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Odd Jeep Start-up Issue

I have a 1995 Jeep YJ with a temperamental start up that has confused me. In most cases, the engine starts just fine. However, if I stop for something quickly, say 5-15 minutes, then try to start it again, it doesn’t want to start. Battery is fine, starter works, engine turns over, but it just won’t start. Now, this could be a lot of things, but here is the weird part: While poking around trying to find the problem, I have noticed that if I manually open the throttle all the way and use something like a bottle cap to keep it open, the Jeep starts right up. However, if I keep my foot on the accelerator, hold it down for a minute or so, and then turn the ignition, no such luck. What could I be affecting when I manually open the throttle that I am not affecting when I fully push down on the accelerator? I’ve already looked and it appears to open all the way when someone pushes down on the pedal. Any thoughts?

I dunno but when you push the accelerator all the way to the floor as in a flooded condition, it tells the computer to shut off all the fuel to the injectors in order to clear the flood. I don’t know if blocking the throttle plate would do the same thing. To me I would think that the mixture is being screwed up for the engine temp so I’d be looking at the engine coolant sensor or such. The only way you can see that though is with a computer diagnostic test when it is hard to start.

The previous owner did put in a thermostat that was about 35° too low for the engine, so I tried replacing that to the proper one, thinking that maybe the other thermostat was throwing things off, but unfortunately it was to no avail. I just don’t really have the money right now for a diagnostic so I’m hoping that maybe someone once had a similar issue.

A thermostat just regulates the coolant to maintain a particular engine temperature after it is warmed up. The engine temp sensor measures the temperature of the engine and tells the computer what ratio of fuel and air to set for starting. Not related.

The next time you know the engine is going to be hard to start, open the hood and then try starting the engine.

If the engine starts, come back and I’ll tell you what the problem is.

And by the way, when you step on accelerator while cranking the engine, the computer goes into the flooded mode, and cuts the injector pulse signals in half to clear the flooded condition.

Tester

My guesses, either a heat related ignition system problem, or the fuel injectors are leaking gasoline into the engine when it is turned off, creating a flooded engine problem. To differentiate between the two, check for spark at a spark plug next time this happens. If there’s no spark, ignition system problem, likely a crank or cam position sensor or ignition module. If there’s spark ok check the fuel injectors.

I’m not s ure what you mean by holding the throttle open with a bottle cap. It may be the bottle cap method doesn’t cause the throttle position sensor to register in the same way it does when you step on the gas pedal.