I've read and reread here and still wondering if changing the factory injectors will help with MPG. The after market offers from several makers the replacement unit with the true injector for each cylinder instead of the master unit that fires all injectors 3 times for each revolution. These fire only for each cylinder at the opening of the intake valve. My 99 has gone from nearly 20 on the highway to 16 on a good day. It had 44K on it when we bought it 4 years ago. The old girl now has 80K and the MPG is the only complaint........

 

The MFI (injector at the end of the lines) spider will not provide any additional mileage benefits over a properly functioning SCFI (injector in the main body and a pressure actuated poppet nozzle at the end of the line) spider. Now if your poppet nozzles are not spraying well enough anymore, then it is possible. It's a gamble as to whether it would make that much of a difference to warrant the cost and I wouldn't cheap out on your selection of manufacturers either.

 

I've been looking at Rock Auto for the replacement injector package. Standard auto brand is $200, AC Delco $227 and Delphi is $340. The local NAPA guy gets $300 for his that is an Echlin brand. There is 5% off coupon available online. Still not cheap but a return to decent MPG for this old girl would be nice. I've though about having a test pipe installed in place of the cat but haven't done it. I've done the BG44K twice and saw no real improvement. When I bought the Blazer I changed all the fluids to synthetic. I use conventional motor oil in these older motors. The brakes seem to fine, not dragging. I think the fuel quality has something to do with these problems and the more alcohol is added the worse things get. There is a station not far from here that you can get alcohol free gas. The problem is the price about $4 a gallon. But would be nice to see how much difference there is.

 

Sounds like about the amount of mileage that you lose when the TCC is not engaging anymore.

What you have been told or read about injectors firing differently is incorrect. All that was done for the upgrade MPFI spider was the 6 solenoids for fuel flow were moved from a central location to the end of the tubes and the poppets were eliminated. As far as the firing of injectors, the PCM determines this and it knows no difference between the two types of spiders. This is why you can upgrade so easily. Functionally they are identical, but the upgrade MPFI spider is more reliable without the poppets - and that is why they went to it.

Both types of injectors will fire sequentially with the firing order unless there is a problem with the cam sensor determining which cylinder is next. If there is a problem the SES light is illuminated and the injectors are pulsed in two banks as a limp in mode.

Do you have a SES light on, or even any pending codes - especially misfire codes? These can and will quite often prevent the TCC from engaging and will hurt fuel mileage. Can you feel to TCC lock up and see the tach drop a few hundred RPM as you pass 47mph at light throttle in OD? If you are driving a steady level 50mph for >10 sec, does the tach go up a few hundred RPM when you tap the brake pedal and the TCC electronically disengages?

p.s. The PCM will definitely be able to recognize any test pipe installed in place of a catalytic converter. It will set a code and turn on the SES light. It monitors converter efficiency using O2 sensors and will not like zero efficiency. There are ways to try to trick the PCM, but IMHO is not a good way to go.

 

Thanks for the reply. The information I had read about the firing of the injectors came from this site I believe, but wouldn't say for sure.
I don't have the SES on and no stored codes that I'm aware of. My son in-law has a cheap scan tool that shows no codes.
The TCC works perfect and may even lock up a little sooner than the magic 47 MPH. I did replace the intake upper and lower gaskets and cleaned everything up about two years ago. At that time I didn't really find any surprises. I also replaced the cap and rotor that only showed minimum wear. I used AZ's finest by Bosch cap and rotor. The one thing that that did make a difference in the MPG was when the fan clutch was replaced. The shop that installed it used a NAPA brand extreme duty, we live near Las Vegas now. That one would never release. So after we were done moving and I had my tools I put on a heavy duty unit. It still hangs in there until about 3000 RPM or 70 MPH. But when it's over 100 degrees here in the summer you need that extra fan lockup to keep things running under 200. That is probably one source of lost MPG.
Any suggestions on a good scan tool would be appreciated. We are getting ready for some long spring trips and would like to get the mileage back to close to 20.

 

How much do you want to spend on a scan tool?. I bought the Chinese Tech 2 knockoff for $430 CDN ..... around $250 USD and it works great on GM vehicles.

 

Sounds like you have things pretty well covered. FYI mine only gets 16mpg with the A/C on at 70mph on typical Interstate highway. I can get as high as 22 if I slow down, turn the A/C off, and have a level road and drive at a steady speed. You might get something out of replacing O2 sensors ahead of the cats, if their calibration has drifted - but know they are what drives the fuel mix on a vehicle that is running properly and controlling the fuel mix in closed loop. If their calibration is off, they would show a skewed fuel trim.

I think that everyone pretty much knows that for a 1999 I would recommend either Car Gauge Pro or Dash Command for Android - Dash Command for iPhone/iPad (they make both), Car Gauge Pro is cheaper and more difficult to use - does a bit more. Dash Command is easier to use and a bit more expensive. $8.95 for Car Gauge Pro and $10 + $10 more for enhanced GM PIDS for your year of vehicle in Dash Command. Either one needs a wireless OBD2 adapter. Bluetooth for Android and WiFi for iPhone. For the bluetooth OBD2 adapter I recommend the BAFX sold on Amazon because of scan speed and price ($22). Wifi, take your pick I have no recommendations.

Good luck and best wishes for a successful repair.