Where is my vacuum going?!

[tobyw]

1977 Ford Bronco, 302cid, C4, with an aftermarket TBI injection setup on an Edelbrock intake. The problem is that just about any time the brakes are applied while in gear, such as holding speed down decending a rocky trail, the engine will rev uncontrollably. It will also rev uncontrollably just about anytime you put it into Drive, 2nd, or 1st (does not seem to do it when going into Reverse...) from Park while the brakes are applied. It will not idle down until it's back in Park and then takes several seconds to calm itself down. You can imagine the fun this causes on the trail, or even in a parking lot...

The only thing I can think of is a vacuum issue causing the MAP to go all whacky, and this was somewhat verified with a vacuum gauge - it reads a steady 14" at idle but slowly tapers off when the brakes are applied while in gear, and the RPM's raise as the vacuum decreases (it holds steady at ~13" with the brakes applied in Park).

So the big question is, WHERE is my vacuum going while I'm in gear with the brakes applied? Does this sound like some sort of issue with the modulator on the C4? Any help would be appreciated!!

 

[WHITE TRASH1]

The only thing that comes to mind is a bad booster.

 

[nevada]

The only thing that comes to mind is a bad booster.

that was my first thought.


pinch off, or remove, and plug the line going to the booster, then hit the brakes, and see what happens.

 

[tobyw]

it reads a steady 14" at idle but slowly tapers off when the brakes are applied while in gear, and the RPM's raise as the vacuum decreases (it holds steady at ~13" with the brakes applied in Park).

I don't believe the booster is at fault, as it holds steady vacuum so long as I'm not in gear... It only happens when I'm on the brakes in gear, so I'm thinking something in the transmission vacuum circuit, which I know nothing about??

 

[nevada]

I don't believe the booster is at fault, as it holds steady vacuum so long as I'm not in gear... It only happens when I'm on the brakes in gear, so I'm thinking something in the transmission vacuum circuit, which I know nothing about??

if you do what i said above you can completely eliminate the booster.



whats the coolant level like? does that have a coolant level sensor? i had a damn van that did that ****. would rev to the moon...as soon as you put in in gear. ended up being low on coolant...for whatever reason, it causes it to rev:eeek:


edit: whoa, 13-14?...i missed that. thats LOW. how do the brakes feel with that low of vacuum? very little assist?

 

[tobyw]

Already plugged the booster line, makes no difference. I have also tried to reroute the MAP signal from every available full-manifold vacuum source - again, no changes. It only shows up when you put it in gear...

Yep, 14" seems awefully low to me too. Granted I took that reading at 8600 feet elevation, but still... Brakes are great, though. It's actually a newer vacuum booster assembly from Navajo Brakes through Off-Again in New Mexico. Good stuff.

 

[nevada]

Already plugged the booster line, makes no difference. I have also tried to reroute the MAP signal from every available full-manifold vacuum source - again, no changes. It only shows up when you put it in gear...

Yep, 14" seems awefully low to me too. Granted I took that reading at 8600 feet elevation, but still... Brakes are great, though. It's actually a newer vacuum booster assembly from Navajo Brakes through Off-Again in New Mexico. Good stuff.

maybe 14 is the threshold for the MAP to work.

in reality you should be closer to 20. id guess 17 about as low as you'd want it.

does it have a big cam?

 

[tobyw]

Nope, stock cam. Forgot to answer your question about coolant levels- no sensor there. I took the manifold reading at every availalbe full manifold source, and they were all consistant at only 14". I also put on a new MAP sensor while grasping at straws, and no change

 

[WHITE TRASH1]

Pull the vacuum line off the modulator, if the issue persists its obviously the booster. If it goes away you found the culprit.

How does it shift? Early, late or normalt?

 

[zukkev]

1977 Ford Bronco, 302cid, C4, with an aftermarket TBI injection setup on an Edelbrock intake. The problem is that just about any time the brakes are applied while in gear, such as holding speed down decending a rocky trail, the engine will rev uncontrollably. It will also rev uncontrollably just about anytime you put it into Drive, 2nd, or 1st (does not seem to do it when going into Reverse...) from Park while the brakes are applied. It will not idle down until it's back in Park and then takes several seconds to calm itself down. You can imagine the fun this causes on the trail, or even in a parking lot...

