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13K views 54 replies 8 participants last post by  Ian6 
#1 ·
Ok, so I inherited this car. It runs good, only has 63,xxxx miles on it. But the thing is eating gas, like 10mpg. I know it's an Olds 307 v8, but everyone is telling me it should be decent on gas. So, I changed the plugs, wires, coil, etc and it runs better. But now it seems to chug while idling and sometimes it'll diesel on me when I turn it off (motor keeps running for a few cylinder compressions). I had a friend look at it, he's a mechanic at a dealership, and he noticed that the carburetor is letting some gas out around the front (think he said the pumper, not sure though) and that the "fuel enrichment solenoid" was unplugged. He told me not to plug it back in because the carburetors are junk on these vehicles and it's probably unplugged for a reason. His advice was to either drive it or buy an aftermarket carburetor, Edelbrock etc, and put it on and disconnect of most of the emission vacuum lines etc. I'm not sure what to do here or if the carburetor is even the reason for the bad MPG. I'm thinking the timing is off and it maybe running rich. The check engine light is also on, I have another friend who thinks he can scan it or manually get the codes to see why it's on. I've worked on a lot of cars, but not many with carburetors, especially electronic carburetors. Anybody have any advice? Thanks in advance.
 
#3 · (Edited)
The first thing you have to do is scan the computer for codes. In the OBD-I reference thread, my post (#7) gives information on how to scan your car.

Other things to check are the Air Cleaner Damper door (THERMAC) operation, Air Filter for clogging, the Vacuum Hoses for leaks, splits, and correct roughing (as seen on the Emission Control Label), Ignition Timing, Electronic Spark Timing (EST) for correct operation, Transmission Torque Converter Clutch (TCC) for correct operation, and check for a restricted exhaust system.

Putting on an after market carburettor may solve your immediate problem, but the engine will never meet the factory fuel economy.

The problem could also be a dirty carburetor and pistons covered in carbon build up. I would recommend using Gumout with Regane on your next fill up (you can get it at any part's store/Wal-Mart) and/or using Sea Foam motor treatment to clean the carburettor and engine.
 
#4 ·
Ya, that is what I was thinking. I'm getting different opinions from people though. Basically I was going to keep this car as my daily driver, but I need to figure out this MPG problem. The carburetor is clean and as far as I know it has only had premium gas ran through it. I'm want to just plug the solenoid back in, but I'm afraid it'll screw the whole thing up then. Not sure what to do yet.
 
#6 · (Edited)
Try the Sea Foam through the Carburettor, it's more consecrated so it works better.

That solenoid that's unplugged, which side of the engine is it on? If it's on the driver's side and it toward the front, then it is the Throttle Position Sensor. If it is on the passenger side, then it Is the Mixture Control Solenoid. If the mixture control solenoid is unplugged, then it goes to it default position; Full Rich. It'll come up as code 23 if the MC is unplugged and 21 if the TPS is unplugged.

The mixture control solenoid plug keeps falling out on my car (I have it zip-tied in place), so it could have done the same thing to your's. Plug it in and see what happens.
 
#8 ·
I ran it through the carburettor, but didn't stall the engine out. Let the engine warm up to operating temperature and then you're going to need someone to hold the RPM's up a little while you pour half a can in the carburettor, and then you're going to need to drive the car for about a half hour to clear the engine.
 
#10 ·
Sure can.


A guy here at work told me to try something like this:

"The best way to clean carbon build-up out of your engine is NOT to take the car out and run it as hard as you can. This only results in meetings with local law enforcement personnel, and doesn't do much for cleaning out carbon.
If you really want to clean the carbon out of your engine follow these simple steps. Fill an empty soda bottle with water. Remove your air cleaner. Start your engine. While using one hand to control the throttle, trickle the water into the carb or throttle body with the other while holding your thumb over the bottle opening. Keep the engine rpm's up so that the engine doesn't die. Make sure that you don't pour the water in too fast! Water doesn't compress and if you pour too much water in, severe engine damage can result! White smoke from the exhaust is normal. If you have a port fuel injected car, you obviously won't be able to use this method, but I have had success with connecting a vacuum hose to a port at the base of the throttle body and sticking the other end of the hose into the bottle of water. The vacuum of the engine will suck the water in, creating the same effect. This will clean out even the most stubborn carbon, by 'steam cleaning' your cylinders."

Or to run transmission fluid through the carb and brake booster line. This guy knows about cars, but he's kind of a hillbilly, so sometime his "methods" scare me.... Would any of this work?
 
#11 ·
I don't know about transmission fluid but I've steamed cleaned the engine, using the method you described, and it worked somewhat. But the problem (rough idle, misfire, and dieseling/run-on) came back a few days later. The Sea Foam follows the same method of introducing the stuff into the intake, but the problem stayed away for over a month. Also it's kind of awkward to use one hand to increase the RPMs while trickling a specific amount of fluid into the engine with the other (plus the engine has to be warm so your hand can get pretty hot).
 
