Hummer H1 (2006+). Manual - part 26

2-10

Engine

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ENGINE DIAGNOSIS

Oil Pump Pressure Test

Remove the number one oil gallery plug on the driver side of the engine block. Install J-21876-6. Operate the engine and mea-
sure the engine oil pressure. The oil pressure minimum hot at idle is 14psi and 42psi at 1800 RPM.

Possible causes:

•

Pump pickup strainer or tube loose

•

Pump pressure relief valve or spring problem

•

Pump gears worn, chipped

•

Pump shaft worn, slipping

•

Oil galley plug loose, missing

•

Main bearings severely worn

•

Cam bearing turned (spun) or misaligned

General Information

The diagnosis charts describe common faults that may occur. Potential causes are listed in order of probability. Although the charts
list frequent causes for specific engine related problems, they do not cover every possible cause. The charts are intended to serve as
guides only.

The diagnosis information in this section is aimed at solving engine mechanical problems with normal shop diagnostic equipment.

 Engine Diagnosis

PROBLEM

POTENTIAL CAUSE

CORRECTION

Hard Starting

1.

Insufficient cranking speed (below
100 rpm cold or 180 rpm hot). Caused by:

• Low batteries

• Bad cable connections

• Serpentine belt or alternator problem

1.

Check, repair cables or connections. 
Recharge batteries if low. Replace batteries
if shorted, sulfated or damaged. Replace
belt or tensioner if damaged. Test alternator
output and repair or replace if low.

2.

Starter problem

• Poor connections

• Relay

• Solenoid

• Brushes

• Armature

• Starter drive damage

2.

Check/repair cable and solenoid connections
if needed. Check starter draw; overhaul if draw 
exceeds 450-475 amps. Remove and overhaul 
starter if brushes, starter drive or armature are 
faulty.

3.

Insufficient fuel to injectors.

3.

Check fuel volume at supply pump and
injectors while cranking engine. If volume
is low, check for restrictions in lines or at
filter. If volume is OK, check pressure at
pumps and injectors and repair as needed. 

4.

Water in fuel (filter indicator light NOT on).

4.

Drain off water at fuel filter drain cock.
Then pump the hand primer with drainback open 
for 5-10 seconds to purge remaining water. If
water continues to appear in fuel coming
out drain cock, purge fuel tank and lines
and refuel vehicle.

7-19-05

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 Engine  2-11

5745804

5.

Glow plugs inoperative (most notice-
able at cold ambient temperatures).

5.

Check fuse and wiring harness to plugs.
Correct open or short as required. Refer to
diagnosis in Section 12 if required.

6.

Air in fuel delivery lines.

6.

Check for loose fuel line connections at
pump and injectors. Correct if necessary
then bleed any remaining air with hand primer.

7.

Fuel return lines restricted or tank
vent inoperative.

7.

Check return lines (from engine to tank)
and correct restrictions, kinks, plugged
lines. Make sure tank vent is OK. Repair
as necessary.

8.

Air cleaner plugged (after extended
off-road operation).

8.

Remove and clean air intake components.
Replace filter element if plugged or restricted.

9.

Vehicle misfueled.

9.

Drain sample at filter drain cock. Replace
fuel load if fuel contains gasoline, kerosene, 
fuel oil or low quality fuel.

Hard Starting 
When Hot

1.

Wrong fuel or poor quality fuel.

1.

Drain and refill tank. Purge rest of fuel system and 
bleed.

2.

Air leaks in fuel supply (suction) lines.

2.

Check lines at fuel pump and injectors for leaks or 
loose fittings. Tighten or replace lines as needed.

3.

Fuel return or vent line restricted.

3.

Clear or replace either line if restricted, pinched or 
damaged.

4.

Low compression due to rings, pistons, 
valves.

4.

Check compression and overhaul engine if 
necessary.

Hard Starting 
When Cold

1.

Incorrect grade of diesel fuel.

1.

Drain and refill fuel tank and filter. Bleed and 
purge lines as needed.

2.

Water in fuel or wax buildup in filter.

2.

Drain water. Remove and clean filter and lines if 
wax formation has occurred.

3.

Glow plugs inoperative.

3.

 Repair wiring or replace faulty glow plug(s).

Engine Will Not 
Crank

1.

Shift lever not entirely in Park (or Neutral 
position).

1.

Check and correct linkage adjustment as needed.

2.

Insufficient battery voltage due to:

• Low Charge

• Corroded connections

• Damaged Cables or solenoid wires

2.

Check batteries and cable connections. Charge 
low batteries and check generator and belt. 
Replace failed battery(s). Clean corrosion from 
cables connections. Replace or repair cables or 
terminals as needed.

3.

Blown fuse.

3.

Check fuse condition at fuse boxes. If blown fuse 
is found, check circuit for shorts, grounds.

4.

Park/Neutral position switch problem.

4.

Test switch continuity with ohmmeter. Replace 
switch if open/shorted.

 Engine Diagnosis

PROBLEM

POTENTIAL CAUSE

CORRECTION

2-12

Engine

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5.

Starter or solenoid problem.

5.

