Fuel injector magic? Bad PCM?

Terrrs01 · Mar 20, 2021

Long story short... my 97 2.5 TJ starts missing once it warms up, then if you wait a little longer is starts missing even more.
I eventually tracked it down to cylinder 1 and cylinder 2 fuel injectors physicals stop spraying fuel ONLY ONCE WARM (or 1-2min after idling). Here is the kicker...I spliced wires into the injector and read the consonant hot wire with a multiple meter and the pulsing ground wire from the PCM with a test light. I did this while the engine was running good until it stated missing and saw no change on either. I immediately (while warm) pulled the rail and to see if fuel is coming out the injector and nothing..

so I assumed it pressure/blockage related. Took the rail off, cleaned up rail, swapped injectors around, and put in my homemade fuel pressure in-line gauge. Fuel pressure is 55, with a new clean rail and injectors swapped around and STILL cylinder 1 and then a min or 2 later cylinder 2 stops spraying.

Maybe it’s electrical again???

-I checked the ground resistance from the PCM to the injectors. That was fine. I

-I checked resistance from the negative battery terminal to the injector to make sure it wasn’t grounded out some where. There was a large resistance (which I wanted to see)

I guess I’m at a loss...
-good fuel pressure
-clean lines
-swapped injectors
-getting correct hot volts
-ground pulsing like it should
-works properly when cold
-then 1-2nin after idle it starts with chandler 1 and then cylinder 2.

any hail Mary’s would be helpful!
No codes either

qslim · Mar 20, 2021

The fact that all your woes happen once the engine warms up is a red flag for your upstream O2 sensor. When the vehicle starts cold the PCM ignores all input from the O2 sensor & runs a pre-programmed fuel map to get the engine up to temp as quickly as possible (a mode called open-loop). Once it reaches operating temperature it exits this program & looks to the O2 sensor input to adjust fuel trim (closed loop).

O2 sensors are a maintenance item on these things as far as I'm concerned. When I got my TJ I was having random driveability issues but no check engine light, at the suggestion of this board I changed my sensors & it cleaned up a lot of my issues.

SvtLdr · Mar 20, 2021

@Terrrs01 welcome to the forum.

There's a wealth of experience here, you're in good hands.

Terrrs01 · Mar 20, 2021

qslim said:
The fact that all your woes happen once the engine warms up is a red flag for your upstream O2 sensor. When the vehicle starts cold the PCM ignores all input from the O2 sensor & runs a pre-programmed fuel map to get the engine up to temp as quickly as possible (a mode called open-loop). Once it reaches operating temperature it exits this program & looks to the O2 sensor input to adjust fuel trim (closed loop).

O2 sensors are a maintenance item on these things as far as I'm concerned. When I got my TJ I was having random driveability issues but no check engine light, at the suggestion of this board I changed my sensors & it cleaned up a lot of my issues.

Thanks for the reply.
That’s actually where I started long before I’ve made it down to the fuel injectors. I was just trying to save the post from being 10 pages long. I replaced it with a NTK sensor and no change. I then had a buddy come over and we used HP tunners to essentially keep the jeep in open loop to see what happens. We watched the 02 sensor lean out bad and stay low and the fuel trim increase (assuming cuz it’s seeing that lean upstream 02). This made us check the injectors and find out that cylinder 1 injector and cylinder 2 were acting up.

qslim · Mar 20, 2021

What do you mean 'essentially' keep it in open loop? I'm not familiar with that HP Tuner thing, can you actually command open/closed loop?

Hows the coolant temp sensor reading?

Terrrs01 · Mar 20, 2021

qslim said:
What do you mean 'essentially' keep it in open loop? I'm not familiar with that HP Tuner thing, can you actually command open/closed loop?

Hows the coolant temp sensor reading?

We disabled the 02 sensors and set the coolant temp threshold to enter closed loop very high (assume it would never reach it) (I attached a photo below) My initial thoughts where exactly on the same lines as you. As far as I can tell all sensors are working correctly. I went down a long rabbit hole of finding out why my 02 sensor would base line lean, and at this point I have checked every sensors’ ground resistance and hot wires on the entire intake. Multiple upstream 02 sensors. Vaccume leak check, exhaust leak test....

-with the injector not firing, that would make sense for the sensor to read lean and then dump more fuel into the rest of the running cylinders. Which is exactly what I’m seeing in the spark plug. Cylinder 3/4 black as black could be. Cylinder 2 kinda mixed, cylinder 1 DRY/white.

-Side note, I did a significant amount of welding on my jeep over the winter with the battery connected. I’m aware you should disconnect your battery before welding but it’s never caused any problems before so... I kinda never did. I’m Kicking my but now, because now I’m wonder if I caused a PCM problem to the injectors.

Terrrs01 · Mar 20, 2021

So just double checking a few things today and I realized I was able to ground out the ground/pulse wire (I spliced the wires) to the injector while it was missing, and by grounding it out I’m assuming its opening that injector fully (I could smell the large amount of add fuel) but then afterwards it would idle out ok for another min or 2..... couple questions?

-Does this mean anything or not really? Does this solidify its a fuel delivery issue, or am I just dumping a ton of fuel and it’s running “ok” cuz it’s running off unspent fuel?

-oddly enough it only help when grounding to the battery. I ran the splice wire straight to the PCM ground/pulse and no luck. So between that and no resistance in the ground pulse I think I can rule out any “bad ground wiring” between the PCM and the injectors.

