Misfire Rabbit Hole

[GeoRoss]

Have you compared the resistance through #4 injector to the others? I would accomplish a compression test before tearing into and replacing anything unless you know someone local who would allow you borrow an ecu to slave in for trouble shooting purposes. That would be a quick, easy way to rule out your ecu.

If you end up sending your injectors out, RC ENGINEERING In L.A. has been used by many on Mud including myself. IMO their price was reasonable and turn around time was fast.

In post #2 I asked, how was it running before you did the tune up? When, exactly, did the misfire start happening?

I only tested the voltages of the injector wiring harness at the ECU. I grounded them to both ecu ground pins the FSM says to test and to the body. All six the same voltage. I should have tested resistance but assumed the harness was good based on the voltages based on the FSM tests.

For a six pack I should be able to borrow an ecu from a 94 to rule the ecu out.

The truck ran like crap before I parked it. It was just after my mechanic said he could bypass the EGR VSV with an after market one. It ran fine after I pinched the vacuum hose to that VSV so I assumed it was leaking exhaust gas into the intake during idle. It is possible that this is a coincidence.

 

[GeoRoss]

Have you compared the resistance through #4 injector to the others? I would accomplish a compression test before tearing into and replacing anything unless you know someone local who would allow you borrow an ecu to slave in for trouble shooting purposes. That would be a quick, easy way to rule out your ecu.

If you end up sending your injectors out, RC ENGINEERING In L.A. has been used by many on Mud including myself. IMO their price was reasonable and turn around time was fast.

In post #2 I asked, how was it running before you did the tune up? When, exactly, did the misfire start happening?

I'll try RC Engineering if it ends up being the injector. I was eager to get it on the road quickly and used a local shop that says they essentially did the same work.

I'll test compression if I can rule out the ecu. That is easier to do than pulling the intake. I can then rule out things like a stuck valve.

 

[ZackR]

Has the injector wiring harness near the EGR valve inspected? I had intermittent misfires due to a wiring short in there. You can't tell just by looking at the outside, it needs to be opened up.

 

[GeoRoss]

I haven't looked at the harness by the EGR I awhile. It looked good when I initially wrapped with heat resistant wrap over ten years ago. It is something to look at, but I think I have mostly ruled out wiring since I have the same voltage reaching the ecu on all the injectors. A resistance check would of course confirm this.

 

[george_tlc]

AGAIN.... Unplug the connector at the injector. Measure the resistance of the injector across the injector pins. That is the correct way to determine if the injector is in spec (should be around 13 ohms) - the FSM outlines this measurement (at least it does in the 97 FSM).

You should NOT apply continuous 12V/GND to an injector, they are NOT designed for continuous duty and can be damaged by doing that.

Please measure the injector resistance...

Verify you do not have a short to injector 4 ground side drive to the body at the EGR pipe and behind the glove box (those are classic locations).

IF you had a short of the +12V to the injectors (common to all 6 injectors) it would be a short to Ground and would blow the EFI fuse, so that is essentially 'tested' at this point.

Do the above before mucking with ECU swaps.

cheers,
george.

 

[GeoRoss]

Thanks for the reply George. I tested the harness as the FSM specifies. Each one was only tested for a few seconds each.

Accessing injector #4 is easier said than done with the throttle body and intake on. I will reach in there and see if I can unplug the injector but I'm not sure I can. I will definitely test the injector resistance when I can reach it. I'm delaying pulling the intake off again as long as I can as I am sure you understand. Even if that means ruling out low probability failure points.

I am not too certain about how to test the ground side. Visual inspection at the common failure points? I thought by testing at the ECU with the ignition on I was testing the whole circuit? Are you saying I could still have exposed wiring and still get 12V at the ECU? If I have a ground side short on injector 4 I guess it would have been running since I buttoned everything back up.

Thanks.

 

[Tools R Us]

Injector referb is simple, the machines make it really easy, I would trust the local shop. Basically they are cleaned then run into a row of beakers, the output measured. There are a bunch of you tubes. They can die at anytime, but they are very reliable, so would be rare. Have you checked the injector resistance from the ECU connector?

Can disconnect and plug the vacuum line to the egr, it will not work, but shouldn't leak. Takes it out of the equation, makes a good test.

 

[inkpot]

If your fuel line pressure is bad and causing an injector problem on #4 cylinder , it would be the same on all cylinders?

 

[GeoRoss]

Injector referb is simple, the machines make it really easy, I would trust the local shop. Basically they are cleaned then run into a row of beakers, the output measured. There are a bunch of you tubes. They can die at anytime, but they are very reliable, so would be rare. Have you checked the injector resistance from the ECU connector?

Can disconnect and plug the vacuum line to the egr, it will not work, but shouldn't leak. Takes it out of the equation, makes a good test.

This was my thinking on the injectors and the local service. On my list of "I whish I had..." is the check the resistance of all the injectors before installation. Either way, there is no clicking on #4. Electrical or a failed injector.

I did not check resistance of the injectors at the ECU. Another thing to the list of "I wish I had ", but easy to recheck. All the injectors had the same +12.03V, exactly the same. If I had a ground side short (ECU side) wouldn't the voltage be less on the bad injector? I'm not an electrical genius, but wouldn't it be "leaking" electrical potential upstream of where I tested or am I still looking at the electrical potential of the whole system and would need to ground it outside of the vehicle to see a difference? I am not an electrical genius, just a dude with a multimeter.

