Hello everybody!
First I have to introduce myself and I am sorry for my bad English!
I am Michael from Germany, 29 years old and I've owned my 4th F-Body in 2014. That car is the reason for signing me up in this board. Unfortunately in Germany no one has an idea anymore. It's a 1982 Trans Am 4-speed manual with a LG4 engine in original condition. Odometer says 29k miles, don't know if the mileage is correct. The interior looks quite good, perhaps the previous owners had the same problems with the car like me and so they couldn't drive it, whatever!
At first please watch my video that I made for showing you the problem:
The engine runs like crap in idle, especially when it's cold it. It needs a few seconds to start up. Then while driving the rev-meter is "jumping" between 1800-2200 rpm. The engine is stuttering in this area when driving in part load. At full load the engine accelerates quite normal. The gas mileage is horrible. It's about 11 MPG.
What I have done since 2014 (the problem came directly after buying the car):
Checked the compression, set the ignition timing to 6° before top dead center, replaced O2 Sensor, Cooland Temperature Sensor (there were relevant trouble codes earlier), carburetor (Rochester Quadrajet 4bbl E4ME from Rockauto), the whole distributor including ignition modul, ignition coil, rotor, spark plugs, ignition cables, installed new original cylinder heads!, map sensor, battery, ported vacuum switch, removed the air-pump, checked for vacuum leaks, checked all electric wires and replaced the 4 wires from the distributor to the ECM, installed a new ECM, even a new PROM. I tried to adjust the dwell but it's not possible! Furthermore I removed the air pump. I am clueless now and meanwhile I hate that engine.
Sometimes the SES lamp comes on after starting the engine for about 10 seconds and trouble code 41 is set.
Next thing is to check the timing chain I guess. What else could I do?
The car was even at a US Car repair shop and they couldn't locate the problem.
Thank you in anticipation!

 

I wasn't able to play the video here on the forum. But was able to watch it on the u-tube site. IIRC, when setting the MCS the meter should be showing dwell, and read as a 6-cylinder engine.

As for code 41, lack of distributor reference pulses (DRP) to the ECM. The ECM uses the vacuum sensor to 'see' that the engine is running, but it isn't getting reference pulses.

This is likely a wiring/connector issue between the distributor ICM and the ECM. Without DRPs the ECM can't calculate the engine RPM, which means that it can't properly control the ignition timing.

Check the DRP (PPL/WHT) and GND (BLK/RED) wires and connections.

Welcome to TGO.

RBob.

 

Thank you very much, RBob.
Replacing the 4 wires between the dristributor and the ECM didn't change anything. What else could I check? Could a bad float level cause the problem?

 

Are there any laws in Germany about keeping the car as a computer controlled carb setup?

 

Yes, there are, but I think the inspectors wouldn't notice any differences because the cars are rare in Germany. As a car mechanic at Mercedes-Benz I can do the emissions tests on my own.

 

Since the carb has been shipped the float level should be checked. It doesn't take much to bend things a bit.

I'm not sure if I am following the timeline. From your first post it sounds like the code 41, tach jumping, and poor idle were there when you first purchased the car. Then replaced a bunch of stuff and these problems are still present?

RBob.

 

Couple of troubleshooting charts. The code 12 chart also applies to code 41.

RBob.

 

Yes, i replaced all that stuff and nothing helped.

Thanks for the troubleshooting charts!
But I thought code 12 says that the diagnostic mode is running. But "no reference pulses to ECM"sounds logical... Thanks for the moment, I will have a look at it at the weekend.

 

I've got some news...
I found a thread in another forum: https://www.camaroz28.com/forums/3rd...ode-lg4-93200/
It seems like that it's normal getting code 12 and 41 when the engine isn't running...
When the engine runs and the SES lamp comes up for a few seconds and I am looking for trouble codes when it's running, there's no trouble code anymore! So I think I've got a carburetor issue.

 

If code 41 trips when the engine isn't running then the vacuum sensor is bad. Testing it is in the code 41 chart I posted.

RBob.

 

Well, it took a bit of time but today I could perform some measurements!
If I remove the hose at the map sensor, the output voltage doesn't change (less than .5 volt change). So I did the testing for trouble code 34.
I checked the vacuum at the sensor with engine idling, it's about 40 kPa, so it's OK.
With engine idling the voltage from sensor terms B to A is about 1,7 V. So I disconnected the jumper from term B, checked voltage from sensor term B to A, it says 3,3 V. ECM term "20" is NOT grounded. trouble code 34 chart says I should replace the ECM. But the ECM is a new one... got the same problem with the other ECM. What else could I do?
edit: I just ordered a new map sensor from AC Delco, I think the one which is installed on the car is from "standard" or something... I will give you a feedback when the new map sensor is installed.

 

So... I installed the new (GM) map sensor and drove about 5 miles, the engine ran like new... no problems so far. but then suddenly the same old problem occurred. The rev meter is jumping at 1800 rpm and the engine is stuttering then. I am clueless for the moment. edit: There's no trouble code anymore!

 

OK, so I'll have a crack at this one. I can't tell you want the problem is, but maybe I can help you diagnose.

The ECM/computer controls do very little on these engines compared to a later model EFI engine. Among the "missing" features is that the ECM has NO WAY TO CONTROL THE IDLE SPEED. There is no IAC valve. It's controlled mechanically by the primary throttles. ONLY the choke linkage hanging up on the fast idle cam or a lack of 12V to the choke element on the side of the carb can cause it to idle high when it's warmed up. Having the idle RPMs suddenly jump up to 1800 can only happen via MECHANICAL means, not because of computer controls (if the tach is reading accurately, which I have my doubts in this case). Are you feeling me on this? If the idle is ACTUALLY jumping up (not just the tach reading wrong suddenly) the problem is MECHANICAL in nature, not ECM-related.

The CC-QJet engines and their related computer controls do not have a MAP sensor. They have two little black boxes that LOOK sort of like MAP sensors, but they are not. They have a BARO sensor that reads ambient barometric pressure (the one with no vacuum line on it but a hose nipple that looks like it should have one). And they have a VAC sensor that reads manifold vacuum (the one with the vacuum hose going to it from the intake manifold). The VAC sensor works exactly like a later MAP sensor EXCEPT THAT THE VOLTAGE READINGS ARE EXACTLY THE OPPOSITE HIGH VS. LOW. If you stick in a later-style MAP sensor (which looks identical) it will hook up and plug in but it will give the ECM exactly reversed voltage readings from what the programming expects. I've run into this problem only once or twice but since you're in Germany I figure just about anything is possible at this point. The fact that it ran fine for a while and then SUDDENLY didn't indicates against this being the problem.

And don't forget about simple stuff like grounds and such. Wouldn't be the first time somebody got tripped up by a bad (or missing) ground somewhere acting like a computer problem.

Hope that helps (it probably won't).

If I had a gun to my head and was asked to diagnose this dead-nuts with no further information I'd say you have an ignition problem. Only ignition problems have this type of "lightswitch" behavior. Works fine one minute, complete disaster the next.