Hummer H1 Service Manual page 186

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quicker. At high temperatures, fluid viscosity decreases and
more fluid circulates through the cooler maintaining an even
operating temperature and a constant pressure in the cooler
(see Section 1 for service procedures).
A separate smaller oil cooler, mounted to the fan shroud, is
used to cool the power steering fluid.
Two fluid returns are used: one from the hydro-boost and one
from the power steering gear. Fluid flow through the hydro-
boost is never completely restricted during braking. The hydro-
boost incorporates a fluid return to release the hydraulic pres-
sures used when applying the brakes (see Section 8 for service
procedures).
ENGINE COOLING SYSTEM DIAGNOSIS AND
TROUBLESHOOTING
WARNING: Do not release surge tank cap when en-
gine is hot. Steam and/or coolant may cause serious in-
jury.
Loss of Coolant
1. Pressurize system and check for leaks at all cooling sys-
tem hoses.
a. Tighten loose clamps, fasteners, or fittings.
b. Replace leaking hoses.
2. Pressurize coolant system and check for leaks at water
pump or around cylinder heads. If any leakage is present,
replace cylinder head gaskets, cylinder heads, or water
pump.
3. Check cylinder block for cracks. Replace if cylinder block
is cracked.
4. Check expansion plugs and block heater for leaks.
5. Check radiator and surge tank.
6. Remove surge tank cap. With engine running, check for
excessive bubbles in surge tank that may indicate leaking
head gaskets or cracked cylinder heads. If bubbles are
present, remove cylinder heads and check for defective
head gaskets, cracked cylinder heads, or cracked cylinder
block. Replace cylinder heads if damaged. Replace engine
if cylinder block is cracked.
Engine Coolant Temperature Gauge Above
255°F (124°C), Engine Overheats
1. Inspect for low coolant. Verify low coolant sensor is oper-
ating properly. (Refer to Electrical Troubleshooting, Sec-
tion 12).
2. Inspect the drivebelt and pulleys for damage.
a. Replace any damaged parts.
b. Check belt tension. Replace if necessary.
WARNING: Use caution when testing thermostat. Hot
engine coolant will cause burns.
05745159
Cooling System
NOTE : Off-road driving in extremely dusty conditions may
result in debris entrapment between the radiator and oil cooler.
Trapped debris may increase coolant temperature.
3. Check for clogged or blocked radiator system. Clean and
flush as required. Cleaning requires the separation of the
oil cooler from the radiator and both units must be
thoroughly flushed with water and compressed air.
4. Check thermostat for
thermostat and place thermostat in container of water
known to be 190°F (88°C). Observe valve. If valve does
not open, replace thermostat.
5. Check radiator for bent fins. Straighten fins or replace
radiator if damaged beyond repair.
6. Check operation of temperature gauge. Refer to Electrical
Troubleshooting (Section 12).
7. Inspect fan blades for damage. Replace fan if damaged.
8. Check fan clutch operation.
a. Check for excessive bearing play. Using fingers only,
press tip of fan blade toward and away from engine.
Tip of blade total movement should not exceed 0.20
in. (5.1 mm). If movement is more, replace fan drive.
b. With engine off and ambient temperature of 50°F (10°
C) or higher, fan should turn evenly with noticeable
drag. If fan turns hard, very easily, or with uneven
resistance, replace fan drive.
c. Fan noise is sometimes evident under the following
normal conditions:
• When the drive is engaged for maximum cooling.
• During the first 15 seconds to one minute after start-
up, until the drive can redistribute the silicone fluid
back to its normal disengage operating condition
(after overnight settling).
Fan noise or an excessive roar will generally occur
continuously under all engine high speed conditions
(2400 RPM and up) if the drive assembly is locked-up
due to an internal failure. If the fan cannot be rotated
by hand or there is a rough, grating feel as the fan is
turned, replace the fan drive.
9. Check for leaking or defective water pump. Replace
leaking or defective water pump.
WARNING: Do not release surge tank cap when engine
is hot. Steam and/or coolant may cause serious injury.
Fan Runs Continuously (Noisy)
1. Check that all fan bolts are present and tightened to 45 lb-
ft (61 N•m).
2. Perform steps 8a and 8b on preceding page.
3. Check for fan cut off (disengagement).
4. Observe fan drive disengagement after 1 to 1-1/2 minutes.
If fan speed does not drop and coolant temperature
remains below 190° F (88° C), replace fan drive.
4-3
proper operation. Remove