peejay Posted February 9 Share Posted February 9 Clear up this confusion for me, please! On these VG engines, when the thermostat opens, does hot coolant exit the engine via the lower radiator hose? And cooled coolant exits the radiator via the upper radiator hose? Link to comment Share on other sites More sharing options...
Slartibartfast Posted February 10 Share Posted February 10 Nope! Other way around. Hot coolant passes from the engine to the rad via the upper hose, and cooled coolant returns via the lower. This is the direction in which the thermosiphon effect is pushing the coolant anyway, so it's how most cooling systems work. (There are reverse-flow oddballs out there, notably the GM LT1, but the VG is not on that list.) This means the VG's thermostat is on the cold side. I don't know why Nissan did this, but it seems to work. There's a flow chart (heh) on LC-6 of the '94 FSM, but this diagram from the '87 is a little easier to understand--at least once you work out that the grey arrows mean something different in the diagram than they do in the flow chart below. 1 Link to comment Share on other sites More sharing options...
peejay Posted February 11 Author Share Posted February 11 Thanks, Slart! And thanks for the REAL diagram, you've been telling us for years to get the earlier FSM's as well, now I see why! They must've omitted the drawing from later ones to hide their illustrious "design" of the cooling system. (Tstat being on the "cold" side is well-stated!) 1 Link to comment Share on other sites More sharing options...
Slartibartfast Posted February 11 Share Posted February 11 It is nice to have a few manuals to play off each other! The '87 in particular has lots of weird first-year stuff that's not in the later manuals, including a just-different-enough-to-cause-problems engine diagnostic code table. Nissan also put the tstat on the cold side of the KA and the VQ, and Chevy did something similar on the LS V8s, so clearly there was some kind of thinking behind it. I have not found anything that explains what that thinking was. My best guess is that mounting the tstat lower means it's not sitting in the hottest water in the engine, so it's opening later, helping the engine come up to operating temp quicker (which is better for fuel economy, engine wear, and of course emissions.) But that's just a guess. Whatever effect they were going for on the VG, I guess they decided there was some meat left on that bone when they drew up the VQ, because it's even weirder. They still put the tstat on the cold side, but they also split up the coolant outlets for the block and the heads, which let them put a second tstat on the hot side of just the block. 1 Link to comment Share on other sites More sharing options...
peejay Posted February 11 Author Share Posted February 11 Your explanation makes perfect sense, my apologies to the engineers! However, had they gone "conventional" they may've been able to avoid that %#%#*@!! bypass hose, so for that, there is no forgiveness! Link to comment Share on other sites More sharing options...
Slartibartfast Posted February 12 Share Posted February 12 LOL yeah, there are still plenty of reasons to question the engineers on these. The steering linkage, the strut rods, access to the #6 spark plug... Link to comment Share on other sites More sharing options...
peejay Posted February 21 Author Share Posted February 21 (edited) Alright, @Slartibartfast, just a few more pieces to the puzzle that I'm missing... (I'm trying to figure out how the Coolant Temperate Sensor responds to the coolant gradually warming up...) When coolant leaves the Intake manifold and reaches the Thermostat housing (and the closed Thermostat), how does the gradually warmer coolant reach the coolant temperature sensor/water outlet, or does it ONLY reach the outlet/CTS once the thermostat is open? But if so, wouldn't the CTS only read "cold" and "VERY HOT" coolant temperatures? And I may be confusing myself a bit here, this circuit is from a '02 Frontier FSM (I'm assuming the VG30/33 cooling systems are identical?), and it makes it look like coolant leaves the Intake manifold, gets to the Water outlet housing, then [if thermostat is closed] goes to By-pass hose, reaches Thermostat housing, and recirculates: (Also, the above circuit makes it look like (to me), that if the Heater is on/selected, coolant will not go to the Water outlet housing, but that can't be right, right?) I added lines which I thought represent the above circuit: Based on that, why wouldn't coolant leave the Water outlet housing as well as the bypass hose, even when the thermostat is closed? *** Last point (and this is WAAAAAY out there, but)... Is it possible, that coolant is "constantly" flowing out of the Water outlet housing and into the radiator, but because the bottom hose is closed off at the (cold) Thermostat, the coolant flow "stops", or better yet "backs up" and seeking the path of least resistance, is diverted into the Bypass hose? I'm sure that is not what happens, but why? Thank you for your time! Edited February 21 by peejay Link to comment Share on other sites More sharing options...