The only thing I can think of is a vacuum issue causing the MAP to go all whacky, and this was somewhat verified with a vacuum gauge - it reads a steady 14" at idle but slowly tapers off when the brakes are applied while in gear, and the RPM's raise as the vacuum decreases (it holds steady at ~13" with the brakes applied in Park).

So the big question is, WHERE is my vacuum going while I'm in gear with the brakes applied? Does this sound like some sort of issue with the modulator on the C4? Any help would be appreciated!!

First of all, what's your timing set at??? Low vacuum (14" is low) is usually caused by retarded timing...if the timing's ok, does the aftermarket TBI have some sort of adjustable IAC (Idle air Control) valve??? Maybe it's allowing too much air through, causing the engine vacuum to be too low, therefore wacking out the MAP sensor, and ultimately causing the RPM increase!!! Also, make damn sure you don't have any other vacuum leaks (intake gaskets,TBI gasket(s), vacuum hoses, etc....)....I highly doubt the trans modulator is causing all this.

 

[Binder]

What TBI system is this?

 

[jeffs_62]

since it only happens in gear, isn't there a curcuit that is supposed to keep the idle up when you put it in gear? don't remember what it's called but it went out on my Dakota, took the guy ten minutes to fix.

 

[super samurai]

do you have a brake controller hooked up? my friends brake controller power was hooked to his starter solenoid and the wire for the trailer brakes rubbed to bare wire and did that kind of thing, it would run rough with the brake on and kill it when the brakes were on, just an idea?

 

[tobyw]

How does it shift? Early, late or normalt?

It seems to shift normally...

First of all, what's your timing set at??? Low vacuum (14" is low) is usually caused by retarded timing...if the timing's ok, does the aftermarket TBI have some sort of adjustable IAC (Idle air Control) valve??? Maybe it's allowing too much air through, causing the engine vacuum to be too low, therefore wacking out the MAP sensor, and ultimately causing the RPM increase!!! Also, make damn sure you don't have any other vacuum leaks (intake gaskets,TBI gasket(s), vacuum hoses, etc....)....I highly doubt the trans modulator is causing all this.

Timing is 8* advanced. If I attempt to advance or retard it, I immediately start to lose vacuum. The TBI does have an IAC, and I have attempted to unplug it when closed so that it cannot react at all, but the problem persists. It seems on this setup, vacuum is the highest rank for fuel delivery. I am certain there are no other vacuum leaks in the engine compartment - the only possible leak would be in the line going to the modulator, so I need to cap that line off and see what happens...

What TBI system is this?

It's a system from an outfit called Turbo City out of Orange, CA. It's been a complete POS from day one, as I have to replace nearly every electrical connection in the harness they supplied, the fuel pump they supplied, as well as the TBI unit itself just to get to this point

do you have a brake controller hooked up?

Nope.

 

[zukkev]

Are the timing chain marks lined up on the cam/crank??? 14" is just too low....and will screw with the MAP... have you tried a different vacuum guage just to be sure yours isn't out of calibration????

 

[tobyw]

I have thought about the cam issue... Very good possiblity, and I will definetly tear it down to verify that. And yes, I have tried two vac guages with consistent readings :booo:

 

[Binder]

It's a system from an outfit called Turbo City out of Orange, CA. It's been a complete POS from day one, as I have to replace nearly every electrical connection in the harness they supplied, the fuel pump they supplied, as well as the TBI unit itself just to get to this point

Is it a GM system relabled? 14" is plenty of vacuum if the system is tuned for it. I would look at other things.

 

[tobyw]

Yep, all standard GM bits-n-pieces...

 

[Binder]

Yep, all standard GM bits-n-pieces...

OK couple questions....Did you install the system and how long ago? Has it always done this? Was it custom tuned for your app- did you supply them any specific info or data from your rig? Is there a wire going from the shifter to the ECM?

 

[tobyw]

OK couple questions....Did you install the system and how long ago?

Yes, I did the install several years back and have been struggling with it ever since. It has always had this high-idle issue, and it's about the last thing I have to iron out. This is not my rig, it's my father's, so I don't have full-time access to it. He's in a wheelchair, so I'm his wrench when I have time...

Has it always done this?

Yes, from the first day I was actually able to drive it under EFI power...

Was it custom tuned for your app- did you supply them any specific info or data from your rig?

No custom tune, just a basic chip for a small V8 application. The instructions told us to disable all advance from the stock distributor (vac and mech), and gave very vague/basic direction on everything else.

Is there a wire going from the shifter to the ECM?

Nope, the harness is only engine related stuff.