L86 Coolant Temp Spike under WOT

[Dominatorstang]

I was logging VVE today and had done a few hard accelerations in 3rd for just a few seconds before I would let out. The 2nd time I did it and then was decelerating when I noticed my temp gauge was near 240. I had never seen it get anywhere near this, or not enough to catch my eye like that. The temp came back down pretty quick while cruising. I looked through the log and found it would go from 203 to 241 within 15 seconds when pushing it like this...

I then looked through my logs from the past couple months when it was cooler out and noticed the engine had been running 196 and would spike to maybe 210 on hard accel. A log from a few days ago going down the highway I noticed ETC's around 205 and spike to 215.

I do not notice any coolant loss, engine performance issues, or moisture in the oil. The coolant system pressure feels pretty normal on the hose. The hoses are factory and in good shape. The radiator active grill shutters are not stuck closed.

After checking all this out I took it down the road again with the heater on high to see if I felt the air coming from the vent cool off from lack of flow or anything at WOT in 3rd for 2-3sec. The heater actually felt to get a little hotter at WOT, but this time the gauge did not move much. I tried this twice with the same result. I then turned off the heater and tried which got the gauge to raise up to 3/4 position or so.

This has me guessing either the active grill shutters are limiting air flow to the radiator too much, or that maybe the lower radiator hose is collapsing? I would think a shutter issue would not show so much temp rise in just 15sec though.


VVE tuning drive highway town.hpl
vve idle tweak two.hpt

[5FDP]

I don't see a problem.

Mine does the same exact thing and I have a 5.3. A few long wide open throttle pulls or just one long one in the summer months will spike the coolant temp way up. It can go from 210 to 230+ real quick and then drop like a rock. I think it has something to do with the way the water pump flows and how slow it is. The temps nose dive right away because 1/2 of the coolant isn't 230 degrees, just the coolant inside the block will spike in temp and that is where the coolant temp sensor is.

Running a cooler thermostat would help with this.

[smokeshow]

What part number thermostat?

[5FDP]

I have a stock theromstat, whatever that number is.

[Dominatorstang]

I am using 194deg MOTORAD 815194 (815-194) from Rock Auto. https://www.rockauto.com/en/moreinfo...t=10337&jsn=13

I hope to find mine is working normal like yours in this way 5FDP, but I am still worried at this point when looking into the log more I find a third spike happen at just 30% throttle and only getting to 3000rpm went from 201 to 219deg in 20sec. It was only 70deg out with this log yesterday and the sun was down. If I can hit 241deg on a quick 3rd gear only rev with 70deg air then I think pulling a trailer would just make it way worse.

Sleeping on it I feel like the only two reasons would be lack of flow between engine and radiator or head gasket. The problem not happening if I am running the heater makes me lean towards radiator flow which I like much better than head gasket too lol. Slow reacting thermostat could cause it which may be why smoke show is asking part number? I will try to recreate the issue today to narrow down results more if it is load or RPM and double check how the heater effects the results. And I may throw a 1500lb trailer behind it to see what happens.

I towed 4000lb a few weeks back for an hour or so run each way and never got temps above 220 is another reason it worries me It was probably ~50-60deg outside that day.

[Dominatorstang]

Here is a snip from the 4k towing log a few weeks back. coolant temp range was 201 to 216 and looks to be about 65deg outside.

cut down 4k quick tow.hpl

[Dominatorstang]

Had to drive to an event across town just now so tried a few things on highway. Temperature will not spike under heavy load if RPMs stay low like under 2000. If I drop a gear cruising to get 2800rpm then it slowly rises to 240 or so and really does not come down until I bring RPM down.

I have heard of water pump impellers spinning on shaft but never seen if happen. Starting to wonder though. I will try a couple other things on way home.

[smokeshow]

Reason I asked is a lot of the 160 thermostats that guys get for LS stuff are shorter where the the bypass valve sits in the bottom of the housing. If its not fully seated, any coolant that makes its way through displaces flow coming from the radiator. Its a small fraction usually but it has led to high temps and overheating. Not sure if the LT thermostat housings are like that though, never seen inside one.

[5FDP]

Had to drive to an event across town just now so tried a few things on highway. Temperature will not spike under heavy load if RPMs stay low like under 2000. If I drop a gear cruising to get 2800rpm then it slowly rises to 240 or so and really does not come down until I bring RPM down.

I have heard of water pump impellers spinning on shaft but never seen if happen. Starting to wonder though. I will try a couple other things on way home.

Now that doesn't seem really normal. I only get the spikes in temp if I am really flogging it. I can make mine downshift or manual downshift and hold a higher rpm but my coolant temps never spike that high. I'd be wondering if that thermostat isn't working all the way.

