jeff351 Posted June 1 Share Posted June 1 Freshly built 4.0 with about 30 minutes of run time on it (driveshafts not done yet, so i haven't driven it) (.030 over pistons, factory compression ratio, 2.5" exhaust) Fires right up with no issues. Initially I tried a larger throttle body but had a very high idle.. I swapped back to the factory throttle body, and while at it replaced the IAC and TPS. After that, the idle wasn't as high, but it was rough. I replaced the MAP sensor which fixed the rough idle part. Now the idle is still a bit high, see below video. If I rev it, it slowly returns to the (high) idle. Checking if you all have any ideas how to get the idle down. Does vacuum look alright? While it was idling I took a propane torch (not lit) and pointed it at all the vacuum lines I could get to, with no change to the idle. 20240601_154413.mp4 Link to comment Share on other sites More sharing options...
jeff351 Posted June 1 Author Share Posted June 1 92532006_20240601_160131(1).mp4 Link to comment Share on other sites More sharing options...
jeff351 Posted June 1 Author Share Posted June 1 forgot to mention...98 XJ conversion into my 89 MJ. Funny thing is when i was driving the XJ, the idle was a little high back then also, but virtually every part of the engine/ignition/fuel system has been either rebuilt or replaced since then. Wondering if once I break it in with some driving if the idle will come down. Link to comment Share on other sites More sharing options...
Gojira94 Posted June 4 Share Posted June 4 Vacuum looks a touch low, ideal would be between 16-18" steady idle. Your throttle blip looked normal but slightly low- vacuum drop and then jump to 20 when it snapped closed. Maybe 22-23" snapped closed with MAP reading in the 20s kPa would be closer to normal. Steady idle MAP about 50-55kPa. For what it's worth, if the OBDII system is intact, you can put a scanner/ code reader on it (the kind that costs about $80 or so) that can give you real time parameter readings from at least some of the various PIDs. Look for what the IAC counts and TPS % are, and what the fuel trim or basic AFR is. You'd be looking for a concurrent lean condition with the high idle. If IAC counts look like they're high (closing off additional air), pintle fully extended, you might try closing the throttle blade stop a bit until IAC counts are in the normal range and idle comes down, with either throttle body. And then adjust the TPS if needed to read 0% with throttle closed. Maybe also see what the CTS is reading just for giggles, in case it's skewing fuel trim. Also make sure you don't have any exhaust manifold leaks. Link to comment Share on other sites More sharing options...
zomeizter Posted June 5 Share Posted June 5 Looking at the video posted by OP, I would ensure that the throttle return spring is strong enough to get the valve closed, may just be a binding issue... Link to comment Share on other sites More sharing options...
jeff351 Posted June 5 Author Share Posted June 5 Thanks for the replies -I do have a SnapOn scan tool, I didn't think about using that for this issue, so I'll find time to check it this weekend and see what shows up. I also wonder if i have a small vacuum leak somewhere..I'll disconnect a few things and plug them to see what happens. -The throttle return seems strong enough, however after I gave the TB a good cleaning I don't think I lubricated the valve after. Will do that. Link to comment Share on other sites More sharing options...
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