Prior to water pump failure this past friday, engine temp always stayed around 190f during normal highway driving. I installed a new water pump (from O'Reilly), new thermostat 192f (Murray Plus) and 4.5 gallons of coolant/water mix. Coolant was filled thru t-stat opening to reduce trapped air and topped of thru radiator and overflow.

Now, after repairs I have noticed a fluctuation of operating temps. Temp gauge reads around 210-215 before t-stat opens up and will drop down to about 180 after. This 180-215 fluctuation occurs constantly during normal highway driving (70mph, duration 45min, outside temp. 60(today)).

I found info for this to be normal on the 2nd gens but no info for the 3rds.

Is this normal for the 3rd gens?
Why the change from stock t-stat to new t-stat temps?
Could new t-stat not be functioning properly or was stock one bad?
What else might be causing this?

Any help or information on this issue would greatly be appreciated.

 

Stock fluctuations should be less then 5 degrees. Sounds like the new tstat is FUBAR.

 

Mine has 94000 on it now. Didn't do it before, but now it acts like yours....

 

Once mine is warmed up, it stays at 188-190 all day long

 

Do no understand the differing temps... Mine is steady at what ever temp it peaks with a slight variation of less than 5 degrees. Since the only varible is the thermostat, I would certainly check there first. Pump would not vary the temp, radiator would not vary the temp, even during the summer with the air on and off my temp does not vary that much...T stat.

 

That usually means air in the system.

Did you also replace the cap? If it's clogged, the system won't burp.

Take the thermostat back out, and drill a small hole in it. That usually resolves the problem.

 

The only autoparts store t-stats I have had any luck with are the ones from NAPA.All the others have never been close to the opening temp and cause large temp fluctuations.I have learned to get them from Cummins if possible.The comment about trapped air can also cause this.Get it up to temp and crack the radiator cap to burp it.May need to do this a couple of times to be certain there is no air in the system.

Drilling a hole in the t-stat will help with peak temps but will allow the temp to drop alot when not under load or idling.I did that to an Autozone one that caused big temp swings on one of my 01 3500's and it brought the temp down some while under load but sitting at a stop light the temp would fall off as much as 30* down to the 160* range.This was with 2 extremely small holes drilled.

 

I had a similar issue after installing my Opie-bypass. Just couldn't seem to get all the air out. So I went back to square one. Pulled radiator cap, cranked truck, set highidle for 1500RPM and waited. It took about 30 min. but it finally came up to temp. on a stone cold truck. Blead the Opie-bypass in and out and finished the job. Don't put the cap back on, just keep it full. Something I've done over 1,000 times. If you pull the cap after your truck cools down and you have air in the radiator, you still have air in the system.

 

I wouldn't crack the cap while at operating temp. The coolant is under pressure, and very hot. Once realesed to atmosphere, a lot of it will become live steam. The risk of a significant burn is very high. Unless you have a cap with the red pressure release lever. Even then, it can still spooge out and burn you. Cover with a heavy towel or blanket before releasing pressure.

I'v not tried this on a diesel engine. But I do this for every gas engine I'v ever replaced the thermostat in. I'v never gotten those temp fluctuations at idle. But, those were gas engines.

It can be difficult to get all the air out of the system. It can get trapped in passages in the head, and behind a closed thermostat.

A system will be "self burping" if there are three simple conditions: Fill point is the highest point 2. There is a working recovery system - cap, siphon tube and bottle. 3. Air is allowed to rise to the top when cold - that's the reason for the hole.

 

Sounds like some air still in system. Try pulling allen plug in right side front of cyl head. This will allow any air in cyl head to escape. Add coolant till it flows out the hole. Fastest way to bleed the air out cyl head that I have found.

 

You ever done an older Nissan Quest van with the bleed bolt on top of the intank darn near 4 inchs higher than the radiator cap. You have to put the cap on and rev the engine to get the coolant to come out the bleed bolt hole and god help you if you drop the copper washer off of the bolt putting it back on cause it was hot as all. Oh what fun and memories.... Never did I let a vehicle get back in a customers hands without everything perfect and that includes no air in the system.

 

Fluctuations leveled out today, so I suppose the air is finally out. Temps at 70 mph are holding at about 205-210 and drop down to 190 at idle. The high side is a little hotter than before. I would only see that during summer months hauling my boat. I might swap back to stock t-stat and see what happens. If I got the coolant to water mix off (too much coolant or water), would that influence the operating temp? Or, is the aftermarket t-stat throwing me off?

 

Yes, too much coolant will cause the temps to be just a little bit higher.

 

I don't like aftermarket thermostats.Have found some to have a smaller opening then original and other variances.Thermostats last just about forever on these engines.Unless a zillion miles I would have left thermostat alone.

 

Guys,
I posted a similar question a couple weeks ago. At about 60k miles mine started to fluctuate. Would drop below normal sitting at a red light, and go above normal while driving. On the highway, slight incline roadway at about 70 it would go above normal and then go back down as the road flattened out. I noticed the coolant level was low so I added some in the resevoir to top it off. No change. Now I drive with the heat on to keep the temp at or under the normal range (luckily it doesn't matter with the temps outside the way they've been). I fear if I don't a blown head gasket and worse could be the result.

I'm actually waiting on a Cummins suplier to call me back with a price on a new T-Stat (OEM). I intend to do a drain/flush/fill this weekend and if nothing changes I will install the new T-Stat.

One thing I read here is that there is a difference between aftermarket and OEM T-Stats in that there are two pins on the OEM and only one on aftermarket types. And read this might be a huge difference. Another thing I read here is some fluctuation is normal on these engines. I don't like it. it never did this before. I'm hoping old coolant alone is the culprit.

Just my .02 worth.
Rob