Help a noob diagnose intermittent starting problem?

Hi folks,

My father bought the vehicle 8 months ago. 1996 Laredo 4.0 with 90k miles. It's been sitting in his driveway most of the time. Recently, it wouldn't start. His mechanic replaced the alternator and battery.

He doesn't need the vehicle and sold it to me. I've been driving it for the past week around town, no problems. Then Yesterday out of the blue it won't start. It cranks and battery gauge shows normal power.

Let it sit about 2 hours, came back and it started up.

Later last night it wouldn't start again.

This morning it started up just fine. I drove it down and dropped it off at the same mechanic, so we'll see what he says Monday morning.

Any thoughts on what might cause such a intermittent problem with it starting up? Especially since after letting it sit for a while it starts up just fine?

Try cleaning out your IAC

I had that problem for 3 months, liked to drove me nuts. Turned out the air intake temperature sensor was bad. It would not start, or had a lot of trouble starting cold, first morning start, but started fine the rest of the day when it was still hot or warm. It and other computer control sensors can be tested with a volt-ohm multi meter.

Get yourself a jeep manual at the auto parts store, like a Haynes, read up here on older threads about starting problems, and get a $10-$20 multimeter if you don't have one. The old style needle, called analog, meters are more useful for jeep testing than digitals.

Could be the CPS, IAC, IAC B+ latch relay, a leaky fuel injector, CTS or MAT going bad.

So it looks like it will probably get a new fuel pump tomorrow.

The pump is located in-tank, I know that much.

Where is the fuel filter located? I'll have that replaced as well.

gotmelk said:

So it looks like it will probably get a new fuel pump tomorrow.

Click to expand...

Why?

Well, because reading around it seems that the fuel pump failing is fairly common on XJ's, and is a likely candidate for the intermittent problem that mine is exhibiting.

That, and the fact that I don't have a multimeter, and wouldn't know how to use it anyway. I'm not even able to locate any of these various sensors. Total Noob.

Not that I'm opposed to getting a Hayes manual, a multimeter diagnostic tool, etc and learning these things, but I came up from Charlotte to New York to pick up this Jeep and drive it back.

It was working fine the whole week, and then literally the night that I go to leave it wouldn't start up. So I just need to get it home first, and then I can tool around with it on my own time.

gotmelk said:

Well, because reading around it seems that the fuel pump failing is fairly common on XJ's, and is a likely candidate for the intermittent problem that mine is exhibiting.

That, and the fact that I don't have a multimeter, and wouldn't know how to use it anyway. I'm not even able to locate any of these various sensors. Total Noob.

Not that I'm opposed to getting a Hayes manual, a multimeter diagnostic tool, etc and learning these things, but I came up from Charlotte to New York to pick up this Jeep and drive it back.

It was working fine the whole week, and then literally the night that I go to leave it wouldn't start up. So I just need to get it home first, and then I can tool around with it on my own time.

Click to expand...

JEEP stands for Just Empty Every Pocket. Even more true if you start randomly replacing parts with doing tests first to find out what is really wrong. Your symptoms are no where to close to a bad fuel pump.

If it drives fine, once started it is not the fuel pump. If it pumps; if you can hear it prime for a few seconds when you turn the key from off to run, not start, and it does not start in start, then it is not the fuel pump.

If it does not start after 2 tries, floor the gas peddle, hold it at wide open throttle (WOT), while cranking the third time. If it was flooded that will unflood it and start it.

Yeah, I've now moved away from thinking it's the fuel pump and from reading other posts/forums with people with same model and year, describing the same problem it seems to be that replacing the Crankshaft Sensor fixed the problem for damn near everybody, so that's what I'll be doing

My money is still on a sticky IAC (just clean with solvent very carefully and oil it), IAC B+ latch relay (need a meter to test, or swap relays to see if the problem moves, or relays are cheap and easy to change, the CPS is b*tch to replace), a leaky fuel injector (acts flooded, so floor the gas peddle to test, that dries it out), or MAT, which makes start / no starts temperature sensitive, and can flood it.

Bad CPS usually causes multiple problems, like rough running at higher rpms, above 2000 rpm, jeep suddenly dies while driving, plus starting problems. When the CPS completely dies, it never starts. IAC is the Idle air controller, it is mounted on the throttle body.

MAT is the temp sensor, 2 wire, brass thingy threaded into the intake manifold, rear area of the intake manifold near the firewall.

MPslayer and Ecomike are leading you in the right direction. CPS no start, come back later and it starts will occur when the jeep is hot, not cold. Also of note is these engines don't like multi-electrode platinum plugs. I had a lot of cold start problems with Bosch platinum +4 plugs. I'd start with a simple IAC cleaning, plug check, and sensor check before dumping $$ to replace every sensor and fuel pump.