low boost

whitechicken

Registered User
I was wondering if somebody can tell me why i'm running about 6-8 psi. less then two months ago it was around 10-11 at wot. Might my problem be in the gauge? my car is a 90 auto and for now she is completely stock. thanks guys
 
Get a can of carb cleaner and spray around all vacuum connections while the car is idling. Areas like ic tubes, vacuum lines, sc top, inlet and return plenum connections and vacuum lines. If the idle jumps up, that indicates a leak, which is a common culprit of loss of boost.

Make sure your sc belt is tight and not slipping as well.

Here's a thead about checking for vacumm leaks a different way.
http://www.sccoa.com/forums/showthread.php?t=81529
 
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Bypass valve may not be closing all the way, or the belts may be slipping. If you had a boost leak that large, you would have an equally large vacuum leak and the car probably wouldn't even stay running at idle.

David
 
David, I would agree with that to a point. It would have to be a big leak to lose that much boost. Although, when I bout my 91, I was only getting about 9 psi. according to the stock gauge and when the headgaskets blew and I put everything back like it was, but made sure the sc top and ic tubes were sealed good and I gained about 2 psi. just from that.
 
I have to agree with David here. Large leaks also affect idle. If the bypass valve is not working correctly you can easily lose 4lb of boost. I know I did. Add to that one loose bolt somewhere in the link and that could easily account for another pound or so which would only show up under throttle.

I think it is very possible.

Ira
 
RadiumSC said:
David, I would agree with that to a point. It would have to be a big leak to lose that much boost. Although, when I bout my 91, I was only getting about 9 psi. according to the stock gauge and when the headgaskets blew and I put everything back like it was, but made sure the sc top and ic tubes were sealed good and I gained about 2 psi. just from that.

David,

I agree that leaks can account for a loss in boost, and if he had only lost a pound or 2, I would also look for leaks first. But to loose 3-4 pounds and not mention any any other problems, sounds more like a bypass valve or belt problem.

David
 
the bypass valve

thats funny that is brought up. i bought this car for a good deal because the rod on the by pass had bent and messed up the diaphram or what ever was on the inside. I have an sc that had been rear ended so i took the good one off of there and replaced it. it just has me baffled because i had more boost not long ago and i did a quick check for a vaccum leak but i guess this weekend will be more thorough. next week is the last week at the track here so it's gotta run her best.thanks for all your input. David
 
well tonight i sprayed some carb cleaner around all the lines i could find. no help i did miss one spot and it might be a major one now that i have remembered it. the intercooler tubes. how ever i did run some sea foam threough the system and as one should suspect it was smoky as usual. i took her for a drive afterwords and my boost did seem to raise 1-2 psi but i'm still only running 8-9 psi at wot. what would cleaning the tb with some barb cleaner or something of this nature do? i know on my brothers mustang it says he shouldn't do this is it the same for us? thanks for any help...

David
 
whitechicken said:
well tonight i sprayed some carb cleaner around all the lines i could find. no help i did miss one spot and it might be a major one now that i have remembered it. the intercooler tubes. how ever i did run some sea foam threough the system and as one should suspect it was smoky as usual. i took her for a drive afterwords and my boost did seem to raise 1-2 psi but i'm still only running 8-9 psi at wot. what would cleaning the tb with some barb cleaner or something of this nature do? i know on my brothers mustang it says he shouldn't do this is it the same for us? thanks for any help...

David

Cleaning the TB with carb cleaner won't do anything to help with low boost. It may also damage the IAC (idle air control)valve.

Did you check the bypass valve and or replace the belts ?

David
 
You can buy throttle body cleaner for it. They don't want you to use carb cleaner because it will remove the coating on the inside of the TB. As mentioned however that won't do anything to cure your low boost situation. What is your vacuum at idle in Park and Drive?
 
as has been stated, if you lost that much boost from a leak then the car would 99% also have a very bad vac leak at idle, and it probably wouldn't hold idle. Ocassionaly leaks do form only under boost though, that won't be there under vaccuum.
I brought my car to a shop for an exhaust job, and the mechanic forgot to retighten the lower IC tube. Where the tube met the IC the bolts were literally on just finger tight. The car, believe it or not, held an idle, and ran fine for the most part. I was grannying the car, so i didn't notice a power loss. That was until one hill on my way to work that i could normally pull in 5th at 1900RPM, i had to downshift to 4th and then even 3rd. My boost gauge was not working at the time so i can't comment on boost loss, but considering the power loss I can only imagine that boost was extrememly low.
 
I've also seen an SC that idled perfect, but had a huge boost leak when put under a load and it sputtered and bucked. Turned out that someone forgot to tighten the three bolts that hold the return plenum onto the intake manifold. They were just started into the holes and not even finger tight.

It's very rare to only leak under boost, but it can happen.

David
 
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