Page 8 (Updated 28-Oct-2008)
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VGE Engine Coolant Flow
- General Overview
- Engine Flow
- Engine Flow Control, Sensors
- Peripherals I & II
- Troubleshooting
Free Service Manual, Free Download.....see FAVORITE LINKS -page 26- FAVORITE LINKS
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General Overview
No other publicly available source on earth explains this coolant flow as an entity; still Nissan 3L V6's has sold millions. After weeks of research I have come to following conclusion, and illustrated it like follows.
The cooling system is rather complicated and may be functionally divided into multiple sections. Some parts are always 'active', some flow is temperature controlled, some user controlled (when no automatic cabin climate ctrl). Main system function is for COOLING while the most complicated parts are for HEATING various 'peripherals'.
Arrow points how Coolant is pumped to the Engine Block main channel,
up into Heads, out to the Lower Intake (not shown)
via oval channels (beside intake air input channels).
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Engine Flow
The Engine Coolant Flow may be functionally divided into three separate tasks;
This 'three phase' -cooling flow is typical for all gas powered liquid cooled engines.
The most important function is the engine cylinder & head cooling circulation.
Engine; see below pics 1-3:
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1. Cylinder Head Cooling
'Main task' is CYLINDERs & HEADs COOLING. For this a heat transfer medium is used (cooling liquid). This cooling liquid is pumped first around cylinders and then to cylinder heads. This circulation is always 'active' when engine runs. This pressurized flow is dependent on engine (and pump) rpm. Heat from hot engine parts is transferred into the liquid heat transfer medium.
2. Heating 'peripherals'
Secondary tasks for the system are for HEATING various entities using this heated liquid. Before returning to the pump, the now hot liquid, is used for heat transfer e.g. to warm the air intake (rather foolish carburettor relic). After this, various peripheral devices are being heated (always 'on') with this hot liquid (most of this is flow is '99% stupidity', just to ensure proper operation in any imaginable freezing weather condition).
3. Return Flow: Cold/Hot
The 'return' mainflow after 'main task' has two operationally different states: COLD / HOT.
- COLD : As long as liquid is cold, flow has shorter loop directly back to the pump, no cooling is needed.
- HOT : After liquid warmup, separate cooler [Radiator] FOR THE HEATED LIQUID is needed. This longer route is opened only after liquid warmup and is used only FOR LIQUID COOLING utilizing heat transfer to the ambient air. This liquid flow is independently Thermostat controlled (blocked when cold).
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All mentioned systems aim into fastest possible warmup time to optimum engine operating temperature. After warmup the task is to stabilize engine & peripheral temperatures regardless of driving conditions or ambient temperatures... just to ensure smooth operation. One exception is cabin heater circuit: driver (=buyer) temp has been prioritized before the engine...
From troubleshooting point of view, pump belt & tightness has to be ok, see Page 23 . Pump itself may have corroded 'blades' reducing cooling capacity. Thermostat may be stuck closed (causes overheating) or open (engine stays abnormally cold). Also the whole system has always to be leakfree & pressurized; leak means air pockets =hotspots, which will ruin the heads. Also be on guard of frying your ECU; hole in the cabin heater may leak liquid into it.
Peripherals; see below pics 4-5:
Two peripheral (somewhat overlapping) HEATING circuits exploiting engine excess heating capacity, takes care of engine idle system etc. de-icing and cabin temp. This excess heat is constantly transferred via the flowing liquid medium into these circuits. See all these details below...
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Engine Cooling [flows always]
1-4:
Following steps 1-4 are always used purely for COOLING purposes.
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Pic 1. Coolant flow inside the Engine - Always Circulating
1. ..........Water pump created vacuum sucks (always 'optimal') coolant from thermostat housing, pressurizes it. Flow is directed into main engine block channel.
2. ..........Block main channel distributes around cylinders.
3. ..........From around cylinders, flow continues to both heads
4. ..........and out of cylinder heads via the oval passages located beside six intake channels.
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Engine Cooling [COLD ]
Closed Loop 1-10.a. :
..........Pic 2.: COLD Engine circulation - Thermostat CLOSED - Small Loop
5. ..........Coolant is directed into the lower intake assembly passage for HEATING the intake.
6. ..........Next the coolant mainflow goes to the manifold front, the Temperature Sensor housing (Ts) with three hose joints. From the manifold's other end, lesser flow [7.].
7. ..........Cabin Heater Core & Upper Intake and Peripherals get heated coolant from Lower Intake.
