Rough idle when hard braking - JeepForum.com
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post #1 of 6 Old 07-12-2020, 08:52 AM Thread Starter
Laggin lycan
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Rough idle when hard braking

96xj 4.0 just little over 200k

just trying to pick some brains on wtf is with my jeep.. had an idoit jump in front of me and I had to lock down on the brakes but when I did the engine started to stumble and smoke and had a good bit of a knock to it.. pulled off the road and switched it off.. switched it back on and used obd2 adapter to check for pending/logged codes and got a code for injector #4 sticking so I poped open the hood and looked at the connector just to make sure and it was fine but i also noticed that the brake fluid was dam near empty and I just filled it a week prior.. started it up after topping off the fluid and it ran fine no knock or smoke .. now I've got some 4port injectors from k suspension and im going to change the rear wheel cylinders to make sure there's no leaks there but has anyone ever seen this before?? Someone said it could be the brake booster and best way to check is to pump the brake firm hold it and start the jeep with it held and if it kicks back or doesn't want to move the booster needs replaced.. well my brake fell nice and smoothly to the floor but it has a bit of a woosh sound and after I continued to hold it down that stumble came back.. it wont do it just average braking at lights and such just when i push the pedal all the way down with force.. someone also mentioned a check valve could be causing the stumble and the loss of fluid..

I have a big road trip in a month and trying to figure this out before then and you folks have had more jeep experience that my local backyard mechanics that work on gms daily lol


I AM NOT A MECHANIC BY ANY MEANS BUT ANY HELP TO POINT ME IN THE RIGHT DIRECTION WOULD BE GREATLY APPRECIATED

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post #2 of 6 Old 07-12-2020, 09:07 AM
jtec
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you could look at the booster vac line ---- is there evidence of fluid in the line?
the whooshing soound only happened when your pedal was at bottom?

With pedal dropping that much there should be some tells, wet and dripping at - wheel cyls, or calipers are first suspects.

There is NO code for what you describe.

Hold off on those POS injectors for now - one problem at a time.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 6 Old 07-12-2020, 09:44 AM
CJ7-Tim
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The brake fluid has to be going somewhere, either leaking out to the ground, or the master cylinder is leaking internally. There is a brake proportioning valve, but if faulty it would not leak fluid or cause an engine stumble or smoke.

Behind the brake master cylinder is the power brake booster. The booster uses engine vacuum in order to make the brake pedal easier to press down. Since the booster is powered by engine vacuum, this means that it is connected to the intake manifold of the engine. If the rear seal in the brake master cylinder has failed, this will allow brake fluid to enter the power brake booster. With brake fluid inside the power brake booster, the intake manifold is able to inhale the brake fluid into the engine.

Symptoms of a Bad Brake Booster or Master Cylinder
.
  • Engine misfire or stalling when the brakes are applied.
  • Leaking brake fluid.
  • Insufficient braking pressure or hard brakes.
  • Spongy brakes or sinking brake pedal.
  • Brake warning light ON.

If you install a new master cylinder, partially fill it with fluid and bench bleed it before you install it.
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post #4 of 6 Old 07-13-2020, 09:00 AM Thread Starter
Laggin lycan
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Quote:
Originally Posted by CJ7-Tim View Post
The brake fluid has to be going somewhere, either leaking out to the ground, or the master cylinder is leaking internally. There is a brake proportioning valve, but if faulty it would not leak fluid or cause an engine stumble or smoke.

Behind the brake master cylinder is the power brake booster. The booster uses engine vacuum in order to make the brake pedal easier to press down. Since the booster is powered by engine vacuum, this means that it is connected to the intake manifold of the engine. If the rear seal in the brake master cylinder has failed, this will allow brake fluid to enter the power brake booster. With brake fluid inside the power brake booster, the intake manifold is able to inhale the brake fluid into the engine.

Symptoms of a Bad Brake Booster or Master Cylinder
.
  • Engine misfire or stalling when the brakes are applied.
  • Leaking brake fluid.
  • Insufficient braking pressure or hard brakes.
  • Spongy brakes or sinking brake pedal.
  • Brake warning light ON.

If you install a new master cylinder, partially fill it with fluid and bench bleed it before you install it.

Yep its in the booster so I guess I'm in for a new master cylinder.. took out the check valve and used a piece of wire kinda as a dip stick and stuck it in the booster and low and behold the dam thing was dripping because of how much is in it.. so here's my question now i know I should replace the master cylinder because its leaking into the booster but is there a way to clean the booster or do I need to replace that too?
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post #5 of 6 Old 07-13-2020, 11:11 AM
CJ7-Tim
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Brake fluid is very corrosive, and brakes are a good thing to have when required, I would replace both.
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post #6 of 6 Old 07-15-2020, 01:43 AM Thread Starter
Laggin lycan
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Quote:
Originally Posted by CJ7-Tim View Post
Brake fluid is very corrosive, and brakes are a good thing to have when required, I would replace both.
Thanks for the input... guess I'm ordering both and a bench bleeding kit and some line wrenches.. now to just put them in and bleed the system in a few days. Guess I'll throw in my injectors while I'm at it ... just gonna drive as little as possible.. now just for the E22 recall/campaign to see if they'll put me a new cat on it at the dealership for free 😀
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