Fluctuating temp gauge after lower intake gasket job

[SensRanger]

96 Ranger 4x4 with the 4L OHV. Blew the lower intake gasket and got a new one installed. They also replaced the thermostat. (Not sure if they put in a Motorcraft one with the air bleed hole though). Other than near-overheating when the coolant poured out (I stopped and put more in immediately), temp has always been pegged around 1/4 or 1/3 of the way up the gauge when running right.

Now, once warmed up, the water temperature gauge sweeps from 1/4, to 3/4 of the way to "H" then back immediately, sometimes hanging out right in the middle for a bit, hot again, cold again, all over the place... but never into the red zone.

I thought it was air trapped in the system. I've tried all the tricks for purging air: parked on a steep uphill with heat on high, idled with the rad cap off (coolant would gurgle out now and then before settling down), manipulated the upper rad hose and heater hoses (coolent would trickle out). Heater blows hot hot, no issues there. No little bubbles in the rad or overflow tank, oil is ok & no white smoke in exhaust: seems to eliminate head gasket issue. Shop said cooling system was pressure tested and was fine.

Not sure what to do next. Any ideas? I've tried the air bleed trick two or 3 times already for 15-20 mins each time. Coolant did seem to gurgle out randomly (air being pushed to the rad?) and the swinging temp gauge would settle for a while, but the wild sweeps kept returning. Might add that the wild swings are seemingly random - not tied to RPMs, can happen on freeway, residential street or stopped at a light.

Possible that the new thermostat is sticking and not opening when it should? Need to bleed more air? Something else wrong? I'll bring it back to the shop if I have to but if it's just an air pocket I don't really want to. (shop is a ways away and a hassle to drop off etc.) Thx!

 

[2000StreetRod]

cheap thermostat

Based on all of your air purging attempts I suspect a cheap, low quality thermostat was installed with a wide fluctuation (slow to open, slow to close). I suggest replacing the thermostat. The shop should be able to monitor the engine coolant temperature via the OBDII port and confirm the temperature is actually fluctuating. If it isn't then you probably have a defective engine temperature sender.

 

[tenny]

I suspect, a cracked head.....or slight head gasket breach.
(exhaust gas in coolant)......I hope im wrong, in your case.

the only way to be sure, is to do a chemical test.

its a blue dye, that is put directly over the rad cap. if there is
exhaust gas, it will turn green (or yellow for FULL breach).

after you've done everything else....have this done.

like I said...haha, I hope im wrong

good luck

or it could be just a fuggin partly clogged radiator, ha

 

[SensRanger]

Thanks for the replies. I've heard of head gaskets causing similar symptoms for sure (and the 4L OHV seems especially prone), the only reason I hope I am safe from that for now is that this issue only started after the lower intake gasket and new t-stat repair. Prior to that the temps were perfect. So I suspect air in system or cheap t-stat... I hope. Wound up dropping it off at the shop tonight after all so we'll see, I'll update.
Probably should also mention that when the lower intake first let go, I temporarily plugged it with a sealant additive. Some of that crap might still be kicking around in the system... but again, things were fine after the sealant but before the intake was torn down...

 

[Runnin'OnEmpty]

I suspect it's a sticking thermostat, especially since
you added the sealant to the system. Sounds like
a ratcheting open/close of the thermostat, rather
than it operating smoothly.

Might be too late since you've dropped it
off already, but it's always a good idea to drill a 1/8"
relief hole in the thermostat if it dosen't have one...

 

[SensRanger]

Thanks all. Shop just called and yup, the thermostat was bad out of the box. They put a new one in and claim the problem is fixed and the needle doesn't go past halfway anymore. I'll reconfirm that when I drive it. I also asked if the t-stat had a air bleed hole and they claim the new ones don't have that anymore. If I need to change it again I'll be sure to drill a relief hole or try and get an original model with the hole.

 

[SensRanger]

Picked up the truck and drove it a while. True enough the temp no longer goes past halfway. BUT it still fluctuates between low and halfway, which it did not do before. Pretty sure this is due to the lack of jiggle valve. I may remove the t-stat and drill it. But no big rush. Of course, on my drive the CEL came on and I have a P0401 code now. Hoping it's a simple matter like a worn vacuum hose going to the DPFE (they would have been manipulated to get the intake apart). It has a new EGR assembly already but the DPFE is older. Anyway, always something it seems lately.
It was a trouble free ride for many years....

 

[tenny]

great news!!!!!

glad to hear it was something very simple.

thats always nice.

 

[SensRanger]

Little update. I decided to spend the cash on an OEM Motorcraft thermostat and managed to install it without dropping any bolts or otherwise screwing up. The previous one did actually have a jiggle "pin" after all, so don't think that was the problem.

I tried to bleed the air out a couple of times after installing the OEM stat. SAME PROBLEM though. It will stay cool for 10 minutes than randomly spike to 3/4 hot and come back down quick, to almost the Cold line. Then hang nicely at 1/3 or 1/2 way. Then a tiny spike. The back down. No rhyme or reason... no difference whether under load or stopped, if the outside air is warm or freezing, or whether heater is on or off. (Heater works just fine). Seems totally random.

I'd be worried about head issues but the shop said they and the gasket tested fine. Plus, I think if there were hot exhaust gases getting into the cooling system, the effects would be more constant and predictable, rather than this "perfectly fine for 5 minutes then crazy temp swings" situation.

Radiator is full and overflow is 2/3 full. Maybe bad rad cap (it's original), still an air pocket, or any other ideas???

 

[2000StreetRod]

monitor engine coolant temperature

Do you have access to a scanner that can monitor the engine coolant temperature via the OBD-II port? Maybe your instrument cluster temperature gauge or your engine temperature sender is erratic.