Thanks for the heads up on the EGR valve. I will pick up a vacuum hand pump to double check it isn't partially stuck open. See if I can get it to stall the engine.

 

[GeoRoss]

If your fuel line pressure is bad and causing an injector problem on #4 cylinder , it would be the same on all cylinders?

That's what I thought too. I was willing to spend $100 for him to check the ignition system, not an additional $300 on what seems less likely. It would be nice to be able to check fuel pressure at the fuel pressure regulator, not at the firewall side of the fuel rail.

 

[Tools R Us]

... I did not check resistance of the injectors at the ECU. Another thing to the list of "I wish I had ", but easy to recheck. ...

Part of the referb would have been an electrical check, if they didn't pass, would not of passed the flow test. A failure would have been obvious. Checking the resistance from the ECU connector will check the wiring and injector, they should all read the same, if so eliminates that variable. If not, could be wiring, connector or injector.

... Thanks for the heads up on the EGR valve. I will pick up a vacuum hand pump to double check it isn't partially stuck open. See if I can get it to stall the engine.

Most of the EGR valves have holes on the underside of the diaphragm housing. By sticking a finger (or tool depending on the size of the hole) and pushing up, should be able to open the valve. With it idling, open the valve, should start missing, maybe die, when released, should return to normal. Easy test.

 

[GeoRoss]

Ruled out the EGR causing the misfire. Applying vacuum to the valve causes the motor to stall. My modulator isn't working, but at low temps it shouldn't matter as the EGR valve is closed.

I then pulled the ECU to check the harness. I have low resistance on injectors 1 and 2. Injector 4 appears to have no continuity. I rechecked the voltage coming in. 12.25V coming to injectors 1 and 2 with the ignition on. About 2mV at injector 4.

Looks like I'll be diving into the harness. I'll pull the TB so I can really look at the injectors and have more room to look at the harness near the EGR valve. Any hints at what I can pull under the dash to make more room? The cooling unit sure is in the way.

I'll recheck things tomorrow as I am pretty sure I had 12V going to injector 4 the other day. Maybe I had the harness in just the right place and today not. Makes me think I need to look very closely at the dash side of the harness first.

 

[GeoRoss]

Played hooky this afternoon. I finally bought a real multimeter to replace my cheap Craftsman. Even sprung for the automotive leads to better test pin outs. The wiring harness leads for injector 1 and 2 have 517 ohms, injector 4 is an open lead. I get +12V with the ignition on between injectors 1 and 2 and pin E01. There is no voltage on injector 4. Just like last night.

So I have something wrong going on with the ground/ECU side of injector 4 or maybe a dead injector. I'm a lazy man so I start off with the easy stuff. I thoroughly inspected the harness near the ECU. It is fine. Then I do some mental arithmetic and decide pulling the throttle body is easier than checking the harness near the EGR. I go to unplug #4 and it is super loose.

The injector has 13.7 ohms so it is good. The ground side has continuity and <1 ohm to the ECU. Good! I firmly reattached the wiring housing. I'm getting the same resistance and voltages across the board. Yeah!

I'll button it back up and see how it runs. I'm going to go ahead and replace the other injector wiring housings and a new tb gasket. I may not have time today though. I can only speculate that the housing was loose and that led to my inconsistent multimeter results.

 

[retrofive]

Played hooky this afternoon. I finally bought a real multimeter to replace my cheap Craftsman. Even sprung for the automotive leads to better test pin outs. The wiring harness leads for injector 1 and 2 have 517 ohms, injector 4 is an open lead. I get +12V with the ignition on between injectors 1 and 2 and pin E01. There is no voltage on injector 4. Just like last night.

So I have something wrong going on with the ground/ECU side of injector 4 or maybe a dead injector. I'm a lazy man so I start off with the easy stuff. I thoroughly inspected the harness near the ECU. It is fine. Then I do some mental arithmetic and decide pulling the throttle body is easier than checking the harness near the EGR. I go to unplug #4 and it is super loose.

The injector has 13.7 ohms so it is good. The ground side has continuity and <1 ohm to the ECU. Good! I firmly reattached the wiring housing. I'm getting the same resistance and voltages across the board. Yeah!

I'll button it back up and see how it runs. I'm going to go ahead and replace the other injector wiring housings and a new tb gasket. I may not have time today though. I can only speculate that the housing was loose and that led to my inconsistent multimeter results.

@gnob Didn't he say that days ago, check the connection.

 

[GeoRoss]

Yes, it was on the list. Like I said, I'm lazy and wanted to put off taking things apart. I also wanted to rule out the EGR. Easy stuff first.

 

[gnob]

Wish I could get paid to fix s*** over the interwebs.

 

[landtank]

Get the code pulled by a parts store see if there is a P0330 or similar along with others. that would be a sensor issue

 

[GeoRoss]

Wish I could get paid to fix s*** over the interwebs.

How much you want?

 

[GeoRoss]

Get the code pulled by a parts store see if there is a P0330 or similar along with others. that would be a sensor issue

Sadly obd1, I had a code 25 that I hope is related to the misfire.

I just bottoned it up and is smooth as butter.

 

[GeoRoss]

Thanks for all the help everyone. Even if it seemed I wasn't listening.