peejay Posted February 21 Author Share Posted February 21 (edited) Okay, question simplified (courtesy of AI) How does the temperature signal change (shortly after warmup) if warmer/hotter coolant only "touches" the coolant temperature sensor (via the Water outlet housing) once the thermostat opens? By my flawed logic, the system would run rich until the thermostat opens. Edited February 21 by peejay Link to comment Share on other sites More sharing options...
Slartibartfast Posted February 21 Share Posted February 21 (edited) Hot coolant naturally rises over cold coolant. So even though there's no flow directly past the sensor when the tstat is closed, the warmer coolant will still make its way to the top of the system, which is why they put the temp sensor there. That sensor location is also why the heater core and intake warming circuits run back to the cold side. Both of those circuits are dumping heat, so the coolant coming out of them will be cooler than the actual engine temperature. If this cooler coolant was introduced to the system at the top, it would mix with the hot coolant around the sensor, giving the computer an inaccurate idea of how hot the engine was. Figuring out the rest of the coolant flow was more difficult than it had a right to be, mostly because flowcharts were a terrible way to try and explain this system, but also because Nissan doesn't seem to understand them either. Now that I'm really looking at them, the flowcharts in both the '95 and '87 manuals are just straight-up wrong. They show the water pump only being fed when the tstat is closed, and the tstat and rad forming a loop on their own, sans pump and engine, when the tstat is open. So, yeah, that ain't right. The Fronty flowchart makes a lot more sense. To confirm that it's the same as the VG30 cooling system, I pulled up yet another manual, this time for the '85 300ZX (which also had a SOHC VG30). That diagram looks very similar to the Fronty diagram, apart from the turbocharger of course. So, yeah, near as I can tell, that Fronty diagram should be correct for this application. It looks like the end of the thermostat is what blocks off the bypass hose. So as the thermostat opens, it's allowing more flow from the radiator, while throttling flow from the bypass hose. (Kinda like the hot/cold mixer knob in a shower.) This one-wax-motor-moves-two-valves thing would've been explained much better in a cutaway than it is in those stupid flowcharts! I did find a cutaway in the '93 300ZX manual, so check that out if you're struggling to visualize this--though that car has the DOHC VG, and I don't know what else changed on that variant, so don't trust it too far. Hopefully that explains it! I did not expect the rabbit hole to go that deep, or the rabbit at the bottom to be that vicious. I wonder if the borked flowcharts were drawn by the same guy who copy/pasted that fuel pressure test. (For those who don't know--the fuel pressure test in the WD21 service manual mentions a fuel pressure regulator control system, which was not fitted to these, but was present in some early 300ZXs.) Edited February 21 by Slartibartfast 1 Link to comment Share on other sites More sharing options...
peejay Posted February 21 Author Share Posted February 21 This is gonna make me download every Nissan FSM, isn't it? Thank you for your explanation, as always. The warm liquid rises thing (ya made me google it)..."convection", I think that's part of the physics puzzle! Thanks for the suggestion on the 300ZX cutaway, it makes it a little better to visualize, but they really need a better team working on those flowcharts! Maybe they don't know how the system works, just that it does! (It still makes it look like (to me) that the outlet is acting as some sort of diverter, but I'll let it go...for now!) If I get bored enough, I'll rig up some wiring adapter between the Coolant Temperate Sensor and the harness, to try and monitor real-time the voltage change engine running, or resistance as the system cools, engine off. Link to comment Share on other sites More sharing options...
peejay Posted February 21 Author Share Posted February 21 Okay, last one, I promise! (assuming this is correct): coolant exits the front of the intake manifold --> enters the Water outlet housing --> is SUCKED down the bypass hose, into the thermostat housing by the centrifugal force of the water pump... why isn't coolant also sucked in from the upper radiator hose (and therefore the radiator) when this happens? if this is what happens... physics? magic trick? Link to comment Share on other sites More sharing options...
