I have an '86 Monte Carlo LS with a 305. It runs very rich. There are puffs
of smoke from the exhaust and it smells pig rich.
I have put the following new components on the engine: plugs, wires, cap,
rotor, O2 sensor. I have also bought a carb kit and thoroughly cleaned out
the insides of the carb. It still runs rich.
At idle if I lean out the idle mixture screws it runs rough, though smells
less rich and less smoke. If I put the screws to where I get smoothest idle
and highest vacuum it smokes out the exhaust and smells very rich. There are
no vacuum leaks.
It may be a problem with the carb, I think. Perhaps I need to readjust the
lean mixture stop? I know in a Quadrajet the idle circuit runs off the
primary jets. Perhaps if the MCS is not adjusting the main jet orifice size
properly it would result in a pig rich idle and all around rich condition. I
would greatly appreciate any advice or thoughts on this. Thanks.
I have set the idle to read between 30*-35* on the dwell meter's 6 cylinder
scale when attached to the green test connector in front of the carburetor.
Is this correct?
With that setting it still idles rich, and slightly rough. It idles
better/smoother and less rich smelling when I advance the timing beyond the
factory 0* specification. At the moment though it is set at the factory
I had a similar problem and I bet the solution would have been buying a
good carburetor. Instead, I experimented with an RC cirtuit to control
the MCS and finally tweaked it so it has full power, no lag and is not
as rich. I originally took the carb apart due to a heavy float.
Find the (charcoal) canister control valve, it's a black plastic
device with 4 hoses running to it, IIRC, it is usually mounted
above and to the center of the alternator bracket, remove it from
the car and shake it, if any gasoline comes out of it, replace it.
Common failure part.
No gas came out, so I would guess gas isn't getting sucked in through there.
What else should I check?? I'm really getting frustrated with this thing. I
should work. I've got the lean and rich MCS stops adjusted like you said so
at a higher RPM it reads 30*-35*. I have the idle mixture screws and air
bleed screw adjusted so at idle it reads the same. Still though it smells
rich throughout the RPM range and the idle is rough. I don't understand.
I tried pulling each plug wire one at a time to see that all cylinders were
firing. They are all firing, though #8 is slightly weaker than the rest, but
I can't imagine that should make such a big difference. Maybe a slightly
rough idle if anything but shouldn't make it pig rich. The car runs nice and
smooth and drives nice once you get going, but idle is rough and I can't get
it smoothed out. If I unplug the MCS and the distributor plug it will idle
more smoothly. I'm feeling like I've done everything I should for it to run
well, but it still doesn't.
I check the voltage from the O2 sensor and it bounces around from .3-.6 and
often bounces up to .7-.8 briefly before coming back down. Also, the air
pump is not pumping air to the manifolds once it's in closed loop so it
shouldn't be getting a false lean reading from the O2 sensor, and from the
O2 sensor readings it is indicating rich if anything. I don't get it.
I suppose that is a possibility. It would certainly make setting the float
level to factory specs inaccurate. The float is one of the solid rubber
types. I never understood why they don't just use brass floats from the
factory. I've never had a problem with them getting sunk/fill or otherwise
out of whack like the solid ones sometimes do and like the plastic ones
always do. I was getting desperate so today I set the float slightly lower
than the specs called for. It didn't seem to make a big difference.
At least the car didn't stall today after my other adjustments. It's only
gone 55 miles and it's already through half a tank of gas. That has included
a good amount of idling in the driveway, but that seems ridiculous. I figure
the tank is about 18 gallons, so assuming the sender is reasonably accurate
that puts it at about 6 MPG. Let's say I burned a crapload of gas in the
driveway and give it 10 actual MPG. That's awful. I've never even seen that
bad of mileage in my Galaxie! The computer is setting the mixture to be so
rich, and I can't for the life of me figure out why that is. If the float
level was high or anything causing the mixture to be rich the computer
should at least be reading a rather high dwell from the green test
connector, but it is right around 30* which I understand to be a neutral
point. More and it is attempting to lean the mixture, less and it is trying
to richen the mixture. The O2 sensor trends up and down of the .45v stoich
mixture reading, but often jumps to .7v-.8v which is rather rich.
I guess what I'm getting at is if the float were too high causing a rich
mixture would the computer not be trying to compensate and give a high dwell
reading?? Shouldn't it be giving a high dwell reading if for any reason the
mixture is rich and it can't bring it back down to where it should be using
I'm struggling to understand this thing. I thought I understood it before,
but it is not behaving as I understood it to from the advice I have been
given and from waht I have read. I think I may try pulling the carb off
again and epoxying the fuel bowl plugs which are said to be prone to leak. I
think I may be grasping at straws here. If I can't come up with something
I'm tempted to pick up an older quadrajet and distributor. I'd really like
to get it running properly as-is though. I was hoping for upwards of 20 MPG
on the highway and about 15 around town. Unfortunately this damned computer
is fighting me every step of the way.
No single control on the ccc sytem could be that at fault to cause that much
fuel consumption, the mcs and o2 sensor cannot correct or cause such a
severely rich mixture, another thought here, your coolant temp sensor could
be bad and sending a very low temp reading to the ecu, but normally that
would prevent it from going to closed loop. Seems like an internal carb
If the MC dwell is varying at fast idle with a fully warmed up
engine, the computer has control of the mixture based on O2
sensor input. I don't trust smell of the exhaust, rich and
missfire are too easy to confuse. Plug color on the other hand,
The EGR valve could be leaking, #8 is the closest to the EGR and
will usually be effected soonest from a leaking EGR valve.
The O2 voltage -should- be rolling high to low.
Rich would be the O2 stuck high.
Check the EGR, check and make sure that the MAP sensor is
properly connected, check and make sure that the TPS voltage is
correct. Make sure that the magnet in the pole piece in the
distributor isn't cracked, this could send extra RPM counts
making the ECM 'think' that the engine is running faster than it
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