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Ts : .......Tri -hose connection & sensor housing; allows free flow in any direction. Contains two coolant temperature sensors: Using this coolant measurement point, two sensors give input to ECU (for Radiator Fan & EGR valve ctrls). Up to this point the mainflow is always the same; from here thermostat regulates if main hose [9.] -or- [8.] is used. The task for the independent Thermostat is to keep the coolant at 'optimum temperature' for the engine.
8. ..........Main hose to radiator. Some minor flow goes through here in all cases. When engine is cold, the closed thermostat blocks main backflow from radiator, and this hose sits 'idle'.
9. ..........As long as engine coolant remains cold, thermostat is closed, main return flow goes through this main hose and via Thermostat housing, to the pump.
10a. ......... The Thermostat leftside housing bypass route is open and lets still cold coolant return through this route to the pump and again into the engine block.
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Engine Cooling [HOT ]
Radiator Loop 10.b. - 13:
....................Pic 3: HOT engine Cooling - Thermostat OPEN - Radiator Loop
10b. ......... The Thermostat's gas activated, heat controlled work-cylinder has pushed its valve open because coolant is hot. Operationally Thermostat has gradually 'switched sides' during warmup, e.g. coolant mainflow 'small loop' is now closed. Thermostat controls the flow and directs mainflow now through the COOLING 'section'. Now Thermostat simultaneously blocks (X) flow via hose [9.] and allows flow via [8.] to Radiator. All this allows effective coolant COOLING. Cooled mainflow returns from Radiator to the Thermostat housing, pump, via hose [11.].
R : ..........Liquid flowing through Radiator is being cooled, heat disspated to ambient air. Based on coolant temp measurement input to ECU, Radiator cooling is fan boosted.
11. .........Cooled liquid main return line from Radiator[used only after thermostat opens].
12. .........Return line from Cabin heater core [when climate ctrl valve is open]
13. .........Return line from Throttle Body TB to the thermostat housing [always open].
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Engine Flow Details:
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Left: Coolant circulation from waterpump to engine block and around cylinders, TOP view. From here water rises up, circling to the heads. Right: Water Pump coolant circulation.
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Engine Flow Control, Sensors
Coolant Temp sensors at the tri-hose joint housing (Ts., refer to picture above) The temperature sensor at left is connected to ECU (2wire), called ECTS.
The temperature sensor, pictured above at right, (schema below) is connected only to dash Temp Gauge (one wire). In the FSM it is sometimes called water temp, temp transmitter (!?) hello etc...
EGR temp sensor in the FSM Schema below is not normally there nor used (it is exhaust temp, only for California).
Refer to page 3 for temp sensors, measurements .
(This simple issue has been really mixed up and made quite complicated --->in the FSM.)
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Peripherals I; Heating Engine Accessories
Flow Always On
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Engine Heated Coolant flow is used for heating:
Fom Engine Heads - Via Lower Intake...
A. to BV -> ... B. to IACV -> ... C. to TB -> ... return to Water Pump
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Pic 4. Peripherals Coolant flow 
5. .........Heated Coolant flow from lower intake to peripherals circulation (red arrow).
7. .........Hot flow to Cabin Heater Core & system Bleed Valve [BV].
11. .........Cooled liquid return lines to Thermostat housing, Water Pump. (Note: smaller tube not thermostat regulated.)
12. .........Cold return flow from Cabin heater core.
13. .........Cold return flow from TB.
. Red arrows and red lining is used to depict hot flow to Upper Intake, BV, IACV and TB. After TB cooled water heat is retuned to main flow. (Yellow BV Bleed valve screw is only opened for service; to bleed air bubbles out.)
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Intake & BV Details:
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Hot coolant is first routed to Upper Intake and BV Bleed Valve.
Coolant is routed inside intake to heat the whole aluminum assembly. BV is located on the highest point, on top of right hand corner of upper intake assembly. [Caution; Hot System is pressurized: Loosen cautiously only for bleeding]
7. ......................Hot coolant to IACV/TB & towards Cabin Heater Core
13. ....................Return line from Cabin heater core
BV .....................Bleed Valve. Valve for bleeding air out from Cooling System.
Coolant IN. ........Hot coolant to Upper Intake & Bleed Valve.
Coolant OUT. ......line to IACV & TB circulation.
PCV. ......................PCV Positive Crancase ventilation Valve for engine ventilation (not cooling)
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IACV details:
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Idle Air Cut Valve - coolant temperature controlled airflap.
Coolant temp controlled work cylinder in IACV chokes idle air channel when warm; here the cutoff valve is seen at +20C temperature position (slightly ajar).
