Vac problem?

[ninety15.0]

OK so i checked my fuel pressure with a guage on the end of the fuel rail and i got about 40 vac off and about 31 vac on. When i punched it with the vac on and put a load on the engine it went back up to about 40, and then very quickly dropped back down to like 31. So i am assuming that it is OK? Does that sound ok? SO i think i might have a vac leak somewhere. I am loosing a ton of power when i either floor it or when the RPM's get over 3000. I have been trying to figure this problem out for forever!!! im getting pissed. How would i check for a vac leak? i know i need a vac guage, but where would i check it? and how much should it be at idle and at 3000 rpm? thanks for the help fellas!

 

[87'GTstang]

All engines are about the same on this category. Between 15" and 20" Hg (mercury) is GOOD vacuum, 12" if you have a turbo. Plug the gauge inline with a vacuum line (there's quite a bit on the plenum) and take your reading at idle. When vacuum testing, it's unimportant about the readings at higher RPM's because it's all about how much force the engine is creating to "suck" the air past the throttle plate. Obviously when you floor it, the plate is no longer closed therefore your vacuum dramatically and almost entirely disappears/drops.

 

[HISSIN50]

i agree with above. also, the vacuum tree might have a plugged off fitting, which works well. while you are testing, if you notice a drop off of vac when holding the throttle at a constant RPM after about 15 seconds, that could suggest an exhaust clog.
good luck.

 

[ninety15.0]

Well thanks for the info about the vacuum stuff...the thing is tho....i donthave cats and i have FMS headers and dumps so i dont think there is any way i could have an exhaust leak. Wouldnt a vac leak near the intake manifold effect performance. I have no idea anymore about what this could be. I will check the Vac but it doesnt sound too promising from what i hear. thanks for the info guys

 

[HISSIN50]

sorry if i was ambiguous. i was talkin about having an exhaust restriction (almost always cats) if the vac drops when rpm's are held for some time....a leak should not make much of a difference. your lack of cats makes that not much of a likely culprit. i did not notice that in your sig the first time around. my bad.

have you pulled codes. if the performance is suffering that much, i would wonder if something is malfunctioning enough to toss one.

have you installed your Crane coil yet? if so, and this happened afterwards, take it off and see what happens. if not, i would put that thing in, and see if you arent losing spark (or a decent amount of it) at higher rpms.
good luck.

 

[ninety15.0]

Thanks, ya dont worry bout it. yea it definately wouldnt be an exhaust blockage. I havent installed the coil yet. Its at my house back home and im still at college, so i will put it on when i get home next week. See the thing about this problem that im having is that it only happens when i actually floor it. It still pulls hard when i just use about 1/2 or 3/4 throttle but the second i floor it i loose a ton of power and it gets even worse at the upper rpm.This crap is getting so frustrating and im sure it is some little sensor or electrical thing that i cant find out by pulling codes or testinf vac and fuel pressure. any other suggestions would be awesome. thanks guys

 

[blown93feature]

I thought that I read that you said that under wot that the fuel pressure went to 40 than dropped to 31? I s that drop happening while your under wot?

 

[HISSIN50]

another idea im having: how is your TPS voltage under WOT? should be around 4.5 volts. i wonder if the thing took a poo, and you have a dead spot...just something to check real quick.
good luck.

 

[ninety15.0]

Well when i was testing the fuel pressure i was punching it real quick but i dont think i was flooring it. And yes it would shoot up to about 40psi and when i got off the gas it would come right back down to 31. Thanks for the suggestion about the TPS i will definately have to check that. Do i do that test with the Key on engine off or with the engine running. I know which wires to probe, and i know the voltage at idle is supposed to be between .94 and .98. I will give it a check tomorro thanks

 

[ninety15.0]

anyone?

 

[87'GTstang]

Is it flat out when you floor it or any point in the RPM band when you floor it? If it's just at the upper RPM's then it sounds like your valve springs are floating and are worn out, aka - they're not closing fast enough to give your engine efficiency worth crap.

 

[87'GTstang]

Well when i was testing the fuel pressure i was punching it real quick but i dont think i was flooring it. And yes it would shoot up to about 40psi and when i got off the gas it would come right back down to 31. Thanks for the suggestion about the TPS i will definately have to check that. Do i do that test with the Key on engine off or with the engine running. I know which wires to probe, and i know the voltage at idle is supposed to be between .94 and .98. I will give it a check tomorro thanks

Well it shouldn't matter as to on or running, as long as there is power going to the TPS (definitely not off), there should be a voltage reading coming from it in the first place however.

 

[ninety15.0]

OK cool thanks for the info on the TPS testing. And NO it doesnt only happen in the upper rpms....its just any time i floor it. U can actually feel the car slow down, when i push it from about 3/4 throttle to WOT. Its like something happens when the pedal hits the floor that just causes the car to feel like a civic!!! :-( no good! But it does happen if i floor it at any RPM. so that kinda eliminates severe engine problems i would think?!!? i did a cylinder balance test the other day when i was pulling codes...and i got no codes and all cylinders were fine! I havent down a compression or leak down yet, but this doesnt seem like a compression issue, it feels like something electrical or a sensor or some crap! thanks for the advice though...i appreciate any help i can get!

 

[87'GTstang]

Yeah it sounds like a sensor that's confused. Test out the TPS for sure first. I don't know how it works but try looking at the WOT relay mounted on the passenger front inner fender. Maybe there's a short to ground and when it senses WOT and tries to pass that signal through the relay - it ends up confusing the ECU.

 

[ninety15.0]

great thanks for the suggestions....i will test all that stuff tomorro....i really hope that works! thanks again

 

[Pod]

Not sure if you've done this or not, but when checking the TPS voltage, make sure it works all the way through the throttle range. It should be around .98 or so at idle and gradually increase up to, I think, 4v at WOT. My old one was fine at idle, but had two areas where it would increase slowly and then drop back to zero, and then jump back with a bit more throttle.
Oh yes, this is much easier to do with the key on, but engine off!