Weird Vacuum readings

[andrewjscott]

my car is stock, usually i'm reading about 15-16 in Hg after its all up to temperature and at idle. If i shut my car off for a couple of mins, turn it back on, all of a sudden i read 19-21 in Hg for a few minutes, after i drive it for a while, it goes back to 15-16 in Hg at idle. If i shut my car off for even a few seconds, and turn it back on i get this kind of reading. Could something be wrong with my boost guage? or any other ideas?

 

[Defiant]

How used-up is the car? You might be heatsoaking something -such as the pistons- that are sealing better when hot. Your normal idle vac is about 3psi or so on the low side.

 

[andrewjscott]

had a rebuild less than 20k ago. my compression is all good. I think i might have a boost leak somewhere, probably somewhere in the welds for my custom UICP but i don't think its a big one. It doesn't seem like temperature has anything to do with it. I'll be at idle around 16 in Hg...shut the car off for litterally 5 seconds, start the car up again, and i'll have around 20 in hg for about a minute.

 

[jtmcinder]

Could it be that the emission systems doesn't activate until after a minute and that there's a leak in that system?

- Jtoby

 

[CarTrader29]

What kind of boost gauge do you have?

 

[andrewjscott]

autometer sport comp

 

[CarTrader29]

how long did you have it for? Also what type of boost gauge line are you using? Rubber or the hard nylon hose?

 

[andrewjscott]

had it for about a month, read 15/16 for about 2 weeks now its doing this for about 2 weeks. I am using the hard nylon hose and its taped into the fuel pressure solenoid.

 

[CarTrader29]

I would check the nylon hose. I used to have a problem at the ends of the hoses but I used a piece of rubber hose and silicone to solve that problem. Then the other day I had a small bend in it which later turned into a leak. I was have the same problem as you. Found the small bend and tapped it up(i know its ghetto) and now its fine again. In the summer I plan on replacing everything with a rubber hose and barb fitting.

 

[andrewjscott]

i'll definatly take a look, i kinked the nylon tube pretty bad and had to replace the section with a capped T fitting. Does it matter what kind of rubber hose you use? I also don't get how this would be related to shutting my car off and having a different reading.

 

[CarTrader29]

i dont know as well but mine did the same thing untill it read less and less vaccum. A kink will eventually become a rip in the hose. see what happens. Hope this helps.

 

[Quasimondo]

had it for about a month, read 15/16 for about 2 weeks now its doing this for about 2 weeks. I am using the hard nylon hose and its taped into the fuel pressure solenoid.

Try tapping from another location. The fuel pressure solenoid stays closed on startup for about a minute or so so that there's enough fuel pressure to crank the engine. Try teeing it off at one of the EGR vacum lines.

 

[Defiant]

I think i might have a boost leak somewhere, probably somewhere in the welds for my custom UICP but i don't think its a big one.

Vacuum only exists between the throttle plate and the valves. Boost is from the output of the turbo to the valves.

 

[andrewjscott]

Turns out it was a split in the vacuum hose to the FPR. Now i'm running 18inHg at idle and 25 on decel.