Replaced Cam in L-81

[KC-21]

Problem:
I bought this car a few months ago and the fellow I bought it from said he replaced the cam & lifters & timing chain. Now on start up it doesn't want to idle (acts like it's loading up) once you run it for a while it will idle OK but if you shut it off for a few minutes then start it again it's back to not wanting to idle correctly.
If I understand what I read, the computer is not recognizing the cam in the engine now and it takes it a little while till it finds it's own happy medium. I read that I may need to replace the computer chip to a performance chip to help eliminate this problem?????? Problem is, I don't know what cam was installed only that it is a little bigger than stock......

Any recommendations, and am I possibly on the right track here??????

HELLLLLLLLP......:confused

KC

 

[UKPaul]

I'll try to help. As I understand it, the problem the computer has with cams is when the cam fitted has enough overlap to give a low vacuum at idle (lots of overlap increases exhaust scavenging at higher rpms, giving more power). If a cam is fitted that gives a low manifold vacuum at idle, the computer reads the low vacuum as the throttle being opened, so it supplies a richer mixture as it thinks you want to go faster. At idle this extra fuel floods out the motor & can pool in the intake manifold so that when you do give it some throttle the motor bogs down due to the sudden inrush of fuel. This could be happening now as letting it idle for a long period may get the intake hot enough to boil off any fuel that's pooled in it (but that's just a guess). Any black smoke coming out the exhausts??
As far as I know the computer hasn't got a clue about the cam fitted & just reads the manifold vacuum (a simple thing to check is that the vacuum hose to the manifold vacuum sensor is in place & not leaking. It's the very thin pipe going to the sensor mounted on the d.side firewall on a bracket alongside a couple of relays). There's been a few people that have fitted different cams to L81's (me, for one!) without any problems.... the cam just needs to be able to give a high enough manifold vacuum.
To find out if this is a compatability problem with the cam I'd hook up a vacuum gauge & measure the vacuum at idle. I've forgotten what the minimum reading should be (15?) for the computer to function correctly, but I'll try & find it again. Also check that all electrical connections are clean & good & that there's no vacuum leaks.
If you can get a manifold vacuum reading I should be able to tell you if it's OK (this problem needn't be a cam/ECU problem).

 

[gedmeyer]

UKPAUL, what cam did you fit to your L-81? I'm trying to figure out what I need to do to mine... What kind of performance gains did you get?

 

[KC-21]

UKPaul,
Thanks for the info, but I've been over this thing from top to bottom and everything is in place and working. After a few minutes of using the throttle to keep it running it then kicks in and idles fine and as long as you run it, it idles and runs fine, but if you shut it off for a few minutes and restart it you have the same thing again. I've talked to several 80-81 owners that are having the same problem after cam replacement and I even saw one that was having this problem after installing a crate motor but used the carb & distributor from the stock engine. If it was a vaccuum problem it should exhist all the time. Not just for the first minute or two of startup, cold or hot it's the same........

KC

 

[UKPaul]

My cam is a Crane CompuCam 2040 with lifts of .440"/.454" & dur of 210/216. It's been renamed to Powermax no.xxxxxx or something like that, but suppliers still recognise the old name of 2040. I think the LCA was 114? What it did was to slightly reduce the bottom end torque, but gave a lot more midrange power. It apparently gives more top end as well, but I haven't had a chance to find out yet (it certainly keeps pulling OK!). The power that I "lost" at the botom end (just off idle) came back after fitting a Hypertech chip. It's not a high performance cam, but for normal road use, where midrange power is what I want, it does the job well. The computer has no problems with it at all & fuel economy is still good. I've got some info on it somewhere that I could dig out if you need it.

KC,
This sounds awkward It's almost as if there's some sort of problem with the choke. Does the computer give any error codes at all? Have you still got all the emission systems in place? If so, is the EGR system working correctly? What stat are you running?

As you say, if it was a vacuum problem then it should do it all the time. What I do know is that I've swapped the cam out & another L81 owner also runs the same cam as me with no problems. Yet another runs a Comp Cams 260H without any problems, so a cam change is possible (you just need to ensure adequate vacuum at idle). Mine didn't give any problems with the stock chip & the Hypertech chip mainly changes the spark advance curve to give more power on a modified engine.
Could the problem be that the computer thinks the motor is cold, so is giving it a rich mixture (in open loop?) until it gets warm? Have you got a workshop manual??? If so, then it would be worth checking that the output from the coolant sensor for the computer (front p.side of intake manifold) is correct.
It sounds like a sensor problem rather than a cam problem, I'll have a look at my manual & see if it says anything about this.

