Bad running after manual swap

[JPVortex]

Those are AFAIK signal outputs from the ICM to the ECU, so I doubt the ECU could affect the ignition through those connections... doubt, provided the "timing" wire was also disconnected.




Remind me: Have you absolved the ICM? And you're confident the ICM's ground through the dizzy, hold-down clamp and engine block is soild?

Regardless, you really need a guy with an oscilloscope to pay you a visit... someone who knows how to use it and also knows what he's doing. This would go oh-so-much-faster.

Ok thanks for clearing that up about the ECU.

And yeah I can rule out the ICM itself, issue started with old dizzy, replaced the assembly with new ICM and everything, issue didn’t change one bit.

Now the grounds do make me wonder somewhat… from what I see it was missing the following grounds,

- frame to engine block
- frame to firewall(underneath heater hoses)
- firewall to back of passenger side cylinder head

Now it seems the shop only put in 1 more ground. He brought a ground from the front of the frame on the passenger side to the block, using the bolt hole for the cutout for a mechanical fuel pump(my 350 still has it cutout, even though it’s tbi)

So that takes care of the engine block ground, but there’s still no ground strap for the firewall or the back of the cylinder head. Now thinking about it, the ICM/coil would probably use the ground on the back of the head…. I wonder if it doesn’t have good ground because it needs to travel somewhat far for one…

 

[1998_K1500_Sub]

Ok thanks for clearing that up about the ECU.

And yeah I can rule out the ICM itself, issue started with old dizzy, replaced the assembly with new ICM and everything, issue didn’t change one bit.

Now the grounds do make me wonder somewhat… from what I see it was missing the following grounds,

- frame to engine block
- frame to firewall(underneath heater hoses)
- firewall to back of passenger side cylinder head

Now it seems the shop only put in 1 more ground. He brought a ground from the front of the frame on the passenger side to the block, using the bolt hole for the cutout for a mechanical fuel pump(my 350 still has it cutout, even though it’s tbi)

Best bet here is follow the FSM. As I remember, there are two ground straps:

- one from the frame to a firewall stud, both connections coincidentally near the back of the engine on the passenger's side, and

- one from the engine head (passenger's side), to the same stud on the firewall mentioned above

Reinstate these grounds with ground straps (braided). Straps exhibit lower inductance than a simple wire; I learned this from a service manual, somewhere.

The battery ground (black cable) connects to the engine block somewhere, i can't recall exactly where (down low, front of the engine block on passenger's side likely).

On my 1998 Suburban there's also a short ground wire (wire, not strap) from the battery "-" to the passenger's side fender. There's also a strap (not a wire) from the hood to a stud near the top of the firewall (this strap might be to mitigate radio interference from the ignition).

Don't go ape$h!t installing grounds everywhere / anywhere, do it like the factory did... at least until this is sorted-out. Some guys think that if one ground is good then too many is not enough. Don't go there.

 

[JPVortex]

Best bet here is follow the FSM. As I remember, there are two ground straps:

- one from the frame to a firewall stud, both connections coincidentally near the back of the engine on the passenger's side, and

- one from the engine head (passenger's side), to the same stud on the firewall mentioned above

Reinstate these grounds with ground straps (braided). Straps exhibit lower inductance than a simple wire; I learned this from a service manual, somewhere.

The battery ground (black cable) connects to the engine block somewhere, i can't recall exactly where (down low, front of the engine block on passenger's side maybe, sometimes to the passenger's side head like on my 1979 Camaro).

On my 1998 Suburban there's also a short ground wire (wire, not strap) from the battery "-" to the passenger's side fender. There's also a strap (not a wire) from the hood to a stud near the top of the firewall (this strap might be to mitigate radio interference from the ignition).

Don't go ape$h!t installing grounds everywhere / anywhere, do it like the factory did... at least until this is sorted-out. Some guys think that if one ground is good then too many is not enough. Don't go there.

Ok so yeah I’m definitely missing a couple grounds.

I do have the small ground wire on the fender, that’s the one that was loose that he repaired. It comes out from the main negative battery cable.

My negative battery terminal wire bolts on with a nut to one of the intake bolts, seems like that is the way it came factory. Definitely not long enough to go onto the block.

Need to get a ground from frame to firewall though, and from firewall to back of passenger cyl. Head.

