n7kme Posted February 28, 2015 Share Posted February 28, 2015 I have a very tired 1990 YJ 2.5. It spent it's life as a pit vehicle at a sand and gravel pit in north Las Vegas. It has had runability issues from day one with rough idle, to very little power. I have changed the tbi injector, cps, tps as well as the map sensor, ignition module, coil, plugs, wires, cap, rotor, and egr valve. I just put a timing light on it to see where my timing was and at an idle it is showing 15 degrees btdc. Is that normal? Or is it possible the timing chain has slipped a tooth? As the idle increases it moves toward tdc and at about 15 to 16 hundred rpm it's at tdc. Any thoughts on loose timing chain vs. bad module somewhere? Thanks for any help. Quote Link to comment Share on other sites More sharing options...
n7kme Posted February 28, 2015 Author Share Posted February 28, 2015 Revision: as the idle speed increases the timing moves considerably further away from tdc. Evidently I was not seeing the mark correctly. It goes so far btdc that I can't see the mark at 1500 to 2000 rpm. (Or could this be a vacuum leak?) Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted March 1, 2015 Administrators Share Posted March 1, 2015 Hi, n7kme...Ignition timing on a Jeep electronic fuel-and-spark management system, including 2.5L TBI, is fully a function of the ECU and its sensor signals. The distributor is simply that: a way distribute spark to the plugs. All spark timing and fuel supply functions are computer (ECU) controlled. The only spark timing verification is base timing. The procedure for setting base timing is in the factory data below, although this factory data makes an erroneous reference to hooking up the vacuum advance hose. (There is no vacuum or centrifugal advance on the 2.5L TBI distributor. Only the 2.5L carbureted engines use a vacuum and centrifugal advance.) I included both the vacuum circuit diagram and the distributor removal and replacement details. The distributor information talks about the positioning and base timing setting. Follow the vacuum diagram first. Then, if the problem persists, consider removing and installing the distributor "by the book": YJ 2.5L TBI Vacuum and Timing.pdf Moses Quote Link to comment Share on other sites More sharing options...
n7kme Posted March 2, 2015 Author Share Posted March 2, 2015 Thank you very much. I'll give that a try. I knew the timing wasn't adjustable and was thinking that 15 or 16 degrees btdc at idle might indicate a slipped tooth on the chain. Thank you for the pages, it helps. Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted March 2, 2015 Administrators Share Posted March 2, 2015 You're welcome, n7kme...If you're concerned about possible timing chain wear or looseness, see my quick check for wear: http://forums.4wdmechanix.com/topic/174-quick-test-for-engine-timing-chain-wear/?hl=%2Btiming+%2Bchain Note: When a loose chain jumps a tooth, the ignition timing will lag and not advance. A worn chain will also cause ignition timing to retard. Think in terms of "late" timing... We also discussed low vacuum and timing chain issues for the Jeep 2.5L TBI engine at: http://forums.4wdmechanix.com/topic/343-jeep-yj-25l-tbi-rough-idle-and-very-little-vacuum/?hl=%2Btiming+%2Bchain#entry1883. The search box works fairly well at these forums. Try a search for "Forums" and use key words like "2.5L TBI", "timing chain" and "ignition timing" (each without the quotes). Let us know how this unfolds... Moses Quote Link to comment Share on other sites More sharing options...
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