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1998 jeep Cherokee injectors won't fire


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Theprofessional23
Novice

Aug 7, 2015, 7:49 PM

Post #1 of 10 (1992 views)
1998 jeep Cherokee injectors won't fire Sign In

Hello all, first time posting here, I have a 1998 xj and it has started throwing codes p0204,5,&6, and I've tried everything, test light showed up fine, cleaned grounds, swapped injectors and codes stayed on those cyl, new plugs, new distributor cap, new pcm, and have narrowed it down to knowing for sure that the injectors on those cyl are not giving fuel, but connectors/harness tests good on everything(voltage,resistance,ground)? Me and about 3 mechanics and 5 very good car people are completely stumped.. Please help!...


Discretesignals
Ultimate Carjunky / Moderator
Discretesignals profile image

Aug 8, 2015, 5:36 AM

Post #2 of 10 (1964 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Is the engine actually skipping while it is running? NOID lights don't pull very many amps which may light up on a circuit that has high resistance or driver issues. Do any of your mechanics have lab scopes? Is the resistance of all the injectors the same?





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 8, 2015, 5:40 AM)


Theprofessional23
Novice

Aug 8, 2015, 4:34 PM

Post #3 of 10 (1877 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Yes , its running very rough, injectors 4 5 &6 are not being told to fire, they didn't use scopes, and yes the resistance is the same on all, I also switched Injectors around and it stayed on the same cylinders. Any ideas?


Discretesignals
Ultimate Carjunky / Moderator
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Aug 8, 2015, 7:31 PM

Post #4 of 10 (1865 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

The puter is seeing circuit problems on those injectors. Have you check voltage with your meter by back probing the injector electrical connectors? Don't unplug the injector. Back probe the dark green/orange wires on those injectors while the engine is running. You should see close to charging or battery voltage on your volt meter connected to those probes. Make sure the black lead of your meter is connected to battery negative, so you have a good ground reference. Let us know what your readings are.

Back probe, so you can leave the connector plugged in, have something to connector your meter lead onto, and not have to punch holes in the wiring:






Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 8, 2015, 7:34 PM)


Theprofessional23
Novice

Aug 9, 2015, 1:01 PM

Post #5 of 10 (1847 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Don't have one of those back probes, wouldn't it be the same to use a test light on it? As I said in the original post the voltage ,resistance, and ground on all connectors are the same and good, is there a sensor somewhere that could cause this to happen? I do use it for mudding and what not so maybe something got fried and I didn't notice?


Theprofessional23
Novice

Aug 9, 2015, 2:01 PM

Post #6 of 10 (1841 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Update: re checked the grounds vehicle running, neg meeter lead on neg bat term, pos in ground of Injector connectors, #6 was reading voltage .25 varying up and down some and #5 was .01 whereas #2 connector(which is firing fine) read .02 steady, what does this mean?


Discretesignals
Ultimate Carjunky / Moderator
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Aug 12, 2015, 4:37 AM

Post #7 of 10 (1807 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

What about the reading from the dark green/orange wire? It should be a steady 12.8 volts or above.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 12, 2015, 4:38 AM)


Theprofessional23
Novice

Aug 13, 2015, 5:11 PM

Post #8 of 10 (1795 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Yes they all get about 13.2-13.5 v while running, also will note that I discovered the signal wire on #6 gives a voltage reading of .35 while all others are around .02-.05 max. So the ground is somehow getting voltage..but other than that evrything tests good... No resistance on any of the grounds etc..how do I pinpoint a short in my harness?


Discretesignals
Ultimate Carjunky / Moderator
Discretesignals profile image

Aug 14, 2015, 4:53 PM

Post #9 of 10 (1770 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

Its important to know that your injectors are getting good supply power. That is important because if your supply power to the injectors wasn't good, checking the ground (control side) would be a waste of time.


What your doing when you are back probing with the meter on the control side of the injector to the battery negative is measuring the voltage drop through the control wire, the PCM circuitry, and the ground circuit of the PCM that goes back to battery negative through ground straps or wiring.

The voltage after the injector is being brought to ground by the PCM when the PCM turns the injector driver on and current flows through the injector. Now, a volt meter really isn't all that great for checking the control side of the injector because the volt meter isn't fast enough to capture the whole voltage event and display it on the screen, so the meter averages the voltage. The injector may only be turned on for a few milliseconds, so your not going to see much of a drop on your meter's screen. What you really need is a lab scope, but most people don't have those laying around less knowing how to use one.

If the injector control side was shorted to ground, the injectors would be on all the time with the engine running. The fuel would be dumping into the exhaust and you would probably see white smoke that smells like raw fuel. The plugs would be gas fouled and you could possibly cause a hydro-locking condition. If the power side was shorted to ground, it would blow fuses, but you know that isn't the case because you show good power to the injectors.

One thing that you might want to inspect is the wiring harness that runs along the back of the valve cover. There is a head stud that the wiring can rub onto and cause issues.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 14, 2015, 4:54 PM)


Theprofessional23
Novice

Aug 18, 2015, 3:34 PM

Post #10 of 10 (1748 views)
Re: 1998 jeep Cherokee injectors won't fire Sign In

I had removed the harness, found multiple frayed wires making contact with each other where the inj harness connects to the main harness, de pinned one at a time and repaired this and a few other frayed wires, but it didn't solve problem, another thing I noticed is that with volt meter I put pos lead to pos bat terminal, neg to the signal wire pin on #6 inj connector with key on eng off it read 11.3 v... Battery dying a little is at 11.7, the original reading on signal wire was 10.9 v when bat was like 12.5-.7, all others read .0x, and #6 reads .0x when I unplug the signal wire harness connector from pcm, does this mean I have a bad PCM circuit? Or is it possible I have a ground somewhere else in the main harness that powers pcm/inj drivers...??






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