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Mysterious engine misfire, no solution in sight...

Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
Some specs:
383 based on 350 LT-1 block, Al heads.
Comp Cams 286XR solid roller, 1.5:1 roller rockers
Accel 7 DFI, sequential MPFI, dual-synch distr.


So here's the story to date: The car started to run extremely rough and did not want to stay running. Eventually the valve covers were pulled and #6 intake pushrod was broken (the rocker-end tip had actually been bashed down about 1/2".) I thought it must have snuck past QC with improper hardness. No wear on the sides of any pushrods. That valve stem tip (and to a lesser degree a couple others) were a bit flattened out as if they'd been slapped hard. I figured I had probably let it go too long without adjustment as it only has a few thousand on it since its build.

So, I replace all 16 pushrods (correct length) and the damaged roller rocker, reassemble, ... and the poor running is essentially the same as before. I did compression and leakdown tests, all cylinders (most notably #6) are stellar. Replaced all plugs, wires are newish and look good.

[[Intermission for several months sitting and not running, shame on me...]]

Today, determined to finally find the problem, I go looking for a bent valve and take the springs off. Nope, travels smoothly. Rotates easily at all points of travel including closed. This was my best guess as being "downstream" in the valvetrain from the source of force it best explained the busted pushrod. Ok, so my final theory is wiped cam or busted lifter. Took off the guide plate and pulled the pair of lifters out through the head ... Nope, lifter looks practically new, cam lobe the same. As far as I can tell the valvetrain is in perfect health.

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At this point I am out of good ideas and moving on to bad ones. As much as House MD has taught me not to accept coincidence as a diagnosis, I'm wondering if the running problem may be unrelated to the mechanical failure.

I have not changed anything in the EFI setup to date but my next plan at this point is to put everything back together and hook up the new work laptop and see what I can see. No idea how to use it but presumably it should put out some useful OBD2-esque codes? Any better ideas anyone?

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SVO

Well-known member
Joined
Jan 11, 2011
Messages
332
Location
Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
Are you sure your valve train geometry is correct? Coil bind with your valve springs? Something chewed up your push rods and i don't think it was mis-adjusted valve lash.
 
Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
Are you sure your valve train geometry is correct? Coil bind with your valve springs? Something chewed up your push rods and i don't think it was mis-adjusted valve lash.

I can't say I'm sure it's correct as I didn't build it. Everything else in the engine has been well done though so I would be surprised. Still, can't rule it out. I'll have to do the requisite checks.

I suspect that's not the cause of the poor running though, even if it did cause the pushrod damage, because that sort of thing wouldn't have changed and the car was running perfectly for several thousand miles before suddenly going south.


Another thought, fouled O2 sensor maybe? Perhaps even caused by the broken pushrod. It is in the header on that side.
 
Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
Well today has been a mixed bag. I got everything back together and went to fill the coolant back up... and it poured right out onto the ground! I thought I'd left open the block drain plugs open but it turned out to be two freeze plugs that had pushed out. One behind the starter (plug AWOL), and one behind the driver motor mount :banghead: The H2O %age must have gotten too low while I was still in NY. It certainly hasn't happened since I've been in CA. So now I have that to tackle...

On the plus side, I got everything back together and decided to run it for ~30 seconds without coolant just to see if anything had changed and to grab some data onto the laptop. Despite seemingly nothing changing, she ran beautifully and throttle response was perfect! No sign of the misfire, though without knowing where it went it could very well come back at any time...

If it wasn't for the damned freeze plugs I would be out driving it now for the first time in nearly a year!! Soon... very soon... Off to the autoparts store now to at least get some rubber type plugs until I can get some proper replacements in!

FYI: From what I've seen so far, these look like the dog's bollocks for hard-to-reach freeze plug replacement:

Dorman Products - 02483


Data logging via CalMap. Unfortunately it limits you to the values seen here, no temperatures or any of the rest it is capable of seeing from the car but apparently unwilling to record... Still useful information though.
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LT4man

Well-known member
Joined
Jun 25, 2003
Messages
2,217
Location
Chicago
Corvette
96 Collectors Edition LT4
I have used those type of freeze plugs many times without any issues.

One tip: Coat the outer edge of the plug with a good sealant. I used to use a "brown" liquid from Permatex which would come in a can with a brush applicator.

I would be very curious as to how the engine will run after warming up to temperature.

SAVE the WAVE! :w
 
Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
You see, the funny thing about things that seem to good to be true is...

Water in the oil.

It "magically" ran so well suddenly when I recklessly ran it for a minute without coolant. I filled it up again and suddenly the oil level doubled. I started it and the oil turned to milk and running was back to rough again. I'm hoping it is just a head gasket, which will see me back on the road in 2 weeks. (Next weekend is a rock festival.)

This explains the blown-out freeze plugs (compression pressure was getting to the coolant) and the rapidly filling oil catch-can. And of course the rough running. This does not explain why I never saw milky oil before, compression and leakdown tests were good, etc. For the compression test it could be that I didn't listen in the right place to hear the air escape as I was mostly expecting piston, ring, or valve problems.

Anyway, another step closer...
 

lars

Well-known member
Joined
Feb 15, 2003
Messages
209
Location
In my workshop bar in Denver
Corvette
64 Black Roadster; 85 Coupe; 09 G8 GT 4-door Vette
Keep in mind that "freeze plugs" are not actually there to protect the engine from freezing. The so-called "freeze plugs" are there to remove the sand from the casting process, and they're just random plugs to plug up the sand removal holes. I've never seen a case where an engine has frozen, the freeze plugs popped out, and the block was not cracked. If the freeze plugs have popped out, the block has likely cracked in the galley area or in a water jacket/cylinder wall area. The freeze plugs do not protect the block from cracking when frozen.

Lars
 
Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
Hey Lars, I went to one of your Tuning For/With Beer days in Herndon, VA back in ~'05 with the old '77, back when I was barely legal for that beer! Hope all is well.


So, that's worrying. I guess when I have the heads off I can take the oil pan off and fill the block with water to see if anything finds its way out the bottom. If so then my original LT-1 block is scrap.

Considering that A) the oil pressure gauge still reads high and B) the oil is milky but not the water suggests that the water must be getting somewhere low-pressure. That is probably bad and less likely to be head gasket...

Does it sound realistic at all to you (or anyone) that cylinder pressure into the coolant circuit could have blown out freeze plugs? (Only 2 popped.) Whenever they went it was not while parked in my possession, though it could have been while on the transport truck being moved out here from NY in December. They didn't report anything but who knows. I never had a puddle or found the missing plug and CA has never been cold enough anyway. If it happened while running then obviously it wasn't a freeze.

Another symptom: when the radiator is filled to the brim, it slowly drains back out again to about 4" from the top. I think this is about head gasket height, though it could be something else high in the engine! Definitely not low enough to be low in the water jacket/cylinder wall.

I will post up what I find when the heads come off. :eyerole

Edit: I've been saying head gasket but it could just be an intake gasket, I will try filling the block with just the intake and oil pan off and see if it leaks before I pull the heads.

-Chris
 
Joined
Apr 7, 2010
Messages
22
Location
San Jose, CA
Corvette
1970 LT-1 Resto-Mod, 1977 L48 (parting out)
Cracked block :hb

Mmm, bisquick:
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Snap, crack(le), pop.
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Fortunately I picked up a low-mile, original bore, 010 '70 Camaro block 7 years ago for $20. Who knows how long it will be until I can get it prepped and the whole thing rebuilt though... On the other hand, the stockpile of original '70 LT-1 parts just became rather redundant for a car that no longer has its original block. Potential funding source?
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