You think missing these 2 grounds could be a potential cause of the problem?

 

[1998_K1500_Sub]

Need to get a ground from frame to firewall though, and from firewall to back of passenger cyl. Head.

You think missing these 2 grounds could be a potential cause of the problem?

Umm... how does the fuel pump current find a return (ground) path to the alternator (engine ground)? Evidently through some mysterious path that the engineers didn't intend.

The list goes on...

Fix that $h!t

While you're at it, check and confirm the location and condition of your ECU grounds on the engine (typically on the intake somewhere).

The service manual is your friend.

Get this sorted-out and then, if the problem persists, we're on more solid ground (no pun intended).

 

[JPVortex]

Umm... how does the fuel pump current find a return (ground) path to the alternator (engine ground)? Evidently through some mysterious path that the engineers didn't intend.

The list goes on...

Fix that $h!t

While you're at it, check and confirm the location and condition of your ECU grounds on the engine (typically on the intake somewhere).

The service manual is your friend.

Definitely am going to fix that up.

Already did some looking at the FSM within the last week, that’s how I came to find out I was missing these grounds.

And already cleaned up and checked out the ECU grounds last week when I was trying to diagnose the issue. that’s one thing I can check off lol.


So at this point it’s either going to be a shorter wire or a grounding issue in my opinion, grounds are easier to install and need to be done even if it doesn’t solve the problem, so I’ll start with those and see what happens.

That’s if the shop doesn’t find a smoking gun.

 

[1998_K1500_Sub]

That’s if the shop doesn’t find a smoking gun.

They might be smoking something themselves...

 

[JPVortex]

They might be smoking something themselves...

Kind of sounds like it lol,

Every time I’ve ever had a vehicle in the shop(almost never now because I do the work myself) Ive had consistent updates almost daily whether there’s actually a change or not.

I went a week without a single update from this shop and it took me going in person today to get information out of them.(only good thing is I got to see my baby, AKA my truck for the first time in a week lol)

You’d think if an idiot like me can see that there’s more grounds missing then a shop with 15+ years experience should see that…. And not just throw one ground in.

 

[1998_K1500_Sub]

Let's talk about grounds...

I know a guy who stuffed a V8 engine into his Dodge (formerly V6) pickup truck.

Poor sap, for over a year it would start but not continue to run for him. He and a bunch of guys looked at it, with no joy, before I got the call.

So I went there (Saginaw, MI) and started looking. Turned out, during the engine swap, his ECU logic ground wasn't reinstated properly to the firewall; it was connect to some random bolt, but the bolt wasn't grounded... it looked like it could be a ground, but it wasn't. Consequently, the ECU's logic (uP, etc.) was instead finding a ground path through the ECU's sensor ground, which happened to be via an oil pressure sender... which enabled the ECU to function for a little while...

Long story short, the engine would crank and start, and immediately die. And it would only do this after sitting a while. But it was repeatable.

When I found and fixed his problem (remember, he had had this problem for over 12mo by this time) he literally cried.

(Edit). I will add details to this later about why an oil pressure sender (not sensor) was relevant to this whole mess.

 

[JPVortex]

Let's talk about grounds...

I know a guy who stuffed a V8 engine into his Dodge (formerly V6) pickup truck.

Poor sap, for over a year it would start but not continue to run for him. He and a bunch of guys looked at it, with no joy, before I got the call.

So I went there (Saginaw, MI) and started looking. Turned out, during the engine swap, his ECU logic ground wasn't reinstated properly to the firewall; it was connect to some random bolt, but the bolt wasn't grounded... it looked like it could be a ground, but it wasn't. Consequently, the ECU's logic (uP, etc.) was instead finding a ground path through the ECU's sensor ground, which happened to be via an oil pressure sensor... which enabled the ECU to function for a little while...

Long story short, the engine would crank and start, and immediately die. And it would only do this after sitting a while. But it was repeatable.

When I found and fixed his problem (remember, he had had this problem for over 12mo by this time) he literally cried.

Wow that’s quite the story! I bet that guy was happy.

Really does show how important grounds are lol.

I’m giving the mechanic
with my truckthe rest of the week to figure something out, if I don’t hear anything by next Monday I’m going to pick it up.