LT1 misfire from Hell

[Glenn]

Yes. Ran fine cold with the bad one. Never threw a code either. Jacked with my issues for many mouths. Changed the ECT and the problem went away instantly.

[Dustin M.]

Hmm. Well it's easy enough to change and I'm bringing a few items to the track. Tonight I changed the AC Delco copper plugs out for some single platinum NGKs. I'll run like that next weekend and see if I can discern any misfires on track. If so, I can change the ECT and if that's no good, I'll throw in my spare PCM. If still no good, I'll change the opti on Saturday night and after that I think I'm about out of ideas.

[Pranav]

Just to re-iterate, I and others watch Glenn's car fall over and die on track multiple times due to a $10 coolant sensor. No amount of icm/coil/opti swapping and checking of fuel system did him any good/improvement until he swapped the sensor.

 

It's hard to believe but it's worth a shot before you throw anymore time/money at it.

 

Especially that brand new opti. You might want to see if someone has a used one you can try out before you swap it for another brand new unit.

'

Definitely understand your frustration of not being able to romp on the car on the street to test out your work, just hate to see you waste more $ if it was in fact that coolant sensor.

 

EDIT: OK re-read your post, ok so you're gonna just swap the coolant sensor and see how it goes. Good call. If you've already bought a second new opti just keep that thing in the box! lol

[Dustin M.]

Well the nature of this problem has me believing the previous used opti is in fact a good unit, thus leaving me with that as a spare. I even went ahead and put red loctite on its rotor screws and RTV'd the thing shut so it's all modded now too. The NIB opti is one from Amazon, I told 'em I got a bad one so I've got until April 15th to send one back and not get charged. The used spare is also an AC Delco unit. I'd jump for joy if it was the ECT, the only money spent twice will have been plugs.

[Dustin M.]

so the ECT wasn't it either. guess I'll throw in another opti after this weekend, if I keep it wound up I seem to do OK on track.

[Dustin M.]

2 other things the car did: random rev cut (only happened twice) and acted like it starved on a banked right hander (on a roval) with a fullish tank (need it to make weight). Almost makes me wonder if there are 2 things going on now. Either way, than random rev cut points toward junk opti IMO. I've got a month until the next event so I'll inspect the wiring from the PCM to the engine as well. I felt I got outpulled on the initial squirt leaving corners, kind of like losing a drag race on the 60' because I felt I was able to get on the gas sooner than those that I chased yet still got outpulled, quite frustrating unless that's just the way it works.

[Dustin M.]

New new opti is in, this one has noticeably tighter bearings than the last new one so at least there's that. This is the 3rd opti I've put in this car, 2nd new one, also the 3rd one I've loctited/siliconed shut. Next I'll strip the split loom off the engine harness and inspect/beautify/properly secure what's there. If I start freight training people at the next event I may have to revisit the dyno.

[cozog1548534733]

You've been concentrating on spark/ign, so what about fuel? Have you changed the fuel filter? Is it the stock fuel pump? I recall reading somewhere about a concern of the stock fuel pump not keeping up and leaning out the afr's, leading to engine failures.

 

Also, the tanks in the LT1 are known to have fuel pick up issues. Some guys can run below 1/4 tank with no issue, but I've read others having engine cutout below 3/4-2/3 tank. Prob what caused your issue at the track.

[Dustin M.]

Well I thought about that, then I thought about the fact that I filled it completely full before the race and quickly ruled that out. Wideband indicates 13:1 at 2,000 RPM transitioning to 11.4:1 at redline. Based on that, I've ruled out fuel.

[Pranav]

Same case when I killed the opti, AFR on teh dyno looked great all the way through.

[Pranav]

Ohmed my plug wires since I had them out. My coil->opti wire had 70k ohms of resistance. That might explain why its been sounding rich/poppy at idle and has a consistent high RPM miss with no codes.

 

Still went ahead and replaced my opti cap/rotor, going to use the "old" msd cap/rotor on the backup/used opti to keep in the spare parts box.

[Dustin M.]

I haven't validated it yet, but I have a high level of confidence that this was my problem.

 

 

See that thick red wire? It feed the ignition switch. See that small yellow wire? That was the return from the ignition switch and fed EVERYTHING in a non-fused manner. So now I get to do the second half of the Painless harness installation that was never done and the car will run like a champ. The bottom left terminal was charred, arcing, corroded, and melted the plastic all around it. It used to have the yellow wire on it with only a washer jumping power to the adjacent terminal. Horrible. My plan is to relocate the fuse box and install (properly!) a new terminal block and mount them both next to the PCM so I can finally have a glove box door so I don't get DQ'd at nats.

