More Failure To Start Issues

[Braxton]

88 4.0, Aw4, 4x4. Two years ago I was having crank but no fire issues, fixed it by running a jump wire to the fuel pump relay. Last week I was driving and the fuse in the jump wire blew for no reason, and kept blowing every fuse I put in. Towed it home and pulled wiring harness. Found a fried fuseable link and reattached the wires. The fuel pump kicked on with the ignition like it is supposed to. Then the next day I was driving home and the wiring that goes from the A/C relay on the passenger side to the driver side of the radiator was smoking and melting. Got it home, and found and installed a wiring harness from an 87 Cherokee, 4.0, AW4, 4x4. Everything hooked up just like the factory harness, but when I turn over the key the fuel pump does not kick on, just like before. Any ideas? I am losing my mind, money and GPA. List of parts replaced two years ago up to now include;

Fuel pump yesterday

Computer last week

Plug

Wires

Ignition control module

Coil

Cap

Rotor

Ballast resistor

Ignition switch

All 4 relays

Wiring harness

Any ideas are much appreciated. I just want to get it running so I can sell it.

[1989 Eliminator]

I say upgrade your grounds on the truck. This topic is from Cruiser54's page that he wrote up.

 

Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal.  All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals.  Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4” socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18” long with a 3/8” lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10” long with 3/8” terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.

If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com.  He makes an incredible cable upgrade for a very reasonable price.

Revised 12-04-2012
I added to this so it would include a ground connection specific to the Comanche.

[Braxton]

So I cleaned the grounds and still no luck. I then found a melted and separated fuseable link which provides power to the fuel pump relay. The link was connected to the starter relay. Replaced with a new fuseable link and was able to get it started. Now it won't rum for more than 45 seconds at a time. And when you hit the gas it dies completely. Any ideas on what it is now? I am running a new Bosch fuel pump that the store says is the correct pressure and flow rate for the truck, do I need to look into a bigger fuel pump?

[ParadiseMJ]

If I'm having a problem with a stock item, I usually replace it with a stock item, unless it's a viable upgrade.  A bigger fuel pump won't solve your problem.  Fixing the stock system properly will serve you better than "running a jumper wire".  A stock pump should supply the right amount of pressure if it's got the right amount of voltage.  I'd say your problem lies in the wiring, not the pump.

[Biotex]

NOTE: To increase fuel pressure, turn adjustment screw inward. To decrease fuel pressure, turn adjustment screw outward.

4.0L MPFI

1) Remove cap from pressure test port in fuel rail. Connect Fuel Pressure Gauge to pressure fitting. Start vehicle.

Pressure should be approximately 31 psi (2.7 kg/cm2  ) with vacuum hose connected to pressure regulator.

2) Pressure should be 39 psi (2.74 kg/cm2 ) with vacuum hose removed from regulator. If fuel pressure is not to

specifications, check for kinks or restricting bends in fuel supply and return lines. Check fuel pump flow rate. Pump should deliver

minimum of 1.06 quarts (one liter) of fuel per minute with fuel return line pinched off.

3) If flow is inadequate, check system for plugged fuel filter or filter sock. Fuel pump flow rate can be checked by

connecting a hose to fuel test port on fuel rail and inserting other end in clean container.

4) To operate fuel pump, install a jumper wire into diagnostic connector terminals D1-5 and D1-6. Pinch off fuel return

line to ensure that no fuel returns to fuel tank. If fuel pressure is still not to specifications and fuel flow is normal, replace

regulator.

 

IMO you have an electrical issue. Whatever is causing the fuseable link to melt is the problem. Since you kept blowing fuses, I'd venture to guess it is a direct short to ground. Look closely at your rear harness that feeds the pump.

[Braxton]

Issue resolved. There is a 3 prong connector that plugs into an assembly on the firewall with a vacume line running into it. Connector was loose.

[cruiser54]

Issue resolved. There is a 3 prong connector that plugs into an assembly on the firewall with a vacume line running into it. Connector was loose.

MAP sensor.

[Braxton]

Drove the jeep for a week without issue. Today I shut the truck off and when I went to start it up 30 min later it would crank but not fire. Checked all my connections and nothing. I fiddled with the map sensor and it wanted to start but would only shutter and die. Finally started but it ran really rough, like it was running out of fuel. When I fiddled with the m sensor while it was running rough it would smooth out for a bit then die. I replaced the MAP sensor with no change. When I hooked up the new wiring harness and was having rough running and die issues I found that I had the map sensor wiring plugged in backwards. Did something get fried and take a week to quit working? Any help is greatly appreciated.

[cruiser54]

It did not run for a week hooked up wrong.

[Braxton]

Which is why I said "after I hooked it up correctly it ran fine for a week". Now when I try to start it, it acts like it wants to start but won't. When it does start it idles really rough at really low RPM, if I give it gas it dies. It is popping and sounds like its only running on 3 or 4 cylinders. Replaced MAP, TPS, IAC and O2 sensor with no change. Anyone have any ideas?

[cruiser54]

Fuel pressure low?

[Oddmodman]

Do autos have issues with their CPSs going out? 

[Braxton]

Gonna have to buy a pressure tester tomorrow and check the fuel pressure out. The pump is only a week old. I'll be more than a little upset if I have to get into the tank for a third time in 10 days. And the CPS is fresh and working as far as I can tell, due to having constant spark. How can I test the fuel pressure if it won't stay running though. Any ideas?

[cruiser54]

It builds pressure when cranking.

[Braxton]

Crappy fuel pump does not have the nipple on the outlet of the pump and the hose slipped off. Had to pull the pump for the 4th time in a week. Put the hose back on with two clamps and cranked em down as hard as I could. Now I have low and surging idle.

[cruiser54]

Now check fuel pressure.

[48MJ]

I have an 88 4.0 auto MJ and it is doing the exact same thing. Went through all the same tests after replacing the fuel pump. It was running great when I shut it off the night before, When I went to start it the next morning it just cranked but would not fire. Anyway diagnosed a bad fuel pump and replaced it then it would start but only idle (bad idle) give it gas and it acted like it would die, let off and would go back to the cruddy idle.My fuel pressure and volume is good and I have parts jeeps so I started replacing sensors and looking for vacume leaks found nothing wrong and nothing I changed made any difference. Checked all the grounds. Still trying to figure it out. I'm wondering if it is the computer now, but don't know how to check it. Anyone have any ideas? tired off throwing parts at it.

[cruiser54]

I have an 88 4.0 auto MJ and it is doing the exact same thing. Went through all the same tests after replacing the fuel pump. It was running great when I shut it off the night before, When I went to start it the next morning it just cranked but would not fire. Anyway diagnosed a bad fuel pump and replaced it then it would start but only idle (bad idle) give it gas and it acted like it would die, let off and would go back to the cruddy idle.My fuel pressure and volume is good and I have parts jeeps so I started replacing sensors and looking for vacume leaks found nothing wrong and nothing I changed made any difference. Checked all the grounds. Still trying to figure it out. I'm wondering if it is the computer now, but don't know how to check it. Anyone have any ideas? tired off throwing parts at it.

 

Define "checked all grounds".

 

Did you dislodge the throttle body to MAP sensor hose or the electrical connector at the MAP?