v accum question

puddin

Registered User
Hey guys im helping a friend of mine with his super coupe.
Its a 92 model. It has a real ruff idol. This motor is only showing
5inches of vaccum. I took the vaccum line off of the fuel pressure
Regulator and plugged it and put my hand held vaccum pump on it
and brought it to 17 inches of vaccum and the car smoothed out.
cannot hear any vaccum leaks at all. Is there some type of vaccum
Control on these that im missing.

Thanks in advance

Puddin
 
Would anyone advise to use the old carb cleaner trick to find vacuum leaks on the SC's? Or is the smoke test recommended?
 
Smoke test...or pressure and listen.

Trying to dose the intake will result in the computer reacting too quickly to stabilize.
 
Sorry but what is the smoke test. Ive never have heard of
that test before. Ive always used starting fluid
 
Sorry but what is the smoke test. Ive never have heard of
that test before. Ive always used starting fluid

Some mechanics have a smoke generator that they plumb into the intake system and let it puff away. It's a very nice way to do it without the risk of fire (or getting caught in the belt), or trying to hear above the engine noise.
 
A smoke generator (sometimes just a soldering iron dipped in baby oil) will let you put a vapor into your intake system, engine off, so you can then take your time looking for leaks. I think Party City sells/rents them.
 
As large as this vaccum leak may be, wouldn't the car have a high idle like around 1000 to 1200 rpm?
this car as ruff as the idle is , is only idling at 750 rpm. After working on grand nationals only since 1985
I have never had this happen before.if you raise the idle to1000 rpm it smoothies out also but Vaccum doesn't change
It stays at 5". And like I said in my first post, if I put more vaccum to the fuel regulator , the idle smoothies out.
it stays at 750 rpm but it idle's right. Is there any thing in or on these motors that control vaccum.

Puddin
 
I think you'll find these cars very different from what you may be used to - don't assume the same troubleshooting methods and keep in mind the computer may be adapting to changes that occur when things go wrong. Know that these cars are all about electronics. That not only includes the computer, but the many sensors, harnesses and grounds. Having a factory shop manual and EVTM are good investments for any owner doing their own work. Don't guess....start by checking the codes. If you don't know how to do that and/or don't have the tools, check for a local auto parts store that can do it for you.

Are you seeing 5" vacuum on the stock gauge? Did you confirm by (a) using another stock gauge/cluster to test or (b) removing the vacuum line that feeds it on the manifold and checking with another gauge there?
 
Ok number 1 this is obd1and yes I checked the codes there are no present or stored codes.Checked all grounds with a Digital volt ohm meter and grounds show up at .002. The worst was .004. checked both lines that come from the back of the intake and one on the plenum with a auto meter vac/ boost gauge.
All 3 only have 5 inches of vaccum. Stock gauge in the car shows the
same.I dont ever trust factory gauges .checked injector wiring with a noid light
All is good.fuel pressure with the vaccum hose off
is 46psi.
 
When you put 17" vacuum to the FPR, and it smooths out, how much vacuum then?

Any work done lately? Plugs? Wires?

How old is the fuel in the tank?

Stock air filter/box or cold-air intake?
 
Fuel pressure 46 psi above atmospheric? That seems too high. It should only be 39 psi above manifold pressure. Idling at 5" vacuum, the pressure should be about 37 psi above atmospheric. Seems like the regulator might be bad ... unless ... here's another thought. Have you checked the boost bypass on the intake plenum for proper operation? If that is stuck closed, it will cause the supercharger to work all the time, instead of idling. Still, the supercharger by itself can't raise the overall vacuum level of the system, since the throttle body plate is still closed. But it can raise the pressure POST-supercharger. Which is where the FPR vacuum/boost reference line is supposed to be connected.
 
w

When I put vaccum to the regulator. The stock vaccum
line is plugged.there would not be any change in the vaccum to
the rest of the motor it would still be at 5. This test was to
see if the low vaccum was the reason for the bad idle.the 46 at the regulator
Is with vaccum line off. That is static fuel pressure.there should be
any where from 7 to 9 lbs difference with the correct vaccum going
to the regulator. So if there is not enough vaccum at the regulator
then the fuel pressure will be to high.which will make the car too rich an idle bad.
it has new plugs and wires. Bypass is working properly.
still has the stock air box.everything on this motor is all
factory. This is the reason I dont understand. If we cannot find
A vaccum leak is there any thing except for the bypass
that would control vaccum on these cars.the reason I keep
asking this question is because it is supercharged.yes at this point
It would make you think that it is the bypass but its not
car gets in to boost fine
thanks guys again for all of your help
 
it has new plugs and wires.

Confirm the wiring/firing order with this diagram:
attachment.php


Be sure all wires are completely/firmly socketed on the plugs and at the coil. Check that none have been melted on an exhaust manifold.
 
The 46 at the regulator
Is with vaccum line off. That is static fuel pressure.

What I am telling you is that the correct static fuel pressure for this car is 39 psi.

yes at this point
It would make you think that it is the bypass but its not
car gets in to boost fine

A failed bypass will always let the car make boost. But only a working bypass can prevent the car from making boost. The can returns the butterfly to closed when vacuum is removed. Only vacuum lets the butterfly open and recirculate air back to the inlet of the blower, bleeding off boost.

So the fact that the boost builds fine is no evidence for a properly working bypass.
 
Sorry didn't make myself clear on the bypass.at idle the bypass
is open BUT not very far.maybe an inch. But with low vaccum that is what
I would expect. When i said it gets into boost fine i should
Have said that the bypass Lowe's. Now my question is and always
has been is there any thing, solenoid, computer control valve, etc
that controls vaccum on these motors.

The fuel pressure thing yes thank you but I knew that 39 was the
correct fuel pressure. I was using the fuel pressure as a test for the idle.
to see if putting more vaccum to the regulator helped the idle.
that by itself tells me the car is too rich at idle. Thats why I keep asking
about the control of vaccum
 
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