and it went into reduced engine power mode. I shut it down and restarted, once I was moving the temp gauge started going down quickly. After that every time I was at a stop light the temperature started climbing up but when moving it was normal (~200°F). 

The next day it was even worse with outside temperature in the 90s, any stop light had the temperature climbing to 220-230 but dropped back down when moving. When hot the mechanical fan was easy to spin by hand so I figured it needed a fan clutch. 

Yesterday I put on a new fan clutch. I let it idle for about 40 minutes this morning with the scan gauge hooked up to monitor temp. When up to temp it was around 200°, when I started running the AC it moved up and was 215-220°. Revving the engine to get the fan moving dropped it down pretty quick. 

I ran some errands over lunch and all seemed well, sitting at stop lights was fine but after my last stop at a stop light it started going up to around 230°, once moving (slowly through a parking lot) it dropped right back down to 200. That seems pretty quick to shed 30° of coolant temp but maybe that is normal? 

I am planning on flushing the coolant, it still has Dexcool in it. It has not lost any coolant that I can notice from the level in the reservoir. 

Could it possibly be a faulty temperature sensor or more likely a water pump not flowing enough at idle?

Can you repeat the test with a bigass fan in front of it?  In theory that should let you determine if airflow or engine speed is what's cooling it down.

In reply to ¯\_(ツ)_/¯ :

Hadn't thought about that. I have a box fan I could stick in the grille to see if it makes a difference. 

My thoughts are leaning towars clogged radiator. Ive had them that are only shedding heat between the inlet and outlet in a more or less straight line  the rest of the radiator core was significantly cooler, as it wasn't shedding heat due to being clogged.

Theae were dexcool gm products, but not trucks/vans.

In reply to Dusterbd13-michael (Forum Supporter) :

The coolant in it is orange and the owner's manual specifies using Dexcool. 

My 6.0 Suburban did the same thing at about 150k.  As this is often used for long tows I loaded the parts cannon and did radiator, water pump, fan clutch, and hoses and flushed the dexcool all out for green stuff.  Doing that solved the problem even towing 7k up mountains. 

Based on old anecdotal evidence from my time working at Firestone shops, I would think Dex turning into milkshake along with a possible worn thermostat that isn't reliably activating.  If it were mine I would do a serious DIY flush with copious water rinse, replace with 50:50 green stuff, and a new thermostat.  Retest and return with results report.

My 2 cents.

In reply to EvanB :

Well, at least you don't have tbe combination mess! 

However, still sounds clogged....

One of the vettes i found this on had brand new appearing dexcool...

Dexcool sucks.

Dexcool is like a snowflake.  It's beautiful, perfect, and so delicate that the first sign of changed chemisty leads to catastrophe.  Dexcool is like dating a Gemini.  Everything is going so well.... until the evil twin shows up and you'll wish you had never met her.

Clogged radiator is a good guess.  Once Dexcool starts turning sour, it has a sandy, slimy, jello-like precipitant that sandblasts water pump impellers too.  Possible your water pump isn't pushing enough because it has been eroded.

I'd take a $1 bet that when you open the draincock on the radiator, nothing comes out.

Something definitely came out when i opened the radiator drain. And it looked very clean, just not as much as i was expecting. I flushed some with water then ran it up to temp and am now draining the water out to flush it more.

EvanB said:

In reply to Dusterbd13-michael (Forum Supporter) :

The coolant in it is orange and the owner's manual specifies using Dexcool. 

So what happens if you use something besides Dexcool in a GM product.  After proper flushing of course.  

In reply to spitfirebill :

I can answer this as I did exactly this with my Olds Intrigue.  Nothing happens which is exactly what you want.  It just works.  You need to check to see if any residual Dex in the system shows up on the bottom of the rad cap or in the overflow, etc but in my experience the swap went perfectly.  You just have to flush the whole system really well.

spitfirebill said:

EvanB said:

In reply to Dusterbd13-michael (Forum Supporter) :

The coolant in it is orange and the owner's manual specifies using Dexcool. 

So what happens if you use something besides Dexcool in a GM product.  After proper flushing of course.  

Blissfully nothing happens, which is more than you can say for Dexcool.

Dexcool is wonderful in theory, but it doesn't take much to turn it into a useless, corrosive, abrasive pile of goo.  One of the things that kills it is (wait for it) .... air.  If you let it run low and it sucks air in a line, the cavitation renders it useless.  I went through 3 or 4 heater cores and two water pumps under the 3 year warranty on my 96 Impala SS.  Every time it was filled with sandy orange jello.

