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P0128 Help, Engine Coolant Temp too low


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Hey fellas, this one has me stumped and I've been fighting it for several weeks.  Been throwing P0128 codes, which indicates that the engine temp is not reaching normal operating temps.  This is on my 04 R50 3.5L and I believe it's been doing it since August when I did a bit of an engine overhaul.  Here's what I can tell you:

  1. Installed new thermostat and water control valve.  Also installed new gaskets, upper/lower hoses, and replaced coolant.
  2. I run 50/50 per FSM.  I refilled with a gallon of concentrate, a gallon of distilled, then topped it off/overflow with pre-mixed 50/50.
  3. I've idled and revved the engine per FSM, put it on ramps, and otherwise attempted to get any air out several times.  Repeated hot/cold engine cycles and top-offs.
  4. The rear breather tube was clogged, so I drilled the crud out and got it to a point where coolant eventually spurted out.  (For those not familiar, there is a vent tube above the engine at the center of the firewall.)  That is the high point in the system.  FSM says coolant should come out that, but seems impossible considering it's a few inches higher than the radiator neck.
  5. The FSM and internet suggest the #1 culprit is a thermostat stuck open.  Again, brand new, but I pulled it for testing.  Was closed when cool; the neck held water.  It opened at 170F as spec'd in hot water.  Closed again in cool water.  Thermostat is working perfectly.
  6. I also tested the water control valve because I had everything apart.  Was closed, then opened at 203F as spec'd, closed at anything cooler.  Water control valve is working perfectly.
  7. Pulled the temperature sensor.  Per FSM, measured resistance at several temps from 68F-194F and it remained in spec at all points.  Temperature sensor is working perfectly.
  8. With engine off, noticed a tiny screech noise when turning the fan clutch by hand, and there was a little wobble when it was warmed up.  Installed new fan clutch, new fan blade, and new radiator cap two weekends ago thinking maybe the clutch had too much load at lower RPM.  New clutch didn't feel much different, but it's clear it's performing differently (better) than the old clutch was.
  9. ECU seems fine.  It reports changes in coolant temp and they seem accurate enough.  I was also monitoring other sensors; ECU responded as expected.
  10. Cycled heating and cooling modes in the cabin; blows hot/cold respectively.


My scanner shows that the truck only hits about 132-140F when idling for about 10-15 mins.  On steady drives, observed operating temp is around 158-166F.  I presume operating temp needs to be above 170F; internet says 195-220F is normal.  Temp gauge reads slightly below 50%, and always has for as long as I've owned the truck.  Ambient temps around me are about 50-70F this time of year, but it was "underheating" during the fall, and it's been underheating on recent long (6-8hr) freeway hauls, too.  Heater seems to work just fine.  Since I've been fighting to resolve two other engine codes, I've needed the truck to hit 158F at idle and I've only been able to do so by obstructing the radiator about 40-60%.  I could only get it to hit 180-190F at nearly 100% radiator obstruction and revving over 2K for a several minutes.


Now, while I'm super pleased that I have an extremely optimal cooling system, the engine code is frustrating me.  Anybody got ideas on how to increase coolant temps beyond blocking the radiator?  Does it seem likely I've still got air trapped somewhere?  I haven't tried tipping the truck sideways, but I might.

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I didn't even know there was a code for that! Shows what I know about the VQ.


i'm used to hearing about air pockets causing the opposite problem. Then again, I've heard of temp sensors reading low when not immersed in coolant, so maybe an air pocket is fooling the sensor. Given your #4, I'd start with bleeding.

If you've got a coolant fill funnel that goes in place of the radiator cap, you could fill that to above the level of the bleeder, take the bleeder cap off, wait for coolant to come out, and then put the cap back. What I've done without the funnel is to fill the rad, put the cap back on, remove the bleed bolt (the VG has a bolt instead of a rubber cap), and then squeeze the upper rad hose until coolant comes up to the threads. Hold the hose, put the bolt back in, release the hose, top up the rad. I assume either trick would work on the VQ. I also assume that if it's got a bleeder, it needs bleeding. Hopefully it's that simple.


What was the bleeder tube clogged with? Seems like a weird place for a clog to form.


Also, long shot, are the thermostats installed with the bleed hole/jiggler at the top? 


And on the off chance there's a connection, what are the other codes you're chasing?


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Yeah, it's a silly code.


The reason I don't believe there's an air pocket around the temp sensor is because the sensor and water control valve are on a crossover pipe that runs between the heads and has an output that wraps around the passenger side of the engine to the upper radiator hose, which exits higher than the crossover pipe. 


In the pic below, the sensor is by the dragon logo, and the WCV housing to its left.





The Y-pipe by cylinder 3 runs to the heater core.




The heater core return line is the black pipe running along the firewall.  The little vertical tube with a rubber cap on it is the vent tube  That is the high point of the system.




