Car has been running extremely lean, even at idle. The car will slowly stumple to a stall when idling once hot. The MC solenoid buzzes always when idling, even after 30minutes and its fully warm. I was getting no check engine lights, until last night when the MC finally stopped buzzing after 20mins of idling and the SES light came on permenantly. Now the ALDL will not allow reading of codes with the jumper method. Is this signs of a faulty ECM? Most of the issues came at higher RPMs under load where it would hesitate and sort of ping. Plugs are compeltely white even at idle. Base timing is like 7*.

Help apprecaiated. This is a 1985 with a 350, CCC carb/dist. The ECm is likely the stock LG4 ecm/PROM. Anyone know the # for this LG4 ECM that I should replace it with, or a better one for my situation (all stock 350).

 

Seems like it to me. The fact that you can't get codes out of it is the most telling sign. The rest you might be able to explain with actual engine/sensor problems, but getting no codes out at all when you jumper the pins tells me she's cooked.

Grab a junkyard ECM (they ahve itnerchange manuals to tell which ones are compatible with your car- many cars used the same ECM just with different chips in them), swap your chip into it and see what happens. Last time I paid for one at the junkyard I think it cost me about $25. Cheap money for diagnosing problems like this.

 

Thanks Damon! Read the good news about your fam! Congrats!

I could get codes out prior to last evening. I was taping into the MC connector with a paper clip and reading dwell on the blue wire to see what the heck was going on with my MC. Then like 15mins of idle later the thing stopped buzzing and the SES stayed on. It will not respond. I am going to disconnect ECM and reconnect it (as well as re-seat the PROM). Also take down the #s on the ECM while I am down there.

I hate ECMs..... My camaro hasnt had any issues since the orignal tune with the Holley and Vac Dist....haha. Oh, man you shoudl see these sweet Recaro seats I installed last weekend!

 

Well it was a blown fuse that was keeping the ECM from being able to give codes and go into diagnostic mode. Now there are no codes, however the car is still hesitating & running lean. This happens most in 3rd gear when under load and trying to have it kickdown. Im lost on this one. Can an exhaust leak cause these issues? It sort of sounds like a nasty raspy noise through the exaust when it starts to hesitate when flooring it under load and at high rpms.

 

Well, I will be checking the dwell tonight. I would assume I am trying to see a dwell of near 30* and one that varies slightly when in closed loop. Would my dwell results indicate if my ECM is leaning the mixture extensively through the MCS? I wish I had time to go buy a used ECM for this thing so I can just rule out the ECM being messed up. What ECMs would work with LG4 sensors/carb that now has a 350 bottom end? Is there a list of compatible ones somewhere on Thirdgen?

 

I'd beg borrow or steal a scan tool- you need to see it how the ECM is seeing it. Just looking at MC dwell can be decieving if you're never getting into closed loop. The ECM throws a pre-set dwell map at the MC solenoid even in open loop. You might think "wow, I'm right in the middle of the range!" but only because it's running off the pre-programmed dwell map becuase it's never getting into closed loop.

ECMs are the same for a LOT of cars of that vintage. It was just the chip that was different. I'm talking 4 cylinder Buicks, 6 cylinder Oldsmobiles and 8 cylinder Chevys, etc. Only the programming on the chip was different for many of those engines- the ECM itself was the same. Junkyards have Hollander Interchange Manuals to know which ones interchange. Parts stores you just look up by application and they can tell you what you need. Putting your chip into that ECM is your responsibility (but very easy to do). You probably won't find a better chip than the stock LG4, besides maybe a same-year L69 chip. The system won't care much about the extra 50 cubic inches you got. Don't know anyone who makes custom chips for these old CC-carbureted engines and they probably wouldn't help much anyway- the chip is mostly there just to run the spark advance curve. The carb is 90% self-compensating for changes in engine airflow by it's nature, unlike an EFI setup.

Thanks for the kind words about the family. We're all real glad he's doing as well as he has been. He goes back to school next week after being out for treatment for almost a year.

 

Well, I purchased an ECM. Exact service number I have. Bought it on Ebay for 19$ shipped. Hope to have it by saturday. I dont have a scanner or access to one.

How do i know when the car is in closed loop?

 

Installed the new ECM and have the same issue, however there is now a code 21, or TPS Volt (hi). I guess I now need to perform the TPS checks with the multimeter. THe SES light came on once, and then went off.

Funny thing is the prom that came in the replacement ECM caused the car to surge to 1500rpm then down to like 350rpm then back up....constantly once it was warm. I stuck my PROM in the new ecm and this stopped.

 

Tested the TPS:

1) with car cold and ignition on: 5.0v on top wire, 2.66v on middle wire.

2) with car running and warm, choke off: 5.0v top wire, 2.5-2.6 on middle.

With the ignitoin on when i move the throttel toward WOT it adjusts the voltage upward fairly smoothly maxing out at 4.5v.

Does this need to be replaced or should i learn how to adjust it?

 

I think that is correct. If you want to force it into closed loop mode you can rev it above 2000RPM for 2 min. This forces the ECM into closed loop mode.