North American Turbocoupe Organization



Unsteady vac?
Harvs Offline
Senior Member
#1
I have noticed that i dont get a steady vac on the boost gauge. I goes to 20 then as the idle fluctuates the vac floots between 15 and 20,the car runs better when i get 20. What's causeing this?
Sandlewood beige 88 coupe 93k/ super 60 T3 at 24 pounds/ ATR 3" no cat or tail pipe/ 255lph high pressure pump/ Kirban AFPR at 42/ Spec stg3/ GN FMIC/ 1g DSM BOV/ 14* timing/ disconected KS/ Tri-ax short throw/ Autometer boost,vac and air to fuel/ 140 spedo/ Ford motorsports 5300-c springs/ tubular rear upper cntrl arms.
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Pete D Offline
Administrator
#2
how rapidly or slowly does it fluctuate. It might be a vacuum leak or it could be some mechanical condition. Vacuum gauge can tell a lot. be specific aboout how it's behaving.

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NATO Member. it's not a vice, it's an obsession
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Harvs Offline
Senior Member
#3
i'll try to elaberate some more. when my car idles, its at 1000, then every now and then it will hesitate and bog like 200 rpms for like a second then it goes back to 1000. It always does it every now and then. The vac moves with the idle, when the car hesitates and goes down a couple hundred rpms, the vac goes up to about 15 16 then goes back to 20 when the car levels out.
Sandlewood beige 88 coupe 93k/ super 60 T3 at 24 pounds/ ATR 3" no cat or tail pipe/ 255lph high pressure pump/ Kirban AFPR at 42/ Spec stg3/ GN FMIC/ 1g DSM BOV/ 14* timing/ disconected KS/ Tri-ax short throw/ Autometer boost,vac and air to fuel/ 140 spedo/ Ford motorsports 5300-c springs/ tubular rear upper cntrl arms.
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Noname
Guest
#4
What kind of cam are you running? A cam with a lot of lift and duration can cause a rough idle and intermittent loss of a few in. hg. of vacuum. I hold a steady 15" at an 800rpm idle, but it'll dip down to about 12" during those occasional misses (and the associated drop of rpm) at idle.

What does your vacuum gauge indicate at a solid 3000rpm in neutral?

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Estoril blue 1987 TC 5spd, 141k miles
PRofec B electronic boost controller, kirban fpr, walbro 255 pump, Bailey BOV, Spearco 26x8x3 FMIC, a237 cam, t3/t4 50 trim / stage III .63, 3" exhaust, eec-tuner, 55pph injectors,
Best 1/4 mile: 13.64 (with 2.19 60' time) @ 102.91mph And that was on 93 octane Wink
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Harvs Offline
Senior Member
#5
My car has the stock cam. what is causing the occasional miss, mine does it a lot. Is it bad?

I cant tell you about when its at 3000rpm cause my car is not running right now, what should it be at?
Sandlewood beige 88 coupe 93k/ super 60 T3 at 24 pounds/ ATR 3" no cat or tail pipe/ 255lph high pressure pump/ Kirban AFPR at 42/ Spec stg3/ GN FMIC/ 1g DSM BOV/ 14* timing/ disconected KS/ Tri-ax short throw/ Autometer boost,vac and air to fuel/ 140 spedo/ Ford motorsports 5300-c springs/ tubular rear upper cntrl arms.
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turbocoupes Offline
Senior Member
#6
if this "bog" is consistent at about the same time intervals it's the computer doing some kind of "cycling" closed loop to open loop if my memory serves...course it does come and go[memory]...winter's coming and the body wants to hibernate!!!LOL!!!every Ford product in my yard does it...and when I was in EECIV school with Ford in 88 every tech and teach said NORMAL...BUT...if it's rythmic then as Pete D is eluding to you could have a valve train problem or an ignition problem[plugs,cap,rotor,wires,coil,computer wiring for the ignition,etc]and if the car has sat for a long time...fuel,fuel pump,fuel filter,injectors...

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