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bradleyheathhays

cranks but won't start

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So I got this '96 Jeep Cherokee XJ that cranks but doesn't start.  It's got around 180k on her but I keep everything in good shape.  Batt seemed strong.

 

I used to drive it everyday but this past year I've driven it only about twice a week.  It's been just slightly slower to start in the last couple of months, but just barely slower.  Was guessing it had something to do with aging plugs or something.

 

Drove it yesterday and everything was normal like always, sat overnight and today it cranked but didn't catch.

 

Any ideas on where to start?

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Gracias twintornados.  That sounds exactly right.  How do I check that the ignition coils are getting a signal to fire.  Is there a tool that does that?  I heard the fuel pump come on so at least that can be ruled out.  To check the fuel I suppose you get a fuel pressure gauge, and if the Hg is in range then fuel can be ruled out?

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Does it have spark plug wires or coil on plug? FYI Jeep 4.0 crank sensors are a common occurrence iirc. If it uses plug wires, take one off and lay it close to something metal like an intake bolt, have someone look while you try and start it and you will see speak arching during cranking.  Another fun way is to pull plug wire have a friend hold the wire end and touch the engine while you crank. If they yell, then you have spark. 

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Another shade tree way to see if it’s getting fuel or spark is take some throttle body cleaner, spray into throttle body while you try and start it. If it runs or spits and sputters, you have spark but no fuel. If it doesn’t nothing I suspect spark. 

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Thanks for the info.  Those sound like simple and quick tests.  Good to know.

 

So far I've been able to run a couple tests..

I hooked up a fuel pressure gauge and first key turn got it up to only 20psi. Second turn got it up to 45. And third and subsequent turns got it up to 48. It only fell to 43 after 2 minutes so I think that means the fuel pressure is good.

Hooked up a spark tester between a plug and wire and it didn't light at all when cranked so I'm guessing I've got a spark problem. I'll start by doing a tune up with new rotor, cap, wires and plugs then see what happens.

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I’m curious, you ask what to test. However, you know how to check fuel pressure and you have and know how to use a spark tester. So you have some idea already of what to do. Obviously spark fuel and air make it run. You’ve identified it has no spark but you are going to replace cap and rotor. Which if you check the spark coming out of coil and there is none, new cap and rotor are not your problem. No spark will be a PIP (profile ignition pickup) problem which could be crank sensor, which is common and relatively inexpensive. Another basic thing would be to take the distributor cap off and make sure the rotor  spins as engine turns to eliminate timing chain/cam/cam gear issue. Cap and rotor would only be replaced after you get spark Out of the coil. 

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On 9/5/2019 at 3:03 PM, fordtech1 said:

Does it have spark plug wires or coil on plug? FYI Jeep 4.0 crank sensors are a common occurrence iirc. If it uses plug wires, take one off and lay it close to something metal like an intake bolt, have someone look while you try and start it and you will see speak arching during cranking.  Another fun way is to pull plug wire have a friend hold the wire end and touch the engine while you crank. If they yell, then you have spark. 

I remember watching an old time mechanic back in the seventies perform a spark strength on a early sixties Valiant with a slant six. He would actually remove each spark plug wire and hold the end in his hand to find the weak one, all with the engine running. Of course ignition systems were only capable of 20K volts back then, but to see his arm spasm was incredible !

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