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Low Vacuum at Idle

HttpWayne

NAXJA Forum User
Location
Auburndale, FL
2000 XJ, AW4

I just finished up some wiring on the passenger side of the XJ earlier in the garage. When I turned it on I could hear a faint uneven sound that sounded like it could be coming from the engine. From inside the cabin I could hear it but outside I couldn't.

While I was near the front of the engine I heard what sounded like light weight metal sitting on a shaking metal surface. It was a very low volume sound and it sounded like it was near the AC compressor. I've heard a lot of noises in my time and pulled the valve cover to make sure it wasn't a loose rocker developing, but they were all tight. I marked the rods with a marker to verify they are spinning just to be sure and they were. I bolted it back up and removed a loose clip I wasn't using anymore near the fan shroud thinking that might be it. It wasn't.

All throughout this time as the engine idled lower is when the sound would present itself. If I just started it up the first 10-15 seconds it's on it idles around 700 evenly and is quiet, then slowly starts dropping and I can hear the sound I mentioned.

That's when I decided to get out the vacuum gauge (I hadn't touched any vacuum hoses except the PCV's which were thoroughly reconnected). It showed the idle was bouncing between 13 and 15 inches. I'm at sea level so that isn't right. Tomorrow I'm going to disconnect all vacuum lines for inspection. There shouldn't be anything wrong since they were all recently reconnected since I finished an engine swap. I've driven it two days with no issues. I didn't think it would be a coincidence that I started tinkering in the engine bay area and suddenly this problem appeared. I suspect I did something.

The vacuum gauge I have says "Late Ignition Timing" right inside that little red zone of vacuum on it. I'm going to hook up my data scanner tomorrow and watch the ignition timing but I don't expect to see any problems there.

Any clues on things I should look for?
 
Also of note, when I give it gas it runs just fine. That's what makes me think it's vacuum but I've got a lot of experience troubleshooting vacuum lines and am usually hyper aware of them any time I touch anything in the engine bay area.
 
I found this. The drifting between 14 and 16 is the closest description to my problem, but these plugs haven't been touched and have been used for several thousands of miles with no issue so that's not possible. I don't have a points system to synchronize so... Not sure what the equivalent problem would be on a 2000 distributorless model.

Engine Vacuum Reading Chart:

vCYXyAa.jpg
 
My cam sync sensor was half plugged in. Fully plugging it in stopped it from varying between 13 and 15 and kept the idle a little higher, but the vacuum is still at 15 inches. I wish it was more. Ignition advance half plugged in was around 18, now it's around 8 and varies as low as 3 and as much as 13.
 
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