87 Dak feedback carb--help, I'm drowning

Yes, I'm drowning in sorrow for my ailing 87 Dak... and because I have to pass a smog test
before the end of the month.

Where to begin?? Well, it started with stalling at stops, first when stopping going downhill,
then stopping level, then slowing down with foot off the accelerator. Adjusting the idle speed
higher would cause the engine sometimes to surge at idle and other times to idle at idle.

I then rebuilt the carb. This is when I discovered the feedback solenoid. I got up to speed on
the system through cruising around the net and learned some diagnostic techniques, including
measuring the duty cycle of the solenoid with a high impedance meter, which I went out and
bought. I also have the Haynes manual -skimpy- and bought a subscription to Alldata's online
manual -less skimpy.

So here is what I have discovered. The solenoid works okay. It responds to 12 volts as it should.

When reading the duty cycle from a cold start, the computer begins controlling the solenoid
immediately and tends to close off gas flow through it as the engine warms up. The duty cycle
percentage starts below 10 and gradually increases to over 90. In between, the duty cycle
fluctuates around 50% for a short time.

This means that the solenoid jet starts mostly open and ends up mostly closed. With foot off the
accelerator, the scale goes to under 3, meaning the solenoid jet is fully opened. This on a zero
to 100 scale. When in warmup mode, I have no drivability problems

At some point after about 10 minutes, the duty cycle goes to 20% and sticks there, plus or minus
1/2 of one percent and this is when the stalling at stops occurs.

One would think that the computer

1. is trying to compensate for a stuck lean condition
3. has fallen back on a limpalong mode
3. is broken

When this occurs, disconnecting the feedback solenoid cures the drivability problem completely.
No stalling, no surging and more power than the damn thing has ever had. I could live with that
except for two reasons: smog test and compulsion to fix things.

Along about this time I sprung for a new O2 sensor, seeing as I could not recall ever having
changed it and I am the original owner. No change in symptoms. I hooked up the meter to the O2
sensor to see what it was reporting to the computer in both "solenoid on" and "solenoid off" mode.

The readings were very similar, if not the same: very high voltage output, about .95 volts, with
the throttle open and very low, almost none with the throttle closed. Of course, these readings
are after warmup. From a cold start until the duty cycle of the solenoid sticks at 20%, the
voltage does fluctuate in a midrange as if the system was attempting to adjust the mix. But this
only lasts until warmup.

The air pump and switch valve appear to be working correctly. I clamped off air to the upstream
and nothing changed so I unclamped it. The engine temp sensor works okay.

Okay, I will end here. If you have any insight, I sure could use some.

Jack

 

I think I missed the original post,,, SBC doesn't keep things around very
long,, sorry but that said...

,, all those readings are telling you is what is going to the solenoid,,
they do not tell you what the solenoid is doing with them..I would suspect
the carb is dead.

Had one of these on an old Horizon unhooked it (the solenoid), ran fine,,
hook it back up, buck, stall , backfire, can't accelerate, replaced said
carb with new Holley (2 stage 2bbl),, think it was about $200 back in those
days,, worked fine for about another year or so when it started the same
crap,,..

Dumped the car.... buretor and bought something I don't think they sell
anymore,, was a bolt on Holley throttle body that was self contained, that
whole kit cost about $300 (from memory,, or what's left of it) and ran it
some more,, don't remember how long, but it was quite a few miles..passed
emissions here in Illinois.

Me thinks your carb is toast,, just an opinion...kind of like getting a
cancer diagnosis over the phone,, probably not overly reliable....
but,,thanks for taking the time to read it anyway...LOL

Ted

 

Well thanks for taking the time to reply, Ted. You didn't miss the original post. That's all
there was.

I can't believe the carb is dead. The solenoid, maybe, but I clean and rebuilt the carb with a
rebuild kit and there didn't seem like anything that could die except the solenoid and maybe the
float.

You are right about one thing. I don't quite know what the solenoid is doing with the juice that
the computer is sending to it. When I had the carb apart, it worked okay when I fed it 12 volts.
I even unscrewed the jet on the solenoid an rebuilt the plunger. Tricky task, that one.

I guess one way to get an idea what is going on is to rig up some kind of way to listen to the
solenoid when it is getting signal. Unfortunately, that only seems to happen when the truck is
moving. and I'm not about to ride the front bumber with a stethoscope

Jack