My boys and I are building an 88 IROC Camaro with 305 TPI 5-speed. Runs pretty good, but has a stumbles/jerks/misses during accerlation. We've replaced plugs, wires, rotor, cap, MAP Relay, Fuel pump and Fuel pump relay (which were bad). We pulled error codes 21 (TPS Volt Hi), 33 (MAP which this engine (F) doesn't have) and 54 Fuel Circuit (which we fixed by fuel pump and relay). The TPS tested fine at .55v at idle and just at 5.0v at WOT and was smooth through range. It starts and idles great and seems to run a bit better after warm-up. One thing I can do to elminate missing during acceleration is to jumper A and B terminals on the ECM. When I do this the system starts out in Open Loop (2 1/2 flashes/second when cold) and switches to Closed Loop (1 flash/second) after a warm-up period just like it's suppose to. Why would it run great with the ECM jumpered? I'm assuming we have a bad sensor, but not sure which one. Any thoughts?

 

Welcome aboard thirdgen.org.

From the Tech FAQ linked from the thirdgen.org homepage:
"33. High voltage (low vacuum) at mass air flow sensor (or MAP sensor)"

I'm not the mostest TPI 'spert, but I'd check the MAF.

 

Thanks much! Just to be clear your refering to the MAF itself right (at engine air intake), not the relay? Does jumping the ECM (A and B) override the MAF and this is why it runs great when jumper? I'm a little baffled on this.

 

Do you have a scanner? I would just unplug the MAF and see how the car reacts. If it runs better like it does when you jump the ECM then ya its the MAF. If not, then obviously its not the MAF and you can rule it out. Maybe when you jump the ECM is goes onto open loop and doesn't use the MAF? The scanner would answer that question.

 

Thanks, I'll try disconnecting the MAF. I don't have a scanner, but it sounds like something I could use. With the ECM jumpered it seems like (according to the book) that it does cycle from Open Loop on start-up (cold) to Closed Loop after warm-up (based solely on SES blinking message), but maybe by being jumpered and the ECM in Field Service Mode it operates a little differently. What is the Field Service Mode for anyway?

 

When my 305 TPI is jumpered to Service Field Mode it runs great. What does having in this mode tell the system to do differently than normal operation?