99 5.9 with 188000 miles. Doesn't pull as well as it should in fifth gear, getting poor fuel mileage, an idles rough on cold starts for 20 seconds or so. Have already changed the fuel filter. Any ideas??

 

Clean air filter? Check for codes? Have you done an APPS reset? What is your FP looking like?

APPS Reset Procedure

Answer:This procedure has shown to help 24 valve ISB engines. It is basically teaching the fly by wire accelerator on these trucks to reread the position of the pedal and it's limits. Most complaints this will help with is poor acceleration, improper shifting, bucking in 5 and 6 speeds. Has been known to increase fuel mileage in some instances.

1. Set parking brake
2. Disconnect negative battery cables on both batteries
3. Turn ignition key to run(to drain any remaining capacitors)
4. Let truck sit for 30 minutes( this is the recommended minimum)
5. Return ignition to OFF position
6. Reconnect both batteries and secure, not a timed event
7. Turn key to run position(not start) and depress accelerator pedal slowly all the way to the floor
8. Allow pedal to return to top position slowly
9. Turn ignition switch OFF
10.Release parking brake

 

New air filter, no codes yet, haven't heard of the apps test. Will try that tomorrow. Are these symptoms of bad injectors?

 

It could be, do you know what your fuel pressure is doing?

 

When I changed the fuel filter the lift pump seemed to prime well. Haven't checked the actual pressure but am going to put a low pressure light on the fuel system. I've read that pump problems usually cause the engine to cut out under load. I haven't had that problem.

 

Well I suggest the APPS reset for now, then report back when you can and we will go from there.

 

Scan with a good scanner, not key trick

 

Thanks

 

The key trick won't work on a 99. Not ragging on you Shayne just informing....

 

could be injectors too when they are going bad they will have poor economy could be a vp issue but would suggest a fuel system upgrade no matter what.