 

[KC-21]

UKPaul,
Thank you ever so much for racking your brain over this. I have replaced the temp sensor and checked it with an analyzer and it is reading correctly, it has a 195 degree tstat in it and it runs between 195- 210, I replaced the choke module and it seems to be working OK. There was a code in the computer because of the bad temp sensor and I'm not sure if it was cleared by the analyzer but I think it was, but to be sure I can put it to sleep for a few minutes to clear the codes if there are any left in there. It does however set a soft code when going down the mountain letting the engine hold the car back but the "check engine" light goes back out after a few seconds. I do know that the vehicle speed sensor doesn't work and my mechanic said it may set a soft code because of this but according to the analyzer there are no codes in the puter.......
All the emission stuff is in place and we have checked it to be sure it's operating, everything seems to be working fine!!! The car runs fine and it will smoke the tires with the new cam in it but the idle at start up is a bit annoying at times. I guess it's not really as bad as I'm making it sound but I want to have my "Baby" running perfectly, that could be asking alot from a mostly stock 22 year old car.

I'm going to check the vaccuum line to the transmission modulator valve, that's the only thing I haven't checked every inch of yet. I don't think there is a leak there but I just want to be sure.

Thanks Again,

KC

 

[UKPaul]

Hi!
There's nothing wrong in wanting it to run correctly I'm wondering if the check engine coming on when on the overrun (what we call slowing down by closing the throttle & using engine braking - what do you guys call it?) is a big clue. I had a read of the manual last night & 3 things leapt out at me as potential causes:
1. The temp sensor, but you've changed that already.
2. The charcoal cannister being purged. Mine has been disconnected & I was trying to get it connected in again (the TVS is broken, I couldn't find one, so gave up!). What I remember about it is that if something was wrong then it could dump to much air into the system. The valves on the cannister on mine had dirt in them causing vacuum leaks which would have been a real headache to find if the cannister was plumbed in correctly (could be worth checking?). The valves pop apart, but you need to get the cannister off the car to do it. From memory, there's a pipe from the fuel tank connected to it, a pipe from the front of the carb (both 5/16" bore?) & then theres some pipes going to the 2 valves on the cannister. When the engine is running one valve is open & the other closed. When the engine is off the valves are in the opposite configuration. To check if the purge system is the cause I guess that disconnecting the pipe "T"'d into the PCV hose and plugging it should disable the purge. For a belt & braces approach you could also disconnect the pipes from the carb & TVS to it & plug them as well. It won't take much effort to do, but at least you'd be able to rule out one potential problem. I'm guessing that the purge pipe "T"'s into the PCV hose? That's the pipe missing on mine.
3. The EGR system. All the fault diagnosis stuff in the manual that refered to a bad idle gave the EGR system as a possible reason (either playing up, or erroneously routed hoses - did I have some fun there!). The EGR should stop exhaust gas recirculation at idle, or when decelerating with the throttle closed. If it was misbehaving it could cause a bad idle..... & problems on deceleration. Suspicious, if nothing else - could the EGR system be pumping exhaust in when the throttle is closed? I can't help you with how to test it as a previous owner had disconnected mine (and left the hoses in a real mess). All I know for sure is that it shares a relay with the TCC lockup function & it looks like the ECU disables both EGR & TCC operation while the engine coolant isn't up to temp. Again, a nice easy check on the system is to disconnect & block the vacuum pipes that operate the EGR system, but I wouldn't have a clue what or where they are.

Does your TCC lockup work OK? If you can get a vacuum reading at idle it would be helpful, even if to just rule out the possibility of the cam causing it. It could be that the vacuum reading is so borderline that the engine has to be fully up to temp b4 it runs correctly? The computer & systems on the L81 are fairly simple, but there's so much "stuff" there that it can get difficult to find what it is causing problems.
Good luck with it, I know exactly how you feel! When I first got mine I had the opposite problem in that it was great until it warmed up. Then it would flash the check engine light regularly when it was just off idle. It took me weeks to find the simple cause of it....
Paul

 

[KC-21]

Paul,
Once again, Thank You!!!! My mechanic and I both think that the EGR & TCC lockup are working OK. One new twist that I just discovered is that at an idle with the trans in Reverse or Low gear I can hear a distinct whistle but it only occurs in these two shifter positions. As of yet I haven't been able to pinpoint the whistle but this is one more reason I need to do a thorough check of the line to the modulator valve on the trans. The valve looks new but I'm not convinced there isn't a rub hole in the metal line somewhere, and until I'm satisfied it's OK I'm not going to go changing or trying to troubleshoot a problem that may be obvious. I will however check the purge tank too but only after I've ruled out the modulator valve.
I'll keep you informed of what I find..........