-I still cant wrap my head around why the problem starts with cylinder 1 and then moves to cylinder 2

-I attached a video of my ground long out the injectors and the changes that happened.

SvtLdr · Mar 20, 2021

@Chris, any ideas on who to tag into this thread? Looking for some electrical diagnostic experience. Thanks.

never monday · Mar 20, 2021

Terrrs01 said:
Here is the kicker...I spliced wires into the injector and read the consonant hot wire with a multiple meter and the pulsing ground wire from the PCM with a test light. I did this while the engine was running good until it stated missing and saw no change on either. I immediately (while warm) pulled the rail and to see if fuel is coming out the injector and nothing..

Electrically the ecu was still tryimg to pulse the injectors while no fuel flowed?

qslim · Mar 20, 2021

Tagging @Wranglerfix to see if he’s seen PCM issues like this before

Terrrs01 · Mar 20, 2021

never monday said:
Electrically the ecu was still tryimg to pulse the injectors while no fuel flowed?

Correct! Only after it idles for a min or so tho. And then it eventually slides into cylinder 2 acting this way.

So this immediately made me think I have a fuel pressure/blockage problem. However I have cleaned/checked/verified fuel. No pressure problems. No blockages. Great pressure.

in my previous post I posted a video of it missing, and I actually grounded out the fuel injector ground/pulse wire (which assumingely opened the injected for a long time) but it added a ton of fuel (I could smell it) and it essentially rested my problem. It would idle normal for a nothing 1-2min before acting up again.

qslim · Mar 20, 2021

Terrrs01 said:
Correct! Only after it idles for a min or so tho. And then it eventually slides into cylinder 2 acting this way.

So this immediately made me think I have a fuel pressure/blockage problem. However I have cleaned/checked/verified fuel. No pressure problems. No blockages. Great pressure.

in my previous post I posted a video of it missing, and I actually grounded out the fuel injector ground/pulse wire (which assumingely opened the injected for a long time) but it added a ton of fuel (I could smell it) and it essentially rested my problem. It would idle normal for a nothing 1-2min before acting up again.

Have you plugged in a noid light to the suspect injectors?

Terrrs01 · Mar 20, 2021

qslim said:
Have you plugged in a noid light to the suspect injectors?

A noid light no. I just ordered a set earlier today, however I used a normal test light to check ground pulsing and a multimeter to check the consistent hot. Both seem to be functioning correctly.

I’m wondering if the pulsing ground from the PCM is the problem. There is no resistance within the line, but I’m wondering if there is some sort of ground current threshold that would be high enough to turn on the test light, but not enough to open/close the injector?

Chris · Mar 20, 2021

SvtLdr said:
@Chris, any ideas on who to tag into this thread? Looking for some electrical diagnostic experience. Thanks.

@Jamison C @Wranglerfix

Terrrs01 · Mar 20, 2021

Chris said:
@Jamison C @Wranglerfix

Thanks for for references. I actually came across wrangler fix just googling and current sending him/that company and email.

Chris · Mar 20, 2021

Terrrs01 said:
Thanks for for references. I actually came across wrangler fix just googling and current sending him/that company and email.

That’s a good idea. He’s the go to go for PCM stuff around here.

never monday · Mar 21, 2021

Terrrs01 said:
Correct! Only after it idles for a min or so tho. And then it eventually slides into cylinder 2 acting this way.

So this immediately made me think I have a fuel pressure/blockage problem. However I have cleaned/checked/verified fuel. No pressure problems. No blockages. Great pressure.

in my previous post I posted a video of it missing, and I actually grounded out the fuel injector ground/pulse wire (which assumingely opened the injected for a long time) but it added a ton of fuel (I could smell it) and it essentially rested my problem. It would idle normal for a nothing 1-2min before acting up again.

Have you moved injectors around to see if the problem moves?
The ecu makes the ground connection to trigger the injector. So what your describing sounds normal.

Terrrs01 · Mar 21, 2021

never monday said:
Have you moved injectors around to see if the problem moves?
The ecu makes the ground connection to trigger the injector. So what your describing sounds normal.

Yes. I’ve swapped them around. Still cylinder 1 then cylinder 2. Then swapped in my buddies, and same thing cylinder 1 and then cylinder 2.

it’s extremely odd. As far as diagnosing the fuel injectors everything is working just as they should, but I can physically see no fuel coming out of the injector (sometimes not always) while seeing my test light pulse with the ground and seeing no drop on the constant hot wire with my multimeter.

ColoJeep · Mar 21, 2021

I hate problems like this ! Sounds like your doing a great job of diagnostics to me.
Hopefully Wranglerfix can offer some insights and corrections!
Hang in there, your in the right place for help.
I wish I had some suggestions to offer you!

never monday · Mar 21, 2021

Terrrs01 said:
Yes. I’ve swapped them around. Still cylinder 1 then cylinder 2. Then swapped in my buddies, and same thing cylinder 1 and then cylinder 2.

it’s extremely odd. As far as diagnosing the fuel injectors everything is working just as they should, but I can physically see no fuel coming out of the injector (sometimes not always) while seeing my test light pulse with the ground and seeing no drop on the constant hot wire with my multimeter.

It’s been years sonce I’ve works on a 2.5L.
where does the fuel line connect vs the regulator. I wonder if the regulator is hanging open

Fuel injector magic? Bad PCM?

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