A bad fuel pump, that has stopped sending fuel to the fuel injectors, will cause a ‘cranks but does not start condition’ on your 4.0L equipped Jeep.
A fuel pump that is working, but not sending enough volume and therefore not producing enough fuel pressure, will cause severe drive-ability issues.
The cool thing is that it's easier than ‘a hot knife cutting through butter’ to test the fuel pump's pressure and/or check a lack of fuel condition causing a no start condition.
In this tutorial I'll show you the 2 basic tests you'll need to do to check the condition of your Jeep's fuel pump.
Contents of this tutorial at a quick glance:
- Symptoms of a Bad Fuel Pump.
- TEST 1: Using Starting Fluid To Confirm Lack of Fuel.
- TEST 2: Checking Fuel Pressure With a Fuel Pressure Gauge.
- Where to Buy a Fuel Pressure Test Gauge.
- Where to Buy the Fuel Pump and Save.
- More 4.0L Jeep Diagnostic Tutorials.
You can find this tutorial in Spanish here: Cómo Probar la Bomba de Combustible (4.0L Jeep) (at: autotecnico-online.com).
The fuel pump, in your 4.0L equipped Jeep, usually fails in one of two ways. It'll either stop pumping fuel or it will pump a volume that's lower than normal.
Obviously, when the fuel pump completely fails... your 4.0L Jeep isn't gonna' start. In this type of scenario, you'll see that:
- The engine turns over but will not start.
- The ignition coil pack is creating and distributing spark to all 6 cylinders.
- The PCM will still pulse (activate) all 6 fuel injectors.
But when the fuel pump sends a lower than normal volume of fuel, your Jeep will start and run but with engine performance problems. You'll probably see one or more of the following symptoms:
- Rough idle.
- Engine starts after extended cranking.
- Lack of power when accelerating the vehicle down the road.
- Back-fires thru' the intake manifold when accelerating your Jeep down the road.
Both of these conditions can be tested with a fuel pressure gauge. Alright, with this info under our belts, let's get testing.
The ‘quick and dirty’ way to check to see if a lack of fuel is causing your 4.0L Jeep to no start is using starting fluid (although it's not the most accurate way of diagnosing a bad fuel pump).
The rule of thumb is that if by spraying starting fluid your Jeep starts, even if momentarily... then this tells you that fuel is missing from the equation (engine start equation: fuel+spark+air=combustion).
NOTE: To get an accurate test result from the starting fluid test, you need to first confirm that the ignition coil pack is creating and delivering spark to all 6 cylinders. You can easily accomplish this by attaching a spark tester to the spark plug wires and having a helper crank the engine (while you observe to see if the spark tester sparks).
IMPORTANT: This is a very fast and easy test but you do have to take one very important safety precaution and this is to reconnect the air intake duct after spraying starting fluid down the throttle bore (although you don't have to fasten it). This will prevent any backfire, that might occur, from scaring the heck out of you when cranking the engine.
This is what you have to do:
Remove the intake air duct from the throttle body. You don't have to completely remove it, since you'll have to reconnect it in one of the next steps.
Open the throttle plate and spray starting fluid down the bore.
As a safety precaution reconnect the air duct after you have sprayed a good squirt of starting fluid (but you don't have to tighten the air duct's hose clamp).
Crank the engine once the air duct is back on and you're clear of the engine compartment.
You'll get one of two results with this test:
1.) The engine will start momentarily and after a few seconds will die or.
2.) The engine will only crank but not start at all.
OK, let's find out what your results mean:
CASE 1: If the engine started and ran for a few seconds: This test result tells you that the no start problem is due to a lack of fuel.
Your next step is to check to see what the fuel pressure is with a fuel pressure test gauge. Go to: TEST 2: Checking Fuel Pressure With a Fuel Pressure Gauge.
CASE 2: The engine did not start, not even momentarily: This usually means that a lack of fuel IS NOT the reason your car is not starting.
Now, remember what I said about this test not being very accurate? Well, I suggest you do one more test and this is to check the fuel pressure with a fuel pressure test gauge. Go to: TEST 2: Checking Fuel Pressure With a Fuel Pressure Gauge.