Fuel Pressures Dont Drop And Bounce, Please Advise

goodbadgirl · March 12, 2009

My 1998 s10 blazer has 4.3 liter CSEFI and woudnt start the other night. It seemed if the fuel pump wasnt working i swapped relays and still no start. I got it towed home and the next day it started fine. I took fuel pressure readings. Key on not running went to 60PSI then settled to 55 and held. Started the truck and fuel pressure was steadly bouncing between 55 and 58 not dropping the prescribed 3-10 psi.... Pressures were within limits listed in haynes manual but the bouncing concerned me. Haynes says to bring to a dealer. Im quite proficient if given good info...any out here?

chato · March 13, 2009

1st possibility can be the csefi, if when the vehicle is started, its missing, stubling, like if the motor is flooded with fuel, then the regulator in the unit is leaking internally, the csefi unit needs to be replaced.

2nd possibility can be the fuel pump, hooked up a fuel gauge, turn the vehicle on, then turn it off , keep an eye on the gauge, see it drop's , some with bad fuel pumps will drop right away while others may take 30mins , an hour , and some are overnight . if it does drop then u have a bad pump

Hope this helps in your diagnosis

Chato

MikeInCtown · March 19, 2009

On the 4.3 engines those years you need to take two readings. The first you did and it worked fine. It settled at 55. After you turn the key back off wait 30 seconds and turn the key back on. It should jump the same and then settle back down. It should not go below 54psi even when running. If the manual is telling you that it may go down 10psi, that better be from the 65 psi max. If you are only getting 50psi you will be in trouble. These testing procedures are straight from the GM service manuals.

Once you have verified that the pump works fine you may need to check the fuel pressure regulator as mentioned above. If you drop a significant amount of pressure in 5 minutes after key off then you may have a weak FPR. The old spider injectors are also known for causing problems. In 2002 GM came out with an updated spider system that will retrofit into older trucks. If you have to have the injectors tested and they tell you more than one is bad just replace the whole setup as it will be nearly the same cost.

One easy way to tell if you have a fuel problem and not an ignition problem is to pull the hat off during a no start, spry a slight amount of starting fluid into the throttle body then put the hat back on as fast as you can and try to start the truck. if you have good steady sprk the truck will fire. if it stumbles then stalls you have a fuel problem. If it starts right up nd continues to run you probably have fuel problem. if it still doesn't start at all you may want to look at an ignition problem.

Good luck.