Jump to content


  • Posts

  • Joined

  • Last visited

  1. Would a sub-standard catalytic converter (cold or hot) cause a signal-based failure to start (ignition switch, not push-start)? I apologize in advance: you may need to review the thread to answer.
  2. Wouldn't a filter swollen from water contamination also prevent a push-start? Recall that even when failing to start normally, a push-start always works and the engine runs normally afterward. Regardless, filters are cheap, so worth a try. Will also fully drain and clean out tank.
  3. Are you stating that the fuel/air ratio and quantity at startup are pre-programmed and not dependent upon initial throttle position? Opening the throttle means more air, so if the ECU holds the fuel quantity constant at startup, regardless of throttle position, then a slightly more open throttle creates a leaner mixture, more easily combustible. Sounds like the ISA continues to move the plunger out in order to increase the engine's ability to fire up. It shouldn't have to work that hard, though. I'm keeping at it. Going to continue cleaning and testing! Thanks!
  4. New info. Just failed to start, but for the first time, I was able to start it with very slight pressure on the gas pedal while cranking. I thought that only worked with 6 cyl., and it's actually 50 to 75% depressed for a hot engine start. I was only pressing about 1/8"! Sounds like the extra air/fuel helped, but throttle might actually not be at best starting position. ISA is definitely resetting after shutdown, but whether or not it's setting the throttle to the best location is the question. Turns out I was right. Read on. If I accidentally turn the key to start only for a fraction of a second (just long enough to turn over once), the idle actuator used to "pump" the accelerator pedal several times when the key was returned back to the Run position, as if to prepare for the next startup attempt. It rarely does that any more. Is that the "ratcheting" that occurs if the ISA motor runs when the actuator is at full extension? (Yes it is!) I am guessing that the ECU interprets a failed start as a shut-down and attempts to re-position the ISA - or is supposed to. Is that correct? (Yes it is!) I'm so close I can feel it! And more: just made some progress. When startup fails, ECU sends power to ISA again, opening throttle a little more. If startup fails again, ECU does it again, until actuator is at full extension, and sure enough, the ISA motor ratchets, shaking the accelerator pedal. Now, rewind back to the the start of this latest post. Stepping on the gas just a fraction simulated the ISA opening the throttle a bit more. On shutdown, does the ECU aim for a pre-set physical throttle position (and if so, does TPS or CTS provide feedback) or does the ECU simply energize the ISA through the B+ relay for a preset amount of time, regardless of throttle position? I just read that last question to my wife, and she just stared at me, then asked, "So, is it fixed?"
  5. Super! Thank you. I hurried outside to resume troubleshooting, and found that my wife drove it to work! Aside from the fact that I can't work on it, she also has no hill to park near in case she needs a rolling start! But I won't ask your advice regarding my wife...
  6. Msongy...Vacuum circuits are always a concern. However, you're sharing that the engine runs great once started, and it starts every time you push-start the vehicle. Of course, you do want the vacuum circuits to be routed correctly without vacuum leaks, as this can affect overall performance. Leaks also include the brake vacuum booster or even the brake booster's check valve. Worth a glance... If you have any suspicions about the vacuum circuits, you have the FSM for diagrams. To save time, there is an earlier post where I added a PDF of the vacuum circuits for a 1987-90 Jeep YJ Wrangler TBI 2.5L: Follow my highlighted comments above, and you will systematically eliminate possibilities. Among those possibilities is the success or failure of starter fluid. There's a fundamental mechanical problem, and you're going to find it! Moses
  7. Thanks! I don't know whether to be relieved or concerned that you didn't mention any vacuum lines. Is that because checking for broken, dirty lines is obvious or because the logic doesn't support it? Also, does the occasional success with starter fluid (key start) indicate anything in particular? I must also add that the frequency of success with starter fluid has been slowly declining over time, indicating either increased corrosion, pitting at contacts or buildup of material in lines, filters or orifices. Or all of the above! U agree?
  8. T-shooting an intermittent starting problem. About 80% of start-ups are perfect. When failing to start, it will turn over and over, but won't fire up. Sometimes starter fluid will cause it to fire up and then it runs fine, but that's only 50% effective. The rest of the time, only push-starting will work, but push-starting does work every time. Most obvious assumption is fuel problem, but fuel system checks out fine (replaced almost everything over time anyway). Ignition/electrical is fine. Also, the problem seems to be completely independent of outside and engine temperature. I started reading about ECU inputs, and discovered that during startup, an engaged WOT switch causes ECU to cut off fuel because ECU thinks it will flood the engine. The WOT switch is worn, but shouldn't prevent starting if disconnected. Would any other ECU inputs prevent fuel flow during start-up if their signals are outside normal limits? Service manual says during "Ignition On", CTS, MAT and MAP sensors begin providing input to ECU, but doesn't list MAT or MAP sensors during actual start-up. See attached page. So which sensors matter to ECU for start-up fuel supply calculations or decisions? Also, during push-starts, the ignition (key switch) is in the Run position, and not the Start position, which changes the ECU inputs, too. My shop has a DRB II diagnostics reader, but unless they hook it up when it happens to be misbehaving, shouldn't everything check out ok? Unfortunately, I've never been able to get it to fail at the shop. I also tried replacing the fuel relay. I also noticed that it is no longer cycling the ISA motor when it fails to start. It used to, but I can't figure out why it did that anyway. What are my next moves? Might be trying to oversimplify this, but what kind of intermittent problem would occasionally prevent a normal startup, but NEVER prevent a push-start from succeeding?
  • Create New...