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Funky Idle, Stiff Brakes, Vacuum Leak?

kdailey4315

NAXJA Forum User
Location
Pacifica, CA
I've had a couple of problems since driving my XJ again after a year of sitting. I've had a stiff brake pedal (all the time), occasional ABS light that stays on for about a minute and now I've got a funky idle. I'm thinking it's a vacuum leak somewhere. Any suggestions on where to start looking?

 
You don't specify a year, so we don't know whether this is the funky Renix era ABS or the later type that uses a conventional diaphragm booster, but the obvious place to start looking is whatever part of the brake system uses vacuum. Check hoses, check valve, and diaphragm itself. Maybe a cracked hose or fitting. The check valve can sometimes leak, especially if you've had a backfire, which can blow it apart.

For a first step, disconnect the brake vacuum line from the manifold and plug or cap the connector at the manifold. If idle now works then start hunting for brake problems.

Do you get any brake boost at all? Assuming a conventional vacuum booster, pump the pedal with engine off, then start the engine while you have your foot on the brake. Does the pedal sink at all? If not, you're not getting any boost. If it does, then at least some vacuum is reaching the reservoir. Check for changes in idle when you pump the brake (sign of booster problem), and also for smoke in exhaust when brakes are applied (booster leak pulling fluid out of master cylinder).
 
Sorry, forgot the Jeep info. 98 Limited 4x4 ABS

It looks like it might be a boost issue. I pumped the pedal 3 times while the engine was off until it was very firm. Started the jeep and it barley sunk at all. 1" at most.
 
Check the check valve at the booster. It is a 1 way valve. To test just remove and blow into it. Should only be able to blow through 1 way.
 
For a first step, disconnect the brake vacuum line from the manifold and plug or cap the connector at the manifold. If idle now works then start hunting for brake problems.

Did this and the engine jumped up to 1500 RPMs and then shut off when I plugged the vacuum on the manifold.

Do you get any brake boost at all? Assuming a conventional vacuum booster, pump the pedal with engine off, then start the engine while you have your foot on the brake. Does the pedal sink at all? If not, you're not getting any boost. If it does, then at least some vacuum is reaching the reservoir. Check for changes in idle when you pump the brake (sign of booster problem), and also for smoke in exhaust when brakes are applied (booster leak pulling fluid out of master cylinder).

No change in idle when I pump the brakes but if I pump it once and hold it for a second when I let go a get a slight jump in RPMs. Maybe 20-30, from 750is to 780ish.
 
Check the check valve at the booster. It is a 1 way valve. To test just remove and blow into it. Should only be able to blow through 1 way.

The valve checks out ok. I jacked up front today and spun the tires. I can hear the brakes dragging on the rotor and there's a groove in one of my rotors. I'm going to tear my brakes apart next weekend and inspect them. I want to completely flush the system and put in new brake fluis but I'm not sure if I can do it with ABS.
 
....You still haven't reported whether or not you have boost.

If you did not test this, try again: With engine off, pump the brake pedal until it's good and firm, with no residiual boost. Now, keep your foot firmly on the pedal, and start the engine. If your brake booster is working properly, the pedal will sink noticeably when the engine starts. If this does not happen, your booster or the line to it is bad.

Try also plugging the vacuum line on the manifold with the engine off, then restarting, to see how it idles without the brake system in the mix. That way you won't be mixing up vacuum loss issues with the attempts of the engine to correct itself for a sudden change.
 
....You still haven't reported whether or not you have boost.

If you did not test this, try again: With engine off, pump the brake pedal until it's good and firm, with no residiual boost. Now, keep your foot firmly on the pedal, and start the engine. If your brake booster is working properly, the pedal will sink noticeably when the engine starts. If this does not happen, your booster or the line to it is bad.

Try also plugging the vacuum line on the manifold with the engine off, then restarting, to see how it idles without the brake system in the mix. That way you won't be mixing up vacuum loss issues with the attempts of the engine to correct itself for a sudden change.

I did. I posted the results a few posts ago (in red).

When I pump the pedal while the car is off it goes down a very small amount. Not even half way.

When I unplug the vacuum hose at the manifold the idle jumps to 1500 then dies when I plug it up.
 
I did. I posted the results a few posts ago (in red).

When I pump the pedal while the car is off it goes down a very small amount. Not even half way.

When I unplug the vacuum hose at the manifold the idle jumps to 1500 then dies when I plug it up.


Maybe I'm not understanding your reply. When you hold the pedal down and start the engine, does the pedal sink? The change should be quite noticeable.

How does the idle act if you do the removing and plugging with engine off and then start the engine? Does it still stall, or does it idle?
 
Maybe I'm not understanding your reply. When you hold the pedal down and start the engine, does the pedal sink? The change should be quite noticeable.

It is not a very noticeable drop. It drops very little. Much less than previous cars I've done this to.

How does the idle act if you do the removing and plugging with engine off and then start the engine? Does it still stall, or does it idle?

Plugging the manifold vacuum and then starting the car there is no change in idle. It acts as if the vacuum line is on there. Once I unplug it the RPMs jump to 1500. If I plug it while it's running sometimes it dies other times it just goes back to normal idle.
 
Thanks for the clarification. I think at least one of your problems is a bad booster.

As one last check to make sure you don't miss something, I suggest you pull the vacuum hose at the booster end, and cap or plug it, and try the idle. If it idles well then, you know the problem is not some hidden problem in the check valve, or something like that. Probably not, but redundant checking never hurts.
 
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