[Dominatorstang]

I would agree that a flow restriction is most probable cause at this point and thermostat would be first place to check. I did try a few more things on the way home and in the driveway with some logs attached below. Here are the results from them.

still around 65 or 70deg today

road tests
-normal acceleration in town still keeping RPM's under 3k caused quick spike to 230 each time.
-cruise down highway with high winds added to load but gauge stayed in the middle pretty well and ~207.
-downshift and keep RPMS at 2800 while maintaining speed which raised temps to stay around 230deg.
-downshift another gear to keep RPMS at 3500 with maintained speed put temps staying around 245deg.
-running heater at anytime would pull 5 or 10 deg out.

driveway tests
-idle at 2k in driveway gave temps of ~215.
-letting out coolant pressure and running at 2k showed no bubbles coming into coolant bottle and did not act like any pressure was being put into the system
-idle at 2k and using thermal camera showed 175deg where lower hose attached to engine. 220deg where upper hose attached.

It is going to start raining any minute and pretty well all weekend, but I will get thermostat pulled out soon and look for any other restrictions or such.

highway drive from home testing temp spike.hpl
testing coolant temp spike on way home.hpl
2k idle with heater on high.hpl

[Dominatorstang]

I am sorry, I said the wrong thermostat. It is a Motorad. I have corrected that above.

[Pro Street]

My 2018 GMC Denali L86 does the same thing since new.

[Dominatorstang]

Do you have a log for yours? Yours is factory 207deg tstate too right? This one is 194deg and fan temps knocked down to match. Climbs quickly to over 240deg when running 2800RPM with very little load cruising and wont drop unless letting RPM's come down? It was not doing this a month ago when I first got it and did the work.

[Pro Street]

Do you have a log for yours? Yours is factory 207deg tstate too right? This one is 194deg and fan temps knocked down to match. Climbs quickly to over 240deg when running 2800RPM with very little load cruising and wont drop unless letting RPM's come down? It was not doing this a month ago when I first got it and did the work.

I have a K&N intake and Flex Fuel sensor. Everything else is stock. I will look and see if I have a log.

[Pro Street]

13wot.hpl

[Dominatorstang]

I see what you are saying. looks like you were running about WOT each time on around 80 or 85 deg day with coolant going from 215 to 232 or so.

I know the title of the thread throws it off, but at this point I am just running 3k RPM with little to no load and I hit 240 deg on a 65 deg day with a 13deg colder thermostat.

It is surprising how much yours spikes too, but I am not so sure mine is doing exactly the same thing. Even though I would like to think it is nothing to worry about. I think the main thing odd about mine is that doing higher RPM low load like running VVE spots then it would not try to hit 240+ deg previously.

[smokeshow]

Let's not confuse temperature regulation with improved cooling system performance. A colder thermostat buys you more time before overheating...but if your cooling system isn't built for it (none of them are, not even stock), then you will overheat.

You should log engine fan status when you drive.

[Dominatorstang]

Good point.

My test above in the driveway while holding 2000rpm showed 175deg coolant temp coming out the radiator and 220deg going in the radiator. Right now if appears to be a flow issue.

[Dominatorstang]

I pulled and inspected both the thermostat and water pump. They both appeared to be in good working order. The thermostat is stamped 194 and about 2 months old. I also flushed out passages with the water hose to make sure a blockage had not developed somewhere.

Next I put the thermostat in a pan of salt water with a cooking thermometer. I took it to 220deg and could not even get it to start opening. I found another thermostat I had on the shelf that was still new but for the LS model and stamped 187. This one in the pan was wide open by 200deg or less.

I can not say I have had a thermostat fail in this manner, but this Motorad thermostat is opening way too late. I had actually done one last road test and in 38deg air with fans running 91% at 60mph and 3500rpm on flat ground it came up to 250F and then stabilized at 248F until I dropped the RPM back down.

Not many options out there for 194deg thermostats either. I like the 194 to run a little cooler than stock, but still hot enough to boil out moisture from the oil.

I had also run across an interesting read yesterday of a similar truck testing cooling with some weight and road grade. https://www.gm-trucks.com/forums/top...h-towing-data/
The cooling fan table on it was not optimal, but they also had a 180deg stat and did not have the NHT radiator that I have in mine.

[Dominatorstang]

Last night the thermostat came in and I replaced it. It was a little colder out, probably 45deg. Cruising at 3k RPM the temperature would come up from 195 or so to 205 and then stabilize around 203deg F. cruising at lower RPM's would get it back down to 196 or so. Hopefully this thermostat lasts much longer than the 2 months of the previous one.