FIXED!!! Bad ECU - Runs rough, CEL and P0304 (1 Viewer)

[Jeff Elliott]

Would be very rare, most common would be clogged, but it would click. If you want to confirm, swap #4 with one of the others, if the code moves with it, it's the injector, doesn't it's wiring.

I feel the same. What I can't get my head around is this - I test resistance at the ECU, wiggle wires, etc. and nothing changes which suggests no wiring problems. That said, the injector tests the same as the others all the way back to ECU which suggests a good injector. Start the truck up, without even driving it, it's obvious it's down a cylinder - and no ticking of the injector...

All of this points to a bad injector??? I will be heart broken if I pull this beast apart again, replace that injector (or all of 'em), put her back together and it does the same thing!

The only other thing I can think of is this - the PO replaced the ECU with one from a '96 as it had a WOT stumble and someone told him it might be the ECU (no idea why). Could it be the ECU is not pulsing that injector??? I read here that someone said they were going to test with "noid lights" but have no idea what that is. Happy to buy them (Harbor Freight seem to have them) but don't get how to test the pulse if the engine isn't running (given throttle body would be off at minimum). Thoughts?

Thanks,
Jeff

 

[Tools R Us]

Agree that you are not getting anything definitive. Can unpin the connectors for #4 and #5 at the ECU and temporally swap them, if the miss moves to #5 it's the ECU, if not it's wiring/injector. Next try swapping injectors or injector plugs (may need to make an extender) to see if the problem moves. This would confirm injector or wiring.

You have a '97 with a '96 ECU, can you read codes, see OBD2 data with the key to run, motor off?

 

[Jeff Elliott]

Agree that you are not getting anything definitive.

Thanks again so much for all the help! I just saw your post but only after I did this...

Agree that you are not getting anything definitive. Can unpin the connectors for #4 and #5 at the ECU and temporally swap them, if the miss moves to #5 it's the ECU, if not it's wiring/injector.

Don't the injectors fire in sequence with the cylinder that needs the fuel? If so and I swapped the pins wouldn't it be pushing fuel at the wrong time?

I did break one connector so I will have to replace that one anyway (It was #4 and it seemed loose to me so that could have been the issue all along).

You have a '97 with a '96 ECU, can you read codes, see OBD2 data with the key to run, motor off?

Yes, it does read with the key on - I have a Scan Gauge installed.

 

[Jeff Elliott]

Also, forgot to say that I did plug the injector back in and wiggle the wiring all along the length and couldn't get the the resistance to change.

 

[Tools R Us]

...
Don't the injectors fire in sequence with the cylinder that needs the fuel? If so and I swapped the pins wouldn't it be pushing fuel at the wrong time?

Yep, they are sequential, wouldn't leave it out of order, but works for a test.

...
Yes, it does read with the key on - I have a Scan Gauge installed.

A benefit to upgrading to the early ECU, all of the '97 rigs we work on have to be running to get data.

 

[Jeff Elliott]

Well, 6 new injectors later... Still throwing a P0304!!!!!!!!

Folks, I live near a lake and am about to drive this thing into it! I've verified resistance at the ECU, moved wires, etc. Everything I know to do to figure out why it's dropping that cylinder.

Can unpin the connectors for #4 and #5 at the ECU and temporally swap them, if the miss moves to #5 it's the ECU, if not it's wiring/injector.

@Tools R Us - If I move the pins at the ECU, can I drive it that way? The only way to get it to throw the codes are to drive it under load - it won't throw them at idle or just being revved.

If anyone has any other ideas, LMK!!!

 

[Red Merle]

Well, 6 new injectors later... Still throwing a P0304!!!!!!!!

Folks, I live near a lake and am about to drive this thing into it! I've verified resistance at the ECU, moved wires, etc. Everything I know to do to figure out why it's dropping that cylinder.



@Tools R Us - If I move the pins at the ECU, can I drive it that way? The only way to get it to throw the codes are to drive it under load - it won't throw them at idle or just being revved.

If anyone has any other ideas, LMK!!!

Erase thread, up your insurance and put it into the lake. Probably for the best...

Could it be a corroded wire somewhere? EC1 and EC2 where they go through the firewall have been known to cause issues, but what you are describing doesn't sound like the 10 amp GAUGE fuse that many struggle with.

 

[Jeff Elliott]

Ok, I've completely bypassed the wiring for injector #4 and still have P0304 and rough running. I ran a new wire from a brand new injector connector from the #4 injector to the ECU (see pics). I jumped the +12v from the #3 injector over to the #4 injector and verified I had +12V before reinstalling the throttle body. At this point, I don't know what else to try... Except another ECU. For the record, so far I've:

• put in 6 new injectors
• Almost every hose under the hood
• Plug wires, plugs, cap and rotors
• New #4 injector connector
• Bypassed all factory wiring for #4 injector
• Verified spark being sent to #4 via timing light
• Verified continuity between front of ECU connector and jumper seen in pic (to isolate that variable as well)
• Checked for HG problem with block tester - stayed the right color

Calling @inkpot , @Malleus , @Tools R Us and anyone else who might be able to help solve this mystery and misery!