As such, with enough drives (I spent a good chunk of Monday driving around to perform the drive patterns to get the truck emissions ready), I'd expect any air to roll out of the crossover tube pretty easily.  The only place I'd expect trapped air at this point would be at the WCV on the intake plenum side if the coolant did not reach 203F to dump it into the crossover pipe and circulate it.  Air on that side is inconsequential, though.


Full system flow:




Breather tube is on the "from heater" line.  "To|From intake manifold collector" runs through the throttle body opening.


I do have a radiator funnel and was using it exactly as you mentioned.  Ran the truck a few times and let it sit.  Eventually when I determined the vent tube was clogged and drilled it out (the tube and pipe is steel, so I'm guessing the persistent coolant level over the years just corroded where sat), I still had the funnel attached and about half full.  I could tell coolant was circulating somewhat because crud I couldn't vacuum out eventually surfaced in the funnel.  


The thermostat is actually a triangular thermostat housing assembly that can only be installed one way; the thermostat part of it isn't replaceable, but yes, the valve was upward.  Mine has a water inlet port for the oil cooler on ATs.  The WCV is a normal thermostat shape, and it's installed flat; it's installed in a manner that allows coolant to flow across the thermostat body without being obstructed by the supporting vanes (as described in the FSM, no less).


Everything said, I have also not seen any leaks whatsoever.  All gaskets replaced, tightened to FSM spec.


I was throwing these 3 codes:

P0128 - this mess

P0171 - Bank 1 fuel injection system too lean

P1148 - Bank 1 closed loop control (faulty passenger upper O2 sensor)


Part of my overhaul in August included new spark plugs, reconditioning the fuel injectors (cleaned, new o-rings, new pintles, new filters), and replacing all 4 O2 sensors.  Long story short is I've been having all sorts of fuel/O2 issues since the overhaul, which is blowing my mind since I set out to prevent all these issues and chase some other code that popped up once a year that I can't recall.  I just tore the engine down again (four times in as many months?!) maybe 2 weeks ago to troubleshoot all 3 codes.


My thought process, given all the codes, is that if the passenger bank was in fact running lean, then maybe half the engine wasn't getting hot enough.  The spiral seemed a faulty O2 sensor causing lean condition causing insufficient engine heating.


P0171 suggests a lot of things, including a leak in the exhaust; that seemed the most plausible while other suggestions seemed implausible.  There's a massive backstory bull$h1t fiasco regarding trying to replace the catalytic converters during the overhaul and even up to last week, but let's just say I finally replaced the gaskets on the cats.  I'll note the passenger cats and exhaust manifold were very sooted over...a far cry from a too-lean condition, but possibly sooted from when I was initially dealing with a rich condition months prior.  I also pulled all the plugs, injectors, and coil packs again, cleaned everything again, and moved them all around to different positions.  The code has not resurfaced.


P1148 was pretty specific to the upper passenger O2 sensor.  I monitored the voltages on all the sensors with my scanner over the course of several drives and observed that the nominal voltage for it was not fluctuating as it should per FSM.  All other sensors were fine.  I'm still bitter about having the most difficult sensor to replace be the one that failed.  The other 3 are easy.  Replacing this one sucks tremendously and basically requires pulling the cats.  New sensor is functioning as expected.  Code has not resurfaced.


One additional notable: I installed an aftermarket transmission cooler during the overall and initially ran it in series with the OE cooler built into the bottom of the radiator.  I eventually bypassed the aftermarket cooler and reverted to using only the OE cooler when dealing with some other performance issues that surfaced.  I still threw the P0128 code in the stock configuration.  Just the other week I hooked the aftermarket cooler back up and bypassed the OE cooler.


All said (and sorry for a long post), only P0128 persists now, fortunately.  I was scrambling to get it emissions ready because my tags expire at the end of the month.  The P0128 code resurfaced as Pending code on the drive to the emissions station, which fortunately doesn't prevent passing.  As far as I can tell, the system is finally in top shape (my test drives on Monday were yielding 14-15 MPG, which is worlds better than it has been and pretty solid for my configuration), so this code feels like a lot of tail chasing.  I may just block part of the radiator and call it a day.  What I'd really like to know is if I can just tweak the coolant ratio to reduce efficiency so the temp will be hotter.


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170F is pretty low temp wise, Nissan seems to run the VQ hot, like 95C hot. I didn't see it mentioned anywhere, but was this an oem thermostat or an aftermarket? I have seen aftermarket ones test fine but be just out of the OE spec enough during operation to cause these problems. The Nissan thermostats can handle varying the flow better vs the aftermarket ones that seem to just open full blast at the rated temp.


My Xterra (VG33ER not 4.0) will barely get up to the factory rated 170F temp in the winter here when driving. I have to cover half the rad if not more with cardboard to get good heat.

