7.4L Vortec hard start, stumble - A TEST
Hi,
I am posting this thread for those of us with 7.4L Vortecs who experience hard starting / long cranking time, often accompanied by an off idle stumble. I have a success story on my 97 Winnebago Warrior / P30 chassis with 7.4L Vortec. My issues were the above, also my rig would go into "limp mode" when under stress such as going up steep grades. I would sometimes get lean condition MIL codes.
The culprit in my instance was a faulty fuel pressure dampener, AKA fuel pump pulsator. This component, the "shiny" one shown in my photo as it is seen attached to the fuel pump motor itself, is located in the fuel sending unit assembly, inside the fuel tank. It sits directly above the fuel pump motor. Many vehicles do not even have one of these; evidently it serves as some sort of over pressure relief valve on the 7.4L Vortecs. It is not a check valve - the fuel pump motor itself does this - but if faulty will drastically affect the driveability of your Vortec!
Here is a simple means of testing this component if you have these symptoms and are going to check fuel pressure, etc. It will help you decide whether you have a bad f.p. regulator (often blamed for this), a bad fuel pump, or the possibility of a bad fuel pump pressure dampener.
First, you need a fuel pressure tester capable of 60 psi or greater. I bought one at an online auto tools store ($29), or you can often "borrow" them from a local auto parts store. Hook it up to the test port located at the lower intake manifold level, just in front of where the upper "Vortec" manifold ends, just aft of the upper radiator hose / thermostat housing. Start the engine. You should have 50 psi idling (I had 35 psi). These things will run with lower pressure, but once too low, you start experiencing lean conditions / "limp mode". When accelerated, you should see the pressure increase to about 60 psi (I had no increase from 35psi). No increase shows a bleed off, which can be from either the pressure side, as I will explain, or from the return side (fuel pressure regulator), depending on the outcome of this test. Turn the key off and observe... If you see a slight drop in pressure, then it holds pressure at say, 45psi: This is normal. If you experience a drop to zero pressure over the course of a short time (in my case, dropped to zero after several seconds), there is a way to test what is causing the drop.
You'll need a pair of "needle nosed" vise - grip pliers. Locate where the fuel pressure and return lines exit the back of the engine. They are just left of the distributor if you are looking at it from the rear. Then follow the lines down until they turn into rubber lines. At that point, start the engine and observe the pressure gauge while pinching closed one of the lines. If the engine begins to die and you see the pressure drop, you have closed the pressure line. Release the pliers and go to the other line. Observe what happens when you close the line - carefully! If the pressure jumps, release the pliers quickly. That shows that the f.p. regulator is ok and pressure is now building against the f.p. dampener. That would be normal. If pressure does not raise, leave the pliers on, then shut off the engine with the line still closed. If the pressure drops to zero very quickly, you have verified that the f.p. regulator is not the culprit - the pressure is bleeding back into the pump! An additional test is to go back to the pressure line and close it at the same time as shutting off the engine - if the pressure stays, you've confirmed the leakdown is coming from the pressure side.
If you've confirmed a pressure side leak, sorry, time to pull the fuel tank and get that fuel pump / sending unit assembly out. The fuel pump motor and fuel pressure dampener are readily visible and laid out per my photo. Once I had my assembly removed from the tank, I literally just blew into the pressure line attach point as hard as I could, and immediately noted that there was a leak. I removed the fuel pump motor and blew into it. No leak. I then capped the open end of the fuel pressure dampener with my finger and blew into the pressure line port again. The leak was coming from the dampener itself. I replaced the dampener and the fuel pump motor with new dealership bought parts and reassembled things. Re- testing showed: 50 psi idling, 60 psi accelerating. shutoff: settles to 45psi within a few seconds, and stays there. I did not re -test via the pliers. Runs perfect. Starts instantly, all the time.
This diagnosis / repair has been very perplexing, and I hope this post does more good than bad, confusion wise. I believe it to be a fairly non -destructive way to confirm whether or not you need to take the upper plenum apart and do a f.p. regulator change, or if you're going to be pulling that gas tank....
Thanks, John
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