VSS - if you are shifting still, probably is the instrument cluster. otherwise youd be stuck in 1st gear i believe, like B-bowtie said earlier... agree probably issue in the cluster. you MIGHT get lucky and pull the cluster out, clean the contacts, reinstall connectors and check see if speedo works before you put it back together. dash bezel doesnt come out easily, if tilt steering, drop the steering wheel to lowest position, you'll have to bend the bezel slightly. bezel retaining clips fall off easily.
you probably have a failing intake gasket at the outside edge, so coolant is escaping - see my pic in thread i linked below - you probably have gasket failure at one end of the intake gasket, where coolant escapes/seeps externally. after engine cooldown, slight suction in cooling system is sucking in air from this leak. its easier to suck in air from the leak, than to suck coolant from the reservoir pass the rad cap.
skip rad replacement. its probably pretty junked up with sludge but a professional flush will help lots. if they sold the big O-ring on the plastic tanks on the rad, i'd have already removed the rad, opened it up and cleaned it out by hand. probably runs a bit hot with the sludge
head gasket probably ok, (its a low mileage car isnt it?) i replaced mine b/c i wanted to rebuild the heads. the old head gaskets looked pretty "soggy" bloated looking aroudn the cooling passages.
pressure check? on cooling system? assume you not talking about compression pressure check. it will probably tell you what you already know... leaky LIM gasket. they will just tell you have a slight pressure loss, not where it's at.
ECM/PCM
i use both terms. same thing, almost. PCM is really only for automatic transmissions (PCM = ECM + TCM). If you have a manual transmission then it's basically an ECM since there is no tranmission controller.
other confusing terms, not all apply to the lumina, or at least i havent run across them yet thankfully:
wss - wheel speed sensor - probably term used in conjunction with 4 wheel abs? Anyway, measure wheel rotation.
vss - vehicle speed sensor - sensor at TAIL shaft of transmission, basically how fast you are moving
tss - turbine shaft/speed sensor (measures the transmission INPUT shaft speed to determine amount of slippage from the torque converter. not sure if we have one of these on the lumina. remember torque converter always slips a little until TCC engages. NOT used to determine vehicle speed.
tcc - torque converter clutch. locks up torque converter at cruise for better mpg.
long time ago. i was in that "i hate all these fancy computer crap on cars" but have embraced it and fairly understand it. i was into hobby electronics before computers were the in-thing, made it that much easier.
not a fan of these side mount engines either, it just seems WRONG. no space to work, have to remove 5 unrelated parts to get to the part you need to replace. my dodge ram on the otherhand, i could fit a couple of bodies under the hood..
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to address the seepage.
cant have coolant seepage from side of engine unless its coming from head gasket.
there are 6 seals on each lower intake gasket:
see pic
http://www.automotiveforums.com/vbul...5&postcount=56
see pic of old intake gasket with arrows. the 3 big openings are INTAKE. the two edge openings are COOLANT.
3 orange seals between each intake runner and the crankcase oil. impossible for these intake seals to pull in coolant unless coolant is in the oil
2 orange seals for the coolant passages that goe into the intake. leak causes oil/coolant to intermix (oil leaks in cooling system on engine cooldown, while engine running, coolant forces itself into engine crankcase and mixes with oil) also cause external coolant leak and definitely air leak INTO cooling system.
1 black plastic all around generally seals the crankcase from open air.
if you look at the LIM gasket pic with the 2 arrows - that arrow on left shows where i had coolant almost POURING out onto the ground (external leak) AND some leaking into crankcase. middle arrow pointing at intake runner, shows where i probably had oil (and crankcase air) sucked into the combustion chamber of #3 or #4 cylinder (vacuum leak)
lower intake gasket failure causes one or more of the following:
1) external coolant seepage at front or back of engine.
2) internal coolant seepage into lifter valley and into engine oil. slight chance to suck in oil into cooling system during engine cooldown.
head gasket leak:
1) leak coolant into combustion chamber, and lots of air (fuel/air/combustion gas) into the cooling system
2) oil into combustion chamber
3) oil/coolant intermix
4) air leak between adjacent cylinders (low compression on 2 adjacent cylinders )
engine running, engine hot: oil is 40psi, coolant is 15psi. this is when oil can get into cooling system
engine off, engine cooling: oil is 0psi, coolant goes from 15psi to a "slight vacuum". this is when coolant can get into oil when cooling system is still pressurized, and suck in oil into cooling system when engine is cool and slight vacuum in cooling system
bubbles - since you dont have the steady stream of bubbles [indicating head gasket leak] then you are getting your air when the car cools - when there is a slight suction in the cooling system, leaky intake gasket sucks in air - instead of the rad cap sucking in coolant from coolant reservior. this air causes dexcool to sludge.
consider ordering parts online... like rockauto. that is where i got most of my parts for my top end rebuild.