At my wits end. Computer issues

[Demonoid369]

97’ 4.0L auto XJ no security/alarm. Manual locks and windows.
Like 8 years ago, I backed my perfectly working XJ into a parking spot to remove axles and do an axle and lift that got put on the back burner because of other priorities. 8 years later(this year) and several relocations(without starting the jeep(battery was dead) it at a shop now and axles and susuepnsionn are complete. BUT the issue now is my Jeep won’t start. They diagnosed it as the ecu as dead, because they got their shop 99’ ecu to start my Jeep. So I bought a 97’ ecu, it didn’t work. Bought another 97’ ecu, still no start, then bought a documented and verified working 98’ ecu, still won’t start. I have switch between the two 97’ and 98’ ecu with none of them working, the engine will only crank and crank. I toss the shops 99’ ecu in and the engine will fire and run. Now I know i didn’t get 3 dead ecus, because one was genuinly verified and recorded working the same day they shipped it. So now I’m thinking something is screwy with my jeep. Why is only one ecu working on my jeep and no other ecu is? What can cause that to happen? I’m at my wits end trying to even figure out where to narrow the search to diagnose this odd problem. I have another ecu coming that is a 99’ to verify if it’s a year specific issue or not. But from a fellow Naxja member(Ken) 97’-99’ all ecus should start any of those years, just some might cause a engine code. What would cause only one ecu to be able to start my jeep and no other ecu will? Where do I even look or narrow my search?

 

[Saudade]

Just guessing here. Are you sure the other ECUs were not SKIM enabled and maybe the '99 from the shop is not? I realize SKIM should still allow the engine to start but not run long.

In any event, the "standard" process is to verify fuel and spark. We know it runs so it has acceptable compression and timing.

Did you buy the ECUs from the same source?

 

[Demonoid369]

Just guessing here. Are you sure the other ECUs were not SKIM enabled and maybe the '99 from the shop is not? I realize SKIM should still allow the engine to start but not run long.

In any event, the "standard" process is to verify fuel and spark. We know it runs so it has acceptable compression and timing.

Did you buy the ECUs from the same source?

I know it has fuel and spark, because the one 99’ ecu proves that by firing and running. I even let it run for 10 minutes to verify everything was working. It has a high idle but that’s because I think the iac is failed after sitting for 8 years.
As far as I was told from each seller, the ecu did not have skim. Which as you said, should have allowed it to start regardless, just not run for a long time. But every ecu besides the 99’ won’t fire, it’ll just crank and crank. I’ll toss the 99 in and it’ll fire up and run immediately l. I don’t know what to make of that honestly

 

[Demonoid369]

Bump any idea why only one non skim ecu will fire up my jeep while all the other non skim will only crank?

 

[MAY POP]

What Saudade is referring to is what are the other ECMs not doing?
With the 99 ECM you have proven fuel/ pump and electrical system/CPS and cam sensor. That suggests something wrong those ECMs. Will they sell the 99 ECM?

Ron

 

[Demonoid369]

What Saudade is referring to is what are the other ECMs not doing?
With the 99 ECM you have proven fuel/ pump and electrical system/CPS and cam sensor. That suggests something wrong those ECMs. Will they sell the 99 ECM?

Ron

No he won’t sell the ecu. All the others ecu’s just crank and crank. That one 99’ ecu is the only one that will actually crank then fire up and run. And the newest ecu which is a 98’ ecu, was recorded being plugged in and firing up a 99’ late model conversion mj to show it would crank and fire. But when I got it and plugged it in, all it would do is crank and crank

 

[markatimber]

any updates?

 

[Tim_MN]

I would suggest two logical scenarios:


(1) The CPS is faulty and sometimes it works with one ECM, and sometimes the other ECM's are not happy with it. ECM failure is very un-common. CPS failure is very common.

(2) If SKIM is engaged, or is faulty, the engine would start, run for 3-5 seconds, and then shut off. Repeated unsuccessful attempts to start the engine while SKIM is engaged can brick the ECM and it will not run the engine. I do not know the exact symptoms if bricked.

The logical first step is to test the CPS, or to swap in a known good CPS. Any CPS that is not genuine Jeep is suspect Chineseium, even if new.


When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.

CPS TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.