No Start/Run Condition Problem......

[NC_John]

Background - LSX swap, 04 blue/green PCM. DBW. Speartech standalone harness. Boyd tank with aeromotive pump. Build has got about 60 miles on it, still working through some new-build bugs but engine has been great so far.

Leaving the house the other day the truck started fine and ran fine but then just instantly died in the driveway like it was keyed off. I figured fuel was too low as it just cranked but would not restart. Pushed it back in the garage. Next day added a few more gallons of fuel to the tank but it would only turn over, no start.

Started digging into it yesterday. Noticed I could not hear the fuel pump cycle on to prime with initial key-on (in-tank pump is controlled by PCM through speartech harness). Got my voltmeter on the power terminals at the tank and had zero volts with key-on. Thought that explained the no-start. Jumpered the fuel pump relay terminals to manually run the fuel pump but still crank only, no start. Ran out of time to play with it for the day.

Today checked that the PCM has voltage, it does. Checked all the fuses (power to pcm, fuel pump, 02 heaters and injectors/ignition). They were good. Manually forced the fuel pump on again and cranked. No start. Checked ignition for spark (old school method- screwdriver in plug wire watching for spark to block). None. Pulled a plug and checked for fuel - none, dry as a bone. So it looked like the pcm was neither calling for the injectors to cycle or the coils to fire.

Went back and checked the signal for fuel pump relay to see if the pcm was even calling for the fuel pump. It was, it read 12 v with key on. Put the relay back in- no fuel pump. Bench tested the relay- was clicking and metered good with my ohmmeter. Jumped the power to the fuel pump again. This time it cranked some and then and fired up. A little more backfiring and popping through the exhaust than I would consider to be normal (its pretty free-flowing exhaust, barely muffled on a built engine). The exhaust smoothed out to a more normal idle within 15-20 seconds. Shut it off. It restarted normally. Pulled the relay jumper wire and put the relay back in. Nothing. Crank but no fire.

Even though it tested OK, I suspect the fuel pump relay is bad (some chinese one that came with the speartech harness) and ordered a replacement (its got a goofy ISO280 base so I don't have one here I can use to test). I'll have a couple relays here early next week.

Here is my confusion. If the fuel pump relay is indeed bad, why was I not seeing spark from the ignition or fuel on the plugs when I was forcing the fuel pump on with the jumper wire? There was no evidence that the engine was even trying to run. I was starting to think VATS had somehow been re-enabled. Do I maybe still have a problem with the PCM (cold solder joint going intermittent or something similar)? I don't think it would take that much cranking to prime the fuel line. I don't have a fuel pressure gauge but did order one from Summit that I'll have this week as well (couldn't do a loaner tool gauge on my fast fuel rails.)

I'm going to be nervous about driving this thing until I know why it tested the way it did. Again, if it is the fuel pump relay, which I will confirm when the new one gets here, why was there no spark or (perceived) fuel injector activity? Is the engine looking for fuel pressure before it tries to start? I don't know anywhere where it would measure fuel pressure from.

anybody know what might have happened/might be happening?

[BR3W CITY]

Check the little grounds on the harness that feeds the injectors and the coil plugs. Might be worth hard-grounding the PCM case as well. The internal ground can be suspect sometimes and case grounding can help. The ground for the injector driver board can break loose off its solder joint, and sometimes (worst case) the injector driver is for some reason fried.

[NC_John]

Originally Posted by BR3W CITY

Check the little grounds on the harness that feeds the injectors and the coil plugs. Might be worth hard-grounding the PCM case as well. The internal ground can be suspect sometimes and case grounding can help. The ground for the injector driver board can break loose off its solder joint, and sometimes (worst case) the injector driver is for some reason fried.

Grounds are all good. Already have a ground strap from the pcm case to the cab. Cab to frame. Frame to battery. Checked all the way along with an ohmmeter.

I’m nervous that the pcm might be on the way out but don’t want to buy one and pay another hp tuners license to use it if that isn’t in fact the culprit.

[verdell]

Subscribed. I'm having similar issues as well. Just finished my swap this past weekend. Drove it to the muffler shop yesterday, had exhaust done, drove back to our shop and it hasn't started since.

[NC_John]

New fuel pump relay didn't solve my problem so back to the drawing board. This truck is the gift that just keeps on giving. Might dig into it some this weekend as the hurricane is going to give me plenty of indoors time.

[Flubyu]

FYI. The PCM will only feed the fuel pump voltage to prime the system at KEY ON. After that the PCM needs to see signal from engine that it is running to run the pump. I learned this the hard way during my swap. Also, if VATS were the issue the engine would start, runs a couple seconds and die. I know this isnt much help but I didnt want you to chase your tail like I did trying to figure out my fuel pump issue.

