Barely Running Believe its a CMP issue.

[Spareparts]

I only had a CMP code once when i changed the ECU. I could not make it trip even if i turned the dizzy + or - 25, just would not set a code and it would run fine either way.
I also never got a code for engine misfire with a plug wire removed and driving around the block.
Never did figure out the misfire

 

[1952Chevy]

So the other day I unplugged the MAP sensor, and within 2 minutes I had a CEL, and downloaded a code for it. So it would seem that my computer is still capable of showing codes.

I have been noticing that while all of my random misfires now(extremely low counts) are on bank 2. Cylinder 2 and 4 are the most common recurring symptomatic holes. While I was looking around then engine bay, there was some soot at the mating flange of the upper and lower intake manifolds, between the 2 and 4 intake runners. I'm wondering if I have a slight leak at that point of the intake. If that's the case, it would certainly make sense for a misfire on those 2 cylinders. Hopefully I'm able to take a look at that this weekend.

Thoughts?

 

[termite]

mating flange of the upper and lower intake manifolds, between the 2 and 4 intake runners.

Unlit propane torch to see if you've got a vacuum leak there?

 

[Keeper]

I swear, vortec 454's should have intake gaskets as a regular service item... every 100k at least. They can fail in so many ways. I'd highly suggest doing the lower intakes also if that upper one is shot.

 

[1952Chevy]

I swear, vortec 454's should have intake gaskets as a regular service item... every 100k at least. They can fail in so many ways. I'd highly suggest doing the lower intakes also if that upper one is shot.

They have less than 2k miles on them, which is the confusing part. Before installing this motor it got a whole gasket kit. But I may redo it anyway at this point.

Unlit propane torch to see if you've got a vacuum leak there?

I will give this a shot. I tried brake clean, but it just evaporated. Didn't notice any change though.

 

[Schurkey]

I tried brake clean, but it just evaporated. Didn't notice any change though.

DO NOT EVER, EVER, EVER use aerosol brake cleaner to test for vacuum leaks.

1. Some brake cleaner is not flammable. It gives no indication of a vacuum leak even if there is one.

2. Some brake cleaners degrade into POISONOUS GAS that can FUKK YOU UP PERMANENTLY. Phosgene gas was used during WWI, and caused lots of heartache.

Use aerosol carb cleaner instead. Or propane. I prefer aerosol carb cleaner.

 

[1952Chevy]

DO NOT EVER, EVER, EVER use aerosol brake cleaner to test for vacuum leaks.

1. Some brake cleaner is not flammable. It gives no indication of a vacuum leak even if there is one.

2. Some brake cleaners degrade into POISONOUS GAS that can FUKK YOU UP PERMANENTLY. Phosgene gas was used during WWI, and caused lots of heartache.

Use aerosol carb cleaner instead. Or propane. I prefer aerosol carb cleaner.

It was carb cleaner not brake cleaner! My bad. Thank you though!

I did try again with propane and also didn't notice any changes.

 

[1952Chevy]

Update. Took the truck camping this weekend with my kids. Did great driving through the mountains, except uphill at low throttle.

During those times, cyl 8 would misfire(noticeably) with a count that would climb to around 60. Add throttle and the misfire goes away.

I am thinking of changing the Ignition Control Module, and Coil. Mostly because I already have them. They came with the truck when I bought it. I'm wondering if the previous owner wasn't fighting similar issues prior to the rod knock. Truck came with new in a box: Plug wires, cap, rotor, coil, ignition control module, IAC, fuel filter, and EGR valve.

So current symptoms are CYL 2 misfire at idle with a count not exceeding 12, and CYL 8 misfire while driving up grade w/low throttle input. Everything seems to be on bank 2.

I don't know how the ICM, or coil, would cause issues on only 2 cylinders. I may try new wires(I have a set for another vehicle that I can try to see if that resolves the issues.

 

[Schurkey]

Agreed--modules and coils aren't selective with regard to individual cylinder(s) misfire.

A distributor cap could be. Plug wires, and plugs certainly are. But the trouble there is that higher voltage firing the plugs should lead to MORE problems (misfires) not fewer.

What gets worse at low throttle settings? Vacuum leaks. The "false air" (NOT my term, that came from GM) is a greater proportion of the total air/fuel mix; and since vacuum leaks tend to lean-out the mixture the effect is greater as the throttle closes.

I don't mean to suggest that this is guaranteed to be caused by vacuum leak(s). But it seems a good working theory that deserves to be tested, possibly by taking things apart and seeing what the gaskets look like.

This is a Vortec 454, right? They're known for injector problems. A leaking injector also is worse at low throttle, as the leaked fuel is a greater percentage of the desired fueling. As the load--and therefore fuel demand--increases, the leaked fuel is a smaller percent, and becomes somewhat less noticeable.