XJ Idle and Brake Problem

1988 XJ 4.0 with Automatic:
We have power brakes but feel like manual brakes when pushing the pedal. Pulled the vac. line out of booster and no difference in idle. Plug the vac. line still no difference.
All the other brake parts are in good shape.

Also idles real rough, when in gear it’s worst. The idle surges up and down a 200-300 hundred RPM’s when in gear.

Any idea on these two problems, or is the idle problem tied to the booster.

Thanks

You may want to check the vacuum reservoir. It is located behind the front bumper on the right side. Vacuum leaks there are common on older XJ's. Your vacuum booster could also be bad. The diaphgram is likely compromised. That would explain the poor vacuum, poor power brakes, and the rough idle. Pull the hose out of the vacuum booster and cover the hose with your palm with the engine running. If you have vacuum, the booster is likely bad.:idea:

Renix era XJ's (like ours) are not well known for stopping power. However, you should go through the brake basics. You can search the forum for the proper method to test the booster.

How is the idle in park/neutral? Is it steady then? What if you depress the brake in park/neutral? Idle could be tied to booster as BigSky says.

Saudade said:

Renix era XJ's (like ours) are not well known for stopping power. However, you should go through the brake basics. You can search the forum for the proper method to test the booster.

How is the idle in park/neutral? Is it steady then? What if you depress the brake in park/neutral? Idle could be tied to booster as BigSky says.

Click to expand...

The brakes were better before (not great, but easy), they now feel like manual brakes. Idle is better in park, not great but better.:us:

Test the booster to see if it is even working.

Engine OFF, pump the pedal about four times and hold pressure.

Start the engine--did the pedal drop under your foot? If not, you have no brake booster.

joe_peters said:

Test the booster to see if it is even working.

Engine OFF, pump the pedal about four times and hold pressure.

Start the engine--did the pedal drop under your foot? If not, you have no brake booster.

Click to expand...

Thanks, will do tomorrow. I did unplg the booster vac line while the jeep was running and the idle went up and if i put the finger over the line the engine shuts off.