[NC_John]

Originally Posted by Flubyu

FYI. The PCM will only feed the fuel pump voltage to prime the system at KEY ON. After that the PCM needs to see signal from engine that it is running to run the pump. I learned this the hard way during my swap. Also, if VATS were the issue the engine would start, runs a couple seconds and die. I know this isnt much help but I didnt want you to chase your tail like I did trying to figure out my fuel pump issue.

Yeah, I know. I don't even get that initial prime sequence with key-on.

I have secured a GM Tech 2 scanner for the weekend so I am going to start digging into the pcm and see if I can figure something out. With this tropical storm/hurricane making all sorts of noise and mess outside it'll be a good weekend to spend some time in the garage.

Thanks for the input.

[NC_John]

Mixed results today but I suspect my pcm is on the way out.

Prior to hooking up the tech 2 I plugged in a fuel pump relay and the truck started normally and repeatedly. Hooked up the tech 2 and was able to start and stop the fuel pump at will. BUT, I kept having communication errors with the tech 2 and at times the idle would get very rough.

That worked until it didn't. Without changing anything, I've since lost proper operation, truck won't start, can't control the fuel pump through the tech 2. I pulled the seat out and have gone through all the wiring at the harness. Everything looks lovely and I can't find any problems.

I'm thinking I have a ground or power issue within the PCM and just going to get another one. I hate to buy one for nothing and have to get another HP Tuner license for it but can't really think of what else to do.

[ls1nova71]

I assume this is a factory harness that speartech reworked? Have you contacted them?

[NC_John]

Originally Posted by ls1nova71

I assume this is a factory harness that speartech reworked? Have you contacted them?

Brand new harness. I have not yet.

[NC_John]

Got the SOB figured out today. Turned out to be an intermittent connection with the fuel pump on signal from the pcm. The issue was inside the base of the fuel pump relay so whenever I probed it with my voltmeter, it would make contact and I could not see the fault. I found it just by luck when I was working my way through the harness and the wired jiggled the right way. I removed the bad base and relay and installed one I had in my toolbox, no more problem.

Glad I finally got this one resolved. I was at the point of calculating how much tannerite it was going to take to send this truck into orbit.

[mongocanfly]

Yeehaw...glad you got it....tannerite is would probably also jiggle that wire loose...haha

[oldman3]

Good deal, the little things will get you everytime...Jim

[NC_John]

Originally Posted by mongocanfly

Yeehaw...glad you got it....tannerite is would probably also jiggle that wire loose...haha

Originally Posted by oldman3

Good deal, the little things will get you everytime...Jim

Thank you guys. Like a friend of mine said - "intermittent electrical problems are the work of Satan himself."

You have to expect some teething problems on a new build but this one was no fun. Something as simple as that being so hard to find. Yikes... Almost makes me want to sell the truck and buy a set of golf clubs.

[oldman3]

Originally Posted by NC_John

Yikes... Almost makes me want to sell the truck and buy a set of golf clubs.

Oh no, don't go to the dark side...Jim

[NC_John]

Originally Posted by oldman3

Oh no, don't go to the dark side...Jim

Nah, I hate golf. That's just how bad this got. I'll tell you one thing- this sure has upped my knowledge of the PCM and wiring harness exponentially. I am confident I could wire up a standalone harness pretty easily now.

[jjzepplin]

That is fabulous! I need a harness! Please spend extra time on the fuel pump relay connections prior to sending, Thanks! HAHA! This makes me a little iffy on whether I should do the swap now. Dangit. I already bought the donor.

[NC_John]

Originally Posted by jjzepplin

That is fabulous! I need a harness! Please spend extra time on the fuel pump relay connections prior to sending, Thanks! HAHA! This makes me a little iffy on whether I should do the swap now. Dangit. I already bought the donor.

Well....

I still wouldn't trim down or build a standalone from a donor harness, at least up to or of a certain age, as they get damaged and brittle over time. Insulation gets hard and damaged, copper starts to corrode and the plastic connectors get brittle and break. I work on enough of this crap at work on trucks with 100k+ miles that I see it daily. That was a big reason why I paid the money for a new one. Figured it would be one less thing to worry about (who knew?).

I don't know what it would cost to build a new harness from scratch with all new parts. I'm sure all the special connectors alone would add up significantly. Might not be economically worth it unless you were buying parts in bulk and semi-mass producing them.

As far as cost- mine went over the top because my "cam only drop-in 6.0" turned into a full blown 600 hp stroker. The only parts I ended up keeping off the complete donor engine I bought was the block (which I used) and the 317 heads (which I am not using but kept) in case I decide to add significant boost later. And then adding all that additional power meant I had to keep upgrading everything down the line so it would survive. It just snowballed on me. Its still going on. I am not done spending money on it yet.

[jjzepplin]

Good to hear it was only a relay! Was it the relay that came with the harness? Are you happy with the harness?

[NC_John]

Originally Posted by jjzepplin

Good to hear it was only a relay! Was it the relay that came with the harness? Are you happy with the harness?

It was actually a connection/termination within the relay base. It was part of the manufactured harness. Other than that, I have been very happy with it.