87 Dakota stuck in open loop?

The "lean burn" computer on is not capable of keeping up with this odd fire engine with some miles on it. There was a reason dodge went to throttle body fuel injection the next year and multi port soon after. I spent many years tiring to keep mine running well. My local dodge dealer drilled out the idle adjustment plugs to get more adjustment and that helped, but what really made the difference was eliminating the computer and getting a mopar performance distributor and a accell super coil. That made it run great til I sold it at 175,000 miles.

KS

stops, diminished power,

the fuel mixture

better than new! seriously

duty cycle when engine

the system is stuck in

also checked it by

engine. Closed loop

see what sensors

many on this vehicle.

closed loop mode and is

Hi Kevin,

I feel that I am getting tantalizingly close to understanding what is going on with this setup.

I confirmed that the mixture control solenoid in the carb is working properly. I removed the worn rubber grommet on the end of the plunger and replaced it with a suitable piece of plastic.

I then disconnected the battery to drain the computer memory and hooked up the duty cycle meter again and started it up. This time, the readings were different. The dwell slowly increased as the engine warmed up and fluctuated with rpm change like it is supposed to. Readings swung about the 50% mark like they are supposed to. This hadn't happened before. Then, after about ten minutes, it stuck at 20%. The dwell reading always went directly to 20% after warmup and stuck there. This seems to be pointing to the fact that someting is throwing things out of whack as the engine reaches a certrain temperature. The only sensors that I know of that affect fuel mixture are the engine temperature switch which tells the computer that the engine is warm enough to go to closed loop operatrion, the O2 sensor and switch portion of the switch/relief valve which sends air fromm the air pump into the exhaust manifold when the engine is cold. This fresh air causes the O2 sensor to read a lean mix and compensates by enriching the air fuel mix for better running while cold.

I just replaced the O2 sensor ($30) and I am waiting for the engine to cool down, have disconnected the computer and I am waiting for the engine to cool down to see if the new O2 sensor has any impact on the problem.

I clamped shut the hose that sends air to the exhaust manifold yesterday, but that did nothing to change things. That was before I got the idea to drain the computer memory. If the new O2 sensor does not make a difference, I will drain the computer memory and clamp the air hose to the exhaust manifold again. If the problem turns out to be a faulty computer, I have a line on a "remanufactured" one from Standard Motor Products through rockauto.com for $99 plus core.

But first I want to exhaust all the other potential causes.

My ace in the hole will have to be something along the line of what you did. The odometer just flipped over to 100,000 miles the other day and the damn thing is in GREAT condition otherwise. It just needs to pass California's smog test, which just got more stringent since the last time.

Jack

Kev> The "lean burn" computer on is not capable of keeping up with this odd fire

Jack,

I'm not 100% sure but I don't think the mixture control solenoid and the computer will do much for idle emissions if thats all they test.

Kevin

going on with this setup.

properly. I removed the

suitable piece of plastic.

the duty cycle meter

slowly increased as

to. Readings swung about

after about ten

after warmup and stuck

things out of whack as

that affect fuel

the engine is warm

of the switch/relief

the engine is cold. This

enriching the air fuel mix

see if the new O2

but that did nothing

memory. If the new O2

clamp the air hose to

computer, I have a line on a

$99 plus core.

did. The odometer just

condition otherwise.

stringent since the last time.

Kevin,

This year California made the test more rigorous. Vehicle is put on a dynamometer and tested at

25 and 40 mph in addition to idle. Otherwise, I think you are right. At any rpm, as long as there is no load and no movement, the system reverts to open loop and feedback mechanism is bypassed.

I've made various measurements under various conditions and the results are interesting.

First of all, after complete warmup, with the feedback solenoid connected or unconnected, the new O2 sensor reads at the upper end of its voltage range, .95 volts, when the throttle is open even a little, which is indicative of low oxygen content in the exhaust gases which points to a stuck rich condition. When the throttle is closed, the O2 sensor reading is at the low end of it's range, almost no voltage at all, indicative of high oxygen content which points to a lean mix.

The funny part is, when the truck is moving under load and the O2 sensor reading is indicating a rich mix, the duty cycle of the feedback solenoid reads only 20% indicating that the computer is responding to a lean condition and is feeding more gas into the mix. This is where I get a drivability problem in that the truck stalls when coming to a stop or even just slowing to round a corner or driving in stop and go traffic. If I set the idle higher to compensate, the engine will sometimes race at idle going up to 1500 to 2000 rpm. BUT, with the feedback solenoid disconnected and FULL gas flow through its jet, it drives with no problems what so ever. Idle can be adjusted for consistent rpm at a reasonable rate and the engine never dies.

The only way that the computer can change the air/fuel mix is through the feedback solenoid. The only other major thing that the computer does is to advance the spark when needed using an intake manifold vacuum sensor in the form of a diaphragm.

The warmup phase is another story altogether.

Jack

Kev> Jack,