wastegate or vacuum pump

[82beast]

All right this problem is on my dad's 96 suburban,mods are K&N, 4inch exhaust, GM-8, complete cooling mod. Ok first a little back story, on a trip to Flordia while towing a ~3500lbs they averaged 16mpg over 2300 miles. About a month ago they made they same trip trailer wieght and all and they only averaged 11.5 mpg over the same 2300 miles. The only thing that changed was the adding of the high flow water pump and dual stats,truck had kennedy clutch with 9-blade fan for both trips. My first thought was to test for vacuum and boost, I put the gauge on the line that enters the wastegate, test drove it got nothing so bad vac pump, right. Now wanting to be sure of my self I take the boost gauge off my chevy put it on take a teast drive. What I found suggested a bad vac pump, only made 3lbs at wot, shut it off for around 5 min. When I start driving again I got boost :confused: ,8lbs spike 5lbs at cruise(65) and when I retested for vacuum I had 18 inchs. Just when I think I found the problem it does this to me. :mad: Anybody with any ideas thank you.

[DChristie]

Sounds like a failing solenoid b/c of where you put the vac gauge. Check the vac again at the pump.
Solenoids: There are three of 'em in a row on the Drivers side rear of the engine mounted on top of the valve cover. I'm not sure how to test for which one is toast. redface.gif I haven't tried to understand which controls what. Apathy. :rolleyes:
You should get 18" or so at idle, after that check for leaks etc. But from what you've described I'd bet on solenoid.

Hope this helps. :D
DChristie

[82beast]

thanks DC, seems likely thats the problem.
I will check the vac again and I think just change out the solenoid as they are cheap enough. I guess because you have a 1500 you have more then one solenoid as my dads only has the one. Thanks again :cool:

[charliepeterson]

Sometimes the plastic vacuum harness can crack.