Mechanics

GMC SIERRA ENGINE PROBLEM

2003 GMC Sierra • 192,000 miles

I have engine light with P0300 code - Random misfire.
I'm experiencing rough idle, hesitates as I accelerate but then runs good at higher speeds.
On cold starts, I have to prime fuel by turning key on 2-3 times but then starts fine but again rough idle.
I performed fuel pressure test per your instructions.
Results were 60 psig with key "on", 50 psig when running, 57 psig with vacuum line to regulator removed.
When I turn key "on" and then "off", the pressure will quickly drop to 6 psig.
May be coincidence, but engine light showed up after replacing plugs and wires. Initially I had replaced with Bosch but returned plugs and wires to install AC Delco.
Checked each coil/wire with HEI spart tester and all had spark.
The vacuum line on the regulator is not leaking fuel.
After all of this, I'm thinking I may have
- leaking injector(s)
- bad fuel regulator since I leak pressure so quickly.
- I'm also suspect of the fuel pump although I performed the test. I find it a little louder than I think it should be and I'm not confident that 50 psig running is the OEM spec.(Please confirm)
Oh, engine vacuum runs about 16 in Hg at idle.
Also, I have a strong smell of raw fuel when idling for a while.
How to test injectors?
Additional regulator test?
Thanks,
Danny
Avatar
Guest
April 5, 2013.




The pressure dropping too quickly indicates a leak in the system pressure and it could be from the pressure regulator or fuel pump and delivery hoses.

Clamp the return hose and test. If pressure maintains, the regulator is bad. If pressure drops as previous, you need to check the fuel pump side if there are no visible external leakages.

KHLow2008
Apr 19, 2013.


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