I know this is long, but I am stumped.

One of my son in-laws has a 91 K1500, 5.7, manual trans, bone stock. He came over complaining it was running bad. We did all the normal maintenance, fuel filter, tune up(plugs, wires, cap& rotor,) inspected dissy, and checked timing. Idled great.

Went out for a drive and at light throttle cruise in any gear it would start acting like egr was wide open. Pulled over and disconnected egr vac line, ran great. Went back and installed a vac gage between egr and solenoid and when the throttle was opened slightly egr went wide open. Then installed a lab scope on the control wire and opened throttle again there was no duty cycle, ecm commanded 99.8% and egr was wide open.

Checked alt for diode leakage ( had this happen on a Impala ss one time) none found, replaced ecm with a test unit, same thing. At this point I opened up the bin (ARHT) and looked at egr tables and constants. There is two tables, 1- %EGR VS. LOAD VS. RPM in this table all cells are filled with 99,6. 2- EGR CORRECTION VS. LOAD VS. EGR DUTY CYCLE I think this table is the final amount of egr that is added depending on load?

What I think I am seeing with this truck is the ecm is not looking at the second table and commanding all erg function from the first table 99.6%.

Could this be a prom problem?

I would turn off egr and adjust VE and SA , but he lives in a emission testing area.

 

I highly doubt it's a PROM or BIN issue, it ran fine before, and the PROM or BIN won't change programming on it's own.

There has to be something else that is causing the EGR to open too much like it is.

What does the coolant temp show? IIRC EGR control is also tied to coolant temp, and a bad sensor here could cause problems. TPS voltage look good?

Other than that I'm not sure of the specifics of how the EGR control works (mechanically through the control device), as I haven't needed to get into changing any EGR function yet.

 

Replace the EGR valve. The prom is supposed to be programmed like that. The backpressure type EGR valve itself controls the EGR flow depending on the intake vacuum and exhaust backpressure. The EGR solenoid simply insures the EGR valve will not open at idle or with a cold engine or overheating.

 

I am not questioning the bin, My question was why if the egr solenoid is to be pules width modulated it is being commanded to a 99.6 % at all load and RPM. Thats not a duty cycle, that a switch on switch off.

Its like its only using the first table %EGR VS. LOAD VS. RPM which commands 99.6 % in all cells.

 

There are two different types of EGR solenoids that can be controlled by the 7747. The first is a simple on/off and is used on light duty trucks. The second is an EVRV solenoid that is pulsewidth modulated and used on heavy duty trucks.

 

After sitting down and reading the GM manual on both systems I see what you are saying. Being this is under 8500 gvw. the valve and system are working as designed.

The conclusion I have come to is that exhaust back pressure is very critical to the operation of the EGR. Too little and the system will have insufficient flow, too much and the bleed hole in the valve will stay closed due to positive pressure and EGR will lock open 100% as long as the throttle plates pass the the vacuum port or manifold vacuum reaches near 0 kpa as in WOT.

Looks like I will be taking a closer look at the exhaust system.

Leave it to GM to create two systems. one primitive and digresses at the first turn of the key. The second ECM controlled via PW that will live till the valve fails. Had to be a bean counter decision.

Will let you know my findings hopfully tomorrow.

Thanks for your help Fast355.