First I'd like to say hello. This is a great forum.

Here is my problem, I have a 99 quadcab with the 5.9L, all stock 93xxx miles. I am averaging 9.5mpg and the truck just doesn't seem right. It doesn't seem like it has very much power, it has a hickup in the idle that I don't like, and I think I can hear spark knock when I accellerate. All af this sounded like the Plenum was bad. I tried a vacuum check and it read zero vac in the crankcase, so I guess it's not the Plenum gasket(which was already replaced once under warranty). It just had a complete tune-up, new catalytic converter put on, a new o2 sensor, and the TB was cleaned. Is there anything else that I should check out? There are no check engine lights on. This would be so much easier if there was. Thanks in advance.

 

Try a bottle of Seafoam, Check the spark plugs, distributer cap, rotor, spark wires. As for MPG. Dont expect 15 or 16 mpg. With a 5.9 10 mpg sounds about right, if you want good mpgs get a Ford or Chevy. lol

 

i get 13-14 in my 5.9 city...and more than that on highway (283 miles to a half tank 26 gal. tank) Ive put ALOT of money into her though, and she has 196K and im getting that Milage...anyone wants to ride with me ill prove it.

 

1. Catalytic converter
2. Plenum gasket
3. Gas brand itself- less likely than the two issues above.

Number one and number 2 will usually go hand in hand with each other in the fact that the bad plenum will destroy the converter over time.

 

I get gas at Mobil only, and i use Premium fuel

 

Like I stated, the cat was replaced less than a year ago and the plenum checks out ok. I ran seafoam through it a little over a month ago, and also had the top end flushed at valvoline. I run 93 octane gas. I know a few guys with the same truck and they are getting around 14 mpg. That is a huge difference from 9.5. Is there another easy way to check the plenum other than the vacuum test?

 

You can "Check" the plenum all you want, but you will never know if it truly is OK unless you pull the mani apart and see for yourself.

 

Pop the throttle body off, use a camera with good flash, put it in night time mode, then take pics down the bore of the intake. On your computer, you should be able to see in good detail whether there is oil in there. Alternatively, in a much more difficult manner, you can use a mirror and flashlight and try to look down there, or crawl up on the core support with a flashlight. Personally I preferred the camera method lol. The tests from the TSB can be non-conclusive however, and if the gasket was just replaced and not the bolts or the plate, the gasket could very well have been done in and in need of another replacement.

 

Thanks Trey, I will take a pic down the TB. I have not really noticed any oil loss, but I guess it wouldn't take much. I am still confused on why I don't get any vacuum at all when I test it. I thought that there would be a little bit at least.

 

dont forget the death flash!!!!!!!!!!!!!!!!!!!!!!