Loss of Engine Power/Chugging
 

Hey guys,

I just picked up my first truck - a '93 F150 XLT with the 4.9L I6 and 4 speed manual heavy duty tranny. So far I've loved it, but today started having some issues I hoped I could find some advice on.

I got to head out for my first long trip with the truck - a 2 hour drive up into the mountains. On the way out, I was cruising down the road, easily maintaining 65mph. Once I hit the steeper grade in the mountains, however, I really started losing power - more than I expected. At points, I wasn't even able to maintain 35mph. Although I was having much more trouble than I expected, I just attributed it to the grade. I really started to get worried later, however, when on the same roads I was cruising at 65 earlier in the day, I was struggling to stay at 55. Also, definitely started to get some strong chugging. Tonight, as my wife and I got close to home, the CEL even came in momentarily (for about 30 seconds).

I have two questions:

1. Am I forgetting anything?
   Obviously my first thought was fuel/air/ignition stuff, so tomorrow I plan to just replace:
   • Air Filter
   • Fuel Filter
   • All Spark Plugs
   • All Spark Plug Cables
   • Distributer Cap/Rotor
   I also plan to run a good fuel injector cleaner through the gas tank, and possibly replace the valve cover gasket.
   
   
2. What should my expectations be?
   I know the I6 is a power engine, not a speed engine. Still, I guess I expected that I'd at least be able to maintain highway speeds to 60mph. Is this a bad assumption?

Thanks in advance for the help guys. I'm really looking forward to getting this truck in tip top shape.

BW


https://cimg8.ibsrv.net/gimg/www.f15...3c06494f1e.jpg


My new rig...

Nice truck. First thing to check is the fuel pressure - with a gauge.
Change the fuel filter first thing (before you run injector cleaner is best).
Could be a spark issue, but you want to identify or eliminate fuel pressure as the problem before you start throwing any parts at it.

Chris,

Thanks for the reply. I did put a pressure gauge on the fuel line and got a reading of 47psi. That seemed pretty good, but didn't know what the spec was. I think I read somewhere today that high spec is 45. Also, after shutdown, it bled very slowly. I'm wondering if it's something in the return line. Seems like a clogged fuel filter wouldn't let pressure get that high.

Do you happen to know if those fuel lines just pull off the filter, or do I need a special tool?

Thanks for the help. I was a mechanic in the Navy, but worked on steam plants and reactors. This whole combustion thing is something I'm trying to figure out.

You need special tool to release the lines, you can get them at any parts store, just tell them what you need.

93 was a transition year for 4.9 fuel pressure. It was about then that they had problems with blown check valves and they changed the system over to run at 30 to 45 psi like all the V8's.
But prior to that change 4.9's required 45 to 60 psi so at 47 psi with symptoms that sound very much like low fuel pressure, I'd take a semi educated guess that you have the high pressure system and your pressure is falling below minimum.
Several things can cause that, first obviously a weak pump or clogged filter; in a dual tank system a failed check valve in the pump not being used can allow pressure to bleed off into the tank you're not using (also creating a cross filling problem).
But the system should maintain it's pressure for at least several minutes after you shut the truck down.
I would suspect the fuel pressure regulator Either not holding any pressure back from the return line or if the diaphragm inside is leaking it could be dumping fuel through the vacuum line into the intake. Check that vac line for fuel smell, it shouldn't have any.
The other test for the regulator is to (gently) pinch off the return line and see what that does for your pressure and after shutdown holding power.
The other places pressure can bleed off is of course through a dirty injector or a faulty pump check valve.

Thanks Chris_1. That's really helpful. I picked up a pressure regulator today. I now have a starting problem I need to figure out, but I'll post that in a new thread. Again, thanks for the help

I had a car once that acted like that, and it was a clogged catalytic converter. I don't know how to check for that. Maybe some kind of vacuum check.