FSM text indicates that the IACV air cut valve is just a safety measure:
"The air cut valve closes gradually as the cooling water temperature rises. This valve restricts the auxiliary air flow after the engine has warmed up and limits the maximum air flow even when there is trouble in the IACV-AAC valve, so that engine overrunning can be avoided." from FSM page EF&EC-14.
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Throttle Body Details:
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TB Throttle Body Heating
From IACV flow goes to Throttle Body TB.
Right pic: Coolant returns back to coolant mainflow from TB to return line 12/13; flow inside TB is from right to left.
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Peripherals II ; Heating the Cabin
User Flow Control
Peripherals II ; Cabin Heater - User Operated Flowcircuit. In Manual and automatic Climate Controls user defines the flow. Automatic control has also cabin temp sensor and add on electronics to control the Flow control valve.
These circuits can fulfill their purpose only after engine heat has been passed to the transfer medium; coolant has become hot enough to release its excess energy. This means engine operation at least for 5 minutes...
Note: this circuit is always open and flows - when the Ctrl valve is open. This is a disadvantage from engine point of view as it lengthens its warmup time. User can exploit this in many ways:
- as long as hot air comes to the cabin, one can be sure angine is being cooled and not in danger. This regardless of what instruments say...
- Using full heat with blower, the cooling capacity is doubled. This may be crucial in extreme hot conditions...
(Note that heat transfer medium = coolant flow is just one part of the cabinet warming process - See the Airflow controls & other Controls on previous page 7)
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Pic 5. Cabin Heater Coolant flow
7. .........The pump constantly 'presses' heated coolant from lower intake to the Cabin heater core, Thermostat has no regulating power over the FLOW. Arrows and red lining is used to depict hot flow to Cabin heater Core. This hose is to be DIY insulated in the engine bay, if maximum heat inside cabin is preferred.
Blue/green hose: Cold coolant backflow from Cabin heater core.
14. .........Heater input hose valve; see detailed pic below.
15. .........Vacuum line from intake to coolant valve actuator.
16. .........The magnetic vacuum control valve is controlled electronically. Electrical wire from cabin heater control panel assembly.
17. .........Cabin Heater Core. Outside air flows through core, cools the cooling liquid. Cabin input air gets heated.
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Cabin Heater control valve assembly
15. ...........Vacuum line from intake; hose contains small oneway valve.
14. ............Magnetic valve for vacuum control: Electronic ctrl (to facilitate automated climate control), vacuum powered.
14b. ..........Vacuum line link from magnetic valve to vacuum actuator.
14c. ...........Vacuum cylinder piston via actuator arm moves the actual water hose (7.) valve.
14d. ...........Inset: (Mechanical) Water Valve is 'hidden' under the assembly.
16. ...........Electronic control wiring from cabin. Note the diode -mark on the connector.
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Note: For various reasons coolant may be low, forming air/steam pockets. These pockets may block some/all flow to heater core, causing reduced heating effect. The remedy is thorough bleeding using BV bleed valve or driving heater hose ajar (small leak) for 1hr above 2k rpm ... after leak source correction. Refer to the Maintenance Manual, see & download via page 26.
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Troubleshooting
Considerations; General cooling system Troubleshooting:
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After 'any' tuneup when coolant has been out, filling procedure contains bleeding air bubbles out. Fillup explained in the FSM section MA-8. With J30 VG30E air bleed is done as front is jacked up (one feet) and bleed valve opened, engine run for a while. Whatever way this is done, all bubbles in the coolant have to be out - or engine will be ruined at some stage.
When filling an emptied system with the VGE 8.3L / 8 3/4 US qt. (explained in FSM), all that amount has to fit in ... if system becomes filled as described there then no 'room' for air pockets. If some cant be poured in (=is left out), it is an indication of a air pocket.
I checked but could not find support from FSM for this raise up -trick for bubble bleeding. However, it is logical that hose etc system with curvy structure, does harbor bubbles: tilting forces bubbles on the move...
Bulletproof way to vent the air bubbles out:
One Peugeot piping system was like Chiraq himself - impossible and always cooking. The only way to get all air bubbles out, was to cause a small leak on the highest part of the water system, then drive one hour (cab heating on) while adding cooling liquid, and then close the bleed.
In my RV's (=complicated tubing) I have always devised a bleed system from radiator into cabin, collection canister. I use clear hosing to see the bubbles, hose can be closed with a hand ctrl valve. Typically only after driving one hour on the highway valve ajar, all mircrobubbles have been bled out. After bleed system starts to work as intended. This also tells that the normal '5minute bleed' is often inadequate. Even VW vanagon (Big 16Liter system) Factory service manual states that driving uphill causes engine to heat, temp gauge needle to raise higher temporarily: this is untrue. Its the Air bubbles - they always cause engine cooling temp to wobble, head to overheat.