 

[Jeff Elliott]

Bump.. Would you try and ECU or something else? Looking for advice / validation / suggestions please!!!

 

[Jeff Elliott]

One more bump... Ordered an ECU from a Mud member but would sure like some validation from this community that that's the next thing you'd try. If that doesn't work, there will soon be a listing for this rig which will be heart breaking but my wife won't let me keep it in the garage much longer.

 

[PDoyle]

Still sounds like its electrical, may or may not be related but check the wiring behind the glove box going into the fender well area.
The loom cracks around that area and rubs on the body.

 

[Jeff Elliott]

Still sounds like its electrical, may or may not be related but check the wiring behind the glove box going into the fender well area.
The loom cracks around that area and rubs on the body.

I agree - sounds electrical to me too. I completely bypassed the factory wiring (see pics above). I've got a pulse wire run directly from the ECU to the injector and the +12V for the #4 injector has been fed from another, known working, injector. This isolates the normal issues with the loom inside the glove box, passing through the firewall and passing the EGR tube. Still scratching my head...

 

[PDoyle]

Is there any resistor you may have missed or any ground wires you can clean?

 

[Irish Reiver]

With the harness bypass that you have done I agree that ECU is the next logical component to focus on.

 

[JeepinPete]

While it seemed like an electrical issue, you seem to have eliminated all possible avenues relating to #4 save the ECU. Might be worth while doing a compression check on that cylinder (cheap and easy) just to make sure there isn't something physically wrong.

 

[Red Merle]

I agree on checking all of the grounds and making sure there isn't a corroded wire somewhere. I would also toss in a different ECU before I sold the truck. Checking the compression is also worth doing to eliminate the possibility of a mechanical issue

You've already spent a ton of time and money on this and there is no way you will come close to making that up if you sell the truck. Nobody is going to pay top coin for a truck in this condition. I feel like you are close here and it's going to be something silly when you find it. Don't give up yet!

 

[Jeff Elliott]

While it seemed like an electrical issue, you seem to have eliminated all possible avenues relating to #4 save the ECU. Might be worth while doing a compression check on that cylinder (cheap and easy) just to make sure there isn't something physically wrong.

Thanks @JeepinPete - I did the compression check as you suggested and I'm getting 170 - 175psi across all cylinders so that's pretty good news!

I agree on checking all of the grounds and making sure there isn't a corroded wire somewhere. I would also toss in a different ECU before I sold the truck.

Thanks @Red Merle - I've got an ECU on the way so will be able to check that next week. In the meantime, I can't think of any grounds to check that would explain this issue - can you? The #4 cylinder is clearly not getting fuel (see pic of spark plugs in order from #1 - #6 and note #4 is clean) but all other cylinders seem to be working fine. The injectors pulse to ground which is what causes them to inject fuel I believe. Given I know I've got +12V on the offending injector the only explanation I can think of is either wiring or ECU and we've isolated the wiring.

You've already spent a ton of time and money on this and there is no way you will come close to making that up if you sell the truck. Nobody is going to pay top coin for a truck in this condition. I feel like you are close here and it's going to be something silly when you find it. Don't give up yet!

Totally agree - just can't see any light at the end of the tunnel. If the ECU doesn't work I just don't know what else to try. At least I can tell a buyer that the mileage is likely accurate with those good compression numbers!!!

Thanks to all and keep ideas coming!!!

Here are the plug pics:

 

[PDoyle]

When the engine is warmed up does it still misfire?

 

[Jeff Elliott]

When the engine is warmed up does it still misfire?

It misfires from a cold start or warm start. It idles like a diesel. I saw another Mud guys 80 last weekend and it idled like butter. Mine shakes pretty badly from idle to about 2500rpms and then smooths out. Classic misfire symptoms based on what I've read here. It won't throw the codes on a short drive unless I get on it pretty hard. If I force a downshift from low speed (call it 25 mpg) to above 4K rpms it will throw it immediately. Other times it takes longer but is always there - warm or no.

Thanks!!!

 

[Red Merle]

Wild guess here because I am not that savvy with the way the '96 looks, but on my '94 there is a big ground wire that is attached where the main engine wiring harness is attached to the firewall and then it runs down to the bracket that holds the metal pipe that goes to the blue silicone hose that comes out of the heater core. I don't see that wire in your pics, nor do I know if you even have it, but it's there for a reason on my truck. Could that be there to ground the wire harness?