Need to Learn Tuning 101 Quick!

[WASyL]

Listen to what engine wants, avoid knock and get WBO2. that is true 101 of tuning.

[sherlock9c1]

Greg Banish has some great books on tuning. Agreed with above - fix the fueling first, then play with timing, then revise the fuel one more time and you'll be good.

[1project2many]

Hi Mark,

I would also suspect the injectors are not delivering sufficient fuel. Whether they are faulty or not they might not be able to operate properly with that much fuel pressure. Oscilliscope curves of the injector opening and closing could help determine whether or not they are the problem.

[Mphiver]

Thank you for the responses!
The TBI has been apart and investigated ad nauseam. The original injectors were sent to RC Fuel Injection in Torrance, CA where they found one dumping fuel and both poor spray pattern. They were rebuilt and bench certified, returned to me and installed.. I was hopeful this is all that had gone sideways causing it to fail SMOG and lending to poor mileage. It didn't change the way it ran. I then sent the entire TBI to another specialist who found one of the 'rebuilt' injectors reacting intermittently and recommended replacement - I agreed and replaced BOTH injectors with the same GM Delco P/N 75# units.

The Fuel Pressure Regulation system is somewhat unique. It operates in vacuum/cruise mode off the TBI integral regulator which has been modified to see direct MAP: 12-13psi up to about 22psi. At that point the auxiliary FPR increases pressure linear with positive MAP/Boost pressure between 30 and 40psi (somewhere in that range; needle is fluctuating and hard to read an actual number). According to Dave Darge, this is correct. Picture is the multi-stage regulator.
Fuel Pressure Regulator04-07-19.jpg

The truck is running very rich in boost. Mileage is down around 6mpg and used to be better than 10 when pulling a load. I used the truck 'as is' to tow a 5-6k lb loaded trailer when moving from SoCal to Prescott, AZ up the mountain grades. Sucked fuel and left a black smudge around the exhaust outlet rear fender. Like a Diesel :)

Another 'Expert' suggested I probably have intake gaskets blown and blowing pressure into my crankcase instead of cylinders... I don't buy that; especially on a professionally built SBC. Engine was built and assembled (long block) by Rancho Performance Machine, Temecula, CA - a very competent racing engine machine shop and builder. I can't imagine the intake manifold to head fit using top notch FelPro gaskets would fail with a 5.5lb boost system. I could be wrong... Still pulling my hair out!

Appreciate all your knowledge and input as I approach learning to look at the fuel mapping and understand what is going on. Should have it running on the new ECU mods later this week. Keep you all posted,
Mark

[1project2many]

Mark, the number one way to fail to correct a problem despite investing substantial effort is to fail to test systems while the problem is occurring. You will find that most of the folks here are hands-on, test and fix types. You will be expected to be directly involved and actively participating in the diagnosis. :)

If the tailpipe is producing black smoke and mileage is down that would appear to indicate a low air:fuel ratio. Set up a laptop with an OBD1 cable and Tunerpro. Drive your truck and record data. The datalogs will store a picture of the engine operating conditions as seen by the computer. Once you have several runs recorded you can familiarize yourself with the data. You may see issues in the sensor readings that can help narrow the search. If you upload some of the logs to this post you may find folks willing to help you learn to read the data.

As a "throw it out there" suggestion, consider there were three different injectors used with the 7.4L TBI engine over the years. The 94-95 7.4 engines received an injector that delivered approximately 74 lbs/hr fuel at double the pressure used for the '92 engines. If you are using earlier injectors with a calibration designed for later injectors they would likely deliver too much fuel, especially at high manifold pressure. The calibration will contain a value that represents a ratio between engine displacement and injector flow. This could be used to attempt to determine which injectors were originally used.