2.5 PROBLEMS

M

Mike Mike

NAXJA Forum User
Location
Tucson Arizona
My kids buddy has an 1987 2.5 MJ with throttle body injection. The rpm will drop till it dies. Doesn't mater if hot/cold, at idle or going down the freeway. New fuel pump and ignition module. Any thoughts??
 
Last edited:
this is an intermittent problem, or happens constantly?

was the fuel filter changed as well? id take a look at the injectors while working(if possible,i have gm exp.) to make sure they are spraying correctly. pull the plugs and make sure they look good.. you might want to post this in oem tech, probably get more responses.
 
Check all the vacuum hoses around the throttle body. They crack.

Also, remove the bolt on the firewall that the braided strap is held on by. Scrape the paint off the firewall, clean the cable terminal off and reinstall.
 
They changed the filter couple months ago. It also has a new injector in the throttle body.
He started having this problem and someone changed the ignition module and it ran good for like 30 days and it started doing it again. The ign module was under warranty so I had him exchange it. It ran fine for a day.
It happens almost every time you start it.
It doesn't miss or shudder or shake. It'll idle for sometimes 30 min then the rpm just slowly lowers till it dies. May take as Long as a minute. I looked down the throttle body and the fuel flow doesn't look like it changes.
I though about posting this in the OEM section, but the usual response to 2.5 problems is to replace it with a 4.0.
 
Start it and run back and sniff the tailpipe while the rpm's drop. Do you smell gas?

Pull a plug and see what it looks like.

I ran that setup for a couple of years and loved it. My thought is either a bad MAP or O2.
 
Mike Mike said:
They changed the filter couple months ago. It also has a new injector in the throttle body.
He started having this problem and someone changed the ignition module and it ran good for like 30 days and it started doing it again. The ign module was under warranty so I had him exchange it. It ran fine for a day.
It happens almost every time you start it.
It doesn't miss or shudder or shake. It'll idle for sometimes 30 min then the rpm just slowly lowers till it dies. May take as Long as a minute. I looked down the throttle body and the fuel flow doesn't look like it changes.
I though about posting this in the OEM section, but the usual response to 2.5 problems is to replace it with a 4.0.
Click to expand...

Just like the 4.0 the 2.5 has crappy grounds. The ones at the dipstick tube stud affect the operation of the ISA etc.

Refresh the ground at the firewall and the ones at the dipstick tube stud first before wasting any more time on this. ALL sensors rely on proper grounding to give the ECU good info. The ignition module also grounds at the dipstick tube stud. Could that be why the new one failed? If it doesn't solve it, it will only circumvent a future driveability problem. This is a known issue and was addressed directly by Jeep.
 
The next time he brings it over I was have him go thru all the grounds. I told him he should do that when he bought it. He's not very mechanical and I'd bet money he didn't do it.
 
Mike Mike said:
Probably!!
Click to expand...

Here you go. Skip no steps and take no shortcuts.



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.
　
　
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 11-28-2011
 
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