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engine miss when brakes applied by wolf63
Started on: 03-18-2014 10:55 PM
Replies: 11 (427 views)
Last post by: theogre on 03-20-2014 08:04 PM
wolf63
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Report this Post03-18-2014 10:55 PM Click Here to See the Profile for wolf63Send a Private Message to wolf63Edit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Report this Post03-19-2014 06:45 AM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
Although I'm no expert in the brake booster system, your diagnosis sounds about right to me... booster failure.
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2.5
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Report this Post03-19-2014 07:20 AM Click Here to See the Profile for 2.5Send a Private Message to 2.5Edit/Delete MessageReply w/QuoteDirect Link to This Post
One other thought. Is it an automatic? Could it be that it only misses at low rpm, under load?
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Gall757
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Report this Post03-19-2014 08:16 AM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
There could be a bad ground on the brake light harness that is changing voltage somewhere else.
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wolf63
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Report this Post03-19-2014 08:17 AM Click Here to See the Profile for wolf63Send a Private Message to wolf63Edit/Delete MessageReply w/QuoteDirect Link to This Post
nope, 4 speed manual.
so, any way to test the booster? is this a junkyard grade part or must i buy new? $100 @ napa
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theogre
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Report this Post03-19-2014 09:01 AM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
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

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Report this Post03-19-2014 09:15 PM Click Here to See the Profile for FadingawayClick Here to Email FadingawaySend a Private Message to FadingawayEdit/Delete MessageReply w/QuoteDirect Link to This Post
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
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Gall757
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Report this Post03-20-2014 09:38 AM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
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).]

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theogre
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Report this Post03-20-2014 09:53 AM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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...



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

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theogre
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Report this Post03-20-2014 09:55 AM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post

theogre

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Member since Mar 99
 
quote
Originally posted by Gall757:
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.


Likely not the problem but check anyway. that ground can cause dim taillights.
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Report this Post03-20-2014 07:59 PM Click Here to See the Profile for FadingawayClick Here to Email FadingawaySend a Private Message to FadingawayEdit/Delete MessageReply w/QuoteDirect Link to This Post
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?
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Report this Post03-20-2014 08:04 PM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
pull part or all of plastic inner fender.
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