Yesterday morning on my drive to work my truck would barely maintain 65 mph. As I pressed the throttle it would bog like it was starving for fuel. I thought the fuel pump may be about to die, but I drove to a local FF place for lunch, same thing. After lunch on the drive back to work it ran better. So when I arrived at work I popped the hood while the engine was running and pulled the rubber elbow to the FPR from the vacuum tree on the intake to check for any fuel in the line, none, so as the elbow was inserted on the tree, I felt a vacuum leak sucking on the tip of my finger as it was pushed onto the barb. Removed it to find a crack in the elbow. Replaced at the parts store that afternoon, problem solved. Sometimes the simplest part can cause major problems

 

Glad you found the solution to your problem. If you have time please post a photo of the part you are referencing. I think I know what you are talking about and many of us have replaced our entire vacuum hose set up for various reasons. The photo is just for any user to might find your post after having similar problems.

 

Great catch. Pretty common issue, I was just looking at the split in mine yesterday.
Did you notice your Idle was a little high after it warmed up?
Did you notice it runing rich at Idle?

 

Cracked Vacuum Elbow

Location
Yes, VJSIMONE I noticed the Idle at cold start lasted a little longer each day, but did not smell gas. I chucked it in the bed, I'll try to find it and post a pic.

 

That's a bit strange,...from my experience, full fuel pressure from lack of vac to the reg runs fine, just poor idle air/fuel. Just for s&g's, i would do an injector leak down test. I wouldn't be surprised if you have injectors that are stuck open which would be exaggerated, causing your symptoms, by a spike in fp. If it tests good and runs good, F it.

 

I ran a fuel pressure test a few months back, and had 42 psi at idle and dropped to 38 at WOT. Prestart pressure never exceeded 32 psi. Pressure after engine shutdown climbed to 54 psi after 30 minutes. So I believe I have a weak pump and/or a clogged sock. Injectors held at least 40 psi for more than an hour.

 

That's good about the injectors, but I think some light is shed on my inclination. Full pressure at the regulator still doesn't build full pressure, so I think you were losing pressure faster with the vac leak, and *that* is how the problem of weak puml was exaggerated.

 

I agree, and I believe that is causing the "Vapor Lock' hot start problem during warm weather. After 15 minutes on a warm engine it won't start and run, wait 30 + minutes it will start and run. I noticed during the leak down test that a lot of bubbles appeared in the clear plastic line to the test gauge as the pressure rose to 54 psi.

 

Ten-cent parts can cause problems but I think more problems are caused by ten-cent minds. Vapor lock too, brains can vapor lock.

 

The 1988 4.9l has a Fuel Injector Cooling Fan to help the problem.

 

Also why the 4.9 has such high fuel pressure, to help the problem of vapor lock. IIRC the early efi models didn't have such high fp

 

Wednesday morning I had some difficulty starting the truck, finally she started and I drove approx 10 miles to work. The engine was sluggish but she made it to work ok, later on lunch break she wouldn't start, I could hear the fuel pump relay click and a couple seconds later click again but no pump whine. Had her towed to the shop I use and today got a quote for dropping and cleaning the tank, new Precision brand pump and reinstall the tank of $270.00. I questioned their choice of pumps, but was told that their supplier warrantied the pump and labor. So I told them to go ahead, waiting for a quote on pulling the injectors, replacing O-rings, and pintle caps.