Lean surge 7.4L with TBI

[Neukam]

1project2many has pretty well summed up what I was about to say.

Hmm... Neukam is spot on. The TBI FPR is a typical diaphragm plus spring type regulator. Fuel is exposed to one side of the diaphragm, the other side, with the spring, is exposed to atmospheric pressure through a hole in the regulator hat. Because of this port, changes in atmospheric pressure will change the fuel pressure. The physical placement of injectors and regulator atop the throttle body and inside the air cleaner ensure the pressure at the bottom of the injector and pressure at the atmosphere port of the regulator remain the same relatively speaking. This is well known science.

I just took a picture of the port on the regulator that is used for reference. I will edit this post and upload it from my phone. Many people don't understand this function of the regulator because it is not as obvious as a typical multiport EFI system with a vacuum line attached to the regulator, but the function is the same.

Understand this is not the OEM method, it's not an "improved" method, and it's not necessary for a stock or mildly modified vehicle. This is a technique designed to allow one to obtain maximum fuel delivery under load while reducing fuel delivery at low load. It reduces the total range of VE values across the fueling tables and can reduce the work required to obtain good driveability. This technique is generally used for higher HP vehicles and its a workaround for the limits of the OE ecm and TBI injectors.

The large peak and hold TBI injectors do not meter fuel well at very low pulse width. As injector size is increased, or fuel pressure is increased, the injectors cannot accurately meter fuel at the reduced pulse width required at low load with the higher flowing injectors. Vacuum referencing the regulator and retuning the VE tables can work well to solve the problem, and in some cases may be the only way to solve the problem, but is technically not the "proper" way to do things. It is merely a hack to a compensate for the limitations of the factory TBI system.

I haven't talked with Neukam much but I have to wonder if the poster spent time at a GM facility or a GM dealership in the '90s. We are hearing of things that could be considered arcane knowledge today.

I've never worked for GM. I started working on cars in 2001 at an independent shop that, do to it's location, catered to many customers with lesser financial means. Most of the cars I worked on were from the mid eighties to mid nineties and many of them were "problem cars". After working there for 2 years, I went to NASCAR Technical Institute in Mooresville, NC. I had a partial scholarship. I helped some of the instructors teach some of the other students that were having problems understanding, and I learned a lot from some of them who were extremely knowledgeable. I believe several of them had worked for GM, and one of them may have done some programming on these systems. After graduation, I worked at an independent dealer and repair shop that specialized in foreign cars. There I worked on everything from Geo to Lambo, but mostly on older German cars. After that I worked at another independent repair shop where I again worked on whatever came through the door. I don't claim to be the most knowledgeable person here, but I did gain a lot of knowledge and experience over about 10 years, before changing careers. I now work with train control signals for the local transit company.

The one wire O2 is really a two wire O2 without the second wire. The "wire" for the signal return is found in the body of the sensor. The sensor is attached to the exhaust pipes which provide a great path for ground. And the exhaust pipes are connected to the block. But the exhaust pipes and engine are hung in rubber which provides a poor signal path to the body. Engineers connected the signal return from the ecm to the rear of the engine block or cylinder head, close to the exhaust pipe without connecting directly to the pipe.

I just want to emphasize that this signal return is (and must be) dedicated solely to the O2 sensor. Assuming the ECM is 1227747, it should be connected to ECM pin D6.

Neukam is suggesting there's a possibility that ground is bad which would cause a false lean condition. Neukam is further suggesting the rare but real possibility that the ground for the heater has been connected to the same point as the ecm O2 return wire, and that the a bad connection to the block combined with heater power unable to find ground could add to the issue.

I'm also suggesting the rare but real possibility that the ground for the sensor has been left unconnected, or broken off after performing other work on the truck. You say the truck is running rich, and O2 voltage is reading low, and BLM is high. It would make sense that there's a problem with the O2 sensor circuit. It is possible, but I don't know how likely, that the ECM is raising the idle in an attempt to heat the O2 sensor and force a response. I don't know enough about $42 control strategy to know if that is the case.

Edit: in the last paragraph I wrote that it was possible the ground for the heater has been left unconnected. I meant to write sensor. I had a brain fart. Also uploaded a photo of the stock regulator showing the opening. Both spring pressure and air pressure act upon the diaphragm to regulate the fuel pressure differential across the injector.