I have a 1986 IROC-Z with 305 TPI engine. The problem is that when the engine is in normal operating temp and is idling the engine rpm becomes erratic and feels as good to turn off. I was checking the fuel pressure when I prime the fuel pump the pressure goes up too about 45 PSI, but in less than a minute the fuel pressure falls to 0. And when the engine is running the fuel pressure is at about 35 PSI. Could the idle problem be associated with the fuel pressure issue?

sounds like a weak fuel pump. make sure you have a clean filter. you can try pinching the return line and see if the pressure will hold.

Yes, I'm suprised it dosnt manifest when it's cold as well.

First thing to check are injectors, look up the resistance values on this forum and check them if you can, that'll atleast let you know you need new ones without much work if they are bad.

Second is isolating the pressure drop, a special clamp on the rubber lines will help you track down the leak. There are 3 basic areas the leak could be. 1 the injectors. They can leak at the tips, 2 is the regulator. It can tear and dumb pressure. If it's so slow I wouldnt think the regulator would be it.
and 3rd is the fuel pump in the tank. Specificaly the pump check valve.

Place the clamp on the rubber line that goes between the chassis and motor, the feed line not the return line. Prime the pump 9before you clamp the line lol) and if it still drops off it's either the injectors or FPR.

If not it's likely the pump.

If it's the former then you will need to identify where exactly the leak is at, begin taking the plenum apart but when the FPR is exposed go ahead and remove the cover and inspect the rubber piece inside, look for even the smallest pin hole. Especialy around the grommet in the center.

When you pull the rails with injectors still installed you can prime the pump again and you'll likely see the fuel drip from one or more injectors.

when checking the resistance of the injectors..if you have any below 8-9 get rid of it. Also to do a quick check to see if one or more of the injectors are leaking pull the plugs, if you have one or more wet cylinders you have leaky injectors then you would benefit from replacing them with 19lb bosch lll's.

www.southbayfuelinjectors.com

Guy, thats for your replies!

First, I replaces the injectors all 8 of them about 6 months ago. So I will try by replacing the fuel filter first and then doing the fuel pump trouble shooting recommended.

Thanks for your help!

What brand injectors did you get? Accel seem to have high failure rates as do the OE injectors used in the 90-92 cars.

I tested the fuel pressure while pinching the return line. I noticed that the fuel pressure was maintained while the return line was pinched but when I released the line the fuel pressure dropped to zero.

Do I have to replace the fuel pump?

from that test, your pressure regulator is bypassing internally.
the fuel pressure dropping after you turn the motor off isn't really a problem unless the drop in pressure is caused by one or more injectors leaking or the regulator leaking into the intake from the vacuum fitting because of a ruptured diaphragm.
pull the vacuum hose off the regulator and check for any signs of fuel.
then power up the fuel pump thur the fuel pump prime wire and check it again.
there should be no signs of gasoline at the vacuum fitting. if there is, the diaphragm is bad and you'll need a new regulator.

a good fuel pump should be able to build enough pressure at key on for the motor to start.
as long as the pressure holds at WOT and the regulator isn't leaking fuel from the vacuum fitting i wouldn't worry about the regulator right now.


have you checked it for codes?
check for any vacuum leaks.
also, check for any air leaks between the MAF sensor and intake.

Dont know if you fixed your problem, but im having the same problem. Rough idle, when hot it dies, fuel bubbling in gas tank, bogging when on the throttle. I have also replaced my injectors with denso 24#, rebuilt my motor, replaced gas tank, 4 fuel pumps thinking that was the problem because it wouldnt hold pressure it drops to 0 psi after 30 seconds. 2 FPRs. Had prom reprogrammed for the injectors and EGR delete due to being discontinued, 2 computers, Ive replaced everything. The only thing that wasnt done was blowing through the lines and take the rail apart to do thorough cleaning and inspection. If you fixed your problem I would really like to know and if anyone else has some ideas I would greatley appreciate.