InDirect Injected Diesel Page


6.9/7.3 IDI/7.3 Turbo IDI tips

This page is for those of you who own or work on the older Ford/Navistar diesel engines. Hopefully the information posted here will be of some use to you.
Vehicle service information can be found at my Service Page, and much of what is posted at my PowerStroke Page can be applied to these engines as well.
Click here if you're looking for IDI Parts.

The most common problems with the old IDI engines is hard/no start or stall after starting, so that will comprise the bulk of the tips section.
In the case of a glow plug system problem, plugging the block heater in for a couple of hours usually will get the engine warm enough to start.
A word of caution about ether or starting fluids: a hot glow plug can ignite starting fluid before you have a chance to crank over the engine and will cause internal damage.

Jump to Diagnosis

How the fuel injection system works
These engines use what is called a hydraulic injection system. Fuel is delivered to the injection pump from the tank(s) by a mechanical lift pump. The injection pump contains a governor which contols the amount of fuel delivered to the injectors. The accelerator linkage basically changes the minimum governed engine speed (RPM). If the engine speed decreases for a given throttle setting, the governor increases the amount of fuel to maintain the desired RPM. The injection pump also contains a transfer pump which increases the fuel pressure inside the hydraulic head to 60-120 PSI. Excess fuel passes through a bleed into the governer housing where it is used to lubricate and cool the mechanical portion of the injection pump before returning to the fuel tank.
The injection pump hydraulic head not only distributes the fuel to each injector, but also opens the injectors. It does this by pressurizing the fuel in the injector lines. From the transfer pump the fuel is sent to a plunger-type intensifier pump which increases the fuel pressure further and sends it through the shaft and distributor rotor to each individual injector line, depending on shaft position.
When the fuel pressure in the injectors gets above 1400 psi (new injectors typically open at 1700 psi), it overcomes the spring inside the injector and the valve (pintel) rises off its seat allowing the pressurized fuel to spray from the nozzel. As the fuel is injected into the cylinder, the fuel pressure drops and the internal spring re-seates the pintel.
Air inside the fuel lines compresses and this is what causes so many performance and no-start problems. Hydraulic pressure can't build, so the injector won't open.

Changing the fuel filter
When ever I replace a fuel filter on a diesel, I try to do it with the engine already warmed up to aid in starting and to ensure that oil is distributed throughout the engine. If the Fuel filter header can be removed from the bracket, I do this because it reduces the amount of fuel spilled on the engine. Changine the filter is just like changing an oil filter--you lube the new gasket(s) with clean engine oil and hand-tighten only. Before installing the filter on the header, I refill it to make starting easier. Use clean diesel fuel, but if you don't have any on hand, you can save the fuel dumped from the old filter in a clean container and strain it before putting it back into the new filter. If I don't have enough fuel to fill the filter, I top it off with some ATF. Since the engine has been warmed up, it will start, but it is going to stall. Sometimes you can keep it running long enough by "feathering" the accelerator pedal to purge the air from the system. But if it stalls, disable the glow plug system (to keep from burning out the plugs) by disconnecting the relay ignition wire, run a jumper from any positive battery source to the cold start solenoid and try to purge the air by cranking the engine. Only allow the engine to crank for 30 seconds or starter damage can occur. Sometimes it may be necessary to loosen some injector lines and hook up a battery charger if excessive air has gotten into the system. As you crank the engine, if it sounds like it is trying to start, tighten the injector lines.

Diagnosis
There are basic things that need to be checked: power to the glow plugs; fuel delviery to the injection pump; air, exhaust and fuel return restriction; battery condition and starter draw/speed; compression; fuel quality/contamination or air intrusion; and fuel delivery to the injectors. With the exception of the glow plug circuit and batteries/starter system, these things are also going to affect performance as well.
Unfortunatly, tests for the fuel system assumes the engine will start.

Starter and Batteries
The first thing you should check for on a no- or hard-start concern is the batteries. Clean the terminals and check each battery individually (one negitive cable disconnected). They both should pass a 15 second load test of half the rated cold cranking amps (voltage drop no lower than 9.6 volts). Starter draw should be no higher than 500 amps with both batteries connected and engine RMP should be no lower than 200 RPM.
If the cranking speed is too low, the engine won't produce enough compression to ignite the fuel.

