I just installed a new 355 into my 91 rs. I start it up and it idles like dog crap when it's cold. it idles like dog crap when it's hot. However when under load it will perform very nicely. When it comes up to operating temp the service engine light comes on. I scanned the system and found a code 42.
EST low voltage or
ESC faulty operation
I didn't get this code when I had the old engine in the car. I disconnected my EST to set timing and since then have disconnected my battery and cleared the codes. this one still comes up and only comes up when the engine reaches 220 on my temp gauge. I never played with the ESC. I did replace an eye-connector on a ground wire for the ECM that bolts to the back of the head on the left (drivers) side. it was a light tan wire with a black stripe. I suspect this is the ground for the EST connector but, I don't know for sure and I don't know if this could be causing the problem.
Could this be a false code due to a bad O2 sensor?
Keep in mind it runs the same in both open and closed loops.
Where do I connect my meter to find out if my EST connector really has low voltage? How much voltage is it supposed to have?
How can I check my ESC module to make sure it is ok?
Lastly, what does the ESC look like? Is it that flat, rectangular sensor located right in front of the relays with about 5 or 6 wires coming out of it?

Help.

 

It wouldn't be your O2. Your car will run okay with a bad O2 it will just use a lot of gas. I am not sure about the ECS stuff and will leave that to the professionals here. What set-up do you have? I take it this is a TBI set-up? If so how big is your cam and what kind of TBI unit are you using and with what tune. I hope you are not running on the stock 305 tune because if your are that is your problem right there. If you post the specifics of your set-up we can help you better. Sounds like a cool set-up so far.

 

it's a 350, 4 bolt, punched 40, cam ... I don't know. looked stock to me but, the guy I got it from said it's a fuelly cam. The car does lobe at idle but, I don't know if it's because he was telling the truth about the cam or if it is because of my problem.
moving forward: heads 64 cc, 1.94/1.5. stock aluminum 2-barrel intake from 305. stock TBI from 305. I wondered about the TBI because of the block bore and everyone I spoke to said to not worry about bore, and that it depends on what size my new heads are. they are 64 cc just like my old ones. only difference is that the old heads were 1.84/1.5 and the new ones are 1.94/1.5. I told them this and they said it shouldn't make a difference. the stock setting and tune of the TBI should work just fine since I am also using the stock intake from the 305.
I think that's pretty much about it.
Under load the thing performs like a champ. it just has a very rough, lobing idle and the Check engine light comes on after 220 throwing a 42.

 

That TBI unit and 305 tune is killing you. Your car will never run right. A 305 with heads and cam won't even run right with stock tune. So the stock setting and tune is exactly your problem. What is the LSA on that cam. Anything lower than 114 is harder to tune. Your cam may be too big to tune for. After tuning you will need to get rid of that TBI pronto. The intake is killing as well.

 

The code is probably being caused do to your 305 esc module. You need to change that and the knock sensor when swapping to a 350. You can find the part numbers by doing a search on this board.
As far as the idle goes-it's a combo of things, mainly your current chip. Like shifty said, the stock tbi and intake are killing your performance more than likely as well, though I'm not going to tell you to put a carb on there unless it turns out that the cam just doesn't make enough vacuum to keep the tbi ecm happy.
Also, if you are sick of that SES light coming on at 220*, put a 170* thermostat in there. Actually, I recommend you do that anyways.