rough idle no codes other than knock low frequency.

[the sad technician]

im a tech working on a 1996 gmc with a 454 vortec engine. engine was freshly rebuilt and installed. ive been fighting with this vehicle for over two weeks now and im lost about what to do. we have installed all new map, maf, distributor, crank sensor, injectors, all gaskets on the intake twice, checked for vacuum leaks with starting fluid and by hand, fuel pressure is 50psi and goes up to 55psi when throttled, new plugs and wires verified cam and crank are aligned. both grounds are installed in front of engine and behind the passenger head. engine is changing fuel ratios and both banks stay pretty consistent. timing is set at 0 degrees at idle. but its still miss firing! ran a compression test and we got 100 psi at a altitude of 7,000 feet across multiple cylinders which is average for around here. I don't know what more tests I can run without tearing the engine down again looking for a problem that may or may not exist. both me and the customer are very unhappy. i hate throwing parts at a engine hoping thats the problem when everything is already new! again no codes besides a knock sensor low frequency which is there with or without them being unplugged. i got 5v going to the sensor with 3 kilo ohm of resistance between it and engine ground which is within tolerance considering 100 kilo ohms is said to be bad so idk why its throwing that code and just a random miss fire code. 7 and 8 cylinders were miss firing more than others but all cylinders were miss firing quite a lot. 7 and 8 have 2000 counts while all other have a few hundred counts. pulled the plugs and wires which are brand new and tried putting the old one back on and still no change. tried advancing and retarding the distributor timing and it didn't help. im running out of time and guesses. hell even gapped the plugs to 45 thousands which is spec. 

[richard wysong]

New cam and lifters? I wonder if maybe the valve lash is a tad on the tight side. Welcome to the forum!

[the sad technician]

Yes a believe new cam and definitely lifters but the valve lash isn't adjustable? It's just a single bolt that I made sure I torqued to spec during the install. That was the only thing the machine shop didn't screw down.

[newdude]

9 hours ago, the sad technician said:

im a tech working on a 1996 gmc with a 454 vortec engine. engine was freshly rebuilt and installed. ive been fighting with this vehicle for over two weeks now and im lost about what to do. we have installed all new map, maf, distributor, crank sensor, injectors, all gaskets on the intake twice, checked for vacuum leaks with starting fluid and by hand, fuel pressure is 50psi and goes up to 55psi when throttled, new plugs and wires verified cam and crank are aligned. both grounds are installed in front of engine and behind the passenger head. engine is changing fuel ratios and both banks stay pretty consistent. timing is set at 0 degrees at idle. but its still miss firing! ran a compression test and we got 100 psi at a altitude of 7,000 feet across multiple cylinders which is average for around here. I don't know what more tests I can run without tearing the engine down again looking for a problem that may or may not exist. both me and the customer are very unhappy. i hate throwing parts at a engine hoping thats the problem when everything is already new! again no codes besides a knock sensor low frequency which is there with or without them being unplugged. i got 5v going to the sensor with 3 kilo ohm of resistance between it and engine ground which is within tolerance considering 100 kilo ohms is said to be bad so idk why its throwing that code and just a random miss fire code. 7 and 8 cylinders were miss firing more than others but all cylinders were miss firing quite a lot. 7 and 8 have 2000 counts while all other have a few hundred counts. pulled the plugs and wires which are brand new and tried putting the old one back on and still no change. tried advancing and retarding the distributor timing and it didn't help. im running out of time and guesses. hell even gapped the plugs to 45 thousands which is spec. 

 

 

 

Plug gap is 0.035 in per the service manual.  

 

 

All the parts you changed, did you use either original equipment or ACDelco parts? 

 

What was the trouble code number you have?  

[richard wysong]

If they decked the block and milled the heads then the pushrods may be too long now, it's a long shot but that's all I got. If you are desperate enough back the rocker arm bolts off 1 turn and see if there is any improvement. newdude's pretty sharp so see what he thinks

[the sad technician]

3 hours ago, newdude said:

 

 

 

Plug gap is 0.035 in per the service manual.  

 

 

All the parts you changed, did you use either original equipment or ACDelco parts? 

 

What was the trouble code number you have?  

P0300 and yes they were oem/Delco parts and ok even if the plug gap is 35 thousandths why would that effect cylinder 7 and 8 more than the rest?

[newdude]

39 minutes ago, the sad technician said:

P0300

 

P0300 is random misfire detected.  You state you had " no codes besides a knock sensor low frequency"

 

So was/is P0300 random misfire detected the only code you had then?  There are knock sensor codes but P0300 isn't one of them.  Should have seen a P0325 or P0327.

 

 

Edited February 7, 2025 by newdude

[the sad technician]

Well p0300 is the only code I'm currently worried about. The knock low frequency code is the only other code popping up but that just means the pcm isn't reading the right values for one of the knock sensors. 

[newdude]

32 minutes ago, the sad technician said:

Well p0300 is the only code I'm currently worried about. The knock low frequency code is the only other code popping up but that just means the pcm isn't reading the right values for one of the knock sensors. 

 

 

And that other code # is?  

[the sad technician]

The random misfire code is the only other code. Just a knock low frequency code and random misfire code p0300

[biscaynekid]

I had a plug with the porcelain insulator cracked in two. It looked fine to the naked eye. It was sliding up and down the electrode, sometime blocking the spark. I was putting the plug back in the engine when I heard a clink. I shook the plug and could hear the clink easily .Pure luck finding it. That was causing my intermittent misfires. 

Even though you replaced distributor cap, the new cap could have a hidden crack causing a crossfire.

Good luck