I have searched the threads and cannot locate my exact symptoms. 96 4.9l, sometimes hard or impossible to start. When it does start, it runs horrible for a few seconds and then smooths out. Replaced nearly all ignition components, sensors, etc. Now I'm thinking its a fuel delivery issue. Pulling two lean codes, 0171 and 0174. The unique thing is, when driving and I switch fuel tanks, a second or so after I flip the switch, the truck stumbles and then runs normal. One time I switched tanks and it started a terrible misfire event. I've pulled the vacuum line from the fuel pressure regulator and plugged the line, no fuel in the line and no change in RPM or idle smoothness. I don't have a fuel pressure gauge and wouldn't know what I was reading if I did. Anyone else with this issue? Fuel pumps bad? FPR? Thanks in advance.

 

You can "borrow" a fuel pressure gauge from most parts stores. You are looking for around 40 psi, it should jump 5 to 10 when the vacuum is off the FPR.

Lean codes are usually vacuum leaks.

Next time it doesn't start right away, try flooring the pedal, this will shut the injectors off. You may have leaky injectors and it's effectively flooded.

 

What would account for the stumble when switching tanks?

 

Air in the line between the pump you just switched to and the selector valve.

 

lean codes can also be caused by low fuel pressure. The engine stumble when you switch is probably from bad fuel pump assembly, one or the other or both with failing return fuel check valves. some times the pressure is fine in the pump, but the return check valves quit working properly, if it is a dual tank setup that will cause fuel transfer between tanks, usually rear to front. I have experienced those stumbles. when you do get be around to the fuel pressure test be sure to do a pressure leak down test to see how fast the fuel pressure drops after engine/key is turned off. It should actually hold good pressure for quite a while. With the engine running tou should have around 45 PSI on each pump, and when the throttle is applied it should increase to 50-55 PSI. Also cycle the key on and off three or four times and see if that makes it start better. If it does that would also indicate the fuel pump check valves are bad.

 

Did a pressure test. Key on engine off, pump runs for a second or so as it should - 0psi front, 10psi rear. Started engine, stumbles and sputters but eventually started. 38psi front, 35psi rear. Key off static pressure 44psi with about 2psi leak down per minute. Removed vacuum line to FPR while running and fuel pressure rises to 43psi. Truck experiences the same difficulty starting on either tank equally. Cycling the key repeatedly to "prime" seems to help one time and not another. As long as i restart engine immediately after shut down, runs fine. Let it sit, sputters and stumbles and may or may not start. Fuel filter look like its been changed recently.

Whats the chances of BOTH tank pumps failing in the same way at the same time? I haven't noticed cross filling of the tanks but I haven't driven it far due to its running poorly.

 

ALSO, I'm assuming I would need to be running from the rear tank to cause cross fill into the front and vice versa?

 

No, it will siphon transfer just sitting from the rear to the front if the front tank check valve is bad. I can't guarantee it will happen every time, but used to happen on mine before finally getting it fixed. if you do change the in tank pumps go for OEM pumps. I bought a set of after market pumps and they only temporarily stopped the problem. The return fuel check valve is in the pump and it is opened by pressure from the running pump, so it doesn't matter which tank is selected a bad check valve leaks back running or powered down. My truck is also a 1996 4.9 XL standard shift 2WD

 

OK, I appreciate it! I sure hope this fixes it. I've been throwing $$ at ignition parts, not that updated all of those wasn't a bad idea.

 

Is it possible to replace 1 pump at a time and keep the fuel from back flowing into the other tank, like, bypass the second tank for a bit?

 

I don't know how you would do it. The fuel line system : the fuel supply line from each pump Y's together then goes through the fuel filter (which should be changed before you get too far into the process) then proceeds to the fuel injector rail and fuel pressure regulator. the injectors use what fuel they can and excess pressure is returned to where the line Y's and goes to each tank where it either enters the tank, or is blocked by the return check valve in the pump. The whole apparatus is more or less an assembly from the fuel filter to the back of the fuel rail. Bottom line, I can't figure any way to block one off. Just try to keep each tank at the half full mark while you change one or the other.

 

Additional info. Today I rode with ac and radio off. Noticed a pronounced ticking same rate as engine rpm. Doesn't do it in park, even when rpm is increased. Only in gear even if truck isn't moving (power braking). Could this be something that could cause my issues starting?

 

I don't know anything about the automatic transmission in the 96, but I'm sure it's functions are controlled by the PCM (powertrain control module), that said, I can't imagine what would cause it tick when in gear, and not in park. Does it do it in the neutral position? Unless you know what is causing it, there is no way to determine if it could cause starting issues. You might take a piece of flexible tubing, one end stuck in your ear, and moving the open end over, under, around and through things, using it like a stethoscope and find where the sound is coming from.