Cherokee 94 Misfiring ECU/injctor unsolvable problem

[Gabo]

My Cherokee 94 has a misfire in the cylinder 3 and 4, the scan shows a "fuel injector issue" in these injectors, if the injector conector is unplugged the engine does not change.. -the injector does not work as the scan says- when the noid light is placed in the these contectors (3&4) guess what.. have pulses... but in anyways I sent the ECU to a "qualified" auto ECU shop repair and they say that the ECU is fine...

I guess that in spite that the ECU has pulse the pulse Amplitud ( voltaje) is not enough may be under 12V.. but they insist that the ECU is fine

Of couse, the injectors were replaced, the conpression test shows over 135psi (normal) and the spark plugs, wires and distributor was replaced too...no chnages here

I swaped the 4&5 and the conector5 makes works the injector5 this proof that the ECM is sending enough voltage and pulse width to makes work the injector. The scan still reporting problems in 3&4 injectors

I checked the injectors color code with the OEM manual and all of the are right

I took off the injector rail with the injectors atttached, unpluged the inigtion coil and after I chanked the engine.. well... all the injectors have gas pulses.. this makes the problems worst because the ECM is working, the Injectors are working too..

The leakeage test passed in the cylinder 3&4 for piston rings and valves... the valves opening and closing properly...

It is the fisrt time in my life that this happen to me, all the test are ok but still misfiring 3&4

The dealer has not idea what is the problem, the technicians have not clue...

HAs somebody how solve these "eat technician" problem I think that this case is a real thread

Thanks

Gabe

 

[SteelHeadz]

What is the DTC for the 3&4 injector?. Seems to me that if all the injectors are pulsing and spraying fuel, ignition is igniting and all the compression tests are conclusively within the specified range then everything should be good. One test you need to look at is a leak between cylinders 3&4 , also check the intake for a leak around the 2 cylinders in question. A hairline crack between cylinders will be hard to find but will sure cause a lot of issues.

 

[54cj3b]

is there a loose wire in plug or is there a broken wire any where


also check plugs in 3 and 4 they are prob fouled out also

try pulling injector out and clean it good may be cloged with gunk or something

couple more things you could check

 

[Bounty__Hunter]

Check the CPS (crank position sensor) and the harness/connector to the CPS. This is a common problem with that model and often shows as injector problems, seen it firsthand in my jeep.

 

[MudderChuck]

May first guess was the same as Bounty Hunter, you may be getting poor signal at the CPS. A quick test is to check the signal voltage at the sync sensor, if it's low it is usually an indicator of a partial short in the CPS.

Something to keep in mind, is the injectors are always powered when the ignition is on (same circuit as the coil and alternator power) and the injectors operate through the PCM on the ground side. All the dark green wires should be hot with the ignition in the run position and have near battery voltage. You may want to back pin a good injector on the dark green wire and check the voltage and then compare it to 3 or 4 while the motor runs. Maybe even use battery voltage and a volt meter, to check the ground side also and compare as you are cranking the motor with the coil disconnected. You'll likely get better results using an old analog meter, at least while trying to check for standing voltage on the ground side of the injectors (may be a waste of time, but then again as it pulses you may be able to get a good comparison with a known good pulsing injector). I'd also ohm check the injectors and compare.
Low amperage through the injector is going to be harder to find and likely is a wiring or connector problem. And ohm test between the injector and the PCM connector may show excessive resistance in the wiring (or splices). You may even be able to ohm test the whole system from the ignition coil power in and the individual injector grounds at the PCM plug for excessive resistance.

 

[Bounty__Hunter]

Mudderchuck I wish you were close for when I have an electrical problem, I hate diagnosing electrical.

 

[MudderChuck]

Mudderchuck I wish you were close for when I have an electrical problem, I hate diagnosing electrical.

Most of it is just understanding the basics (volts, amps and resistance) and sticking with it until you find the problem, after awhile instinct usually kicks in and things get easier.
I've found a whole system (or sub system) approach to be more fruitful in most cases, don't get hung up on the components, but think of each sub system (power, wiring and component) as a whole (sub system), then divide it into bite sized pieces and systematically troubleshoot. Jumping from one piece to the next, without a systematic plan, is often a waist of time and money. Cutting corners and going straight to known trouble spots, may or may not work out for you. I usually go with a systematic approach and try not to assume too much.
For every bad sensor I've ever found, I've typically found at least three wiring and/or resistance problems. Some can be a real pain and almost impossible to find, like the ones that only occasionally act up.

 

[Gabo]

Ok folks Thank you very much for your help.. ok this is the way how I solved the problem..
yes.. I started from cero separating each single system that receives signal from the ECU one by one looking for problems.. the CSP conector had 700 Ohms in pin#3.. Bingo... y replaced and problem gone...

very tricky issue but now that is solved seem prety simple... in this model the DTCs are not releiables.. and the best way is go point for point after all the regular tests dont give you anything useful..

Thanks a million for the help, just share the experience

Gabe