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@adamzan Aftermarket unit.  It did test fine with water in a pot on the stove measured with thermometer and I ran a few tests to confirm open/close.  Seemed gradual enough each time.  But I think main thing here is that it stayed closed and sealed (confirmed) below 170F.  The code suggests a stuck-open thermostat would cause the problem.


I can see covering the radiator as necessary where you're at, but winters here are usually 50-70F/10-20C.  Shouldn't be that detrimental here.  Drove the truck for hours straight through freeway mountains over the past few weeks and still threw the code.


I wonder if poor coolant circulation could cause this.  FSM makes absolutely no mention of the water pump for this code, and I absolutely don't want to mess with that unless I'm 100% certain it's a problem.  Maybe I'll temporarily replace a hose or two with some vinyl tubing and see how it flows.

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It's really weird because you'd think that would cause higher than normal temps... My parents r50 when the water pump went it would overheat (but not completely boil over) at speed because the impeller was slipping.


You say the sensor tested good as well, is the wiring ok? No crazy resistance from the connector to wherever it ends in the harness? Sometimes moving old wires around makes problems lol. Have you thought about installing an after market temp gauge for testing?

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So get this: I just took a closer look at my previous thermostat, which I had no reason to check previously because FSM says it's supposed to be a 170F unit, but it's a 180F unit.  I guess my solution is to use a 180F unit.


As far as I can tell, all the wiring is solid.  I specifically inspected the loom for any damages the other weekend because all 3 of the codes conveniently relate to all three of the harnesses in the same wiring pigtail.  I presume the wiring is fine because I was closely and constantly monitoring live data on my scanner and everything always seemed reasonable at all times.


Now I need wrap my head around this.  I'm guessing the previous owner installed an aftermarket 180F unit; looks like one on RA.  The OE 170F unit fits 00-04 R50 and 03-08 350Z.  FSM specs closure at 160F, which could explain why my temp seemed to regulate in the low/mid 160s in steady driving conditions.  The 05-08 Frontier/X/R51 uses the 180F unit on its VQ40, but the housings is identical to VQ35 unit.  I'd be using the R51 unit, essentially.  Just trying to figure out why my ECU may ultimately need the 180F unit if it's supposedly "programmed" for a 170F unit.  When researching the P0128 code online, I did see occasional mentions about some auto manufacturers needing to flash ECUs after thermostat changes, but that seemed like a bunch of fluff.  But maybe that's the case here and my ECU now expects a 180F unit?  I do see a few Nissan TSBs specifically about P0128, but nothing that pertains to my truck.


Oddly, I see the VQ40 doesn't have a water control valve.  I noticed the JDM R50 VQ35 doesn't have it either.  Makes me wonder if there's any benefit to capping the intake manifold passage to the WCV.  Such a crap design, too...spills coolant all over the top of the manifold around the knock sensor that's also buried in there.  WTF Nissan.

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Guessing so.  I know wasn't me in the 9 yrs I've owned it.

Part arrived today, I'll get around to installing it eventually.

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On 1/2/2023 at 2:42 AM, Slartibartfast said:

Maybe the previous owner fought this too, and the 180 thermostat was their fix. I don't know why else you'd put a higher-than-stock-temp thermostat in something. 

Try living where I live, where it routinely gets to -30. The 170F stat in my 95 just would never cut it so I put a 180 in it. Just enough difference to actually de-ice the windows.

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  • 3 weeks later...

I see P0128 codes all the time right now. If coolant is full, you can rely on it being an issue with the thermostat. I gave up testing the things and just replace them. I prefer oem parts but most aftermarket t-stats work in Nissans. I suggest either getting a new one or warranty the one you have. 


Code P0128 is set when it takes too long to reach operating temperature or does not reach operating temperature. It looks like your thermostat is not able to restrict coolant flow enough to reach operating temperature. It could just have a weak spring and the pressure from the water pump could be opening the thermostat a bit causing it to open more than desired. 


Try a new thermostat, they are cheap and will likely fix your problem. It does for me at work. 

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On 1/21/2023 at 11:35 PM, Mr_Reverse said:

I see P0128 codes all the time right now. If coolant is full, you can rely on it being an issue with the thermostat. I gave up testing the things and just replace them. I prefer oem parts but most aftermarket t-stats work in Nissans. I suggest either getting a new one or warranty the one you have. 


Code P0128 is set when it takes too long to reach operating temperature or does not reach operating temperature. It looks like your thermostat is not able to restrict coolant flow enough to reach operating temperature. It could just have a weak spring and the pressure from the water pump could be opening the thermostat a bit causing it to open more than desired. 


Try a new thermostat, they are cheap and will likely fix your problem. It does for me at work. 


A new 180F thermostat will for sure fix it. The new 170F unit that was installed is confirmed working exactly as it should, as well as the temp sensor and rear thermostat. The ECU just thinks it’s too cold still.

I have a new unit in box. It’ll happen eventually. 

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