Thank You very much!!!!!

KC

 

[KC-21]

Paul,
One more question... At 55 mph (not sure of the metric conversion) what RPM is your engine turning???????
Mine is at about 2300 RPM, does this sound right or may my torque converter not be going into lockup????? If I remeber correctly that RPM was about right in my camaro with 3.73 rear gears but I'm not sure it's right in the Vette with 2.87 gears....

KC

 

[22229stingray81]

KC,

I believe the whistle you are hearing in reverse, and when you manually put it in 1st or 2nd is normal. I asked this question a year or so ago and many '81 owners and other th-350 equipped Corvette owners have the same sound. Hope this maybe relieves some stress.


-Tatortot

 

[KC-21]

TatorTot,
Yes it does, but I'm still gonna check everything over very closely.
I have never heard this sound from any other TH-350's and I've owned quite a few of em but Thank You very much. I feel better that I'm not the only one that can whistle while in traffic.....LOL

KC

 

[silver 80]

I had a hole in the rubber line at the modulator and it wouldn't shift on it's own I had to start out in first and then shift it through the gears. replaced the hose and works fine now. i don't think it's going to be a hole in the line, but then I've seen stranger things.

 

[UKPaul]

KC,
I can't remember what rpms it was turning at 55mph (what's metric?!) but with 2.87 rear end my TH350C was turning 3000rmps at 70mph with the converter unlocked & about 2700 with it locked. A simple check on whether it's locking is to get up to around 55mph upwards & very slightly brake. The rpms should increase by about 300 as there's a switch on the brake pedal that cuts the power to the TCC.
I never noticed a whistle from the th350, but my exhausts are so loud it could have been singing & I wouldn't have heard it
The way I eventually found my vacuum leak (which gave all the symptoms of a carb/ECU problem) was to block off entire vacuum systems one by one & see if it improved eg. the vacuum lines to the EGR, then EFE, then purge......
Could your mechanic measure the manifold vacuum at idle?
Good luck, hope you find something.
Paul

 

[KC-21]

Paul,
I checked the vacuum reading at the carb and at the manifold, at 800 rpm the vac reading is 17-18 inches of vacuum in either place. Which according to my guage is just on the low side of normal.
The problem is with the electrical connection on the drivers side front of the carb, when I start it up I can hear it clicking which is normal I think but the idle sucks. After a couple of minutes the idle smooths out and it idles fine or if I disconnect it the idle is fine. Does this unit control the metering rods in the primary end of the carb????? Do you have a suggested fix for this problem????
I think the RPM's you said are about right for mine at 70 mph also, the metric I said about is that funny KPH stuff that I don't understand at all... :L (old Pennsylvania backwoods REDNECK here)

Thank You Again!

KC

 

[KC-21]

Said the heck with it and my girlfriend and I went cruisin' today, bad idle and all!

Check out the Shark at the Eternal Flame at Gettysburg.

Drive On!!!

KC

 

[UKPaul]

