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






IronJen

IronJen

The Iron Lady
#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

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#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

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#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

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#7
#7
If its the LS motor, pull the vacuum line off the regulator. If fuel comes out, replace it.
 
IronJen

IronJen

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#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.
 
IronJen

IronJen

The Iron Lady
#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 $$
 
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