I've done a lot of work on my '82 in the short time I've had it, but there's still a problem. I'm hoping someone might have an idea for the most likely culprit(s). Here are the symptoms...

When I first start the car cold, it begins to surge. Engine RPM will run up to about 1,100 RPM then fall until it nearly dies, then repeat. Each cycle takes about four or five seconds. It will do this for about half a minute, then it stops surging, settles to a reasonable cold RPM (about 800) and starts to chug. The whole car shakes when it does this, just like it's got a couple dead cylinders on one side or the other. Also, the exhaust smells very rich. After three or four minutes like this, something fires off in the system and it suddenly starts running smooth and right. I can drive the car as long as I want after that, and none of these symptoms return.

So far, I've replaced the following:

Fuel pump ('85-'87)
Fuel pump strainer
Fuel pump relay
Fuel filter
Oxygen sensor
Intake plenum gasket
TBI base gaskets
Distributor cap & rotor
Spark plugs
Vacuum hose, intake to PCV
Vacuum hose, TBI to MAP sensor

I haven't replaced the TPS or CTS yet, but it's not clear to me that either would cause this problem. I also haven't tried blocking any of the vacuum lines. I'll just add that heater controls and headlights work OK. The vacuum canister for the headlights was removed a long time ago and replaced with a three-way tee.

So, what would be a good "to do" list at this point?

 

Surging would indicate a fuel delivery problem so I suggest teeing a 15 psi pressure gauge into the fuel hose ahead of the throttle bodies and see what the pressure is showing. It should show a steady 11 psi at idle. As the fuel filter has fuel hoses connecting it on both ends that would be a dandy place to tee in. Let us know what you find.

 

If everything smooths out after 3 or 4 minutes, that means once it hits closed loop all is well. My initial thought is that you have an issue with open loop operation, meaning that the computer isn't controlling fuel delivery for the first 3 to 4 minutes until the O2 sensor gets warm enough for the system to enter closed loop. Test your CTS, mounted in the front of the intake, it may not be reporting the correct voltage to the ECM in open loop, causing your surging. Also, have you cleaned the pintles on the IACs? Doing that eased a lot of my surging issues. I'll do a little research and get back to you, and I'm sure Dub knows more about this than I do, and he'll chime in soon.

 

I had a chugging problem and it took two months to figure out my harmonic balancer was off by 6 to 8 degrees. So I would advise verifying top dead center on the engine reads the same on the timing marks. Before I replaced the balancer I used my butt Dyno to time the engine. I kept advancing the timing and making hard runs, long medium loads going up hill listening for pinging. Eventually I hit that threshold and backed the timing off 2-3 degrees. The idle was a little up and down for about 50 miles until the ecu settled everything into its happy place. It runs great ever since and I kept that timing after replace the balancer. It's about 10 degrees advanced at idle. Stock is 6 I think.

 

I didn't realize the CTS could be involved in this, but I do have a new one. I'll install it this weekend. I'm not at all familiar with the IACs, so I'll study up. Yes, if you think of anything else, let me know. Thanks!

 

At the risk of being ridiculed by at least ten people, I'll admit I haven't checked the timing on this car yet. There have just been too many things to do at once, and not enough time to do them all. A few other members have complained about their balancers being off. I didn't realize it was such a problem on SBCs. I'm not sure there's enough room to get my dial gauge into the #1 hole, but I'll give it a try. Thanks!

 

Mine does that too....like you said for a short period then quits. I think it's one of those things we just have to tolerate on the 82. Seems like when it does go to closed loop the ECM knows what to do. I'll bet if you unplug the battery (so the ECM has to reset itself) you might find that it surges for a longer period until it "relearns". I might try that on mine as well.....just out of curiosity...

 

follow this to the t, or just r

and you will be amazed


http://www.superchevy.com/how-to/468...system-tuning/

 

So what does the ECM look at in closed loop VS open loop? Is the O2 sensor the only difference?

 

The ECM looks at the O2 and the temperature sensor to determine when to go to closed loop.

 

Right, but what does the ECM look at in open loop to control the motor? At the very least, it needs input from a temperature sensor. What else?

 

The ECM defaults to open loop when the sensors are cold and then is forced into closed loop when the sensors reach operating temperature.

 

You're not getting my question.
In other words, in open loop, the ECM accepts input from certain sensors and compares the data to a lookup table in order to control the motor. After it switches to closed loop, it accepts additional input(s) and uses that data in place of the "canned" numbers. One of those additional inputs is apparently the O2 sensor. Is that the only additional input when it switches to closed loop, or are other inputs added as well? To put it another way, which sensors/inputs are "locked out" during open loop operation?

 

The PROM in the ECM has default values and those values are used for the engine timing and air/fuel ratios UNTIL it gets additional inputs from the sensors. So it starts out with "X" timing and air/fuel ratio then switches to "Y" timing and air/fuel ratio when the sensors start sending their data to the ECM. The PROM is the "brains" of the ECM so when you go to an aftermarket PROM the values get changed.

In the case of the 1982's the default is RICH and as the sensors heat it goes to LEAN unless the MAP and TPS call for more timing and fuel. Imagine for a minute the throttle bodies being RICH carburetors and when the sensors get heated the fuel delivery of the carburetors gets pinched down to lean them. So the default (open loop) is RICH and the closed loop is lean. Or something like that...................

 

The PROM can't substitute for all inputs. Data from sensors is required during both open and closed loop operation. Even in open loop, the ECM can't control timing and A/F unless it knows at least RPM and throttle position, and probably several other things.

So, does anyone know which sensors are used for each mode?

 

I was tuning and burning prom chips on my obd1 with moats gear, when i had my built 383 superram 89 c4

the guys here are helpful, but since our 82's we limited to one year, the vast majority are in the dark with the ecm related issues

post on the c4 section and I know the boys over there will get you over the hump


http://www.corvetteactioncenter.com/...open-loop.html

 

The TPS and MAP sensors would certainly play a role in the open loop position. Don't you have a factory shop manual?

 

CTS, TPS, MAP on open loop. Fueling is controlled once the O2 warms up enough to show counts (crossing stoich) to the ECM. Without the O2, fuel trim isn't possible. Everything I've found so far on your issue points to CTS, and everyone says to replace with a two wire CTS with the weatherpack connector.

 

OK, I spent some time with the shop manual this evening. I think these two symptoms might indicate two different problems. The surging doesn't last long from a cold start, so for now I want to concentrate on the open loop chugging. I'm sure this is the result of the TBIs running rich, and according to the manual only two sensors are used at idle in open loop to set TBI pulse duration. They are the CTS and the MAP. The IACs set idle speed, but mine is OK once the surging stops. My CTS is already a two-wire with the weatherpack, so it might have been replaced by a PO. I'll change it out tomorrow anyway, just to be sure. If that doesn't fix this, I'll start hunting for vacuum leaks. I assume leaks could affect the MAP response. The fact that it runs OK in closed loop would seem to eliminate a lot of the other possibilities, like electrical grounding or a bad connector.

Good idea about the C4 forum.

One more question... is there a way to keep the engine in open loop for troubleshooting purposes? I won't have much time after it's started to see if clamping hoses helps this situation.

Thanks for all the info and advice!

 

You can test your MAP sensor in closed loop. As the engine is running, pull the vacuum hose, if the engine doesn't change, replace the MAP sensor. As far as finding vacuum leaks, it won't matter if the engine is in open or closed loop, if you find a vacuum leak you will know because the idle will change.