I have a '97 jeep cherokee 4.0L I6. It occasionally will crank over and over without starting. The battery and alternator are fine. Otherwise it does not die or stall, though it does run slightly rougher than when I bought it. What could cause this, and how do I check/repair it?

Check the NSS, does it start right up when in Neutral? Same thing was happening to mine, a quick clean of the NSS and I was all set.

Oh and check out:
http://www.pirate4x4.com/forum/forumdisplay.php?f=67

-Bill

Neutral safety switch... good thought. Problem is it will crank forever one try and start right up the next, so it's hard to say if one thing fixed it, or even if it's fixed. Anyway, is it easier to access the NSS from underneath or from above?

It not putting out any codes. Actually it's not even acting up right now. Any thoughts?

With the hood open and it cranking but not starting, do you smell gas?

I had a similar problem, although worse. One day I went out, and my battery had died. No big deal, get the jumpers and hook it up to another car. Engine turns and turns and turns and turns but doesn't start, you can smell the gas getting dumped in. So ok, another easy problem, no spark. Pull the plug wire and touch it to ground, and it's sparking fine (or what seemed to be fine)

Put the plug wire back on, tried more and more and more, still nothing. deduced the spark wasn't enough to fire, and hooked it up to a superduty pickup to jump it, and it turned and turned and turned, and I finally (rather pissed off) just put the peddle to the floor and it turned over. Idled rough for a little and then was fine. To this day, haven't had a similar problem.

I saw no signs of a fuel leak.

The pump is putting out 45psi average, though swinging wildly between 42 and 48.

My pressure gauge also has a outlet port for testing flow. Every time I check the flow it is proceeded by a burst of bubbles. Is there a way the pump can suck air or cavitate? Air in the fuel would make for hard starting and rough running. Is the pump a rotor or diaphram type?

put this on www.naxja.org its an XJ site only. I've gotten alot of answers from them.

Crank sensor perhaps, located on top of the bellhousing and plugs into the harness at the rear of the engine. That'll give the crank -> no start problem when it starts crapping out.

And just because no one has said it yet, this is so hardcore.

I disagree, intermittent problems are harcore, in my opinion. It's hard to fix something that isn't broken when you go to diagnose it.

Surely this rates higher than:
"how do I change my oil" - if you have to ask, you shouldn't be on the road.
"how much are humvee rims worth" - ever heard of a google search?
"fan shrouds" - no way, you can make a metal box with a hole in it too?
"on board air" - bolt it in anywhere it won't be destroyed or in the way.
I'm not intentionally flaming these guys, just pointing out that eveyone has their own level of competence and knowledge in each area. I'm sure one of these guys could diagnose the engine in half the time I can. My lack of specific knowledge with the jeep 4.0 makes an intermittant problem hardcore, for me.

I had this problem, MY ECU plug needed to be cleaned. I unbolted the plug and it looked like every pin was a different length due to the dirt on them. A year later, still no problems.

I ended up changing the crank positioning sensor becasue that is what everyone said to do, took me 6 hours to do and did not change a think.

Ecu pins will corrode if moisture gets in there. I had the same problem. 2 pins where almost completely eaten away. I replaced them(pain in ass) and sprayed a moisture barrier in there and never had a problem again.

No experience with a '97 XJ, but I doubt fluctuating fuel pressure and bubbles in the fuel flow are normal. As I recall, the '97's fuel pressure regulator is part of the fuel pump module in the tank. So you need to search for info regarding testing that particular set-up. On my '88, it would be hard to suck air into the pump. The pump is mounted directly to the filter sock, at the bottom of the tank; however, it would be possible to loose fuel pressure between the pump and the tank outlet, as a short peace of hose is clamped in place that could come loose or split. In fact I have read posts where the hose split, causing loss of pressure.

Fuel pump relay. I had similar prob and this fixed it. Here...http://www.pirate4x4.com/forum/showthread.php?t=128000&highlight=4.0+fuel+pump+relay

A little more info...

All problems are in 'cranking' or 'warmup' mode as defined by the service manual. These are both open loop modes, meaning they do not use the oxygen sensors to correct the air fuel ratio, only the stock fuel tables. Since the majority of time is spent in closed loop modes 'warm idle' or 'cruise' without issue, I'm thinking this may be significant.

My current theory is that there is a intermittent vacuum leak. The O2 sensors can correct for this, except when the engine is not using them, in which case it runs lean and dies. Thoughts?

Is this post REALLY in "Hardcore"? :shaking:

Is this post REALLY in "Hardcore"? :shaking:
When you can tell me what's wrong I'll agree. As you can't apparently it's still too hardcore for you.

OK guys here's the deal as I see it and how things are going to work. It may be named something different now, yet the new forum is going to be general Jeep tech, including newbie. So everything that's not "hardcore" belongs in the std. Jeep tech forum.

What's "hardcore"?

Things like fabbing 4 link suspensions, Jeepish buggy's, Big tires, big axles, fabbing trans adapters, fabbing hardcore parts. a common theme is fabbing.

What is non hardcore?

Lift kits, typical springovers, bolt ons, drivability, identification, and countless other things.

So here we are two months later...

Intermittent problems are not hardcore. They're annoying little bitches that just might require some work with a multimeter. What's the resistance reading on your crank sensor?

Engine coolant temp sensor.

Sensor goes to crap and the computer thinks it is way cooler than it is. Floods on startup and pushing the pedal down to the floor turns off the injectors so it can clear up and fire.

My 91 had the exact same symtoms. It was not turning on the CEL but the computer did have a code that was "engine staying cold to long".

The sensor for the computer is in the thermostat housing and (at least for my 91) only costs $20 or so. You can measure the resistance to check it or used a scanner like I did.

What's the resistance reading on your crank sensor?
Infinite to ground on all 3 posts. red wire to black w/yellow strip is 3.75M ohms.
engine off, key off.
Engine coolant temp sensor. You can measure the resistance to check it or used a scanner like I did.
14K ohms @ 65F. 11.4K-13.6K is the normal range. Is that off enough to be a problem?
Engine off and cold.
Still no codes. Dealer didn't find any(not my idea), my scanner didn't find any.

When you pull the coolant temp connector you get a code 22... I know, well duh, but in conjuntion with a 1.2K ohm reading after running for 45 seconds wouldn't that indicate the sensor is working?

I finally got an interesting result on the fuel pressure gauges.(yes gauges, 2 of 'em, one electric, one mechanical)
Key in on position, pump whirrs for 2 seconds then stops. 0 PSI
Crank engine, little slow to start, but starts. 2-3PSI
Engine idles for 20 seconds. <5PSI
Engine idles and fuel pressure creeps up to 45 PSI.

That seems wrong to me. Shouldn't it hit 45 PSI very quickly and stay there? I didn't think it would run at <5 PSI.

Well, you can ignore the fuel pressure bit. The schrader valve adapter tip bent and wasn't opening the schrader valve, hence the funny gauge readings. I replaced the coolant temperature sensor as it was out of spec.