i have an 84 4 cyl. runs smooth most of the time but now when i apply the brakes it starts to miss. release the brake and it smooths out again. I'm assuming a vac leak in the power brake system. does the fact that it only happens when the brakes are applied mean a booster failure? anything else to check, emissions is in a few days so i welcome suggestions.

Although I'm no expert in the brake booster system, your diagnosis sounds about right to me... booster failure.

One other thought. Is it an automatic? Could it be that it only misses at low rpm, under load?

There could be a bad ground on the brake light harness that is changing voltage somewhere else.

nope, 4 speed manual.
so, any way to test the booster? is this a junkyard grade part or must i buy new? $100 @ napa

Could be dying booster. Without Driving, Just pull and plug the vac line and push the pedal. Push pedal can tell if booster or power problems.

quote Originally posted by Gall757: There could be a bad ground on the brake light harness that is changing voltage somewhere else.

Yes, If you have power/ground problem... 84 has 6 brake lights.
OE bulbs total amp load is ~12.6a. Enough iffy wiring and/or alt can cause engine problems.
Alt amps is Peak amps. At engine idle, most alt's making much less. Low power + any possible wire issues can cause problems.
See my Cave, Watt Story and Alternator Sense
If replacing alt, see CS Alternator

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I read this post and decided to do a test and found I have the same problem! If I hit the brakes it misses more than usual (I do have other problems in sorting) but it I just let it roll on idle mines an auto and hit the gas No miss at all. I tried the test suggested by the ogre and its not the booster so what grounds should check? Anyone have pictures or specific locations to check for bad ground I know I have one blinker that does not blink. The stock ground straps to the block were gone so I added two. Also when I step on the brake pedal the lights dim but the voltage doesn't drop and the meter always reads 15 volts

There is some question if all Fieros have this ground location. It shows up on 86 and newer, but some have reported that 84 and 85 are different. You don't need to use this location, however. You can splice into the black wire and ground it in a new place.

[This message has been edited by Gall757 (edited 03-20-2014).]

quote Originally posted by Fadingaway: Also when I step on the brake pedal the lights dim but the voltage doesn't drop and the meter always reads 15 volts

What meter? Dash meter often lie. Use a real meter and check alt output etc.

Check volt often lies too.
Volts can be fine but alt at engine idle make low amps. Problem is amps drawn by car from alt is to high for most meters to read. most read 10 amps max. amp shut can blow it's fuse. unfuse shut can fry.

Carefully Read "Watt Story." (I'll add charts to cave later.)
measure alt and crank pulleys. crank/alt = y, y x engine RPM = alt RPM

Look at 2000 RPM vs output amps. Is close to pulley ratio (Most are 2.5:1 - 2.6:1) at 900 engine RPM
weak wire anywhere just adds to low power...

Power chart of 12SI alternator


CS alt reach peak power sooner than 12SI but a never found cs130 chart. cs130d is close...



------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 03-20-2014).]

I'm going to look into this ground tomorrow as I realized I do not have any tail lights my brake lights work but I have no tail lights. Does anyone know the best way to get to this ground without taking off the rear clip?

pull part or all of plastic inner fender.