Too rich mixture on a 84 Chevy

Hi All,
I have a 1984 Chevy Celebrity station wagon with a 2.8L GM V-6 with a computer-controlled carburetor on it. The engine was rebuilt about 25,000 miles ago and passed
emissions inspection about 10,000 miles ago. I also replaced the EGR valve at that time and cleaned out the EGR passages (in order to get it passed)
A few weeks ago I bought a used OTC 4000E scantool with the idea to use this on both the 84 Chevy and a 95 T&C that I own. When I got the scantool I decided to make a baseline of both vehicles.
The 95 Chrysler came out fine, lots of fun with the scantool testing out it's features, lots of fun statistics and info from the engine and transmission off the CCD adapter.
When I plugged it into the Chevy, however, while I got lots of fun statistics and such out of the engine there, the one thing that caught my eye was that the scantool insists the engine is runing rich. It even has a red light on the scanner that illuminates marked rich.
Anyway, before I go digging, is there anything obvious that anyone remembers about these particular vehicles that I should check first? My gut feel is it's in the carb somewhere, as the carb was not replaced when the engine was rebuilt.
Other than that the Chevy runs fine, although it has a lot worse fuel mileage than I expected on a 2.8L, that is why I think that the scantool is right.
Ted
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[snip]
What is the MAP sensor voltage, key on engine off, and with engine idling? What is the MC dwell reading at: idle, fast idle (2200 or so), during light cruise at 40-45 MPH? What is the O2 sensor doing? Does anything change if you disconnect and plug the canister purge hoses (plug the carb side) What happens if you ground the MC dwell lead (green single bare connector underhood) mainly to O2 sensor and RPM? TPS at curb idle? Both choke vacuum breaks working? What color are the sparkplugs?
The early E2SE carbs had problems with the rubber tip of the MC solenoid pintle breaking off and then not having adaquate seal when energized to go lean, there is a test in the service manual for checking this with a mity-vac and a piece of W/W hose and some jumpers. The A/X body carb chassis in general were sensative to having the charcoal canister get saturated with fuel from people trying to pack the gas tank when filling up. The main metering system in this carb runs the M/C solenoid in parallel with an adjustable main jet, someone may have monkeyed with it. The idle mixture -is- adjustable and should be adjusted to around 35* varying slightly + - 5*. Carb gaskets can not leak at all, mainly the base gasket and the throttle body to main body gaskets, vacuum leaks here really wreak havoc. IIRC the base gasket was actually an electrical EFE heater grid, they liked to melt, many were replaced with a plain gasket, check the service manual as to what belongs there, the only way to cover up a failed/missing EFE grid is to over compensate with a richer mixture. Carb is somewhat sensative to float level, easily measured with a special gauge that drops in where the mesh screen is for the bowl vent on the carb top, or can be measured the conventional way once the carb top is off. Gauge should still be available thru Thexton, Lisle or one of the other specialty vendors. You have to use an AC Delco PCV valve for proper PCV metering, aftermarket PCVs only screw things up. Leaking needle and seat. Cracked fuel bowl or other monkey handed mechanical malfunction.
Although often claimed to be the left half of a Quadrajet, these carbs really were crap.
Oh yeah, ignore the little red light on the scan tool, the carb when set up correctly is biased slightly rich. Everything is working fine when the MC dwell is varying between 25 and 40 degrees under the conditions I described above.
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wrote:

marked
the
MAP sensor voltage is .52 with key on and engine off, going to 3.3-3.7 volts with key on and engine idling.

3 degrees at idle, about 20 degrees at fast idle, about 29-35 degrees during cruise

This is the odd one. At idle at about 625-650 RPM the O2 sensor voltage seems to stay under .20 volts, but it fluctuates throughout that range. But at anything above idle, ie: cruising fast idle, whatever, the O2 voltages are constantly all over the map. The scanner appears to get an update every second, and the O2 sensor can swing from .08 volts in 1-2 seconds, to .78 volts in the next second, then to .45 volts in the next second, then to .12 volts in another second then back to .52 volts in another second. And it was still doing this even after a 45 minute freeway drive.

The rest of these tests will have to wait until the weekend, unfortunately. But, is the O2 sensor reading normal? I'm kind of thinking that it isn't. Certainly the exhaust stream couldn't be that variant?

I have the service manual and I'll look this one up.

Now that one I know about as the shop that dropped in the rebuilt engine discussed that with me and he said the same thing. With my engine, when they separated the carb that heater/gasket was not in good shape, I guess the gasket must stick to the manifold and carb. But, the heater part of it still did work. So when they put it back together the mechanic did reuse the heater/gasket and added form-a-gasket sealer and such to make sure it was sealed well. He did warn me that any attempt to separate it again could destroy the heater/gasket.
When I cleaned out the EGR passages to get it through emissions, I did have to lift the carb. (the passages were completely plugged and I actually had to use a hammer and long wood drill bit with a sharp tip to clear the impacted carbon from the passages, I had a shop vac going in the manifold throat to catch any pieces) Fortunately though the sealer seemed to have the action of instead of the gasket being permanently glued to the manifold and carb, it actually came apart quite easily without damage. I did make sure when I put it back that the carb was fully seated. It also passed emissions then so I am pretty sure that gasket was sealing then.
Ted
Carb is somewhat sensative to float level, easily

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Okay, that's normal for your system which actually uses a differential pressure sensor instead of a MAP sensor.

Three degrees is too low, the computer is seeing a lean condition and commanding the MC solenoid to richen things up. If you can correct it to be 35 degrees by backing the idle mixture screw out, things should be okay. If not, start looking for vacuum leaks with the carb to manifold base gasket being the prime suspect.
about 20 degrees at fast idle,
Close enough to correct given the idle dwell.

Damn near perfect, a tick low but again, could be influenced by whatever is driving the dwell low at idle.

Expected given the MC dwell readings, shows that the system is trying to make a correction, 3 degrees (6 actually) is the absolute limit on a rich command.

Good, that's what supposed to be happening.

Your ECM is a very slow baud rate unit, what is displayed on the scanner is apt to lag behind anything you might observe live on a DVOM or labscope.

The GM CCC systems do not try to target a supposed -ideal- O2 sensor voltage, things swing around a bit just like they do in an EFI system. If the system goes lean periodically, the cat-con is assured a supply of oxygen, if the system goes rich periodically, the cat-con is assured enough HC to keep the fires lit. So, yes, the exhaust stream will vary WRT O2 sensor voltage, when it doesn't, that's when the spend money light gets turned on.

The problem with re-using the gasket and gooping it is that you can still suffer vacuum leaks thru the laminations of the gasket, but before you do anything else, try adjusting the idle mixture richer (out) to obtain a MC dwell of around 35 degrees slightly varying.

Shame on the shop that installed the rebuilt engine for not cleaning the manifold properly, it's in both of your interests that chunks of carbon do not break loose and jam a valve open, plus they should have verified the EGR operation to insure that the ring lands and bearings in your new engine didn't get the shit pounded out of them from the inoperative EGR.

I'd do a little sniffing with propane around the base of the carb to see if in fact there is a vacuum leak there. But honest to god, aside from the low MC dwell at idle, this thing is pretty close to spot on.
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