Bleeding system from radiator, clear tubing, valve, collection bottle...
After using one hour my bleed system on the highway, last bubbles come out. Only after this, the cooling system starts to work perfect, as all microbubble 'traffic' stops. Thereafter no overheating, temp gauge needle always stays 'like glued'...
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How cabin heat works: Engine water circulates always in cylinder heads and into cabin cell tubing’s ... if temp ctrl lever =valve is open. This coolant is cold until engine really warms, first 'five' minutes after startup. First heated coolant is routed into cabin heater. Next the thermostat opens and engine coolant will be routed into the front radiator.
However, if the engine thermostat cant fully close, the engine stays always cool and cabin heater will never get hot (lukewarm, possibly). Engine water circulation heats the Cabin heater core cell, and air blower sucks, pushes outside air through it.
The cabin heater core is an heat exchange device: blower pushed incoming ambient air gets heated as long as the hot coolant flows. Cooled liquid returns into the engine. If coolant flow becomes reduced (normal adjustment or via problem: low liquid level, air pockects etc), the incoming air temp drops fast.
If the incoming coolant flow from engine is insufficient or ambient temp is low, the air may never become hot. Revving engine revs the circulation pump, increasing coolant flow, resulting in more hot coolant flow into cabin heater ---> incoming air is hotter.
Flow control: Cabin heater valve is electronically controlled, vacuum powered valve inside engine compartment. It is located on engine bay wall, under the hood. It gets it operating power from vacuum diaphgram, via vacuum tubes from engine (check for leaks). Electrically control: check valve coil resistance and its control voltage from cabin ctrl module. Disconnect and check that it is mechanically operative.
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To locate a block in the hosing:
Figure out how tubing is routed. Start cold engine and track hoses by hand how the water temp that raises gradually, seep through the tubes. If they all get evenly hot, then the problem might not be blocked hosing but air circulation.
Tubing might collapse and form block. This may not be visible, collapse only for the hose inner part. As already mentioned, more probable is gas bubbles formation on some curved/pinched hose where bubble 'collections' form total blocks in water circ systems.
Air circulation:
One poor cabin heat possibility is in the air flaps, the bypass cool air -door. If it never truly closes [temp lever at right end], cool air will always come and bypass the cabin heater cell. (Something broken or bad adjustment?... Ctrl rod/wire loose, broken flaps mech.?)
Heater core waterflow is almost impossible to become totally blocked, leak is known to happen in maxima. (Btw. that fries the ECU).
Heater core AIRFLOW is typically rather restricted if the car has spent 10 year in the outside. Check previous page for reference... If this is suspect, vacuuming the system is the answer. With an long narrow extension hose, one may reach the heater core and suck it clean.
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If engine is 'cooking':
If bleeding does not help, change Thermostat. If that does not help, somewhere is something blocking the coolant flow, and that has to be found, otherwise you ruin your engine. Do not drive long trips before this is corrected. Do not accept bubbling noise in any circumstances.
Pump blades might be corroded; then pump is not really working.
If the engine is cooking -be it due to thermostat or not - the temp gauge should show it.
- Is needle working normally [cold engine zero, warm halfway up?] If not, most probably the engine not cooking - but still your loud bubbling noise tells of local overheating...
Loudly cooking tubing’s: touching Loudly cooking tubing’s one can feel the bubbles. -Scary- Exploding closed cooking water tubing’s will burn severely [not typical in new cars - if the tubing’s are well installed] Take care to use gloves not to burn your hands.
Aluminum engine heads bend easily due to cooling problems and might later [after warranty is no more valid...] result in blown gasket and broken engine.
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Noisy hoses -Note: Heated water makes rubber hoses softer.
The noise tells that some flow still gets through to half empty pipes. Full pipes without air =no sound.
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Glycol to Electronics -relation:
Most coolants contain glycol. If this is splashed around [radiator leak, anybody?), water evaporates, glycol will not. Now if car history contains 'this', there are microsize glycol chrystal nuggets all over engine bay. Sunny day its no problem. Wet day: glycol rips water molecules from air, and electrical shorting problems start: bad idle, no start, whatever... Only way to get rid of this, is thoroughly clean all, take ign wirings to bathroom...
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Check also Nissan TSB NTB94-087.pdf for cylinder endbolt leaks.
TSB's see for http://maxima.theowensfamily.com/tsb
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Johns view of pure flow: "And he shewed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb."
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