I flushed and filled with good old fashioned green stuff and aside from testing it every 5 years or so, I never had a single issue again.  For its remaining 22 years with me, I never needed a single cooling system repair.  I did replace the water pump because I was replacing the timing chain and didn't want to put a 20-year-old water pump back on, but otherwise, zero maintenance required.

In reply to spitfirebill :

I did a good flush forward and backwards, then filled with just water and a surfactant additive.  I ran it for a few days, drain, flush, then refill with 50/50 green.

A proper flush is definitely good.

My earlier comment about opening the draincock and getting nothing was because when I pulled the block drains on the LT1, I got nothing.  Looking in the hole I just saw compacted orange sand, and that was after 36k miles.  I got it flowing using a blowgun on the air hose, but what a mess that LT1 was from Dexcool.

After flushing a few times and running with water in it I filled it up with water. It ran for an hour with the AC on and the temp stayed steady at 200. Hopefully a few more flushes will get all the dexcool out and I can fill with green stuff. 

Good call.

Don't stress about these living at 200 degrees, either.  Stat is probably a 190, and the temp sensor is after it leaves the heads.  200 is my happy place in the van with a 5.3L.

... which, by the way, the previous owner kept Dexcool in, so I'll be doing the same exact thing here in the next couple months.

Yes, these engines are engineered to live at 210-220 in stock form.

I did have a customer who delivered for Landstar who outfitted his 6.0-engined box van with a bed and stuff, actually really similar to your rig except box van divided laterally instead of vertically.  There was a 2 week period in Texas where he never shut the engine off because he wanted A/C while sleeping.  His coolant temps got up to 265F at one point.

We installed a pusher electric fan tied in to the A/C compressor circuit for signal, problem was solved.  I've seen it on a plow truck where we had to convert to electric fans because you need airflow through the radiator to get the fan clutch to engage to pull air through the radiator.  (Which is why we NEED Eddie Van Halen...)

I am 99% certain that if you knew someone with HP Tuners (ahem) and some spare PCM pins (ahem ahem) you could add the pins for electric fan control, turn on fan control in the PCM, and have a pusher fan tied to engine coolant temp and A/C high side pressure, as a more elegant solution.  I have worked with two van PCMs (LS swaps) and one 4.8 truck PCM (plow truck) and they had the hardware in the computer even though the vehicles had belt driven fans.

You guys and your Dexcool hate.  It's not that bad...

Anyway, have you looked between the radiator and condenser for a nice pillow of garbage, bugs, grass clippings and other airborne swarf?  It's worth a peek.

Open the rad cap, there's 1/4 cup of brown sludge hanging off the bottom of the cap, and Streewiseguy says "that's actually good".

In reply to pres589 (djronnebaum) :

No, that's not what I said.  I said, it's not that bad.  My implication was, if you pay enough attention to notice when your coolant level is low, and actually top it up, you don't wind up with wads of sludge caused by a lack of maintenance.

A lot of dexcool hate comes from the simultaneous introduction of self destructing intake manifold gaskets by GM.  Why don't people whine about them and the thousands of engine failures caused?

Dexcool isn't that bad, if you pay any attention at all to your automobile.

In reply to Streetwiseguy :

Self destructing intake gaskets are only bad on GM because OMG DEXCOOL.  When it happens on a Ford and the manifold face layout means that your first clue is an S shaped #4 connecting rod and knocking noise from the piston slapping the crank....  that's just those rascally old Fords, they do that.

In reply to Streetwiseguy :

I don't complain about GM self destructing intake manifold gaskets because I refuse to buy a POS GM product.  I had too too many coworkers that bought GM minivans that ended up with leaking intakes and a huge repair bill.  

Dexcool works great if you don't have leaks. Air is the killer. Dad's got multiple 250+k mile trucks with dexcool. Works fine. Our 06 Sierra is still on the original dexcool. No sludge, no overheating. Also no leaks. You can hate on dexcool all you want but it's a decent product when people don't f it up. 

And those van engines (2.8/3.1/3.4/3.5) haven't been made in close to 20 years so basing yuor knowledge base on those isn't very accurate. 

In reply to bobzilla :

3.5s moved the coolant passages to the end of the head, the intake gasket is dry.