Vacuum gauge reading help

tyler2136 · October 29

Hi, I have a 2005 Silverado 5.3. This is my vacuum reading right now at idle with the truck up to temperature. The gauge pulsates at around 15 then drop to 13 the back to 15. Would anyone be able to identify what might be going on? I had a lean code that I was able to fix by changing out the spark plugs. The truck still has a misfire at idle but goes away once I accelerate. I bought the truck recently and the guy who had it before me installed an aftermarket cam and never was able to get it tuned. I got it tuned after I bought it but The tuner has recently moved across country so he is no help. Could the cam be causing this? Should I find someone to try and tune it for a better idle loop? TIA

CamGTP · October 29

The misfire is causing the pulsing of the gauge and the fact that it has an aftermarket camshaft.

A longer duration and a different lobe separation angle compared to stock will result in less vacuum at idle compared to a stock camshaft. I bet most stock camshaft would pull close to 18-20in of vacuum.

You still need to fix the misfire at idle though. Not knowing what camshaft it has does not help you at all. Knowing those spec is kinda critical in making sure that you have the correct push rod length so the valves are opening at the right times. Also depending on the size of the camshaft the misfire detection settings in the tune need to be changed so that it doesn't say it's misfiring at idle because of the different cam profile. Also you'd need to make sure that idle timing and idle fueling at correct, there is a lot that goes into it.

tyler2136 · October 29

15 hours ago, CamGTP said:

The misfire is causing the pulsing of the gauge and the fact that it has an aftermarket camshaft.

A longer duration and a different lobe separation angle compared to stock will result in less vacuum at idle compared to a stock camshaft. I bet most stock camshaft would pull close to 18-20in of vacuum.

You still need to fix the misfire at idle though. Not knowing what camshaft it has does not help you at all. Knowing those spec is kinda critical in making sure that you have the correct push rod length so the valves are opening at the right times. Also depending on the size of the camshaft the misfire detection settings in the tune need to be changed so that it doesn't say it's misfiring at idle because of the different cam profile. Also you'd need to make sure that idle timing and idle fueling at correct, there is a lot that goes into it.

I have all of the information on the cam, valves, and rods that were put in. The misfire at idle really started after the tune was done. Could it be just a crappy tune and I just need to give all the cam info to someone to get it re tuned?

CamGTP · October 30

I guess without seeing the tune for myself and the data logs to go along with it, it's hard to say how the tune was done. It's possible the person who did the tuning just didn't go a good enough job.

Some camshafts depending on size do require quite a bit of work to dial in and can't just be done in a 1-2 hour tuning session.