Slartibartfast Posted February 22 Share Posted February 22 9 hours ago, peejay said: why isn't coolant also sucked in from the upper radiator hose (and therefore the radiator) when this happens? For coolant to leave from the top of the rad, it would have to enter from the bottom of the rad. If the tstat is closed, the lower rad hose is blocked, so suction at the top of the rad would have no effect. Like you're sucking on a straw with the end blocked. If the tstat is open, the pump is sucking coolant through that bottom hose. So now there's someone else sucking on the other end of the straw, and they're better at it, and man that got weird fast. So there's either a deadhead or suction at the bottom of the rad, and an unrestricted supply of coolant coming out of the engine at the top. The datalog could be interesting, but leave the two-wire sensor alone and use the one-wire sensor for the dash (with the wire to the dash unplugged) instead. Two circuits trying to check resistance across one sensor could end badly, or at least give weird and unreliable data. And yeah, at this rate I'm gonna end up with a full backup of Nico's service manual collection on my hard drive. Sometimes it's worth it, though--last night I stumbled on the "body sonic system" in the '84 ZX manual. Looks like it had kickers in the seats, hooked to the radio. Butt subs. Oh, and RE convection. Some early cars and tractors did not have water pumps, and relied entirely on thermosiphon cooling. Heat rises in the block, cold falls in the radiator, coolant circulates. It even kinda works, mostly. 1 1 Link to comment Share on other sites More sharing options...
peejay Posted February 22 Author Share Posted February 22 More physics, I see! (Got weird, slartifbartfast??? ) I tested the CTS/ECT (disconnected), all seems good there, slight corrosion on the positive side, gonna clean that just for peace of mind. I did download a few more FSM's, didn't know about the body sonic system, Nissan was ahead of it's time, I do remember the digital dashes and what nots in the early Maximas and Z's. Thanks as always for your help! 1 Link to comment Share on other sites More sharing options...
adamzan Posted March 9 Share Posted March 9 On 2/11/2025 at 11:06 PM, Slartibartfast said: LOL yeah, there are still plenty of reasons to question the engineers on these. The steering linkage, the strut rods, access to the #6 spark plug... I gotta say, when people complain about the #6 plug I can immediately think of some engines that are way worse. Old GM V8s that you had to remove the A/C compressor/brackets (I wanna say pontiac or caddy from the 70s), any transverse mounted gm v6 in the 90s is hell to get to the back 3 plugs. Or how about the maxima with the VQ35, you have to remove the intake plenum to get 3 of the plugs out. Thankfully if you use modern platinum plugs they last 100k easily. 1 Link to comment Share on other sites More sharing options...
peejay Posted March 9 Author Share Posted March 9 For me, it's the fact that it's just that ONE plug! If they made that entire bank hard to get to, maybe I'd only be half annoyed! 1 Link to comment Share on other sites More sharing options...
Slartibartfast Posted March 9 Share Posted March 9 LOL yeah, that could be it. Five easy ones to really make the tricky one stand out. My beef with #6 comes mostly from trying to get a compression tester into that hole. Link to comment Share on other sites More sharing options...
adamzan Posted March 10 Share Posted March 10 5 hours ago, Slartibartfast said: LOL yeah, that could be it. Five easy ones to really make the tricky one stand out. My beef with #6 comes mostly from trying to get a compression tester into that hole. Yeah I could see that being a pain. Fortunately I haven't had to do that lol. Link to comment Share on other sites More sharing options...
gamellott Posted March 15 Share Posted March 15 I've never really had a problem with that specific plug. It's the vacuum line that seems to give me the biggest problem... But I have small hands compared to most adult men Link to comment Share on other sites More sharing options...
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