Rough running 7.4 Vortec

[TexasAggie]

Final update (on this issue at least) - Truck ran good all weekend.... After I got it running last week, I checked the connections at the disti and a couple of plugs and she smoothed out. I think there is still a small issue going on, but I can't be for sure at this point. I need to learn out to use the scanner and see what it tells me. As far as the no start, I think it had something to do with the passkey or whatever the GM security system is. I'm getting a Black Bear tune in a few weeks and will have them disable this functionality which will hopefully solve that issue for good.

 

[stutaeng]

Cool. Keep us posted on the update.

 

[618 Syndicate]

Final update (on this issue at least) - Truck ran good all weekend.... After I got it running last week, I checked the connections at the disti and a couple of plugs and she smoothed out. I think there is still a small issue going on, but I can't be for sure at this point. I need to learn out to use the scanner and see what it tells me. As far as the no start, I think it had something to do with the passkey or whatever the GM security system is. I'm getting a Black Bear tune in a few weeks and will have them disable this functionality which will hopefully solve that issue for good.

As I read through your no start post I figured it was the security system. That start-run-die scenario is a dead giveaway. Glad you figured it out.

 

[8T7K5]

Final update (on this issue at least) - Truck ran good all weekend.... After I got it running last week, I checked the connections at the disti and a couple of plugs and she smoothed out. I think there is still a small issue going on, but I can't be for sure at this point. I need to learn out to use the scanner and see what it tells me. As far as the no start, I think it had something to do with the passkey or whatever the GM security system is. I'm getting a Black Bear tune in a few weeks and will have them disable this functionality which will hopefully solve that issue for good.

I've got a 99 big block I'm currently trying to get running and sold one that had a similar issue as what you're having. Rough idle, rough through the rpm's. Replaced plugs, wires, cap, rotor, nothing helped. Finally replaced the coil and ICM (Ignition Control Module) and that fixed the issue. I'm sure there are ways to test them individually but that engine was in such bad shape I figured they both needed to be replaced. They sit on a little bracket above the distributor on the back of the upper intake, two bolts hold the bracket on, one connector goes into the ICM.

 

[TexasAggie]

The miss is VERY slight now and I probably wouldn't notice it if I wasn't already paying close attention. I'll look at the coil and ICM and see about swapping them out.

Can these be had as one unit or do they come as separate pieces? Pricing seems to be all over the map on these.... will the cheaper knock offs perform as good as the AC Delco versions?

 

[stutaeng]

You can check the resistance of the coil with a DVOM...I have not found a place that check the ICM locally. They can be bought separately.

As far as brand, it think it's a toss up as well. AC Delco was thought to be be preferred brand...but some question the quality now. I bought about 3 ICM (and probably warranted another 3) from a no-start when damp on my 4.3. Probably lost track honestly. My other truck with the 5.7 actually still has the original ICM and coil. I used this one lately on my 4.3 to confirm it was not the ICM, when it did not want to start.

Most of the sensors for my trucks I buy now come from AAP and they carry BWD. I have not had to warrantee anything yet.

Rockauto has the best prices for most sensors and mostly everything really. If I'm not broken down and need it ASAP, I'll order from them, but otherwise locally for warantee purposes, just save your receipts...just in case.

 

[TexasAggie]

Gonna try a $12 ICP from Amazon and see how it goes. Might replace the coil later on as well.

 

[Carlaisle]

If you had a coil or ICM problem it would not be confined to a single cylinder. A single cylinder issue needs a single cylinder fix. Plug wire, plug, injector, cap, etc. Your 52 psi of fuel pressure at idle is low. It should be about 58 - 62. I would be more suspicious of the fuel pump given the mileage, but it could be the fuel pressure regulator instead or both. Pressure issue could be as simple as damage to that 2" long piece of vacuum hose that runs the fuel pressure regulator. If those are the stock injectors they were junk when the factory installed them. It's unlikely they have improved with age.

 

[TexasAggie]

All good points. Is there any way to check any of that without pulling the upper intake off? As it is, she's running pretty good, so I'm not inclined to take her apart to investigate these things just yet. How hard is it to drop the tank? How much do a decent set of injectors cost? Does programming need to change in order to take advantage of new injectors?

 

[chevman_88]

All good points. Is there any way to check any of that without pulling the upper intake off? As it is, she's running pretty good, so I'm not inclined to take her apart to investigate these things just yet. How hard is it to drop the tank? How much do a decent set of injectors cost? Does programming need to change in order to take advantage of new injectors?

Have you watched any live data while driving to see what your fuel trims are doing? Even if there aren't any codes you might be running lean which could cause a miss. I will say that an AC Delco FPR that I bought new only had 52psi KOEO. I ended up swapping out for the 5.7 variant which has a higher pressure rating and now I sit at 58psi KOEO. I'm still chasing a lean issue, but it has improved with each little thing I find and fix and I can verify it is helping by watching fuel trims while working on it.