Replaced Cam in L-81

[KC-21]

Tom.
I thought the EGR valve was working OK but now that you say you've had this same problem, it sure sounds logical and it WILL be checked!! We don't have the emissions checks here yet but I imagine they'll be here all too soon!
I'll keep you posted on what I find, thank you.

KC

 

[UKPaul]

KC,
I'd definately follow what Tom said (or just disable the EGR temporarily) as this does sound like an EGR problem. If the carb still has the anti tamper plugs in it then the chances are that it's adjusted correctly as it should still be set at the factory settings, but it could have some dirt/gunge in it? Let us know what you find with the EGR system.

Mine was originally maroon metallic &, looking at the original paint under the carpets, it looks very similar to yours.

Mike,
The "gift" hasn't arrived yet, but there's an unofficial post strike on, so nothing has arrived (including bills ).
Did you never have to pump the throttle from cold with the original carb? Mine needs a couple of pumps before it will start :confused

 

[KC-21]

OK Gang, the EGR valve is working just fine , loose as a goose. I was thinking (yeah, yeah I know, that's dangerous) but if it were the EGR valve it shouldn't do it when it's warm cause it shouldn't be closing when the engine is at operating temp ????????? Byt, hey, I'm going to block it off and give it a try anyhow...

Thanks again guys!!
KC

 

[tazdevl35]

Hmmmnn.....Several things come to mind. One is a dead o2 sensor with a shifted range that only works when very hot.....causing it to load up at idle...and unless its completely dead the prehistoric c3 computer wont know its dead. Another thing to check is the thermal vacuum switches for the efe and canister purge circuits in the vacuum harness. Vacuum leaks at idle will do the same thing and "fool " the o2 sensor into a false lean making the carb richer at idle and cause a poor idle. Also is your mc solenoid working? You Should hear a click if it is from the carb before startup and at idle. I have spent a lot of time under the hood of early Corvettes during my time working at chevrolet and if you have any questions please feel free to contact me and perhaps i can get you going in the right direction


Taz

 

[KC-21]

Taz,
Thanks for the info. The 02 sensor may have a bearing on the problem, not sure, but it is not setting a code if it is. All the other circuits, thermal vacuum, EGR and M\C are all working as they should. I'm not too sure about the purge circuit though can you give me a little more info on that?
Keep in mind this only happens at start-up and only lasts a couple of minutes.......every start-up, cold or hot. If I unplug the M\C connector it idles just fine...........

Thanks
KC

 

[tazdevl35]

Hello,

Well if you have checked all of the vaccum circuits to the charcole canister, efe and such then my money would be on the o2 sensor...at least in part. Also when its running rough is the choke all the way open? All unplugging the mc connector does is set the solenoid inside the carb full rich. However you have said when its warm and plugged in it runs fine at idle. Another area to check out would be the coolant temp sensor. If its out of range it will do the same as you describe. Have you pulled any codes from the computer? You can read the codes using the check engine light and a paperclip to activate the self diagnostics. The connector is under the ash tray. By shorting the two side by side terminals you can watch the flashes of check engine light, it will flash 3 cycles for each code ...code 12 being the first. Code 12 is always there. To read the light it will flash once for the one (1) then pause for a sec then flash twice for the two (2) thus code 12 is read. Each code will cycle 3 times before moving on to the next code. By the way, this must be done with the key on and the engine not running. Check this first and let me know what you find. If you post the codes , if any , I will translate them into their meaning and tell you how to check the system they affect.



Keep us posted, we'll get it figured out and remember even though we love our cars, they are still only metal , fiberglass and wires. Any problem can be fixed given enough time and using the proper approach! Once its fixed you'll feel as if you climbed Everest!


Taz

 

[KC-21]

Taz,
The only code that is set in the computer is for the Vehicle Speed Sensor which is not registering, I forget the code number but I think it's like 24....... The coolant temp sensor has just been replaced and is within a degree or two of the actual temp. I thought I read somewhere that on startup that the computer was in either open loop or closed loop till the temp sensor said it was warm enough, but that either way it is it takes the o2 sensor out of the picture till it warms up, is this true????? If not, then the next step is the o2 sensor. Yes, it runs and idles just great after the first minute or three until you shut it off and restart it then it's the same thing all over again, hot or cold........ The choke is set properly and everythin is working fine in that department, I just replaced the electric choke module too....

