I have been trying to check the MCS with my dwell meter. All I get is a reading fixed at 10-11 on the 6cyl scale.

No matter what I do with the throttle plate screws or the IAB, it will not change the reading.

Do I have the dwell meter hooked up wrong? I connect green clip from my dwell meter to the green plug over by the A/C. Red(+) to battery (+) and Black to ground. I have also tried with the (+) hooked to the MCS wire by backprobing it and still get the same reading.

If I understand it correctly, this low reading indicates a rich condition? The idle is dead steady @ 750 and I've already checked for vacuum leaks, have none.

Here's the set-up to help: '87 CC Quadrajet, 350 w/ AFR 190 heads and a Weiand Stealth intake. Had to use an adaptor for the carb to fit the square-bore intake, it's about 3/4" thick.

Any suggestions would be greatly appreciated.

Tim

 

^^

I could use any help or suggestions......

Tim

 

The dwell meter you describe is usually used to measure ignition dwell. Ive never heard of it being used to measure dwell of the mcs. So i dont know what a good reading would be
Are you having bad fuel economy/drivability problems? What kind of readings are you getting from the o2? The mcs isnt really a good place to check for rich/lean readings as it will only tell you what the cpu is trying to do and not how the engines running.

 

Actually, a dwell meter will measure the dwell angle of anything; ignition, carb, injectors, whatever. Think of it as a "duty cycle meter".... all it's doing, is measuring the percentage of time that something is grounded vs the remaining percentage of time that it's open.

Anyway, the dwell should change, in fact should be constantly changing. You should be able to adjust the IAC and watch it go from near 0% to near 100% (60° on the 6-cyl scale). Something is not right.

The ECM grounds the MCS lead more of the time (higher dwell angle) to lean the mixture, and less of the time (low dwell angle) to richen it. If you're seeing a constant low number, then the ECM thinks the mixture is lean. If the engine is running right, then it will ordinarily be somewhere near 50% (30° on the 6-cyl scale of the dwell meter) when just sitting idling. I like to adjust them so that their equilibrium point is a little lower, like 20-25°, so that the carb has more room to go rich. It's entirely lean enough most of the time when it's all working right.

Sounds rather like it's stuck in open loop.

Does the Check Engine light work? Check for codes.

Try replacing the coolant temp sensor.

 

You may have the polarity wrong. Try swapping the wires and give it another shot. Some meters are affected by polarity, some aren't.

 

I also think it may be stuck in open loop. The dwell meter came with instructions on how to hook it up for this application so I know it's correct.

Is there any way to determine if I'm in open loop without a scan tool?

New O2 sensor and CTS hasn't helped. My CTS is in the water neck, could a stuck thermostat prevent it from heating up and cause the open loop? My upper radiator hose doesn't get very stiff and I'm think the new thermostat may be sticking or the cap isn't holding pressure, but the temp is fine so may not be either.

Any suggestions?

Thanks
Tim

 

Get it fully warmed up. Hook up your dwell meter. Then manually close the choke plate a little (or hold your hand over the primaries for a similar effect) and see if the ECM reacts by changing the dwell as you manually "choke" the motor to force the ECM to try to change the mixture.

 

I got the dwell to read. Turned out to be bad wire. So fixed and now the dwell is about 45 on 6cyl. scale. So here's what I've tried:

Idle mixture screws 5 turns out

IAB 4 turns out

The dwell is fixed at 45 with no effect from adjusting IAB. I can get it to lower to swing from 30-35 but the car doesn't want to idle worth a crap. And the dwell is very sensitive to the Idle mixture screws. I thought they didn't affect it much from reading but mine does.

I got a 45 code last night while test driving so I think it I re-adjust the lean/rich stops maybe that will help. I had the lean stop at 4 turns out and the rich stop has to be all the way up to achieve 4/32nd's travel, so I'm thinking turning the lean stop down some and then adjusting the rich stop to still be 4/32nd's and try to adjust dwell again.

Any suggestions are more than welcome. And thanks for the replys already posted.

Tim

 

I had to set my idle screws at 4 turns out. To get the IAB to do something to the dwell. I found that out because the dwell wasn't moving at idle so i pulled the throttle slowly back by hand and then pausing (repeat), and the dwell began to move. So I knew that the dwell meter was hooked up right and the adjustments were off slightly. Try to start the IAB at the very top and turn it quarter turns and the wait a full minute until you see the ECM will start adjusting the dwell.

my 2 cents.


https://www.thirdgen.org/techbb2/sho...well+AND+turns