Jeep 4.0L.....Engine cutting off

[NewGuy]

Hello. I have a 1990 Jeep Comanche with a 4.0L engine. I revived it from my Dad's barn (it was my first truck) and now I am driving it again. It has had ARx (2 treatments), a new thermostat, fuel filter, plugs, O2 sensor, tires and brakes. It runs great and I like having it around for hauling things.

Just this weekend it began dying when I slow down hard and take a turn (like in a neighborhood). It doesn't sputter or idle rough or give any indication that it is going to die. I can drop it into Neutral and crank it again easily, but I can't understand what is making it die.

I have been using FP60 and I might change the fuel filter again, but I am hoping that someone might have some insight on what is happening and how to fix it. This old truck isn't worth anything to anyone but me, but I want to keep it running as long as possible. Any thoughts or suggestions would be appreciated.

 

[adam123]

try the Crankshaft position sensor ( my jeep had this same exact problem) I had to purchase 3 of them to finnanly find one that worked. You may also want to check the coil,the Distrubutor cap,Rotor, and the plug in for the coil (Heard of this one corroding ans causing this same issue many time).

Good luck let us know how it goes.

 

[Gary Allan]

Won't this have the internal diagnostics? That is, the key ON-OFF-ON-OFF-(leave)ON and count the flashes from the CEL??

It should tell you what took a crap ..at least if you do it right when it occurs.

 

[adam123]

its hit or miss. My computer still says that the crankshaft position sensor is bad. But It runs just fine and has run on that same sensor for around 40,000,But it could work I guess my jeeps computer is probably getting worn down from so many miles and years too. Hopin this thing will go to 500,000.

 

[TheLoneRanger]

Newguy, you might try cleaning the inside of the throttle body and maybe the IAC. Carbon buildup there will cause a poor idle, and maybe make it quit.

As Adam says, the CPS is also notorious for causing trouble on those older Renix' Jeeps. That's a possibility, as is a bad fuel pump. It also might be a crack in the neoprene hose that connects the fuel pump (inside the tank). I mention this because of the engine quitting under a hard stop. (Fuel might be sloshing and uncovering the pump and hose).

Unfortunately the Renix Jeeps didn't store trouble codes, so you'll have to fix it using real mechanical diagnostics. Guesswork and luck.....

 

[Gary Allan]

Ah ..the notorious Renix managment system (assume Don Adams/Maxwell Smart voice)

 

[NewGuy]

Thanks for all of the advice. I am going to test the CPS this evening after work. My Chilton manual says that it can be tested using a voltmeter. I have checked around at the Jeep dealership has them for $44.99 and AdvanceAuto has one for $31.99.

If it isn't the CPS I will tackle TheLoneRanger's suggestions in order. Hopefully it isn't a cracked hose in the fuel tank. That might cost more to fix than the truck is worth. I will update after I check things out. Thanks again.

 

[NewGuy]

I replaced the Crankshaft position sensor with replacement from AdvanceAuto. It seemed to help the Jeep start easier, but it still stalls if I have to hit the brakes and slow to a stop quickly.

I will try to clean the IAC and the throttle body this weekend (these are the simplest and least expensive alternatives). Then I will check the coil, distributor cap and rotor. How much trouble and how expensive is it to have the fuel pump replaced? That sounds like a major surgery.

My old Jeep runs great (lots of power, smooth, etc.). I hope I can solve this stalling issue. Thanks for the advice.

 

[TheLoneRanger]

Newguy, if the Comanche tank is like the Cherokee, it's easy to service the pump. It's accessible from under the vehicle, without removing the tank. It's held in by a retaining ring. Tap it with a brass or plastic hammer, 1/4 turn CCW to remove. Then you can lift out the pump and gauge sender as a unit.

The pump is connected to the mounting assembly with a neoprene hose and clamps. I suspect that's loose/cracked, and causing your drop in fuel pressure and stalling.....