96 454 Vortec complete rebuild - unknown misfire?

[Texas454]

Hello there, I have searched the forum and am not sure what our next step should be. I was hoping you all will help in guidance please?

I had my 96 454 Vortec completely rebuilt by a very good local rebuilder. I had them rebuild my 99 350 Vortec a handful of months ago and it is running very good. Back to the 454.

I picked it up from the rebuilder and had my plan "B" mechanic working on it. Mechanic "A" didn't want to do the swap. So, Mechanic "B" installed the 454 into my dually and had an issue getting the distributor timed properly. He doesn't have a computer or scanner able to tune it so he installed it as good as he could and it was running really really rough. He said the catalytic converters were bad and needed replaced and it needed tuned better with a scanner or computer.

At this point I paid him for the install and drove it (BARELY) to mechanic "A" so he can use his Snap On scanner to tune it and remove my catalytic converters and replace them with exhaust pipe.

He diagnosed it as having a consistent misfire on cylinders 2 and 3 and he can not figure out why. I will list everything that has been done.

- New distributor with cam sensor, rotor button and cap
- Even though he removed the O2 sensors, he unhooked the exhaust from the manifolds to make sure the bad converters wasn't causing the misfire
- Fuel pressure (good), spark (good), compression (175), injectors (good), plugs (new), wires (good), timing (good)
- Checked for pinched wires, bad grounds

I have a friend who knows a lot about various mechanic work and he said his next step would be to "flash the ecm", I don't think mechanic "A" has done that yet.


After knowing all of this, does anybody have a suggestion on what we should do next? I would sincerely appreciate any help at all, this has been very frustrating and I just want to get my 454 running good again. I will even compensate the person who is able to direct me to the issue and help the mechanic with the knowledge to get it running properly.

Thank you in advance.

 

[TeveTorbes]

Do you know if you have a rich misfire? at idle? I am having a similar misfire problem, although I have a 1995 GMC K3500 w 454. Yours is OBD2 mine is OBD1 but I think some of the potential misfire causes could be the same. The MAP sensor and ECT sensor are likely candidates for causing a misfire. I didn't see those two in your writeup of what you checked so I would check those...and their wiring...which is where I'm heading now with my truck..or to the ECM as the cause. The Engine Coolant Temp sensor (ECT) could be giving a false reading which would let the ECM think the engine is not at operating temp and therefore give a rich mixture, the MAP sensor could also be giving a false reading which could case a rich mixture.

 

[Spareparts]

I feel your pain man.
Are you sure the distributor is timed properly with #1 cylinder?
The crank cam relearn will need to be done. RPM must be above 1K rpm to adjust it and the setting is ± 2 degrees. You will need some way to read it for adjusting it.
Whats your fuel pressure? Vortec 454 fuel pressure needs to be close to 58psi.
Are your injectors working properly? Could be dirty or just plain bad and not working. You say they are good but are you positive they are?
Bad spark plugs. Are they new?
How do you know your converters are bad?
Why remove the o2 sensors? Without the o2 sensors being in place and working properly your ecu will have no idea what to do for fuel.

 

[Texas454]

Do you know if you have a rich misfire? at idle? I am having a similar misfire problem, although I have a 1995 GMC K3500 w 454. Yours is OBD2 mine is OBD1 but I think some of the potential misfire causes could be the same. The MAP sensor and ECT sensor are likely candidates for causing a misfire. I didn't see those two in your writeup of what you checked so I would check those...and their wiring...which is where I'm heading now with my truck..or to the ECM as the cause. The Engine Coolant Temp sensor (ECT) could be giving a false reading which would let the ECM think the engine is not at operating temp and therefore give a rich mixture, the MAP sensor could also be giving a false reading which could case a rich mixture.

Thank you very much for your response! I'm sorry you are having a similar issue, it sure is frustrating. That is odd you mention that about the rich misfire, technically I don't know the answer to that question, but I told mechanic "A", who has the truck now, that when I had it running at my house before bringing it to him to get it tuned I did smell a fuel smell. The MAP and ECT sensor have not been mentioned to me by the mechanic so I honestly don't think they have been checked. It's possible he has and didn't mention it but my guess would be no he has not.