KC, nice picture What color is yours? It looks a lot like the original color of mine which is showing through in a few spots (whoever resprayed it didn't like bending down! The paint is very thick on the top of the car, but is wafer thin on the underside edges with some parts not even covered).
Something I did on mine that improved the low speed running was to increase the base timing. Normally the timing is set by disconnecting the 4 wire connector going to the dist & then setting the timing to 6*BTDC. I set mine to 11BTDC which made pickup better. I could have given it a bit more advance before running into pinging problems, but played it safe. Is yours set to the stock 6BTDC?
There will be 2 connectors on the carb. One will be on the front d.side vertical face & will have 3(?) wires going to it. This is the TPS (tells the computer the throttle position).
The other is more towards the centre of the carb, but on the top, & will have 2 wires going to it. This is the mixture control solenoid. This is a solenoid in the carb which pulls a needle down (the dancing needle) whenever it's energised by the computer. When the solenoid is de-energised the needle pops back up due to a spring under it. The top of the needle has a "wing" on it, the outer edges of which push down on the needles in the primary jets (which also have springs under them so that they go back up). By energising/de-energising the solenoid constantly, the computer controls the position of the primary needles in the jets, to give the required mixture. The clicking you hear is the solenoid working. The overall limits of movement of the primary needles is determined by the fully down (weak) or up (rich) position of the dancing needle. These positions are set by the rich & lean stops, which are adjustable, but have anti tamper plugs fitted over them at the factory. Doug Roe has written a book on Q-jets that also covers how the E4ME works, which will give a better explanation. On cold start, or at WOT, the computer goes into open loop, which means it stops reading the O2 sensor & de-energises the M/C solenoid to give a full rich condition. If this is the connector that you are disconnecting then that is what you're doing ie. giving it a full rich condition, implying that it is running slightly lean as it runs better unplugged.
There's also idle mixture screws & an idle air bleed valve for controlling the idle mixture.
If you look at the wiring harness that runs from between the carb to the A/C compressor there should be a green(?) wire coming out & just hanging there. It'll have a connector on it & this connector is used for measuring the dwell. What you need to do is hook a good analogue dwell meter on it, set the scale to 6 cyl, & you'll be able to see what dwell the computer is kicking out. This is very useful info & is the best way to setup or check the primaries operation. Can I mention another forum?! Rather than typing all this up, check out this link:
http://forums.corvetteforum.com/zerothread?id=478251&postid=5186468#5186468
Somewhere in the archives are a couple of really good posts on setting up E4MEs.
17-18 inches should be fine! I think the computer needs a minimum of 15 to work correctly. The next thing to look at would be to see what the dwell reading is for the m/c solenoid. It could be that the carb just needs a minor adjustment.
KPH? Is that some sort of sensor?
Paul

 

[KC-21]

Paul,
The color of my Vette is Dark Claret, it has been repainted sometime in the past but the color is almost an exact match with just a bit more metallic in the paint than the factory paint. Whoever painted it did an almost perfect job but it's due for another paint job because of a few stress cracks in the paint and the usual stone chips but it still looks great from about 10 feet away and it still shines like a mirror.
The idle problem is definately the mixture control solenoid, it seems to be running lean on startup because when I unplug it the idle smooths out and idles fine. If I unplug the mixture screw adjustment holes and adjust the mixture a little do you think this would solve the problem??

Thanks
KC

 

[mikejpss]

KC-21 and all :
Reading your postings on the carb troubles reminded me of the chase I gave on mine for 3 months. I finally said to heck with it all and replaced with a 1980 non computerized Q-jet and a vacuum advance distributor. Best thing I could have done , wish I had done it sooner. Idles great, secondaries open just in time to pin you back in the seat, etc., etc.
I am not sure what it has done to the torque convertor lock-up, but after reading here I will look today as my bride and I will make its maiden cruise since restoration was completed last week.
The only "problem" I see with my new setup is starting the motor when cold. The accelarator has to be "pumped" 3 or 4 times before the ignition is turned to get an intial burst of fuel into the intake. After she runs thru the warmup cycle, it restarts without touching the accel. pedal for the rest of the day. :dance
UKPaul- did you receive my gift yet? Hope that slow boat to China (UK) gets there soon.
My latest pic:

 

[KC-21]

Mike,
WOW!!! What a nice paint job! She looks goooooood!!
I'm seriously considering doing the swap to a standard (pre-electronic) Q-Jet and vacuum distributor myself. That may be a project for the off season. (winter here in PA)

I'm not sure my torque converter is locking up either, I can't feel it if it is and I'm not sure it wasn't changed to a non-lockup converter at some point in time but I don't think it was....

Later,
KC

 

[Tom Bryant]

KC,

I know you have checked this but it sounds like the EGR valve is sticky to me. If it has a lot of carbon build up on the shaft it may take it a while to close after you start the car. The rough idle is exactly what you get when one is sticking open. I have forced them closed with a screwdriver then put a small ball bearing inside the vacuum hose to block it off. End of problem and not visually detectable and a better idle than even a new EGR would give. Not sure if it would mess up an emissions check if you have that there.

I suggest removing the EGR and soaking it in GM Top Engine Cleaner (submerged) or other carbon desolver, untill all of the carbon is desolved away and it works freely again. If that don't work and it acts sticky I'd replace it. You mentioned some '80 owners with the same idle problem so it has to be something they both have in common. That eliminates anything computer related.

Tom