I have a 95 Z28. I bought the car with a vortech supercharger with 6lbs boost. The car is having problems running, and starting. There is a T-Rex inline 50gph/70psi fuel pump installed. The fuel pressure at idle is 35psi. When I turn the key to the on position, I can hear the inline pump run 2-3 sec and the fuel pump relay kicks on and off during that time too. I can't hear the intake pump turn on which makes me think its not working.

Am I getting only 35 psi because the inline pump is doing all the work?

What would my pressure by with a stock pump, the T-Rex inline pump and stock injectors.

I have looked at the Vortech install guide and with a 95 LT1, it doesn't have the injectors changed. Any thoughts on this.....THANKS!!

 

Your fuel pressure is determined by the fuel pressure regulator, not the pump rating or how many are running. As long as either or both of the pumps is capable of developing more than 35psi, the fuel pressure regulator will open up to reduce the pressure to its setting.

The stock fuel pressure regulator is not adjustable, and is set to hold the differential pressure (difference between rail pressure and intake manifold pressure) at 43.5psi +/-2.5psi. That means with no vacuum (vacuum compensation line disconnected from the fuel pressure regulator, you should see 41-47psi (factory specs). When you reconnect the vacuum line, the fuel pressure should drop in proportion to intake manifold pressure. 35psi would be within that range.

Have you measured the fuel pressure with the engine idling, with and without the vacuum line connected?

How does you S/C system add the extra fuel required to support the boost? Does it have an FMU or a boost reference FPR?

To determine if the in-tank pump is running, with the engine idling, disconnect the power to the in-line pump. If the in-tank pump is not operating, the engine will stall.

 

I do have a vortech FMU. I also have a stock fuel regulator. The pressure with the vacuum line off was around 40. I replaced the intank fuel pump today and things seem to be running 100x better today. I took it for a short drive and pressure never got above 40. I haven't gotten on it all the way yet but I also didn't want to get stranded too far from home. I will have to drive it a bit longer. With having 2 pumps, and an FMU and a stock regulator, why is my fuel pressure still within stock pressures? Do I have bigger injectors? I don't know if the injectors have been changed. Thanks again for the help.

 

You have to understand how the fuel pressure regulator controls the pressure at the rails. The FPR is located on the return line to the tank. It has a spring loaded diaphragm that opens up, or closes down on the return flow, so that the fuel pressure is always 43.5psi (minus the vacuum compensation). Your FMU works the same way. When it starts to see boost, it starts to close down on the return line flow to raise the fuel pressure.

I ran the stock pump and an inline 205LPH pump with my dry nitrous kit, and the stock fuel pressure regulator had no problem holding the stock fuel pressure.

But 40psi (if measured with the vacuum compensation line removed) is not high enough. Do a complete fuel pressure check. If you don't have a pressure gauge added to your dash, get a "test gauge" with a rubber hose long enough to allow it to be taped to the outside of the windshield, so you can see it while you are driving. At idle, with no boost, and the vacuum line off, look for pressure in the range of 41-47psi. Reconnect the vacuum line, and the pressure should drop proportional to intake manifold vacuum, or a 6-8psi drop with a stock cam.

Then take it out on the road. Without boost, you should see the pressure fluctuate with throttle movement, between the two readings you already measured (with vacuum and without). With boost, you have to verify that your FMU is raising the pressure like it should. The FMU should have a rating on it, like X:1, meaning for every 1 psi of boost, it boosts the fuel pressure by X psi. With a 7:1 FMU, you would see fuel pressure raise 42psi with 6 psi of boost. Note that the stock RP ball and seat type injectors do not like to operate at 80psi, and may lock up, causing the engine to run lean and risking serious damage on boost. Hopefully, whoever did the setup switched to a pintle type injector.

 

I finished up with the in-tank fuel pump replacement (trunk access) and most all my problems have gone away. The idle is back to normal and the car does not die, but it backfires at higher RPMs. I took it out for a longer drive and got on it and my check engine light came on. It stayed on until I turned it off and restarted. I still have to get the computer scanned to see if I have any codes. Any ideas to the backfire? Bigger injectors? How do I know what size I have? What about the check engine light on a hard acceleration? Any input would help.

 

Without knowing the code, we're just guessing. But its not likely you need larger injectors. That's the purpose of the FMU.... it raises the pressure in the fuel system as boost goes up. Higher pressure makes your injectors act like they are larger.

What kind of injectors do you have? Stock injector is a Rochester ball and seat injector, that does not like to operate at excessively high pressures. What does your fuel pressure peak at, with max boost from the blower?

 

The fuel pressure was around 42psi, from what I could tell with it taped on the windshield. I don't know what type injectors I have, wether stock or not. How would I find out without taking the fuel rails off and removing and injector?

I tried to get the codes scanned but advanced auto couldn't manage. The check engine light does come on after about 2 minutes of driving but the car doesn't seem to act funny. I will try again tomorrow and repost when I find out. Thanks for the info.

 

We need to know what the fuel pressure is doing as your boost reaches it highest point. If you see 5-6 psi you should be seeing a much higher fuel pressure than before you went wot. Before I removed the fmu and went with a mail order tune my fuel pressure would go as high as 60 psi when I got to 5+ psi of boost. You will need to retune your computer if you get bigger injectors and I would say your close to maxing out the stock ones. You can look at one of your injectors and what a stock one looks like and see if it has been changed to an aftermarket one.

 

I'm willing to bet Advanced Auto saw the 16-pin ALDL connector, and hooked up an OBD-II scanner to it. Won't work that way, since your PCM is OBD-I, with a funky "one-year-only" 16-pin OBD-II style connector. You need an OBD-I scanner, with the correct 12-pin -> 16-pin adapeter plug, or use Shoebox's method:

http://shbox.com/1/xraycable.jpg

 

Well, got the code off at Advanced Auto. I guess someone knew what they were doing. Its code 29 Output Driver Module (ODM) Fault, EAS circuit. The check engine light comes on after 2-5 minutes. The car runs O.K. but gets rough on idle after coming to a stop. Finally got new tires so I can more driving time in. Any ideas??

 

Found some posts on here about code 29 ODM fault EAS circuit. I looked into the air pump and replaced the fuse. It fixed the check engine light. It still feels a little sluggish but I think that could be fixed by getting the PCM tuned. Thanks for all the help.

 

When you get the codes, use Shoebox's LT1 specific code list.

http://shbox.com/1/Dtcs.htm

The AIR pump does not affect how the engine runs. Its only used for a few minutes at startup, to help the cat(s) heat up faster; Once it shuts off, it won't run any more until you restart the car. That's not related to your "sluggish" problem.

Without knowing anything about the injectors, and how the FMU is working, it makes it almost impossible to diagnose the problem. A data log, using PCM scan software like TTS DataMaster or FreeScan might shed some light on the problem. Both those programs are free downloads. You just need a laptop and a cable.

http://www.ttspowersystems.com/

http://www.andywhittaker.com/

http://www.akmcables.com/

 

I read about how the air pump works. That is why the check engine light would come on a few minutes after start up and its fixed with the fuse.

I believe the rest of my problems can be fixed with a good tune. Thanks for the website links.