I had some problems with my engine last year. (1982 with a 305 with a Rochester E4ME).
Carburettor was leaking and not running good at all so I tried to overhaul it myself, but not quite happy with the result so when I just recently went to the US I bought a reconditioned carburetor from Summit Racing.

Having installed this last week I discovered that it was running rough. It starts quickly and runs fine at 2000 rpm, but when it finally idles down to "normal" idle it runs from 1300 to 500 and eventually stalling.
I can adjust the idling screw and then it idles from 1000 to 1500, but still rough. In addition the service engine light is constant on.
I don't know what triggers this light and with a new carburettor I thought should be good to go but, now I feel like I'm back to where I started with an engine that runs poor.

I have installed a pressure reduction valve and have approximately 4-5 PSI of pressure into the carburettor. However there was no change when I started it at first without this reduction valve.
Am I having a vacuum leak or could it be some ignition problems, maybe timing issues since I cannot see that it should be able to idle at 1500 rpm with the idle screw all the way out.
I am considering just buying new distributor cap and spark plug wires just to have these things eliminated.
I have also been thinking about the fuel return line from the pump since someone have messed with both the return and the supply line at an earlier stage.
However these does not explain the service engine light.

 

Make sure there are no vacuum leaks in the system.

Also sounds like you need to try to get a baseline on the adjustment screws (all the way in the back out X turns, but I don't recall off hand the starting point).

Jump the appropriate pins in the ALDL connector under the dash to get the Check Engine light to blink out the code number(s).

 

I got myself a vacuum meter to be able to see whether vacuum is constant or varies and it does vary. It was jumping up and down around 20 in Hg. I was expecting to see it stable, but not.

The error codes after the 12 had flashed three times seem to be 15 and 23 and then 12 again.

15 seem to be Low temperature indicated at engine coolant temp sensor
and
23 Low temperature at manifold air temperature sensor OR Throttle position sensor error

Does any of these two affect the mixture or the rough idle of the engine?

I did a test run to the shop this afternoon and it sort of backfired when I switched off ignition. Does sound like something running rich.

 

Coolant temp sensor does, for sure.

Code 15 usually means that it's disconnected. Which, if you still have the original "coaxial" looking connector style, would be no big surprise. You need to GET RID OF that POS. When you buy a new sensor it comes with a new pigtail, which is INFINTELY better than that old one. Just go get one and swap it all out and be done with it. "Low Coolant Temp" will make the ECM think it's like -40° out, which will make it run STUUUUPID rich. The car has no chance of running right with that going on.

TPS is another story altogether. The computer has no way of knowing that it's actually "bad", in the sense that it can look out there and determine that with any kind of accuracy; butt, what it DOES do, is take the aggregate of ALL sensory inputs, and if they conflict in some way, it sets the code for the MOST LIKELY one that's reading wrong, FOR A BRAND NEW CAR, back in 1982. So for example if the ECM sees high TPS voltage (which means wide open throttle) at the same time as high MAP (high vacuum, which means low to no load on the engine) and low RPM, it's programmed to tell you that the TPS is fornicated, since it's the "most likely" cause of that glaring inconsistency (2 readings that agree and one that's out to lunch comparatively), IN A NEW CAR. For a car with a half century of life and a million miles on it, its guesses aren't always spot on though.

Your car doesn't have a IAT sensor so ignore that translation.

Start with the CTS, as far as fixing the "codes".

It sounds like you have a LARGE vacuum leak though. Check all the vac lines carefully.

Not sure about "return line" and "pressure reduction valve". Sounds like maybe at one time or another in the car's life, that big nut that goes between the front left seat and the steering wheel, backed off and came loose, and started banging around under the hood. Might be worthwhile to catalog the damage that it did so you can put everything back the way it came.

 

Thank you for your help, much appreciated.
I will try to get a new CTS tomorrow and hopefully get it swapped out the same day.

Agree with you on that big nut, will try to sort one thing out at a time and put it back the way it was ;-)

 

Checked the temperature sensor in the left cylinder head and it reads 3400 ohms at cold engine, that is 22 degrees celsius.

Let the engine run varm and measured again, (it was a true challenge not to burn my hands on the exhaust manifold) and it measured approximately 200 ohms.
So I guess the sensor is ok, but it seemed that the connector was a bit corroded so I scraped some of it off and put it back on.
At least the service engine light is now off, but the engine still idles at 1000-1300 rpm.

I've made myself a tool for adjusting the mixture screws and will try to set them at a starting point to see if things change.

 

I managed to adjust the mixture screws all the way in and then 1,5 turns out. The vacuum level on the gauge dropped a little bit as well as the idle, but still runs erratically.

Next up will be to check for vacuum leaks.

 

The "idle mixture screws" on a computer Q-Jet have almost ZERO "authority". Which is to say, they do about as close to NOTHING as you can get, and it still be detectable as SOMETHING. But just barely. They're not the right tool for making that adjustment.

Put them at about 6 turns out and leave them alone.

There are 4 main adjustments to that carb that actually DO something: fuel level, lean stop, rich stop, and idle air bleed. They must be adjusted IN THAT ORDER. The reason why is, the first one affects ALL the others, the 2nd affects the 3rd & 4th, and the last affects nothing else but is affected by EVERYTHING ELSE. IOW, if you think "oh my idle sucks I need to adjust the idle air bleed", and get it as perfect as possible, butt then you have to adjust some one of the others for whyever, your IAB setting will now be OUT TO LUNCH, and you'll have to start all over with it.

