98 Suburban 7.4

[Spareparts]

I loaded the last tune this morning and i really can't tell if better or not.
I was watching the scanner misfire count and #7 has a consistent misfire at idle.


I pulled that plug and re gaped it to .045, spec is .060, These are nearly new plugs and set #2
Misfire is nearly gone and way less on #7. The rest have random misfires but it is minimal
I can also now guarantee the EGR is not leaking.
I did not log it after changing the gap as it was already warmed up but here is the log from the current tune from cold to warm mostly idling.

SiriusLog6.hpl

[SiriusC1024]

No I wouldn't expect it to be better. It's setup to tune base running airflow is all. The values will change based on log idle trims. I'm out of town on vacation for a week and didn't bring my laptop.

[Spareparts]

No I wouldn't expect it to be better. It's setup to tune base running airflow is all. The values will change based on log idle trims. I'm out of town on vacation for a week and didn't bring my laptop.

Vacation always good. ENJOY IT!

[SiriusC1024]

Back from vacation.

Your last log is missing STIT and LTIT channels. Were you not able to find them? That's what I need in order to adjust Base Running Airflow.

[Spareparts]

Well actually i did not look for them yet.
I figured you were on vacation and i did not want to bother you by posting stuff you could not do anything about anyway. Better for you to enjoy a vacation as you should!

Anyway i took the week and tried a few things with my truck.
I was reading some things and discovered guys with higher compression run less plug gap. All my plugs are now .035
Essentially i moved the heads closer to deck by .035 when rebuilding the engine. Don't know if it really makes that much of a difference but idle was improved for the most part still a little rough but better.
Also, my CEL came on and code was for egr. Looking at the tune my EGR was not totally disabled only partially. I disabled everything about it. No more CEL from EGR.
Over 3 or 4 days i plaid with the idle moving it up and down a little at a time.
Now once hot it idles better than ever but that pesky #7 hole is still misfiring and i plan on taking the valve cover off and checking the pushrods and stuff. Want to make sure the valves are working properly.
I can post a log tomorrow after work if your still interested in trying to help.

[SiriusC1024]

Still interested for sure. I'm glad to hear about your progress.

[Spareparts]

Here are pics of the plugs when i re gaped them. I already had #7 re installed when i thought about pics but it looked the same as the other 3.

SiriusLog6b.hpl

plugs1.jpg

plugs.jpg

[SiriusC1024]

What plugs are those and where did you buy them from?

A little improvement on the misfires.

That #7 misfiring makes it a little difficult. Misfire throws unburned fuel/air into the exhaust, lean condition is sensed by O2, airflow responds by closing IAC. This can be seen near the end of the log at 13:35.

BRAF adjusted. Still no STIT logged, so I have to make an educated guess.

Experiment with Idle Spark, also.

[Spareparts]

I totally forgot to look for STIT but im willing to bet my ecu does not have it.
Plugs are NGK TR55IX i think. Rock Auto. Been thinking of just trying some plane ACDelco 12's
Hot idle is for sure better today i was watching it with Torque Pro on the way home from work and with the AC on there were no misfires recorded at a stop light (idling) so i think things are improving.
I have not been able to do anything with it really but drive it as i have been working since last Monday and will not have a day off till this Thursday.
Ill load this tune at lunch tomorrow and post a log of the drive home. I'll look for the STIT also.

[Spareparts]

I looked for the STIT and as soon as the truck connects it disappears. Im guessing unsupported.
I think between your tuning skills and my trying to get #7 firing properly progress is being made.
Your latest adjustment did smooth it out for sure.
Friday i will be able to get a good look at #7. I have looked once at the pushrods and im sure they are not bent but im going to check again to be positive. Cranking compression is good so im suspecting a weak valve spring. That would explain a good cranking compression check but misfiring when running i believe.
The CEL came on again for the EGR. I went in and disabled it again this afternoon. No CEL on the drive home from work.
Anyway here is a log from this morning and the current tune state.

vats disabled idle turnedup_SiriusC1024_Full_6c.hpt
SeriusLog6c.hpl

[Hondaeater]

Go into Exhaust tab and raise the EGR Min VSS speed and Diag Ebnable speed to 200 or more since THIS IS FOR OFF ROAD USE ONLY.
P0401, 404 and 405 no error reported, you did block that EGR off right? It is still looking for it.

[SiriusC1024]

Thank you, Hondaeater.

So about your issue. I think it's ignition. Two reasons:

1) Iridium plugs have are harder to light off. That's why you noticed improvement with a tighter gap. In fact, GM recommends for the LS similar to what you did - .060 platinum .040 iridium. RockAuto spec'ing those plugs of yours at .060, same as the other types, is questionable.

