Need some help on this one, I have a rebuilt stock 355 in a 91 RS, I'm on the home stretch on finishing I hope. I replaced the computer prom for the motor changed out the fuel injectors to 350, changed out the ESC module, and of course the knock sensor. I've also replaced the fuel pump with a Z28 350. With the map sensor plugged in the motor runs kinda rough has to smooth it's self out but the ses light comes on when it does. The problem is that when I go to plug the map sensor to the back of the TBI it runs for a 15-20 sec then dies out. I've replaced the map sensor and oxygen sensor(down stream). The only thing that makes sense, from a suggestion from another person is the fact that I didn't change out the hose that's connected from the fuel pump and the sending unit. Fuel could be spitting right back into the gas tank if the hose cracked because of the different pressure. I am curious if anybody would have any other or the same suggestion, looking for a second opinion I guess. Thanks.

 

Have you checked the fuel psi? I had a customers truck that he had just put a new fuel pump in. He could do the same thing by unplugging the map and drive it... not very good but it would run. I checked the psi and it was very low.

edit: and yes you should always change the hose. It can swell up or split and cause future problems. Also was it a TBI 350 or a TPI 350 Z28 fuel pump you got?

 

I bought the fuel pump new, asked for a 350 Z28 fuel pump (not knowing that 350s only came out in TPI in Zs) at least in thirds. So it is a TPI pump. I haven't had a chance to check the fuel psi, I don't have a gauge. At this point I can't drive the car to a shop yet. How much do they go for anyways maybe I'll just pick one up?

 

First off you need to put a TBI pump in there. You have too much pressure in the fuel system. The TBI is a low pressure pump v/s the higher pressure TPI pump.

 

you can rent a fuel pressure gauge from autozone or the like. I think they get about $100 bucks for it but you get your money back when you return the gauge.

 

chevymec, I bought a high performance computer prom from tbichips.com (Brian Harris) something like that and he was suggesting adding additional pump onto a regular 350 pump to match up with what the prom is programmed to run the motor as. So he said the TPI pump should do fine. On the other hand like I said above I didn't replace the hose in between the pump and the sending unit. So it might not be actually getting all the fuel it is needing. The motor runs even with the map sensor completely disconnected to comp. and TBI, but when I feather the gas to much it sounds like it is losing power, or not getting enough fuel. Another thought is that even if a tpi pump has more pressure than a tbi wouldn't the regulator (regulate) how much is put into the motor? I am gonna get a hold of a pressure tester and find out what is reads and I'm pretty sure I'll know more from that when I get it. Thanks

 

It may work but I'm not sure how much psi the regulator can withstand. I would check the hose that you didnt change once you get a psi reading.

 

check the short piece of hose that connects the fuel pump to the sending unit. They are known to split and cause all sorts of strange problems. My 91 TBI RS was throwing fit and not kicking any codes. That was the only problem! I spent three days throwing money at it like a broken government program. Changed the MAP sensor, IAC, plugs, wires, cap and rotor, fuel filter.

 

If it helps at all, You can use a DVOM to monitor O2 sensor voltage once you verify fuel pressure of 9-13 PSI. IIRC, your 91 has a heated O2 so it should come on line within a minute of the engine running. If oit is single wire you'll need to be sure it's on line by watching voltage. An O2 sensor that hangs around at .450 is generally cold and offline. O2 voltage will tell you if you're lean(low O2 voltage) or rich(high O2 voltage).

If you have access to a scan tool, look at Integrator and Block Learn (short term and long term fuel trims) to see if the system is adding or subtracting fuel. This is a major clue to a lean or rich condition.

 

As long as your pressure regulator in your TBI unit is in good condition, it doesn't hurt anything at all to run the TPI pump. If your regulator is good, it keeps the pressure in the correct range for a TBI system. I ran a Walbro 255 pump on my stock 305 TBI car for several years and it never caused any problems. Lots of people have done it. If you think you're dumping a lot of fuel back into the tank and the car's running too rich, try replacing the regulator diaphragm in your TBI unit. They usually come in the basic TBI rebuild kits, and they're not too hard to replace. Even with a TPI pump pushing 45-50 psi of fuel pressure through the system, an intact regulator diaphragm will keep it right between 13-17 psi at the injectors.