ACR

I'm a long time reader of this forum but a new member ... with a question that I'm hoping this group can help me with.

2001 Silverado 1500 HD 4X4 with 6.0L and just over 160,000 miles.

Great Truck - purchased about a year and half ago for my daughter - who's a barrel racer - so she uses it to pull a horse trailer.

She recently complained that she wasn't able to get it up to 70 mph and that it was just suddenly much worse on gas mileage (as if it could get any worse).

Took it for a test ride and discovered what sounds like a "pulsating" vacuum leak at around 3000 RPMs - as it passes through 50 mph during an acceleration. Also noticed that it was knocking pretty bad.

SES light was blinking. Ran the DTC codes. One was P0101 - problem with the Mass Air Flow Sensor and P0300 - that we were getting misfires on multiple cylinders. Tried cleaning the MAF sensor without success. So replaced the MAF sensor and installed new plugs and wires - but still heard the "sucking" sound at speed.

Had the local AutoZone guys put a fuel pressure gauge on it and found it to be at 50 PSI at idle with no apparent increase when you rev it. Plus, fuel pressure dropped to zero almost instantly when you turned the switch off. Suspected a weak or bad fuel pump.

So, spent an afternoon pulling the bed and installed a new fuel pump. But still no joy. Getting the same "pulsating" "sucking" sound around 3000 RPM.
If I really jam on the acceleration - I can still get the knocking and then the SES light starts to blink. If I clear the codes and drive it gently up to speed I can get it up to about 50 or 55 without the vacuum leak sound or knocking. But, that will never work when trying to merge on to the interstate with a trailer and 1,500 pounds of horse meat. Daughter has a competition next weekend and I'm really in a jam trying to get this solved - so she'll have a way to get her and the horse to an event about 2 hours from our house.

I would really welcome any suggestions or ideas on where I should be focusing my trouble shooting next?

Thanks in advance for anyone that can offer a suggestion.

ACR

I should also add that it idles just fine - and no sound or indication of a vacuum leak at idle.
Also - replacing the fuel pump made a huge difference in starting the truck. It used to take at least 3 or 4 tries to get it to start - now it lights off on the very first try.