I really do appreciate your response and I sent a screenshot of the 2 responses to my mechanic so he can let me know if he has checked those sensors and the wiring and if not then he can look into it.

I can't thank you enough, I will report back if those were the issues. Good luck in getting your issue solved as well!

 

[Texas454]

I feel your pain man.
Are you sure the distributor is timed properly with #1 cylinder?
The crank cam relearn will need to be done. RPM must be above 1K rpm to adjust it and the setting is ± 2 degrees. You will need some way to read it for adjusting it.
Whats your fuel pressure? Vortec 454 fuel pressure needs to be close to 58psi.
Are your injectors working properly? Could be dirty or just plain bad and not working. You say they are good but are you positive they are?
Bad spark plugs. Are they new?
How do you know your converters are bad?
Why remove the o2 sensors? Without the o2 sensors being in place and working properly your ecu will have no idea what to do for fuel.

Thanks for your reply! It is so frustrating, I know we've all been there before on different issues.

Yes, I am sure he has the distributor timed properly with #1 cylinder. When he got the truck after mechanic "B" installed the engine, the distributor was off 12 degrees. His snap on scanner told him that and he used that scanner to time the distributor properly.

I do not know the fuel pressure readings, I will get that from him, but he said they all pressure tested "good" and he said he watched them all spray "good" when he has somebody else starting the truck while he watch the injectors to see if they were spraying good. He also said the harness to the injectors are good too, I forgot to mention that in my original post.

The spark plugs are new, they are Bosch platinum (can't recall the # offhand) and even though they are new he checked all of them and they all have good spark.

The converters were diagnosed as being bad from mechanic "A", who installed the engine, because he could hear it rattling when he hit it with his hand. And mechanic "B", who has the truck now, also diagnosed it as having a bad converter (or both), with his scanner and by visually tapping it and listening to it rattle around inside. Because those are bad I wanted to remove them and do without converters and use straight pipe to the muffler, since I don't have to do emission testing with this truck anymore.

I believe the O2 sensors were removed because they came up in his scanner when he first got the truck and he ran the check engine codes initially. I'm assuming they were not working correctly.

It sounds like you suggest the O2 sensors be installed during testing/diagnosis for the misfire?

When I initially told him I wanted to remove the converters and run straight pipe to the muffler he said we could do it and could do without the O2 sensors past the converters since we have the sensors before the converter location. Do you know if I can remove the converters and run it properly without them?


Thank you very much, I really appreciate your help. I have sent a screenshot to my mechanic and haven't heard back yet, I will respond back here when I get the fuel pressure readings.

 

[Spareparts]

If cats are removed the rear o2 sensors are not needed as they only measure cat efficiency but you will have a check engine light unless the rear o2's are tuned out.
I have been going through pretty much the same thing with my rebuilt engine except my misfires are cylinders #4, #7.
Read my thread. Should give you more ideas to check then you ever thought possible.

My 454 Rebuild

Been watching marketplace for the last few months for a core Gen6 454 and today was my lucky day (maybe) Picked it up for $300. Looks like a good deal. Guy said it was making a ticking noise sometimes. He said the truck was getting a diesel swap and he just wanted it gone. OK. Let's take it apart!

www.gmt400.com

 

[Texas454]

If cats are removed the rear o2 sensors are not needed as they only measure cat efficiency but you will have a check engine light unless the rear o2's are tuned out.
I have been going through pretty much the same thing with my rebuilt engine except my misfires are cylinders #4, #7.
Read my thread. Should give you more ideas to check then you ever thought possible.

My 454 Rebuild

Been watching marketplace for the last few months for a core Gen6 454 and today was my lucky day (maybe) Picked it up for $300. Looks like a good deal. Guy said it was making a ticking noise sometimes. He said the truck was getting a diesel swap and he just wanted it gone. OK. Let's take it apart!

www.gmt400.com

Your thread was one that I had skimmed through before posting mine lol. I didn't read all of the pages but I will go through it more so I can see what all was said. I appreciate that.