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Vacuum map sensor code #34 T/S

Joined
Apr 8, 2005
Messages
274
Location
Charleston, SC
Corvette
1981 Dark Blue Metallic, 2003 AE coupe
OK folks, After enjoying six years of relatively trouble-free crusiing, the old girl is beginning to finally show her age. Here's the history:

About a year ago: new plugs, wires, dist cap, replaced a broken exhaust manifold bolt (driver side), plugs that were in the car were just a tad colder than factory, new ones are correct.

Shortly afterwards, began to get check engine light when warm and idling (clears when driving). Don't drive the girl much (bought an '08 durng this time and she gets the attention) and wrote it off as ECM adjusting to new plugs and corrected possible exhaust leak. Car sat for months at a time between drives. Noticed she was beginning to hesitate while trying to light accelerate on primaries, secondaries kick in and she bolts (open loop?).

Fast forward to today: Finally pulled the codes, 12, 13, 44, 55, 12
Replaced O2 sensor (Bosch exact fit) and discon. battery to clear codes.

Test drove and still the same. Will not accelarate smoothly until secondaries open. Light on at idle and clears when driving. Pulled codes again: 12, 34, 12

Did I just not drive her enough to let the ECM re-learn? I know the 81 ECM is relatively slow, but I'm scratching my head (I do that alot).

I'm not a big motor-head, so go easy on me.
 
Joined
Apr 8, 2005
Messages
274
Location
Charleston, SC
Corvette
1981 Dark Blue Metallic, 2003 AE coupe
No, but it looks like its time to get one. I really haven't had the need until now. Everything in the past has been cosmetic or strictly mechanical.
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Mike,

The codes are not as bad as what they seem.

1st, the code "12" is the computer telling you that it is running fine.

2ndly. code 34 is only the code collected. The second code "12" is just the process looping over. So only code 34.

Stay tuned. Let me check the book.
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Mike,

Code 34 is the vacuum sensor circuit low.

So perhaps is a simple as a disconnected or leaking tubing. The tubing is that black plastic tubin connecting from the rear side of the carburetor at the base. The other end connects to the vacuum sensor which is to the right of the brake booster and under a metal shelf that bolts through the firewall (you can access the hardware in the wiper drive cavity). The plastic tubing is notorious for becoming brittle and tend to crack. It connects at both ends with short rubber tubing.

If it is not as siple as what I explained, then let me know, and I will scan the page and post it for everyone's benefit. :)
 

Tim81

Well-known member
Joined
Jan 3, 2010
Messages
114
Location
North Canton, Ohio
Corvette
81 Black on Black Corvette
Once again Gerry is all over it. Hope its not the sensor itself thats bad..
 
Joined
Apr 8, 2005
Messages
274
Location
Charleston, SC
Corvette
1981 Dark Blue Metallic, 2003 AE coupe
trouble shooting continues

Gerry, I appreciate the insights. I'm currently scratching my head again. I pulled the cover off the firewall with the map sensor in there and the vacuum hose is just a short, cleanly cut stub not going to anything. If this is apparently how it has been since I bought the car, why the problems just starting now. I will look for the other end from the carb and see what I find. It seems I may have to consider a "bubba" factor in my troubleshooting.
 
Joined
Apr 8, 2005
Messages
274
Location
Charleston, SC
Corvette
1981 Dark Blue Metallic, 2003 AE coupe
small success

I found the thin vacuum tube from the carb and reconnected to the map sensor. I think what may have happened is that it got removed during the valve cover gasket replacement (one of many things in the way) and it being so small, got overlooked. Thanks goes out to the CAC folks for the help.

There also may be a little "Christine" syndrone, since all this tempermental activity started when the For Sale signs went in the window.

All codes are clear and no check engine light, BUT...she still idles a bit rough and sputters through take off. Actually stalled when I put her in reverse at idle. This has never happened before. The car has never stalled, ever during my ownership. Well, progress has been made and time to move on to other adjustments.

Any suggestions on smoothing her out a bit?
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Some codes only set once, and do not flash again. If you have had the computer disconnected for more than 5 minutes, it may have reset, and when it re-encounters the condition, it flashed the problem by lighting up the "Engine" light.

Was the plastic black tubing in the general area, though? There should be a small rubber tubing section between the sensor and the black tubing. Then the tubing itself , and yet another small section of rubber hose between the plastic black tubing and the rear of the base of the carburetor.

I would also recommend that when you fix the pneumatic connection between sensor and carburetor, you clear the codes by disconnecting the battery for about 5 minutes. This will also force the computer to sample new atmosphere pressure readings and store them (automatically).

GerryLP:cool
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Yes, see if you can check the air valve 's position with a dwell Gage. Many posts in here talk about how to do that check. Do a search in the L81 forum for "E4ME" or "carb tuning". :D
 
Joined
Apr 8, 2005
Messages
274
Location
Charleston, SC
Corvette
1981 Dark Blue Metallic, 2003 AE coupe
Gerry,

Part of the problem was that I expected to see a vacuum tube similar to what was coming out of the sensor and by the looks of what was there, I thought it had been cut. When you pointed me to the carb, I saw the short rubber tube with the narrow plastic tubing running out. The other end was buried down beneath the dist. shield open to atmosphere. I then realized that it needed to be fished out and rerouted to the map sensor. I did not disconnect the battery since restoring the vacuum line. I will try that to see if it changes the ECM response. I kind of feel bad that I neglected her for the '08 for so long. Thanks.
 

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