#12 ·
t the "fuel enrichment solenoid" was unplugged. He told me not to plug it back in because the carburetors are junk on these vehicles and it's probably unplugged for a reason. His advice was to either drive it or buy an aftermarket carburetor, Edelbrock etc, and put it on and disconnect of most of the emission vacuum lines etc.
Your car is in loop mode, with the Mixture Control Solenoid not hook up it will drink gas. If you did put a Edelbrock on it, you will have to replace your CCC distributor to a vac advance one. The q-jet is a great carb. The main venturis are small and give you good MPG. Hit it to the floor and the huge secondary venturis open up. And most were 750cfm.
 
#17 ·
Ok, so I just got home from work. The solenoid that was unplugged was on the passenger side, so it's the Mixture Control Solenoid. I plugged it in and started it and it just instantly died, then wouldn't even attempt to start. It just turned over. Does this mean to just replace the solenoid, or is there a bigger problem?


There was also another plug that wasn't plugged into anything and I couldn't find what it would got to. It was a green plug with a clip, it didn't have a rubber shrouding on it like the one for the Mixture Control Solenoid. It was taped over, so maybe it is for an option I don't have?

Thanks for all the help.
 
#21 ·
Chugging can be caused by anything from vacuum leaks, to off timing, to poor float adjustment, to contaminated gasoline.

I looked in my service manual and found very little information on idle adjustments. I found a description of the Mixture Control Solenoid that said it is pulsed on and off ten times per second, and then when it's getting power, it forces the mixture Lean. There are adjustments for the Mixture Control Solenoid, the Idle Mixture Needles, and the Air Bleed Valve in my manual, but they are really vague. They don't mention any special tool, but I know you need a gauge, gauge (to determine the proper spacing) and that you need a 6 cylinder dwell meter to see what the computer is doing (you use the green plug and turn the idle screw to obtain a dwell of 25*).

These computer controlled carburettors are not user friendly. To change the Mixture Control Solenoid, you need have two special GM adjusting tools. Being that you're not fluent in carburettors, I recommend you take your car to a GM dealership and have them professionally diagnose the problem and replace the Mixture Control Solenoid.
 
#26 ·
It's a very good carb. You just have tune it. Their are alot of videos on youtube to help you. If put your in test mode to read codes it should be clicking while the car is reading out codes. Also on my 84 electra i have 2 ECM fuses. See if your car has both, and of course make sure both are good

[YOUTUBE]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/obeiD_-gWLc&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/obeiD_-gWLc&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YOUTUBE]

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#25 ·
I called one shop today and the guy actually told me, "If you got it running, I wouldn't even mess with it. They're not worth it." This car only has 60 some thousand miles on it. I plan on keeping it. I talked to a guy here in town who knows about the car and he told me that it probably quit running and a local mechanic just unplugged the solenoid because the owner wouldn't know the difference.
 
#27 ·
Here is the procedure for changing the solenoid (procedure begins with the solenoid plunger removed):

1. Remove the air horn and gasket

2. Remove the solenoid connector attaching screws

3. Remove solenoid adjusting screw using J-tool 28696-10

4. Remove the primary metering rods and springs

5. Remove the float bowl

6. Remove the Mixture Control Solenoid

Make Mixture Control Solenoid adjustment using the following procedure (found the specific procedure in my book):

1. Position J-tool 33815-1 (gage) over the throttle side of metering rod jet guide and temporarily install solenoid plunger

2. Install solenoid adjusting screw and rich limit stop (engage the first 6 screw threads)

3. Hold the solenoid plunger down. Use J-28696-10 to turn the solenoid adjusting screw in or out, until plunger contacts both the solenoid stop and gage

4. Turn screw inward, counting turns to the bottom

5. Remove solenoid adjusting screw, rich limit stop, solenoid plunger, and gage

6. Install float bowl

7. Install primary metering rods (do not force them; damage can result)

8. Install solenoid plunger, solenoid adjusting screw, and rich limit stop

9. Use J-28696-10 to turn the screw inward to bottom. Back out number of turns counted in step 4

10. Install air horn with new gasket

Following a Mixture Control Solenoid replacement/adjustment, the Air Bleed Valve must be adjusted. The procedure is a s follows

1. Insert J-33815-2 gaging tool through air horn vent hole, to contact solenoid plunger. Locate upper end of gage over cavity next to valve

2. Hold gage down, so plunger is against solenoid stop, then rotate gage toward the valve. Turn valve in or out until gage contacts top of valve

3. Remove gage

4. Check idle mixture with the engine running

Idle mixture check Procedure is as follows:

1. Set parking brake and block drive wheels

2. Remove the air cleaner and plug the vacuum lines to the THERMAC

3. Connect a tachometer to the distributor and a dwell meter to the green M/C solenoid dwell connector

4. Check ignition timing and adjust if necessary (ask me for the procedure later; I'm tired of typing)

5. Disconnect and plug canister purge hose at canister

6. With engine running and at operating temperature, transmission in drive, and AC OFF, observe idle dwell. It should be between 10 and 50 degrees and varying. If idle dwell is fixed below 10 or above 50, STOP and determine cause of lean of rich condition. Adjust idle mixture if necessary (ask me for it)

7. If idle dwell is varying between 10 and 50 adjust the Idle Load Compensator if necessary (ask me for the procedure)

8. Reconnect all vacuum hoses and install air cleaner
 
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