My 454 Rebuild

[Spareparts]

I'm not worried about the compression it is lower than expected but still within spec.
It's the slight miss that is a bigger concern.
I guess i don't understand the value of a leak down test as all cylinders have the same compression and should be firing the same i would think.
I also swapped back to the old stock black box ecu with no tune thinking that might be a problem.

 

[Spareparts]

Ok I broke down and ordered a 1time use otc leak down tester. Should be here Friday.

 

[BeXtreme]

I'm not worried about the compression it is lower than expected but still within spec.
It's the slight miss that is a bigger concern.
I guess i don't understand the value of a leak down test as all cylinders have the same compression and should be firing the same i would think.

Maybe, maybe not. Its just a diagnostic tool. You don't know what you don't know. Since you don't know what the problem is, assumptions can bite you. Between a compression test and leak down test you will be able to VERIFY that you don't have a mechanical problem in the rotating assembly or the valvetrain.

 

[Spareparts]

Ok did the leak down test and the engine is loosing between 2 and 3 percent at 100psi. It is a PITA to do by yourself but it's done.
We know the compression is even between all 8 cylinders. Checked again today.
We know it has a rock steady 19inhg vacuum.
I had the valve covers off and all rocker arms appear to be moving properly.
All spark plugs appear to be firing normally. They are all gapped to .050 I have tried two sets of spark plugs.
The plug wires are new AcDelco but using a timing light i found two not firing properly and changed them out. Now using a timing light they all have a steady flash, flash, flash with nothing erratic looking.
tried a new and used distributor cap and rotor.
Injectors appear to be working properly.
It seems to be worse when cold and not so pronounced when hot.
I have no clue where to look. In well over 50 years i have never ran into a problem this perplexing
Thinking a new complete set of plug wires and trying that.

 

[yevgenievich]

What about smaller spark plug gap?

 

[Spareparts]

I would not think that would be the problem. Recommend is .060 I might try going down to .040 though
I'm kinda thinking sense i had a couple bad new plug wires maybe there is something wrong with the rest of them not letting full voltage to the plugs but still firing the timing light ok.

 

[L31MaxExpress]

If you still have it on the engine, an EGR valve not closing all the way could cause a misfire that is most prominent with a cold engine. There is not an engine that I know of that likes a vacuum leak of exhaust into the intake manifold at idle especially when it is cold.

 

[Spareparts]

I have swapped EGR valves. 0 Difference

 

[Piratehunter]

I used ACDelco 41-993 plugs out of the box gapped at .060. Is it possible to watch your engine running in complete darkness to see if you can spot any arcing on the wires?
but the miss is more when the engine is cold verses hot. Does it totally smooth out when hot?

 

[Spareparts]

No, it is always slightly rough hot or cold. You can feel the miss more when cold vs hot when driving.