snipaondeathrow

1994 grand cherokee laredo 4.0 I6
it will start fine in the morning, then after i get where im going and turn it off within a few minutes it wont start again until it sits for around 30 minutes then starts and runs fine.

list of things ive done
------------------------
new battery- battery cables
new spark plugs
new ignition coil
new spark plug wires
new fuel pump
new fuel filter
new fuel pump relay
----------------------------
runs fine until i shut it off, then will not start again until it sits for half hour then cranks right up no problem then drives just fine.

Any Ideas??? cant figure out what i could be missing, starting to get frustrated

97grand4.0

Cranks but wont start , or won't crank at all?

Bustedback

Crank and cam sensors are known for heat related failure.

snipaondeathrow

it cranks and starts fine the first time then if it runs for a bit and i turn it off and try to start it again it spins but wont start, then after about 20-30 min it starts right up

Bustedback

Engine is warm when you shut it off, heat soaks while sitting because of no coolant flow,, cools down after 30 minutes, engine will restart. Sounds like a heat related failure to me.

97grand4.0

Fuel pressure tests. If good try cam sensor, crank sensor, and start and fp relays, (both) in that order. Also check dizt. rotor for bearing play

Priv8Grass

Get new thermostat to make sure engine is cooling properly. Then replace sensors known for heat related failure.

snipaondeathrow

replaced the coolant temp. sensor, and am able to start it back up most of the time now, still runs rough maybe timing issue going to look into it. it still feels like it runs hot and will die on long trips with lots of stop and go. does fine if i dont have to stop very much. also unrelated replaced air filter and oil cap with one that breathes. that doesnt look to affect temp at all(didnt expect it to, just thought it might help oil pressure)

97grand4.0

Crankshaft position sensor. Just went through this myself take it from me DO NOT DO NOT DO NOT buy anything but a genuine MOPAR sensor. Preferably right from, yes, The Dealer.
Did I mention DO NOT buy aftermarket? Here is the skinny, the original sensor has lasted this long, and it is still 3 times stronger than a brand new Duralast. There are many posts about the aftermarket sensors just not working on here, for a reason, they don't. Pay attention.
Take a small jacknife blade and hold it close to each, you will see the difference. Even the original old mopar will be a multiple times stronger magnetic force than the new dontalast.

The sensor is critical in providing ignition information, spark duration to the coil (pulse width) , its all about how the magnet picks up the metal posts on the flywheel as it is spinning by below it. Weak magnet, weak signal ! confused PCM, erratic spark, backfire starting up, no starts, starts when cooled off.
Footnote, the dealer parts guy felt sorry for me, when i asked about the Distributor Switchplate aka cam sensor, he priced it for me, then confided, in the 25 years he'd been there he sold only maybe a dozen of those. It's usually always the crank sensor.

snipaondeathrow

Just wanted to post an update
replaced the crankshaft position sensor and hasnt died again and i can start it right back up again and agreed mopar part only.

now just to deal with my idle problem.......
it idles rough - especially with A/C on
it feels like a miss but its not missing, like its running rich,
would a bad upstream O2 sensor cause this?
or what are the symptoms of a bad upstream O2 sensor?

dave1123

Disassemble and clean your Idle Air Control, or replace it if you like. Again Mopar. The PCM (computer) uses it to control the idle air bypassing the throttle plate. Dirt can build up over time and disrupt airflow.

97grand4.0

*Agree* but I tried cleaning and only way to fix mine was replace it. So, for that I sinned and got an Ebay oem..for $14. Works like a champ though. Other half of that job is check your vacuum hoses CAREFULLY esp where they connect, they get loose and we don't want vacuum leaks.

snipaondeathrow

do you happen to have a part number for the ebay part?
and im going to replace the upstream o2 sensor too i got black soot in the tailpipe so might do that as well maybe i can get a little better mpg

97grand4.0

Take your pick, there seem to be a ton of them on there now. Trying to find OEM is a little trickier, but, IMO I think the IAC is somewhat less critical to find a MOPAR than say a cps or ckps. I would never use anything but mopar on those applications. I also got a O2 sensor from there for similar pricing. But, you have to watch those, the california cats wont work with most of them..there is a solution which is pretty simple but we don't discuss that here.