• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

Help me figure out what's going on with my Suburban




VegasJen

VegasJen

Well-known member (45 ACP)
#1
#1
The last couple weeks of driving and I've noticed a rapid decline in my 99 Suburban, culminating with it leaving me stranded in Vegas the other night. It's been running rougher, and rougher, feeling like a slight miss at first, then progressing to feeling like a severe miss, then to more than one cylinder. Prior to leaving me stranded, it was running terribly rough, and unusually high RPM just to maintain 50mph.

I think the problem is in the fuel injection. I think this because:
1) I have been noticing a distinct smell of gas more frequently.
2) I pulled the O2 sensors just to see what kind of shape they were in and all four were covered in a thick layer of black soot.
3) My mpg has gotten even worse than usual.
4) Despite having a known oil leak, my oil level has actually gone UP in the last couple weeks. Plus, there is a distinct volatile odor to it.
5) I pulled plugs and found several actually wet with liquid fuel.

I rented a fuel pressure test kit from Autozone to see if I could figure out what's up, but I'm not sure exactly what I should be seeing. So here's what I found while checking:
  • When I turn the ignition to the run position, fuel pressure spikes up to 61psi for 1-2 seconds, then drops to zero.
  • When I turn the key off, pressure again spikes up to 61psi for 1-2 seconds, then drops to zero.
  • When I crank the engine, fuel pressure maintains a very erratic 35-50#.
Now I suspect one or more injectors is just constantly leaking fuel into the cylinder, just basically wide open. But I'm not exactly sure what I should expect with a fuel pressure gauge to verify this. Anyone have any suggestions?

The truck hasn't run, I haven't even attempted to start it in two days, but I just pulled plugs 5 and 7 and could literally pour fuel out of the plug well.
 
Last edited:
LongRange

LongRange

Well-known member (45 ACP)
Forum Supporter
2022 Supporter
2023 Supporter
2024 Supporter
Trading Post Subscriber
#3
#3
This could be several things but I think your on the right track…the auto parts stores use to plug into you ECU and was able to tell you what the problem was not sure if that’s still a thing.
I am not sure about the fuel pressure.

Have you checked to make sure you have spark at the dead cylinders?
 
JR3

JR3

Active member (5.56mm)
#5
#5
I'm guessing this is an LS motor? 5.3?
Static FP should maintain at 55-60 with the key on.
Running it should be around 50-55 psi.
Could be a fuel pressure regulator, or a bad fuel pump. I'm going to say a bad fuel filter isn't going to cause the pressure to drop.
Gas engine FPR is easy to replace but the FPR on a flex fuel is part of the in tank fuel pump assembly.
 
Poena238

Poena238

Peanut Butter
Forum Supporter
2020 Supporter
2021 Supporter
2022 Supporter
#6
#6
JR3 said:
I'm guessing this is an LS motor? 5.3?
Static FP should maintain at 55-60 with the key on.
Running it should be around 50-55 psi.
Could be a fuel pressure regulator, or a bad fuel pump. I'm going to say a bad fuel filter isn't going to cause the pressure to drop.
Gas engine FPR is easy to replace but the FPR on a flex fuel is part of the in tank fuel pump assembly.
Click to expand...
I want to say that the 1999 Suburban was still being offered with the 5.7 motor, but that's a good question. The FPR sucks to replace in the Vortec 350 IMO, having to remove the intake up top etc.
 
Harley

Harley

BANNED FOR LIFE+35 Because he dared me to do it!
Forum Supporter
2020 Supporter
2021 Supporter
2022 Supporter
2023 Supporter
#7
#7
If its the LS motor, pull the vacuum line off the regulator. If fuel comes out, replace it.
 
VegasJen

VegasJen

Well-known member (45 ACP)
#9
#9
LongRange said:
This could be several things but I think your on the right track…the auto parts stores use to plug into you ECU and was able to tell you what the problem was not sure if that’s still a thing.
I am not sure about the fuel pressure.

Have you checked to make sure you have spark at the dead cylinders?
Click to expand...
I have not checked spark. I think I need to drain the oil first. I've had a lot of fuel leak down into the crankcase so I'm moving a lot of oil/fuel mixture when I try to crank it. But I don't want to change the oil until I figure out what's going on. No sense putting fresh oil in only to have more gas leak into it.

I do have a scan tool and pulled P1345 (previously known code about timing) and a P0330, knock sensor code. I figure the knock code might be a result of how poorly it was running.
 
VegasJen

VegasJen

Well-known member (45 ACP)
#12
#12
JR3 said:
I'm guessing this is an LS motor? 5.3?
Static FP should maintain at 55-60 with the key on.
Running it should be around 50-55 psi.
Could be a fuel pressure regulator, or a bad fuel pump. I'm going to say a bad fuel filter isn't going to cause the pressure to drop.
Gas engine FPR is easy to replace but the FPR on a flex fuel is part of the in tank fuel pump assembly.
Click to expand...
Negative. Last of the 1st gen small blocks. Vortec 5.7.

I am hesitant to condemn the fuel pump. I know it's easy and tempting, but when I cycle the key to the run position, it shoots up to 60psi instantly, and again when I turn it off. So I'm getting pressure, even if it is only for a few seconds. It seems there is something, somewhere else in the circuit that is bleeding that pressure off, hence my suspicion of an injector.
 
JR3

JR3

Active member (5.56mm)
#17
#17
VegasJen said:
Negative. Last of the 1st gen small blocks. Vortec 5.7.

I am hesitant to condemn the fuel pump. I know it's easy and tempting, but when I cycle the key to the run position, it shoots up to 60psi instantly, and again when I turn it off. So I'm getting pressure, even if it is only for a few seconds. It seems there is something, somewhere else in the circuit that is bleeding that pressure off, hence my suspicion of an injector.
Click to expand...
Spider injectors $$
 
You must log in or register to reply here.