I have a 1988 300-6cy, with manual trans and 3:08 gears. I installed a mild cam, it improved the top end a bit, I find the engine runs lean the plugs are white, I think more fuel would help but I am confused about two things. 1) installing the 19#injectors and a adjustable fuel reg is no problem I have it all but Iam concerned about doing it because wont the ecm just slow done the injector pulse to lean it out with the bigger injectors, because isn't it a speed density injection on 88's? I want to open up the exhaust too but I think my problem isn't that the injectors cant supply enough fuel, its the engine wont let it because the ecm isnt aware of the extra air flow (because of the cam) so Iam scared that a less restrictive intake and exhaust will only glorafy my existing problem and the bigger injectors would help but the ecm wont let them?????
2)I checked my fuel preasure today and at idle its with in spec, but when you crack the throttle the gauge needle starts to bounce and wont stop wether its at idle or hi rpm. then no matter what you do the gauge continualy bounces.

Please Help!!!!

 

What you need to do is put the truck on a dyno and check the air/fuel ratio before you start trouble shooting a problem that may or may not exist. I chased what I thought was a lean condition on the top end for 3 years. Then when a dyno opened in town I strapped it down, checked the a/f ratio and was very surprised to find it rich above 3000 rpm. That said I'll move on to the questions you asked.

The way the PCM controls fuel flow is very complex. It is true that if the PCM detects a rich condition it will tune out the extra fuel. However, at WOT it does not use the O2 sensor to control the mixture. At part throttle it monitors all the sensors and calculates the required fuel flow. At WOT it uses the things it measures at part throttle to guess how much fuel to inject given the air density it calculates and the rpm. This means it is easily tricked at WOT. With the stock 14 lb injectors and regulator, the part throttle fuel flow is roughly 14 lb/hr through each injector because the fuel pressure (about 45psi) is close to the pressure the injector is rated at (3 bars, or 43.5 psi). At WOT (no vacuum to the regulator) the fuel pressure rises to 60 psi and the injectors flow a lot more. The flow can be estimated by mulriplying fuel pressure times flow rate, then dividing by the pressure at which the injector is rated. (60x14)/43.5=19.3. So we have an injector that flows 14.5 lb/hr at idle and part throttle, and flows 19.3 lb/hr at WOT. If 19 lb injectors are installed and the adjustable regulator is set to provide the same fuel flow at idle, the regulator would be set to about 33 psi. When the vacuum drops it increases the pressure by about 15 psi to 48 psi. At 48 psi the 19 lb injector flows about 21 lb/hr. The PCM adjusts the pulse width expecting only 19 lb/hr of flow, but gets 21 lb/hr so the mixture is richer. This is a very crude explanation, but basically how it works.

It isn't uncommon for mild modifications to be compensated for by the PCM by with no programming or fuel system tricks. When you change the intake, exhaust, or cam (as long as idle vacuum is drastically changed) the engine becomes more efficient. Before you say I'm nuts, read all of the remainder of this paragraph; the next sentence will not make sense at first. A stock 300 probably makes about 600 hp. A large portion of the power escapes the engine in the form of heat through the exhaust and cooling systems. After that heat is lost, you might be left with 300 hp. From that 300 hp we have to parasitic losses from the engine (oil pump drive, friction, etc.) and pumping losses. Pumping losses are the power it takes to pump gasses through the intake and exhaust systems, maybe about 100 hp or so. Pumping losses, friciton, etc. are present and approximately equal at all practical engine speeds and throttle openings. At idle the big pumping loss is getting air past the TB, which is why there is a large vacuum behind it. At some throttle opening and rpm combination the TB is no longer the restriction, so the engine accelerates until the losses are equal to what they were before. Oops, I'm getting off subject. If you make the engine more efficient without changing it's effective operating range, it will be more efficient at all throttle openings and rpms than it was initially. What this means is the engine will make more power per degree of throttle opening than it did before. If it flows more air at 10% throttle opening than it did stock, the PCM will see this as a lean condition via the O2 sensor at part throttle and make a corresponding adjsustment to richen the mixture. It assumes that since it had to richen it up at part throttle that it will have to do the same at WOT.

That's why I say put it on the dyno. If the PCM has to add more fuel at part throttle, it will also add it at WOT. If it does, the larger injectors are of no use.

What was the fuel pressure when you checked it? Pull off the vacuum line to the regulator and see if it still bounces. If it doesn't there is something wierd happening in the intake system causing vacuum fluctuations. If it still bounces there is a problem in your fuel system. The regulator might not be able to control the pressure once it increases.

 

wow...