runpasthefence

iTrader: (2)

Pretty frustrated since I had an appointment to have the exhaust built for this car tomorrow, but now I have a no start condition. Here's the rundown if y'all wouldn't mind helping me think through this....

96 Roadmaster, L92/LSA/4L80e. No miles since swap. Car has run well in garage previously. Problems began yesterday. I ran the car up to temperature just to ensure the cooling system was well bled. Turned car off and would not start for the rest of the afternoon. Got home from work today and the car started and ran just fine. Turned car off after about 2 mins of running. Car now won't start. I started trying to diagnose the issue this afternoon. Here are some details:

1. Noid light shows no injector activity
2. Spark check shows no spark
3. Plugs pulled - completely dry
4. Grounds were checked; nothing has changed. I've got 2AWG wire or ground straps going everywhere they need to. They are tight.
5. Harness is from PSI (and obviously worked fine up until yesterday). I've verified 12V to their fuse panel (for FP and ignition etc) at key on and during cranking
6. Ignition relay is functional (swapped with FP relay)
7. HP tuners reads 170ish RPM during cranking (presuming this rules out crank sensor issue?)
8. No obvious damage to crank sensor harness or other parts of harness for that matter
9. All sensors are new (not that that means anything)
10. Fuel pressure at rail is 70psi during cranking
11. 12v is present at injectors and at coil harness
12. PCM plugs are secured well

What am I missing here? What would cause a sudden loss of injector and spark trigger? Could the brand new PCM be dead?

the_merv

iTrader: (88)

I'd try another Crank Sensor. Preferably a GM one.

runpasthefence

iTrader: (2)

Yeah, I’ve got a new one arriving today. I hope it’s that simple.
The one in there now is brand new GM with probably less than an hour of runtime and is showing RPM while cranking.

the_merv

iTrader: (88)

Even with no Cam Sensor a Gen III Engine will start, just will take a bit to learn position but it will start.

runpasthefence

iTrader: (2)

New crank sensor, same story.

Same thing as before. Car starts immediately on first try. If I shut it down, it goes into a no spark/no fuel state. Starting to wonder if I have some weird VATS issue or maybe a bad PCM.

Reviewed all of my grounding (again); looks good.
Unplugged and re-plugged in my PCM plugs. No pins were bent/pushed that I can tell.
Re-checked fuse panel; all good.

Had to walk away. Will be interesting if I wake up tomorrow and it starts just fine again.

the_merv

iTrader: (88)

Put your laptop on it with the scanner open and see if you can read the PCM for any codes. What PCM is it?

runpasthefence

iTrader: (2)

E38 PCM

It showed no codes Sunday and Monday when I checked, but I hadn’t checked since trying the new crank sensor, so I did that this morning…

First start attempt - no start and no DTCs.

So I setup the right coil bank on a spark checker and number 2 cylinder injector plug on a noid light. Setup my phone to video both.
Turn the car over to film and it starts/runs on 4 cylinders
Video showed good spark/injector signal, so I hook everything back up to see if it’ll run.

Next (extended) start attempt - no start with some backfiring
Now I have a cam position sensor DTC

Got called into work right after this.

I know a bad cam sensor will cause extended starts, but that’s not what I’m experiencing. I either get immediate start or no start.

the_merv

iTrader: (88)

Yea you'd hope it wouldn't be the Cam Sensor. Behind the Waterpump is a pain to deal with.

Random thought, is there anything in the fuel system that the PCM gets any kind of pressure reading, as in is there any sensor the PCM reads Fuel Pressure? 70psi is high, I keep mine around 60psi. 58psi is the factory spec.

Almost makes me wonder if there is a ground issue since you unplugged things to get your checkers in there, and it ran... kind of.

runpasthefence

iTrader: (2)

Nah, no PWM and nothing telling the PCM what the fuel pressure is. I've found that cranking/starting pressure is 70ish, but running pressure is more in low 60s

I hadn't thought of that. Just tested the theory - I clamped on a set of 2AWG jumper cables (as opposed to the 12AWG ground on the spark checker) directly from the battery ground to the block and tried to start. No change - same behavior as yesterday. Seems like it's still getting spark and injector signal, but it just farts/pops very occasionally. Almost seems like it's out of time.
I've triple checked the grounding on this thing bc that's what it seems like, but I just don't see anything wrong.

10AWG to body panel <- Battery -> 2AWG -> engine block -> 2AWG to alternator + large ground strap to frame
PS head -> ground strap -> engine block near firewall -> ground strap -> DS head -> ground strap -> firewall

The thing that's kicking me is this is a pretty clear change. I had started and run easily and immediately in the garage for the past couple of months. The only thing I did between when it was running well and this no start situation is install a catch can. Seemed VERY unlikely that a catch can install would cause this, but I went ahead and removed that stuff anyway with no change.

Hoping I can put some more serious time into it this weekend.

the_merv

iTrader: (88)

I'd be checking the main wiring harness, make sure nothing is burned, crushed, or a mouse chewed on something.

runpasthefence

iTrader: (2)

Yep, I put eyes and hands on everything I could today and didn't see anything. Unfortunately, a large portion of the harness is inaccessible without partially pulling the engine. Partially pulling the engine requires pulling the LSA lid. Pulling the lid is an absolute pain in my ***. But I guess I'm running out of "simple" solutions. Might just start by checking the resistance of everything at the PCM connectors and go from there.

the_merv

iTrader: (88)

Yup electronics 101, the Multimeter. I've been there and done that.

runpasthefence

iTrader: (2)

Well, I started Saturday with a no spark condition and managed to do the following since then.

1. Added 2 10AWG grounds direct from battery to driver head (where PSI harness connects for ground); unnecessary, but I wanted it ruled out
2. Checked resistance of all grounds (all good)
3. Checked resistance of all relevant PCM systems
   1. Coils
      1. all have 12v at run and crank
      2. all have good continuity on "signal wire" to their appropriate PCM pin
      3. low reference wires traced and ohm'd back to pcm
      4. grounds appropriately checked
   2. ECT - good continuity to pcm and low reference pin
   3. Crank sensor - all wires have appropriate resistance to their PCM pins
   4. Injectors
      1. Started to run out of time, so I only checked a couple bc they were getting good signal (via noid lights) on Saturday
      2. Everything looked good on the 2 that I checked.

I'm sure I checked more, but I got to a point where I decided button things back up and triple check the fuse box/ign relay wiring — everything still looks good there.

Attempt to start — same as before. New development; I've lost injector signal again according to noid lights, so back to no spark and a no fuel.

Time to start throwing away all of the matches and lighters in the house...

the_merv

iTrader: (88)

At this point only thing left is the PCM if the wiring and everything else is sound.