4bbl swap on NA 3.8L, MC solenoid problem

[84ZZ4]

Finally finished my 4bbl swap into my Oldsmobile. I'm now running an E4ME Qjet instead of the original dualjet.

The motor doesn't really want to idle down, I'm sure I have a vacuum leak, but other than that it runs good at part and full throttle.

While watching the scanner, however, I noticed the the MC solenoid duty cycle is doing some WACKY stuff.

It'll sit at 5% at idle (probably cuase of the vac leak) for a little while, then it will suddenly jump up to 54% and sit there for a few seconds, and then jump right back down to 5%. You can hear the motor change idle speed and sound when it does this, so I have to think that it really is happening. The motor also wants to stall when it's at 54%.

Why the HELL is it doing this? the O2 sensor is still reading somewhat lean when it's sitting at 5%, so there should be no reason in the world why it should suddenly decide to pull fuel. When my Camaro was in a similar situation, it sat at 5% and eventually tossed a lean exhaust code, exactly how it should behave.

Also, the ECM does not throw a code when I disconnect the MC solenoid connector . It will throw one for EGR malfunction though .

I'm leaning towards it being something wrong with the ECM. I've never seen one of these cars randomly change the MC dwell like that without falling out of closed loop.

 

[86cut]

If the ecm was working properly before the swap, then I doubt the ecm would be the problem now. I went throught he same exact problem, actually . Make sure you have the mixture needles adjusted correctly and the idle air bleed set correctly. BUT FIRST FIX THE VACUUM LEAK!!!!! or you will get nowhere.
The idle air bleed (IAB) works in conjunction with the mixture needles. try setting the mixture needles about 3- 31/2 tuns out from lightly seated, then adjust the IAB a few turns out and get it as smooth as you can. Just be patient and wait a few seconds after each adjustment for the ecm to learn.
Also, try unplugging the m/c solenoid and then see if it idles steady- if it doesn;t then you probly have a vac leak.
P.S.> make sure your throttle return spring is strong, mine wasn't and my idle would race at times b/c the throttle wouldnt return all the way.
If you need more help, just email me- R.ck@verizon.net

 

[84ZZ4]

Well, surprise surprise, it was the computer.

I fixed a WHOLE bunch of problems by fixing the vacuum leaks, epoxying the bowl plugs on the Qjet, and replacing the ECM. Got everything running great except at WOT. Right now it's bogging occasionally and backfiring out the intake on rare occasion. I'm running an N hangar and CN rods. Someone following me said the exhaust smelled rich.

I'm thinking the backfire might be related to timing, the base is at 10* and the emissions label clearly calls for 15*. I'm going to fix that and then see where things are.

It was a HUGE power gain, far more than I expected--maybe a good 20hp. That'll teach me not to listen to the people on here.

Of course, my TH-250C is less than pleased with the power increase. It's only been back on the road a week and the fluid smells burnt. I found a TH350C in an 82 Olds Delta 88--planning on picking that up to replace the 250 when it dies.