Later
KC

 

[UKPaul]

Hi KC,
How old is the O2 sensor??? It could be that it's on the way out & needs to get very hot before giving anywhere near reasonable readings. The computer seems to be quite forgiving of bad readings & only sets codes when the readings are way out.

The purge circuit includes the TVS that is mounted in the d.side of the thermostat housing (waterneck?). Mine was causing a vacuum leak & I found it by pulling the vacuum pipe to it from the carb & plugging the carb vacuum port. There are also vacuum lines to the purge cannister that should be blanked off as well for testing. There are two valves on the cannister. One is closed with no vacuum present & the other is open. When vacuum is present they swap over. What I found on mine was that the TVS had a crack in it which was letting in air & both valves on the cannister also leaked. The valves come apart (once you get the cannister off the car) & there's a seal in each one. Mine had dirt in them & just needed careful cleaning & re-assembly to fix the problem.

Personally I'd suspect the O2 sensor, especially if it's old. A lot of L81 owners find that a new O2 sensor makes a big difference.
Good luck!

 

[KC-21]

Paul,
Hello!!! The TVS has also been replaced and everything seems to be in working order there too.... My next step is the o2 sensor, it looks like it may be the original one and probably should be replaced anyway.
The search continues......
I even toyed with the thought of dropping a new ZZ4 crate engine in it, then I decided I wanted the driveability and not the excess power (that by the way will get me in BIG trouble) I've built many sprint car engines and I know how cantankerous they can be at times, so I figured I'd be better off just finding the problem with the L-81 and fixing it. My girlfriend suggested saving the ZZ4 for the next project......(hers I think)(hopefully a 68 to 72 Vette) ahem....wishful thinking I suspect..... I also had a dream of a 572 big block but that's another story..
I'll continue the search for the MINOR problem with this one for now.

DRIVE ON!!!!!

KC

 

[UKPaul]

Hi KC,
Hopefully a new O2 sensor will sort it all out.
Have you got a part number for the purge TVS? I've been trying to get one for ages & nobody seems to know what it is (I've now got 2 EFE TVS's!!).
Meanwhile, if you've got a ZZ4 motor getting in the way I can help you out & take it off your hands
Let us know what a new sensor does for it?
Good luck!

 

[KC-21]

Paul,
I'll check for a good # for you. I don't have the ZZ4, just looking at them........
I'll keep you posted on what the new o2 sensor does.....

Later!
KC

 

[Vader]

Taz is probably on the right course.

Another thing to consider is that before the engine is warm (and O² is active) the CCC is in open loop mode. The factory lookup tables are probably not appropriate for the cam profile, but shouldn't be so far off that you actually have difficulty starting and running the engine until closed loop is achieved. I've got a mild cam (.443" /.465" - 214°/224° - 112° LSA ) and ported 14014416 heads for 9.5:1 static compression, and I know the factory calibrator is way off in open loop (and am still looking for efiting software!). I still get it to start and run well cold, with just a hint of a rich exhaust on startup (with no cats). It's not even enough to darken the plugs. It holds about 16.8"Hg at idle. I can completely disable the MC and still adjust it to run acceptably. You should have no problem doing the dame with just a mild cam swap and a 114° LSA.

I believe that you have carburetion issues, and may need to adjust the lean stop and MC solenoid duty cycle. Obviously, if it runs acceptably once warmed, the MC is probably active, but you may have a lean stop that is too rich for cold starts. You should also verify the correct TPS voltage, since mine seemed to be very sensitive to TPS after the cam/head swap - especially when cold. About all the antiquated CC has for input in open loop is the CTS, TPS, MAP, and RPM. If one of those is skewed a bit, it can really throw off the fuel control. If the carburetor is not clean and properly adjusted, it just compounds the problem.

Incidentally, if you're holding 17-18" Hg at an 800 RPM idle, you probably don't have a significant vacuum leak.