In a thread I posted that is now on page 3 or 4 I described the problem that the pcm wasn’t providing a ground signal to injector # 4 . After endless searching of the internet I finally found a thread that said the OP was getting a P0204 code but his engine was running fine . After checking he eventually found that a previous owner had installed a jumper wire from # 1 injector { ground side } to # 4 injector . His pcm wasn’t providing a ground signal to # 4 either . After thinking about it I wondered what engine he had & what the firing order on that engine was . I looked & looked but couldn’t find that thread again .
On my engine # 5 fires right before # 4 so I reasoned that running a jumper from # 5 to # 4 would fire the injector as close as possible to when it is supposed to fire . Is my reasoning correct ?
On a 4 cylinder a common firing order is 1-3-4-2…but it really depends on the maker of the unit…they can set it up however they want really…All has to do with the cam profile and or what the designer likes…and crankshaft arrangement or style really. Plenty of possibilities. Sometimes on vehicles like the 98’ Audi A6…(Which I just gave back to my buddy all repaired) They used 3 coil packs… One pack per 2 cylinders…now I don’t know what kind of funky funky algorithms the PCM was running to fire those packs…but when its running properly it runs as smooth as butter. The logic was that one plug would fire when it was not needed…but I imagine it was also RE-Fired when it was needed… That vehicle is only able to fire 2 plugs at one time when the signal is sent from the PCM…So it very well may fire when it is time…when it is not time…and then again when it IS time. That is until Audi went with coil over plug…a much better system to troubleshoot and also to live with in general.
Blackbird
On my engine # 5 fires right before # 4 so I reasoned that running a jumper from # 5 to # 4 would fire the injector as close as possible to when it is supposed to fire . Is my reasoning correct ?
Sounds reasonable and worth a try. The worst you can do is fry the pcm, which appears to be bad anyway. If that doesn’t work well, you could try jumpering to #1.
You need to know 2 things
The firing order
The cylinder layout . . . there are lots of different cylinder layouts, even among the same manufacturer. Just because you know the Ford layout, for example, don’t assume jeep is the same.
The firing order of my jeep is 1 6 5 4 3 2 . In my reasoning I was thinking that # 4 { my dead injector } would be firing right after # 5 so if I was to jumper # 4 it should be to # 5 . The cylinder layout is 2 4 6 on the right bank & 1 3 5 on the left bank . Standing in front of the engine bay the right bank is on the left & the left bank is on the right . Standing in front that puts # 4 in the center on my left & # 5 in the rear on my right .
The cylinders are numbered front to rear in the order above .
Ha ha . . .
I just remembered that on some engines, the firing order is cast onto the intake manifold
And some engines even have the cylinder layout cast onto the engine
but that’s not so common, nowadays
Interesting idea, 2 grounds for the price of 1
I don’t know about your Jeep, but on my 87 Ranger the injectors are batch fired 3 at a time and I can see where your idea would probably work just fine on the 2.9.
On the 2.9 injectors 3, 5 & 6 are spliced to pin 59 of the PCM
1,2 & 4 are spliced to pin 58 of the PCM.
a jumper added between 1 & 4 or 2 & 4. should do the trick on a 2.9 anyway.
+1 to @87_Ranger. Many cars batch fire injectors. It makes the PCM cheaper to build and the software easier to write. The fuel just hangs around until the valve opens. Not as good as sequential fired injectors.
IThe best info I can find suggest the PCM is sequential, not batch but it doesn’t say what type injector drives it has. If the injector is a “high side drive” the PCM provides 12V every injector command and there will be a constant ground at one of the wires. Just grounding the injector anywhere will make it run smooth. If its a “low side drive” it completes the ground for each injector fire so one wire of the injector with have a constant 12V. Using another injector ground likely won’t work very well. Either way it loses the ability to detect injector circuit failure so the code will still show.
The paired spark coils do that as well. No harm firing the spark plug into something that won’t ignite.
On my jeep the injectors have a constant power source & get a ground signal from the pcm to fire them . They are not batch fired , each injector fires independent of the other injectors & yes by adding a jumper wire I’m getting 2 grounds for the price of 1 , so to speak .
By doing this the injector is firing an instant before it’s actually supposed to but at least it’s firing . As stated before , I have read other problems can cause the pcm to not fire an injector & when I’m fully convinced it’s actually the pcm that is bad I will buy a new pcm .
The main purpose of starting this thread was to determine if my thinking that injector # 5 would be the best one to jumper from .
Update , I was hoping that if I fixed the # 4 injector problem that the no rev problem would also go away . I cleared codes , connected alternator wires & ran a jumper wire from # 5 injector to # 4 injector . Started engine & it ran smooth as silk . Still wouldn’t rev over 2200 rpm’s .
Just to satisfy myself that # 4 cylinder was indeed firing I pulled the coil wire off of # 4 & there was definitely a drop in rpm’s & the engine ran a little rough .
I plugged # 4 coil back in & it smoothed right out , I then pulled the coil wire off # 5 & again detected a drop in rpm’s & a little roughness & I plugged # 5 back in & again it smoothed right out .
Wanting to see if it would rev smooth with the alternator unplugged I cleared the codes I had created by pulling coil wires , disconnected alternator wires & started the engine . Now it’s running rough . I pulled the coil wire from # 4 again & it isn’t firing . I pulled the coil wire from # 5 & it’s not firing either .
Now I’m not getting a ground signal to those coils . I didn’t check the grounds to the injectors . I removed the injector jumper wire & # 5 still didn’t come back to life . I ordered a new PCM . We’ll see what happens when it’s installed .