Corky

Nice heat shield...... Sounds{and looks} like the tach is reading correctly...

MJ/XJ Owner

Nice looking Jeep! Nice to see it all stock under the hood. Hard to find them that haven't been altered. So you don't have an Idle flare up issue? Just high idle? Then I would follow what Cruiser54 is saying, you're pulling air from somewhere.

lawsoncl

For giggles, I pulled the audio up in Audacity, filtered it a little to see the rpm peaks, and I counted a smidge over 1000 rpm. :}

MJ/XJ Owner

@arpunk On start up the engine races to usually around 2500 rpm's and stay's at that.

On the early 4.0 liter with the renix system, this was caused by bad connections (usually caused from moisture intrusion) or a TPS that was faulty, also a result of water intrusion. This was a big problem back when I worked at the dealer (80's and 90's). But we had a scan tool to look at the TPS live readings and it was easy to identify and fix.

Your idle should always be around 750. If your concern is only idle at 1100 and this all started after you cleaned the throttle housing, I would question if you have a vacuum leak or does this throttle housing have an air bypass screw on the housing as your facing it from the drivers side fender? Has the plug been removed at some point and adjusted to compensate for a dirty throttle housing? and now that you've cleaned the housing the idle is high? My comanche is a 91 and my cherokee is a 99 so I haven't looked at a renix system in 20 years and don't remember.

MJ/XJ Owner

@arpunk Start up to 1800 and back down is normal. when it stays at 1800 is when you have an issue.

I have never adjusted a throttle plate (butterfly, I don't even remove the throttle housing when I clean them. I just use carb spray a an bottle brush with the engine off, once cleaned I start and spray more to make it shiny clean, just don't do it inside a garage. Never had a problem with O2 sensors, (In case you wondered). I do it on my jeep and my 91 and it has the original O2 sensor @ 250000 miles. I had to replace my 99 once but for another reason.

Just keep original parts, when you start adding aftermarket parts you can create new problems.

lawsoncl

I used the EQ curve to filter out the high and low frequencies, and bumped up some mid-range frequencies by trial and error until I could see distinct repeating peaks in the time series. Then counted 26 peaks over roughly 1-1/2 seconds, which x60 = 1040 rpm. Audacity might have a plug-in to do demodulation, which would make it more obvious.

With the right processing, and a well place mic you can pick out the individual gears and bearings in a transmission.

arpunk

@lawsoncl youre saying that you were looking at the waveform and counting the spikes? If you wanna go nuts I can put a microphone right up to the engine. I have a Sennheiser MKH60 as well as a Schoeps MK41. All the rest are omnis.

arpunk

After spraying everything down with carb cleaner to check for vacuum leaks, tightening down intake manifold bolts, and trying different TPS output voltages, I finally gave the throttle stop plate that rests against the screw the TINIEST little bend and now it idles perfect at 600 or so as per FSM.

dmoe69

I know it's super tight back there, but can you take a picture of what you bent? And if I recall, you bent it because the adjuster screw is stripped.

arpunk

Adjustment screw was stripped, yes. I used a pair of pliers to give the plate where I have drawn the red stripes just the *slightest* twist in the direction of the arrow. It was an almost imperceptible movement. If I had moved it too much my plan was to glue a very very thin piece of sheet metal to the plate or perhaps add a dab of JB weld to the end of the screw.


The next picture is a close up of the area in the yellow square

I rotated the red striped area just a TINY bit counterclockwise

74Plb

Success! Where there's a will, there's a way

arpunk

This was my last resort. I didn’t want to touch that but I don’t know if the PO messed with it previously so I figured I’d give it a shot!

lawsoncl

Did you check the clearance per Cruiser #14 tip? (which was coincidentally posted in #14 of this thread)

As mentioned in Cruiser's Tip and your thread on Reddit, after cleaning out excessive buildup or a worn throttle body or if a previous owner might have dorked with it, you might need readjust the set screw to get the right clearance.

arpunk

This problem has been solved. I wrote it in a post a few comments up as well as in the original post. The 3/32” set screw was stripped so I went through every other option before bending the plate that rests against the set screw. Ultimately, that solved the problem.