Interesting issue with my XJ has surfaced since I parked it 5 months ago: When I start the car cold it will idle up to about 1200 RPM and then gradually idle down to ~800. When driving the idle in drive at a stop is also ~800. However, once that car is fully warmed up, if I return it to park, the idle will jump to a steady 1200-1300 RPM. If I get out of the car and lightly touch the assembly, it will bring it down to about 900 but that still seems a tad too high (although very smooth!)

I've already replaced the IACVs about a year ago and followed a YouTube video on adjusting the throttles last night (not the method mentioned in Kirby's book) to no avail. I noticed the cruise cable was a little tight and I loosened it up which helped a bit, but being midnight, I did not get the chance to fully warm up the car to try it again.

I'll give it another go this afternoon/evening afterwork, but if its back, where should I start? Vacuum leaks? Throttle return springs? The Kirby throttle adjustment method? All of the above?

 

You should have two concentric throttle return springs on each throttle. Last year, with an idea to reduce throttle pedal effort, I removed one spring each side. The result was a high idle. So I put them back in! Both return springs are needed (at least on our early versions) to keep the idle vacuum from pulling air past the closed throttle plates.

The U-shaped hoses from the IACV to the intake manifold must be airtight of course. The lower parts were a bit degraded on my car.

 

So I found the issue and it was 2 fold actually:

1. My driver's side throttle butterfly was not adjusted properly.
2. The vacuum line that comes off the driver's side intake manifold was disconnected deep down near the transmission!

I suspect this hose was failing/coming off when I first bought the car as its always had a little bit of a high idle/intermittent rough idle condition. Simply reattached the hose (easier said than done, required a borescope and multiple attempts to get it on) and now not only does the car idle at the correct speeds cold and hot now, but its now as smooth as legend has always said!

 

#2 Wow that's a nice find. I couldn't remember that hose there, had to go check for it. Found it. It is shown on the underhood vacuum diagram label. A definite vacuum leak if damaged or not connected. It is for the purge valves. Mine is ok, I've never disturbed it. Connection #1 below (although its at the bottom of the LH V).

 

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