Check for bad or loose connections at starter and 
solenoid. If OK, note if starter surges when cur-
rent is applied. Problem may be with Bendix or 
armature.

6.

Hydrostatic lock. Combustion chambers 
filled with fuel or coolant due to faulty injec-
tors, or blown head gasket. (Remove glow 
plugs, crank engine and note fluid coming 
out of plug holes).

6.

Test and replace injectors that are stuck open. 
Replace blown head gasket or replace head if 
cracked.

7.

Flywheel ring gear teeth or starter Bendix 
damaged.

7.

Remove transmission access cover and inspect 
ring gear and bendix. Replace flywheel or starter 
Bendix as necessary.

Engine 
Stalling

1.

Air leaks in fuel lines.

1.

Check lines from fuel pump to injectors. Replace 
lines that leak or have damaged fittings. Tighten 
loose injectors. Bleed afterward.

2.

Fuel filter partially plugged by wax forma-
tion, dirt/sludge or partially water filled.

2.

Drain off filter contents at drain cock. Clean out 
fuel tank and lines if filter contained sludge. Clean 
out filter if full of wax or dirt.

3.

Misfueled, wrong fuel or low quality fuel.

3.

Drain and refill tank. Drain fuel filter and bleed 
the fuel system.

4.

Restricted air intake due to plugged filter 
element, air cleaner, intake duct. Generally 
caused by extended off -road operation in 
dusty areas.

4.

Replace filter if necessary. Remove and clean out 
air cleaner body, intake duct, cap, hoses and dust 
unloader.

5.

Fuel drain back or return lines restricted/
damaged.

5.

Inspect and repair or replace lines, clamps as 
needed.

Low Power, 
Poor Accelera-
tion

1.

Vehicle overloaded.

1.

Lighten vehicle load.

2.

Low grade diesel fuel.

2.

Drain tank and refuel.

3.

Sludge or wax buildup in fuel filter (low fuel 
supply).

3.

Drain and clean filter.

4.

Air in fuel supply system.

4.

Check lines from fuel supply pump and to injec-
tors. Replace leaking, damaged lines as needed. 
Bleed air at hand prime pump.

5.

Restriction in air intake system.

5.

Check air intake cap, duct, air cleaner, hoses and 
air horn. Clean/repair as needed. Replace filter if 
plugged.

6.

Turbocharger, related wiring fault may have 
occurred. May also be caused by exhaust 
gasket failure.

6.

Check wiring and ECM with scan tool. Check for 
intake/exhaust leaks. Replace turbocharger or 
related components as indicated by diagnosis.

7.

Injection pump or injector failure.

7.

Diagnose and replace pump as needed. Replace 
damaged injectors.

 Engine Diagnosis

PROBLEM

POTENTIAL CAUSE

CORRECTION

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 Engine  2-13

5745804

8.

Engine problem:

• Piston, ring or valve wear.

• Worn cam lobes.

• Damaged piston(s).

8.

Check compression. Then disassemble engine and 
repair as needed. Remove engine completely if 
diagnosis indicates crankshaft, rod or piston prob-
lem.

Exhaust Smoke:

Blue Smoke

1.

Worn piston rings, valve stem seals or valve 
guides.

1.

Remove glow plugs and check compression. Then 
repair valve train and cylinder heads or perform 
piston ring replacement.

White Smoke

1.

Low quality fuel.

1.

Drain and refill tank with better quality fuel.

2.

Engine timing is retarded.

2.

Check and correct timing.

3.

Engine coolant entering combustion cham-
ber through blown gasket, cracked head, 
loose/damaged head bolts. May be accompa-
nied by low coolant level and overheating.

3.

Verify problem and repair as necessary. Replace 
cracked head, block or head gasket.

4.

Low ambient temperatures (below freezing).

4.

Normal condition caused primarily by water 
vapor. Not a concern unless it continues after 
warm up.

Black Smoke

1.

Low grade diesel fuel.

1.

Drain and refill tank.

2.

Overrich fuel mixture:

• Drain/return lines restricted.

• Injector(s) stuck open.

• Engine speed or throttle sensor fault.

2.

Test, diagnose and repair as required.

3.

Black smoke at wide open throttle.

3.

Restriction in air intake system, air leak at intake 
or exhaust manifold.

Gray Smoke

1.

Long idle periods.

1.

Try to avoid prolonged idling.

2.

Low grade fuel.

2.

Add proper quality fuel. (not necessary to drain 
and refill tank).

3.

Fuel tank vent restricted.

3.

Clear vent and line.

4.

Fuel return line restricted.

4.

Clear restriction. Replace line if kinked or 
pinched.

5.

Engine problem (low compression excessive 
blowby, worn rings or valve stem seals).

5.

Diagnose and repair as indicated.

Excessive Oil 
Consumption

1.

Prolonged high speed, high load or overload 
operation.

1.

Reduce overload or speed as needed.

2.

Incorrect oil grade used for high ambient 
temperature operation.

2.

Switch to C1-4/CH-4 oil with SAE viscosity rat-
ing of 15W-40, 10W-30.

 Engine Diagnosis

PROBLEM

POTENTIAL CAUSE

CORRECTION