Note also, that the ECM uses the O2 sensor output as its "eyes" on the fuel mixture. If that is fornicated then the ECM is blind or at least severely myopic. I'd suggest just launching a pre-emptive first strike on that before you start. Correct operation is IMPOSSIBLE except by an occasional raw accident if it's not telling the ECM the truth.

Fuel level is obvious. To set that, remove the carb's air horn and turn it upside down, and WITHOUT PUTTING ANY PRESSURE ON The NEEDLE VALVE, bend the little arms that the float is mounted to, until you achieve the desired distance between the top of the float and the surface of the casting, with the weight of the float resting on the needle valve. Ideally that distance should be around ¼". A taste higher than the factory spec in any case, but not so high that the float can ever hit the gasket. In fact if you have the kind of gasket that doesn't have a "window" above the float, cut one in it, so that it can't ever possibly interfere with the float.

Next, set the lean stop correctly. This is done by raising or lowering the entire mixture control solenoid assy, by way of the screw down in the middle of the fuel bowl. It is part 226 in this diagram. There should be a real stiff short spring under the MCS "frame", then the MCS, then the screw goes through the MCS, pressing it down against the spring. Needless to say, it can't work right if the spring is on top of the MCS; that will make the rods go about 1/8" or more too far down into the jets, and it will run like crap under any kind of driver demand for power. It should be set such that the primary rods just barely have their tips fully inserted into the jets, with the straight portion just behind the tapering tips just reaching the jet. Not the unmachined part of the rod; it's the same diameter most of the way down, then there's a step, then a smaller straight portion, then the taper, then the straight very small tips. You're looking to get that first machined but straight portion just barely into the jets with this setting. It's easiest to do this with the carb completely apart, and just the parts needed to accomplish it, installed; hold the MCS plunger thing (part 215 in the blowup) all the way down in the MCS with your finger, and set it to where you can see the right behavior; then screw that screw all the way down, counting the turns from where it was; remove it afterwards to properly reassemble the carb; then when reassembling the carb, put it all the way to the bottom again, and back it out to that same # of turns up from the bottom. It is also accessible with the carb on the car and running, through the small hole just in front of the air cleaner flange, if you knocked the anti-tamper plug out.

Next, the rich stop. It's about the size of a dime, screwed into the air horn. Part 72 in the blowup. It limits how far the solenoid plunger can retract upwards, withdrawing the rods from the jets, and closing the IAB. This one is tougher: you can't really set it with the carb apart. It's best to set it after it's fully assembled, and made to approximately idle correctly using the IAB. To adjust it, watch the signal on the bright green connector hanging out of the computer harness somewhere between the blower motor and the pass side valve cover, with a dwell meter. Ideally you want to eventually get the dwell (duty cycle) to about 50% or maybe a shade more, with the motor fully warmed up and running at around 2000 - 2500 RPM (well into the operation of the main system), so set it there via the IAB. You'll have to make sure the IAB is backed out far enough that it doesn't act as the limit to the plunger's travel: back it out until the dwell goes as high as it will, then run it back down until the dwell just barely starts to decline. Its "correct" setting will be where it gives about 30 - 35° on a dwell meter on the 6-cyl scale. Then adjust the rich stop inward (turn it CW) until you see the dwell start to rise, indicating that the stop is now too low and the ECM is fighting back, and the rods aren't able to withdraw fully from the jet; then back it off until the dwell bottoms out and quits going back down, indicating that withdrawing the rods any further has no effect because they've already reached the point where only the straight small tip of them is still inside the jets. You may have to fiddle with the IAB during this part of the process to keep it within range. This adjustment is in the small hole in that little "tower" feature just behind the air cleaner flange. Again, the factory covers it with a plug, which you should have removed.

Last is the IAB. once you have those other adjustments right, simply set that for the 50% or slightly higher (higher dwell indicates that the carb is richer than the ECM wants to target it; factory settings are optimized for emissions, which is too lean, which is why you want to set all of these adjustments just to the rich side of "factory optimum") with the engine at idle.





Once you have that done, you can go out for a cruise and finesse it. It should be close, within a half turn or so, on all of them. You can tweeek the lean stop for best cruising attitude: look for good throttle response with no surging or stuttering with a very light slow application of throttle during a cruise at about 1800 RPM in high gear; the rich stop for best response to the full application of throttle using only the primaries; and of course, resetting the IAB to 30 - 35° after every adjustment to anything else.

Note that AT NO TIME will you make any further change to the "mixture screws", because they don't really do anything. TBH I'm not really sure why the factory even put them there at all, their effect is so miniscule.

If the engine is still bone stock, you should end up with at least 18" of vacuum when you're through. Best (highest) vacuum should occur over a broad range of IAB adjustment centered on the point where the dwell meter reads 30 - 35°. It should PARTICULARLY start to drop when you adjust it such that the dwell is LOWER than ideal, indicating a lean condition, and the ECM struggling to richen it back up.

And of course, definitely make sure there are no vac leeeeks. That kinda goes without saying.

 

Thank you for your thorough answer sofakingdom. The knowledge of people on this site never stop to amaze me.

Is there any way to check the O2 sensor or should I just buy a new ?

 

No real "checking" method that I know of except replacement.

OTOH, think of it as something a bit like spark plugs... a limited-life component exposed to harsh operating conditions whose actual in-operation performance is difficult to measure, at best. Even modern electronic controls with hundreds or thousands of times the processing power of what's in these old things have trouble with it. A gross fault such as disconnection can be recognized; contamination or the like, not so much.

When in doubt, swap it out. (say that 10 times as fast as you can)

 

Thanks I will check if one of my local shops have one in stock or can get me one

 

Shouldn't be a problem; it's SOOOOPER common. Can't imagine it not being on hand. Maybe $25 - 30.