[Dustin M.]

This seems to have fixed it.

 

 

And now the car looks like less of a work in progress.

 

 

The instant the car fired up it was different. It's never been right, so I had nothing to compare it to. Every little thing it does is different: no lean stink when cold, much better response, transition to/from DFCO is smoother, no bog between shifts, pulls much harder now. Everything on the car is now fused too instead of pulling power right off the terminal board receiving power from the next one over with a ghetto ass washer.

[Pranav]

Just an update on my motor, the misfires were gone for the 7 laps I ran it. Then I spun a rod bearing so bad it stacked up on itself and started tapping the piston on the head. Blew out the headgasket on that cylinder and flooded crank case with water.

 

Luckily I was only out that rod and piston, two more rods for spun bearings, and a crank. Heads and block survived.

 

Start cutting your oil filters open and make sure those misfires haven't beat up rod bearings. Not sure if this was the case in mine, but pretty much the only thing I did wrong with this motor for the three events I ran it.

[Dustin M.]

Were your misfires violent? I'm about to change my oil anyway, so may as well cut the filter open. Mine was more just gutless. The only time it audibly misfired was on the dyno. On track it just seemed "off" for the most part, though it would stumble here and there.

[Pranav]

I'd say so

[Dustin M.]

Just under 3 weeks to go, you gonna have it back together? I imagine you could use the crank from a 5.7 Vortec truck motor. I know for a fact they used the same powdered metal connecting rods.

[Pranav]

I've got all the parts together and ready at the machine shop.

 

My heads and block were still good, cleaned/honed up and ready to go.

 

Machine shop received a used LT1 crank and some 95+ PM rods last week that I ordered off ebay.

 

They're hoping to squeeze me in this week, which would mean I could have the motor back in the car this weekend and have time to dyno it AND shake it down before committing to Hallett.

[Dustin M.]

Well best of luck to ya. I got a new trailer to travel with now and combined with the proper running car I'm feeling pretty good. You will too once all this is said and done. It's somebody else's (i.e. neither of ours) turn to struggle!

[Dustin M.]

Seems I spoke too soon. The car runs MUCH better and it was noticeable from tip in on the dyno, but it's still down on torque by quite a bit. I'm red and unrestricted vs. a known good LT1 in blue running a 42mm plate. Car burns no oil, the thing that catches my eye is the jaggedness of mine. I believe that's caused by poor combustion, so with the known good AFRs that I have ignition must still be weak. One thing I found odd was that the torque got stronger as we made more pulls. I expected to see hp creep as the oil heated, but torque was creeping as well. It settled right around the 260whp/high 290s wtq mark with a significant chunk missing under the curve.

 

[suck fumes1548534743]

Have you done a compression/leak down test yet? Your bottom end could just be really wore out. I'm assuming all the internals haven't been touched since you bought it? Cause when it was acting up it could have been detonation from bad gas or something. I would look at all the plugs really closely and see if any are peppered with piston residue. You should always go through the motor every couple of seasons anyway.

[Dustin M.]

No, I did ask about knock retard and he said it'd puff out the back. I also know it'll leave a big sawtooth in the dyno, i.e. it'll dip off very sharply then make a slow return to where it belongs. It also burns no oil, so between that and how incredibly shitty it used to run it never even crossed my mind. They had difficulty picking up my tach signal, so I have to wonder if that's why my graph is so jagged. Kind of trying to decide how to decipher the dyno to decide what to go after next. Compression and leak down have climbed farther up the list though.

[suck fumes1548534743]

If the Dyno has a dip in it at the same rpm in the same gear every time it is detonation. You better pull the plugs and look at the bottom of them and look for shiny silver specs.

[Glenn]

How old are the valve springs? Could be valve float. How tight is the lash?

[Dustin M.]

Haven't checked valvetrain yet. It's crossed my mind among other things. There is 1 more segment of wiring for me to investigate, and that is the stuff under the hood that doesn't require anything more than plugging in. After hallett I'll strip the covering off to see if there's isn't some bullshit going on in there too. I saw ARP head bolts, so the heads have been off the car and they're clean enough to eat off of. If the heads have been off, surely the valve lash can be fubar. Here's my 3 pulls from the other day, no big consistent dips anywhere.