Glow plug circuit
Check each of the glow plug resistance from its terminal to ground with an ohmmeter--the glow plugs should be less than 2 ohms when cold. Another way to test the glow plugs is with a test light (not a LED/circuit tester). Connect the alligator clip to the positive battery terminal and touch the probe to each (unplugged) glow plug terminal. The test lamp will light brightly if the glow plug is good.
Even one open glow plug can shorten the controller on time sufficiently to affect starting.
Next connect the glow plug harness and check for voltage at the relay "hot" terminal with the key off, then check for a voltage drop at the glow plug terminals (86 and earlier) or at the controller/relay "hot" terminal (87 and up) with the relay energized.
If the voltage is low or drops below 10 volts with the relay energized, check for loose connections at the glow plug relay, starter relay or engine harness connector, or bad fuse link(s). Also check for voltage at each glow plug terminal with the relay energized.

Glow plug circuit diagrams:
Early 83
84-86
87-94

Injection pump solenoids
The only other electrical check is to make sure that there is power reaching the injection pump fuel solenoid and the cold timing advance/high idle solenoids. If the engine is cold and the high idle solenoid activates with the key on, it's good. If you have no high idle, check for voltage at both terminals of the cold start switch just behind the thermostat housing.
The cold advance solenoid also aids in purging air from the injection pump.

Fuel system
Next check for fuel delivery through the fuel filter. On 6.9 engines this is checked at the 3/16" hose barb on the fuel filter outlet elbow, and on 7.3 at the fuel filter orifice/return line fitting or restricted filter sensor port. If checking either at the filter return line fitting, plug the return hose. Fuel pressure here should be 1 psi minimum at idle. Lift pump pressure is taken at the fuel filter bleed schrader port and should be 2 psi minimum at idle. Fuel volume is taken at the same point and should be at least one pint in 30 seconds at idle. Inspect the fuel taken during the volume test for signs of water or contamination.
If the engine won't start, these specs may be lower as idle speed is 675 RPM and cranking speed will be less.

Fuel supply and return line restriction is normally tested with the engine at 3300 RPM. For supply restriction a vacuum gauge is tee'd into the lift pump inlet hose and should read less than 6 in/Hg at 3300 RPM. For a return line restriction a pressure gauge is tee'd into the hose running from the rear of the engine. This should read less than 2 psi at 3300 RPM. But a return line restriction can also be detected by installing a clear hose in place of the return line hose at the fuel filter. Watch the flow of fuel while cranking and if it flows towards the filter, inspect the fuel return circuit.

Installing a clear line at the filter return can also be used to detect air intrusion into the fuel supply system if this is suspected, although at this point the symptom would most likely be a stall after starting followed by a hard start, in which case by changing the location of the clear line you can pin point the area of the air leak. If air is getting into the injection pump, this would cause hard start concern. A clear line can be installed at the injection pump outlet to detect this.
If the concern were a rough idle, the clear lines can be used to detect aeration of the fuel. Install the lines and start the engine. Run the engine at 3000 RMP for 2 minutes, then watch the clear lines for signs of air. Some bubbles less than 1/16" is normal, but if the bubbles are larger, constant, or if the fuel appears to foam, there is air being pulled into the supply system.

To check the transfer pump pressure inside the injection pump, a special adapter which replaces the transfer pump cover lock screw and attaches to a 160 psi gauge (the cover lock block has to be in place). Transfer pump pressure is taken at 3300 RPM and should be 90-110 psi for 6.9; 90-120 psi for 7.3.
If the engine won't start, all other checks are normal, and there is no air inside the injection pump, chances are the transfer pump is not producing enough pressure to start the engine. This can be checked by opening one or more injector lines while cranking. If no fuel is reaching the injectors, suspect an internal injection pump problem.

Compression test
To perform a compression test on this engine, you need to have a diesel compression gauge and a glow plug adapter. Remove all the glow plugs to check any cylinder. Crank the engine at least five revolutions and the same number of revolutions for each cylinder. It would probably be a good idea to have a battery charger hooked up during the test. Ford lists no compression specs for these engine, only stating the lowest cylinder should be no more than 25% lower than the highest cylinder, with a low to high comparison chart showing a range of 195 to 440 PSI. In reality the compression should be at least 350 psi.

Timing these engines depends on what equipment you have. Luminosity probe timing gauges depend on fuel cetane level, engine and ambient temperature and can read as much as 10 degrees retarded from the actual injection time.

Cylinder missfire
To detect a missfire, you could slowly loosen each injector line to see if the way the engine is running changes, but an easier method is to disconnect the glow plugs and measure their resistance with the engine running (the glow plugs have to be in good condition). As the engine gets hot, glow plug resistance increases. A cylinder that isn't firing will be colder than one that is. After starting, glow plug resistance usually rises to 15-20 ohms on a operating cylinder, but stays below 10 ohms on a weak one--even lower if the cylinder is dead. If you own an infrared temperature gun, you can detect the missfiring cylinder by measuring exhaust port temp. If the miss cannot be isolated to any cylinder(s), then there may be an injection pump or fuel quality problem.

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