2) There is improvement with lower idle timing. See, I did that in order to light the plug at a higher charge density. Compensated for weak spark by increasing flame front speed. Don't want to go too low, however. Can get to the point where the combustion event will still be in its initial stage while the piston is traveling downward on the expansion stroke.


Most of the cylinders show misfire, but 7 especially. Try a copper plug in #7 as a test. Copper plugs have really good ignitability. Better than anything except ruthenium. Plus they're cheap. Only downside is they wear out fast. The wear rate is why OEM's switched to Pt then Ir. If misfires decrease or go away entirely there are a few options:

1) Ruthenium plugs. Best ignitablility (better than copper) and longevity (better than iridium). Excellent flame kernel. Expensive.

2) Install a Hitachi coil. They're available for your vehicle on RockAuto. Hitachi bought out a company that had the patents some special coil design. You know how spark plugs normally sound in the air, right? Tick-tick-tick-tick. These Hitachi's make the plug sound like snap rocks that kids play with on the 4th of July. Loud with a bright blue spark. Very powerful coils, yet still meet OE requirements for circuit protection.

3) Low resistance plug wire. Not the solid copper kind, though. I like 8mm Taylor, but there are plenty of suppliers.

4) Aluminum cap and rotor.

[Spareparts]

Yes the tune is off road only and the EGR is blocked off when off road. Thanks!

Interestingly enough i do have Taylor 8mm wires. They replaced crappy new AcDelco plug wires.
The distributor was replaced less than 1k ago and it did have Aluminum contacts also.
Im sure willing to try different plugs and coil and have ordered the Hitachi coil and AcDelco plugs R44LTS6
Any recommendation on gap to start with?

[SiriusC1024]

That's kinda funny you are already are running the Taylor wires. A fellow man of culture.

Looks like what I was saying about copper having better ignitability was flat wrong. This is from NGK. The nickel one is what is commonly called "regular copper" plugs.
plugs.png

The finer electrode allowed by the exotic metals makes for better ignitability. If the plugs you ordered make a difference it's because the ignition was designed at a time when that's what was available. I've heard copper works better in those applications. The only reason I've found that kind of makes sense is distributor vs coil-on-plug. Idk why exactly it works like that though.

Sorry for the confusion. We'll see what happens when you get the parts in. Gap the plugs at .060 then adjust if needed.

[Spareparts]

Already ordered so no choice really + i was already thinking of trying old style plugs gapped at about .045.
I have suspected ignition as the problem for a while so i have replaced every part of it. New Distributor, Different coil (used but came off a good running engine), New ICM, New wires, New plugs.
I'll be kinda p1ssed if a new coil and regular plugs fix it the misfires but glad also.
I do have a 0411 that i had in the truck (Black Bear tuned before the rebuild) but put the old black box ecu back in trying to eliminate what could have been potential cause of the misfire problem's.
Just need to wait for the new parts to get here now.

[Spareparts]

I managed to get the plugs and coil changed today.
I gaped the plugs at .045 No change really.
Im at a loss on where to proceed myself. Every ignition part has been replaced with new less than 1.5k on all the new parts. Nothing really changed.
Here is a log for your viewing pleasure.

SiriusLog6D.hpl

[Hondaeater]

Is the misfire felt as well as showing up in log? Do you get a DTC code?
In your scanner, can you delete the spaces between channels in the list of you haven't already? Hard to watch data and scroll.

Do a child start up. Make sure WB is reporting in list. Once up to op temp, go for a short drive, do not be to go over 3-3500 just light throttle, below 40% ok. 20-30's will be good. Post the log and tune used. Is this repeatable and at a specific circumstance of rpm, or speed, map or TPS? I know you mentioned it in previous posts, but if you could update what and when it happens.
CPI?

[SiriusC1024]

Idk why I didn't think of this before. Idle spark advance is 18 degrees. Have you verified this with a timing light? If it's off then the entire spark range will be offset as well.

Also, should try the GM recommended .060' gap. I know it's getting to be a pain, but still.

Finally, go into Engine>Idle>Airflow>Base Running Airflow and try some adjustments. 40kpa idle MAP is probably a little low. I've been adjusting it, but you're there and can get a better judgment of the effects. If IAC counts (warm - engine at operating temp) start to get near 200 then you'll have to adjust the throttle open a turn, perform a throttle relearn, lower BRAF back down and restart.

[Spareparts]

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.

[SiriusC1024]

Maybe the electronic module in the distributor itself? Electronics tend to fail either new or very old. Reliable in between.

Check the ignition control wires for corrosion or looseness in the terminals, too.