Inaccurate AFTERMARKET boost gauge??

[Black Knight]

I have a Autometer boost guage I bought for the Sy a couple months ago and only used it once in while to check it out, when I hooked it up the first couple times it registered everything as normal. At idle I would have about 22 hg of vacuum and boost would run solid up to 15 psi. I got bored the other day and decided to install it in the truck permanently, since then it idles with only about 2-5 hg and sometimes under full boost it will peg the gauge at 20 psi and sometimes only go as high as 10 psi, the truck seems to run exactly the same as usual. I thought the gauge might be defective so I took it back to where I purchased it and got a new one which I have since installed and it reads exactly the same.

I have checked for vacuum leaks and found none, double checked the fittings on the gauge and the intake plenum, its T into the plenum with a brass setup similiar to the ones you see forsale here. Any help or thoughts would be appreciated, nothing I seem to do has any affect on the gauge.

 

[Flyin Ryan]

Re: Inaccurate AFTERMARKET boost gauge??

Did you try the bike tire trick to make sure that you don't have any vac leaks? how bout leakage thru the throttlebody?

I had a bad autometer gauge before.. brand new lunar boost gauge which I installed in my mustang and right off the bat it was reading 2 psi more then it should have but it never changed it was always two lbs too much so I eventually uninstalled it and returned it for a new one so I'd have to say from my one experience that i'd have to rule the guage out and look else where.

 

[QUICK STORM]

Re: Inaccurate AFTERMARKET boost gauge??

make sure that you didn't "T" into a vac line with a check valve

 

[Black Knight]

Re: Inaccurate AFTERMARKET boost gauge??

Did you try the bike tire trick to make sure that you don't have any vac leaks? how bout leakage thru the throttlebody?

I haven't yet, but I might have to. I just checked all my lines which I replaced about 8-9 months ago and they all were hooked up and look fine, might have to do them again.

make sure that you didn't "T" into a vac line with a check valve

No I "T" d into the plenum before the brake boost hose.
Thanx guys

 

[SY2932]

Re: Inaccurate AFTERMARKET boost gauge??

First, I would check the boost gauge against a KNOWN value. That will tell you right away where the problem lies. I have a Mityvac that I use for this purpose. Works excellent for checking both vacuum and pressure.

You mentioned hooking up the boost gauge once in awhile to "check it out". How did you plug the port on the brass fitting when you didn't have the boost gauge installed on the truck? Check the inside of the brass fitting that the line on the boost gauge connects to and see if it is messed up or not. If it is messed up, the compression sleeve on the line that goes to the boost gauge won't seal properly.

 

[Black Knight]

Re: Inaccurate AFTERMARKET boost gauge??

Thanx Mike i'll have to check into getting one of those. When I plugged the fitting I just put a little rubber cover over the end and put a small zip tie on it to keep it from leaking, i'll check it but I cant imagine it getting damaged from that.

I ran up to the store real quick tonight and noticed that at idle now its at 8 hg and at full boost its right at 15 psi, I just don't understand why it wont register much vacuum. Even the stock gauge registers more than that. From all this I would have to figure the problem lies with the gauge setup, because if I only had 2 or 5 hg of vacuum at idle the truck would idle pretty high, right??

 

[Black Knight]

Re: Inaccurate AFTERMARKET boost gauge??

After about 2 hours of checking vacuum and pressure testing, checking hoses, lines, fitting everything, turns out they gave me the wrong lights with the gauge. The 161 light that came with it hits a small piece on the inside and messes it up. With the truck running you can take the light out and it instantly drops to 22.5hg, put it back in and boom 8hg. So I swapped the light for a 194 which is like a 1/32 shorter and problem solved. Talk about a PIA, Thanx everyone for the help and ideas!!!