98 Suburban 7.4

[Hondaeater]

No not a single DTC for misfires but misfires are registered on the scanner and felt.

Never thought to check the timing with a light but i can do that for sure. CMP is set to 0.8, Spec is ? 2.0 so i assumed the timing can't be off as the distributor has very little movement and needs to be in the correct spot to set the CMP.
Can't re gap the plugs till tomorrow morning when the engine is cooler.
Later in the day.
The timing appears to be spot on @18 (ish)
With the timing light on and engine running the flash is not always a regular flash, flash, flash like you would expect for #7 and sometimes #1 i did not check the rest.
#7 has lots more flashes than it should and sometimes #1 also but way less and not always. So something in the ignition is not correct. What, i have no idea as its all new.
I need to get that figured out before we continue i think.

DTC P0300 is set to no error reported so I would not expect it to have a CEL lol. Might want to enable that.

Where is your WB channel? and you are not logging the AFR/ EQ commanded channel? A lean misfire?, the WB could help here. Bank2 seems to be the side with the issue for the most part in log C.
Look at the Seriouslog6c and watch the cold start. You are in OL, but the O2 volts are low on bank 2 during warm up and the most misfires. That is consistent. Now I woud astart looking at the injectors. These are the stockers it looks like. Test the resistance, easy. If you have a noid light set or AutoZone borrow them, You may find the injector is getting signal or maybe not or intermittent signal. The injectors themselves may need serious cleaning. Buy one of the cheap kits from Amazon and replace the little filters inside the top barrel. Get different o-rings though. If you go back and watch that c log it does not have misfires while driving, only at the lowest rpm, Duty Cycle of injectors and high vacuum. WB would be very helpful here. You can switch it from side to side in the NB positions to verify the cold start fuel condition that is showing up in these logs.
1,2,4,7 and 8 have lingering misfires on cold start. On log d they seem to be about equally misfiring from bank to bank. But again what is interesting is as soon as you give it gas the misfires stop. Only under high vacuum which is different, in vacuum the fuel is literally sucked out of the injectors. I would say this is pointing to a sticky set of injectors or weak winding. Swap out 5 and 6 injectors with the most common misfire cylinders, I would say 1,2,7,8 are the worst and swap them with 5 and 6 and watch if the misfires follow each the injectors. Inconvenient yes but basically free just time and effort. I did not dive too deep into the logs and tune progress as you already had some help there.
Good luck and stay tuned.
Homdaeater.

[SiriusC1024]

That's an interesting thought. A leaking injector would explain the difference of 5% fuel trim. B1 STFT is pulling fuel, and the cylinders there are 1, 3, 5, 7. A leak in 7 might be the cause.

[Spareparts]

I dont have a wide band yet.

I don't believe its injectors. 2 different set's nothing changed and they are all within 2 percent when performing a injector balance test using a snap on scanner. Spec is 4 percent. FP does not bleed off after sitting overnight.
Current set is reman stock style. First set were new 5o injectors i ran in the old engine for about 2k before rebuild and worked perfect.

I am going to change the cap/rotor again and install new AcDelco parts see what that does.

[Hondaeater]

Delco is good. I use MSD cap and rotor.
Curious, how do you have the plug wires supported from cap to plug along motor?

[SiriusC1024]

Hopefully it's as easy as cap and rotor.

[Hondaeater]

Are you using the Taylor wires? The 10.5 wires?? If the holders are not sized for those wires or you use some aftermarket goody that runs along the valve covers, the clips/clamps/ holders can pinch/ crush the wires and cause this misfire.

[Fast4.7]

See if you can find an AC Delco NOS Marine cap and rotor that has brass terminals. Next to the MSD that I have heard has become hit or miss, the marine cap was a much better piece.

That being said I have seen a similar issue from a cracked porcelin as well as a wire bleeding spark energy.

One other note, it is important to follow the wire routing and position in the service manual as well. My 97 van has SMP 8.5mm silicone jacketed wires in an aftermarket loom along the head. The loom was designed for 7 or 8.5mm wires but requires lightly trimming the plastic to hold the larger wire, which I found was easiest to perform with a drill bit, slightly smaller than the plug wires . The aftermarket looms space the wires farther apart which is a good thing when the wires run side by side for most of their run. Less chance of electromagnetic induction crossfiring.

