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Over the winter I replaced the NOM in my '75 corvette with an LS6, the details of the engine are below. I have been driving it and working on the tune, but have not been able to get it to run as well as I had hoped. It starts and idles fine and it runs as I would hope for the first minute or so, but then once it warms up it 'stumbles' in certain conditions, mainly at low RPM when I am off idle, so when I start from a dead stop and when I am on the highway at 55 in 4th and give it just a bit of gas, I can hear/feel it misfiring. It does record misfires on cylinder 4, approximately 400 of them over the last couple hundred miles, but not as many as I would think based on the performance, not sure what counts as a misfire. The other day when tuning I saw after a brief stop for gas and re-start, the temp was high - about 240, which went down to about 208 once I got it on the road. I checked the coolant when I got home and it was a couple quarts low. The oil looks perfect so it is not leaking coolant there, but the coolant is milky (I have been running distilled water since I finished the swap in the spring) could it be a leak into the #4 cylinder through the head gasket? I pulled the #4 and #2 plugs and they look identical and there is no "steam" that I can tell in the exhaust, but it has not been cool enough in MD to really tell I guess. Is there anything else I should check / consider before planning on pulling the heads?
Thanks, Scott
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2002 Camaro Z28 LS1(6) / 4L60E
~115k original miles, swapped into a '75 Corvette - the leakdown tests on all cylinders were good
Engine CamMotion LS1 Stage 2 Camshaft (214/220-115+3) Duration at .050 - 214/220 Lift with 1.7 - .553/.553 Lobe separation 115 Intake center line 112 New valve springs Original LS6 intake Original injectors New knock sensors New oil pump/pan/pickup New timing set Basic intake using original MAF - located partially behind the radiator so the intake air is warmer than I would prefer for now. New IAT sensor 97-99 Corvette manifolds into 2 1/2 dual rear exhaust Radiator - 98-02 F-body with dual cooling fans
Upgraded to LS6 Lifter Valley Plate - I believe this may allow more air to be pulled from the valve covers into the intake
Transmission No modifications, pulled the pan to make sure it looked ok and replaced the filter and fluid. Original torque converter
I started the swap during the winter and did not get the car on the road until this spring and it has not been below 32 since I filled it so I don't think that is the case.
Oh good, that one just made the hair on my neck stand up! I had a friend do that with a boat last winter, he filled it and drained without opening the drain ports on the block and forgot to add coolant.. cracked in several places where water was trapped. Was very sad to look at..
Its possible you've lost the head gasket by the top end oil feed port and right now its not leaking bad enough to show up in the cylinders...
I can't see any option but pulling heads. if you do I'd just drop the coin on taking them to a decent head shop and have them pressure tested while you have them off.. I did this on a honda a while back is was like 85 bucks, I ended up having them take a pass on it to make the head flat it was marginal on the flatness of the gasket surface. My total out the door was about 120. The shop I use here in PDX for this stuff is dedicated to Aluminum heads and the guy has a test plate for like every head made.. Luckily gm is easier.. Most shops should have that one..
I got the engine from a reputable dealer that sells a lot of engines ( https://leecparts.com/collections/ls...es-drivetrains ), it came with leak test results that looked good. As I recall when I was pressurizing the cylinders to replace the valve springs the #4 cylinder did his a bit more than the others, I attributed it to some carbon on a valve or something, but perhaps there was a head gasket leak at the time. Hopefully it is the gasket, the heads are not a big deal to replace, and if it is the block I will probably just bite the bullet and put in a used C5 motor, most of the hard work for the swap has been done at this point, a new motor is relatively easy. I can repair/rebuild the existing motor and put it in my '69 for the next swap!
So if I am pulling the heads to replace the head gaskets, should I consider swapping out the heads with refurbished 243 heads since my heads have 115k on them? And if so, is there a reputable source for them? I see plenty of heads out there, such as these: https://www.ebay.com/itm/163925753331
This of course all is contingent on the issue being with the gasket and not an issue with the block.
I have pulled off the passengers side head, here are some pictures of what it looks like. I do not see any glaring problems with the head or head gaskets but I am not certain what I am looking for, any guidance would be appreciated!
Did you do any compression tests? Leakdown tests, check the coolant for exhaust gases before you took it apart?
Was the coolant system being pressurzing instantly when you started it?
Was it using any coolant?
Did you do any compression tests? No. Leakdown tests No, check the coolant for exhaust gases before you took it apart? No.
Was the coolant system being pressurzing instantly when you started it? Not sure
Was it using any coolant? Yes
I took the heads to a local machine shop and they cleaned/pressure tested/milled them a bit and everything looks fine no. I have a set of Felpro gaskets and ARP bolts on the way to start the reassembly next week.
09-25-2021, 02:06 PM
LS6 with milky coolant - must be head gasket? 
I had a friend do that with a boat last winter, he filled it and drained without opening the drain ports on the block and forgot to add coolant.. cracked in several places where water was trapped. Was very sad to look at..
if you do I'd just drop the coin on taking them to a decent head shop and have them pressure tested while you have them off.. I did this on a honda a while back is was like 85 bucks, I ended up having them take a pass on it to make the head flat it was marginal on the flatness of the gasket surface. My total out the door was about 120. The shop I use here in PDX for this stuff is dedicated to Aluminum heads and the guy has a test plate for like every head made.. Luckily gm is easier.. Most shops should have that one.. 
