Author Topic: The Never Ending Misfire  (Read 1475 times)

S298WP

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The Never Ending Misfire
« on: September 30, 2017, 06:53:16 PM »
Okay. I reek of gas right now, and am sitting down with a delicious whiskey drink to lay all this out.
It's been about a 2 month battle to this point.
Preface....1998 Winter Package with a 6G74.

Flash back to February when I pulled the engine to drop in a junkyard engine from LKQ. Long story short...don't get engines from them. Total nightmare.
So my engine ended up being completely rebuilt.
Fast forward to about 7 weeks ago. Pretty hot day. Up until now, she was running like a champ. Zero issues.
The lady and I saddle up and head to Home Depot. Literally 2.4 miles away. Truck starts and runs fine all the way there.
We do our thing for about 25 minutes, come back outside, and upon startup, it's misfiring like crazy. The drive home felt a lot like limp mode.
I pull codes, and it's a P0300.
I know that there's a few very small vacuum leaks, so I address those over a few different intake pulls. In the middle of all that, I get a cylinder 2 injector code.
Intake back off, and decide F-it. Bought 6 new injectors.

So new injectors are in. P0300 still, with an intermittent P0302.
As a quick test, I start pulling the signal plugs off the back of the coils. The 1-4 coil shows no change with no signal. Bad coil? Let's change all 3.

So now. 4000 miles on a rebuild, new injectors, new coils. Zero vacuum leaks. Sitting at 16"Hg vac. (I'm in Denver...so yeah that's about right I think) Still P0300.

Okay...FSM is out. Slowly lean towards ignition control module. Test it, and sure enough...it's shot. Add new ICM to the list.
So now it's running better, starts right up, all cylinders are getting spark, but still a terrible misfire.
FSM back out...start checking voltages at the sensor connections. 30 milivolts at the TPS. FSM says ECM.
Have it tested....yep. Shot.
Ebay to the rescue with an used ECM. Starts better than ever now. Idles better. Drives like shit. Zero power.
Intake back off...new plugs. New coolant temp sensor.  Clean ALL engine grounds, but it all back together meticulously.
Still lagging and misfiring under load and acceleration. Still no codes.
Just did the fuel filter. Hence me smelling like gas and giving up for the night. Same issue.
Earlier today we pinched the return fuel line after the FPR. Sprayed gas everywhere. So I'm assuming there's good pressure, but I have no way to do a proper fuel pressure test at this point.

I'm full on stumped. And officially broke from changing bad parts.
What could have caused so many things to go wrong all at once? What am I still chasing?
« Last Edit: September 30, 2017, 07:01:15 PM by S298WP »

MudRaider

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Re: The Never Ending Misfire
« Reply #1 on: September 30, 2017, 09:18:13 PM »
All at once? Who said they all went bad at once? They may have gone bad, but you said it's been running better with every change. Just hasn't take care of the misfire. Your running out of parts to change. Next will be wires and electrical connections. The ECM has enough capability to operate with parts in the bad  margin of reading.

S298WP

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Re: The Never Ending Misfire
« Reply #2 on: September 30, 2017, 09:50:25 PM »
True.
FSM us saying fuel pump.
Just need to figure out how to get a legit pressure test on it.

Justice

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Re: The Never Ending Misfire
« Reply #3 on: October 03, 2017, 05:54:19 PM »
Most all  24V V6's use the same "power Unit" I think the FSM calls it the Ignition power module. Since they are plentiful and easy to get, i would try putting one in. I had a misfire on my 99, and took the power unit out of a 94 just to test, and that was the source of the misfire. Its the small black plastic box with  a plug going into it on the top of the plenumn.

JohnnyBfromPeoria

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Re: The Never Ending Misfire
« Reply #4 on: October 03, 2017, 09:13:31 PM »
Most all  24V V6's use the same "power Unit" I think the FSM calls it the Ignition power module. Since they are plentiful and easy to get, i would try putting one in. I had a misfire on my 99, and took the power unit out of a 94 just to test, and that was the source of the misfire. Its the small black plastic box with  a plug going into it on the top of the plenumn.

I'm taking an extra one along on next week's trip.

John B.
AZ Crew/East Sider/Former 14th St Crew
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S298WP

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Re: The Never Ending Misfire
« Reply #5 on: October 04, 2017, 05:11:38 AM »
Same as what I referred to as Ignition Control Module.
Swapped out already.

MudRaider

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Re: The Never Ending Misfire
« Reply #6 on: October 11, 2017, 07:51:12 PM »
Since connectors were brought up on Facebook, here's a place with Mitsu connectors;   http://connectors.sheridanengineering.com/index.htm

Something not mentioned yet, but JBfP and his "wiring tune-up" may be an option, with merit