[Fast4.7]

One other thing to check is distributor shaft end play. I check 4 units, including a very low mileage OE unit that someone stripped the cap mounting holes on early in its life that I had laying around the shop. They were all 4 off badly in end play. That causes the CMR and rotor phasing to drift around as well as drive gear wear. Once the drive gear starts to be eaten by the harder cam gear the shaft starts oscillating even more with wider swings in rotor phasing. If the heads and or block were milled the distributor body will likely need to be shimmed as well. The OE 454 distributor had a thick thermal washer under the base rather than a gasket and the distributor body length was setup to compensate for it.

https://www.gmt400.com/threads/distr...ay-fail.62255/

[Spareparts]

Cmr or is it Cmp?
My Ckp is at - 0.3 and does not very much.
Rotor is not hitting the cap. 0 side to side play. There is a little movement if I pull the rotor up. I can't say how much up movement there is but not a lot.

[SiriusC1024]

Well...what if we went the other way with the timing?

[Fast4.7]

Cmr or is it Cmp?
My Ckp is at - 0.3 and does not very much.
Rotor is not hitting the cap. 0 side to side play. There is a little movement if I pull the rotor up. I can't say how much up movement there is but not a lot.

CMR or CMP or Cam Offset as HP Tuners calls it. All terms for the same thing. The distributor cam sensor timing offset in relation to the crankshaft.

[Hondaeater]

See if you can find an AC Delco NOS Marine cap and rotor that has brass terminals. Next to the MSD that I have heard has become hit or miss, the marine cap was a much better piece.



The loom was designed for 7 or 8.5mm wires but requires lightly trimming the plastic to hold the larger wire, which I found was easiest to perform with a drill bit, slightly smaller than the plug wires . The aftermarket looms space the wires farther apart which is a good thing when the wires run side by side for most of their run. Less chance of electromagnetic induction crossfiring.

That's exactly what happened on my 1994 Silverado. I used an anodized loom and it looked great but.... The MSD Super Conductor 8.5mm originally had less than 200ohms resistance and seemed fine. When I started having misfires, I found hot spots inside the wire at the wire holders. Resistance was infinity.
Definitely make sure the wires are not pinched. Good luck.

[Spareparts]

Sorry guys. My misfire problem is a mechanical problem and until I solve that not much sense in trying to tune it out.
Once it's fixed the tune will be a lot different I'm sure.

[SiriusC1024]

Depends on what kind of mechanical problem it is. Either way I'll be happy to continue on if you want. Good luck with it.

[99k2500]

I read this whole thing from start to finish. This is amazing! I also have a rebuilt 7.4 in a 1999 suburban. Mild comp cam. Switched it to the 411 pcm. Since there is no real tune file for it, I went through a company to get a computer flashed with a tune in it. I had it to 2 dyno shops and it still is kind of crappy at idle and starting. I've put 3 sets of injectors in it and #4 has the misfire like his #7. I've replaced literally everything hahaha. I'm not at my computer and I do not expect any help, but if you want to look at it and have me log some runs, I wouldn't say no.... I use it for my volunteer fire dept as a first response unit, so it gets some abuse. before the motor ceasing and the computer swap I was getting 10mpg most days with 14 ish on the highway... Now I am at like 6-7 and 10 on the highway... I didn't change that much on the motor to make that much change in fuel mileage. I dunno, its been a few years and I've just been dealing with it....

You guys are pretty freaking awesome in my opinion!
Thanks for everything you guys do on here.

[SiriusC1024]

Sure. Start a new thread and post the tune and log.

[gtstorey]

I read this whole thing from start to finish. This is amazing! I also have a rebuilt 7.4 in a 1999 suburban. Mild comp cam. Switched it to the 411 pcm. Since there is no real tune file for it, I went through a company to get a computer flashed with a tune in it. I had it to 2 dyno shops and it still is kind of crappy at idle and starting. I've put 3 sets of injectors in it and #4 has the misfire like his #7. I've replaced literally everything hahaha. I'm not at my computer and I do not expect any help, but if you want to look at it and have me log some runs, I wouldn't say no.... I use it for my volunteer fire dept as a first response unit, so it gets some abuse. before the motor ceasing and the computer swap I was getting 10mpg most days with 14 ish on the highway... Now I am at like 6-7 and 10 on the highway... I didn't change that much on the motor to make that much change in fuel mileage. I dunno, its been a few years and I've just been dealing with it....

You guys are pretty freaking awesome in my opinion!
Thanks for everything you guys do on here.

Best to start your own post with details of build with part numbers ( especially injectors and where they were sourced from) along with tune and a log showing problem area.

[Spareparts]

I recently sold that Suburban without ever solving the problem.
Have a different one now 2001, 6.0.

[bk2life]

I recently sold that Suburban without ever solving the problem..

boooooo