1999 Jeep Wrangler Idle heartbeats in sync with AC Compress

TAZZ_2112

MEMBER
1999 JEEP WRANGLER

Engine Performance problem
1999 Jeep Wrangler 6 cyl Four Wheel Drive Manual 100K miles

Hi - my Jeep idle jumps between 800 & 1100 RMPS in a cycle. It's usually at 800 RMPS for 7 seconds than jumps to 1100 RPMS for about 4 seconds. I looked under the hood and the idle heartbeat appears to be in sync with the cycle of the AC Compressor - (it actually heatbeats in sync with the black disc inside the wheel that is driven by the belt ) Do you know what would be doing this?

FYI - I recently replaced the serpentine belt. Before I replaced the belt the jeep had trouble starting, struggled just after starting, and would sometimes stall when I hit the clutch - i.E. High RPM's to low RPM's - but with the new belt I have not experienced any of these problems. The only issue that it had before replacing the belt that still remains is the idle heartbeating. I just wanted to mention the previous issues in case they are related.

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

THe AC does place a heavy load on the engine, so some of that is normal. Has the check engine light ever come on? Have you checked for vacuum leaks that would cause the engine to drop to a lower RPM? Has the check engine light ever come on?

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Monday, June 29th, 2009 AT 10:48 PM

TAZZ_2112

MEMBER
2 POSTS

The check engine light has not come on. I have not checked for vacum leaks, but that seems like good advice and it makes sense. I haven;t done that before - is it diffcult? Can you usually see a hole in a hose with water or something or is it generally more complicated? Thanks for your reply.

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Tuesday, June 30th, 2009 AT 9:55 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

They can be hard to find. Some mechanics will spray starting fluid on them with the engine running. IF there is a leak, the RPM's will increase. However, if a part of the engine is hit and too hot, it can catch fire. So if you choose this route, be careful and have a fire extinguisher near by.

Other than that, look for disconnected lines, dry rotted, or damaged hoses. With the engine running, you may actually be able to hear a hissing sound from a leak. Just be careful around moving parts.

Let me know what you find.

Joe

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Tuesday, June 30th, 2009 AT 11:16 PM

AIRSOFTSOLDRECN9

MEMBER
348 POSTS

Does this problem completely not occur when the AC is not on?

Part of this is normal and part of it not. The idle should jump approximatly 200-400 RPMs with the additional load placed by the compressor. However,
the compressor should not cycle that often. Usually the compressor should cycle once at most every 20-30 seconds. Have an AC technician check the refrigerant level in the system.

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