Intermittent WOT misfire on cylinder #7

[BigMike42]

Hello, I have been chasing a misfire for months now on my C7 h/c/i. Tried replacing spark plugs, wires, and coil pack. Someone suggested it could be a bad injector, so I replaced all the LT4 injectors. The car has a 32% fuel lobe, lt4 HPFP, lt4 injectors.

It will happen randomly on the street during pulls from a roll, but at the track it happens just about 100% of the time.

My tuner suggested it could be too high of fuel pressure and we tried lowering the high pressure desired down from 20 to 18. I didn't get a chance to test this setting at the track, but I was able to do a lot of 20-120 mph runs last night and it did not misfire. However, as I understand a setting of 20 is just stock LT4 settings so I am still very skeptical that the issue is fixed. Also, the car was fine for a year with those original fuel settings.

This all seemed to start when I swapped to the BTR conical valve springs, so another idea was to just put the old springs back on that cylinder and go to the track again. Although, I also ran plenty of perfectly fine mid 10 second passes with these conical springs, so I don't know if it is that either.

Attached both tunes so you can see the fuel pressure difference. The misfire starts at 01:16

[Estep40511]

Ive seen stock LT1s and LT4s do this when they had a weak or broken valve spring. Might go down that path and see....curious to see what you find.

[BigMike42]

I am going to pull the valve cover tonight. I figured if it was a broken spring I would have had catastrophic failure by now

[BigMike42]

Spring was not broken, but I am putting on TSP/PAC dual springs tomorrow and I will report back after I go to the track this weekend.

[GHuggins]

I've never seen a lt4 injector stick at 20. They're usually good to 21, but can start sticking there. Still don't think that was your issue as for whatever reason they usually do it in multiples when they do stick plus I assume the miss stayed on the same cylinder after swapping them, which pretty much just means that cylinder right?

[BigMike42]

I've never seen a lt4 injector stick at 20. They're usually good to 21, but can start sticking there. Still don't think that was your issue as for whatever reason they usually do it in multiples when they do stick plus I assume the miss stayed on the same cylinder after swapping them, which pretty much just means that cylinder right?

Yeah plus the fact that it was fine for a year, I doubt it's the pressure.

I am putting on TSP dual springs which are PAC racing. Will report back.

[BigMike42]

It wasn?t the springs it was the install height. Every single spring needed min .030 in shims and up to .050. Cyl 7 needed the most. I guess since the heads were ported with different valves the valve stem height was different. Back to my normal 133 trap speed with no misfires! That?s what I get for just dropping new springs in without measuring

[GHuggins]

Good you caught even that. The majority of people wouldn't. Also would think the shop that did the head work would have?

[BigMike42]

No the whole thing was my fault. The heads are LME ported factory heads and they shipped with BTR duals and had correct install height. I wanted to try the conical springs and just dropped them in without shims and car was very inconsistent for months. Bought the BTR micrometer and was shocked how off each valve was. I measured each valve several times and used calipers on the micrometer just to be sure.

[blindsquirrel]

Any machinist worth a shit would have put the seats at the same depths, to ensure every valve had the same installed height. I thought CNC-everything would have eliminated Lazy Machinist Syndrome by now, surely? It's not like the old days, using stones, where you just grind each one until it looks like it's cleaned up and whatever the valves end up at is what you get. Modern engines with net lash won't tolerate the old ways.

What I'm getting at is, if all the valves are a consistent length (if they aren't, they are garbage), and all the spring pads are the same location (if they aren't, the head castings are garbage), and all the retainers and locks are consistent (if they aren't, you know...), then the only thing that can create variable installed heights is the depth of the valve seats all being different. I would be super extremely pissed off if I had high-dollar ported heads and found they were like that.

[BigMike42]

Interestingly enough, I did see variation of .005" depending on which locks I used. I ended up using the leftover BTR dual spring locks I had because they were the most consistent.

The springs are these, https://www.texas-speed.com/p-5440-t...-seatseal.aspx. Listed at 1.900" install height. I got them installed slightly lower than that across all cylinders because I just didn't have thin enough shims and wanted the car back together.

Every valve measured between 1.972" and 1.990" without the TSP spring seat, which itself measured somewhere between .025" and .030".

[blindsquirrel]

A range of 0.018" is massive. That is not normal. Anyone worth their salt and using any relatively modern set and guide (edit: seat and guide) machine would have held them to 0.003" or less. I would be worried that the valves have stretched, that's probably more likely than somebody getting the seats that wrong.

[GHuggins]

A few of my customers running titanium valves have had to replace them due to abnormally fast wear like within 10000 miles of use. Oil analysis will show that though.

[BigMike42]

A few of my customers running titanium valves have had to replace them due to abnormally fast wear like within 10000 miles of use. Oil analysis will show that though.

I don't think stock valves are titanium right?

A range of 0.018" is massive. That is not normal. Anyone worth their salt and using any relatively modern set and guide (edit: seat and guide) machine would have held them to 0.003" or less. I would be worried that the valves have stretched, that's probably more likely than somebody getting the seats that wrong.

Damn, well I was planning to pull the motor this winter to change the cam out. It is a big cam, 235/250 114, and I believe is making the car much slower than what it should be. I will send the heads back to LME while it's apart for them to inspect and repair if necessary.