Hey all. I have a 99 gmt800 ecsb z71 5.3l auto. 267k miles on the odo (engine still pulls about 18-19" hg of vacuum)

I'm getting poor mpgs (12.5 on the freeway). It takes two cranks to start if it sits longer than an hour(its been progressively getting worse). If I don't sit and idle for about 5 or more minutes then it's completely gutless. Half throttle and quarter throttle is good when I idle for a couple minutes after start up. WOT is completely gutless.

This has been going on for a couple of months now.

I have codes p0430, and p0332
(cat sys. Efficiency threshold bank 2 and knock sensor bank 2 bad ground. Respectively.)

I hooked up a fuel pressure gauge (cheap one though, it would drip fuel at a consistent slow rate) and it reads a consistent 43psi at idle (well fluttering from 41 and 43. I ruled it out as inj. Pulse) and smoothes out to 41psi at high idle (3000rpm and higher. Still sounds very sluggish though).It has a new fuel filter and regulator done within past 2k miles.

Any ideas? Pretty sure the fuel pump is on its way out.

I've put about 20k miles on it in the past year since I got it. It was very powerful and got 21mpg on the freeway when I got it.

Thanks guys.

 

I would replace both knock sensors, the harness and intake gaskets to start. The knock sensor code isnt doing you any good at all.

You should do a KOEO test of the fuel pressure, that could tell you if the fuel pressure regulator is bad and causing longer starts.

 

KOEO test was about 31-32psi and dropped quickly.

All of these issues were pre-existing to me replacing the fuel filter and pressure regulator. When I replaced the FPR, it did help a little though.

The knock sen. Code was always there. I don't believe it's the cause of my problem.

The problems I'm having, just started at the end of winter and have progressively gotten worse.

 

Fuel pressure with a proper working regulator should not drop quickly.

It should hold the roughly the same pressure a pretty long period of time, like a several minutes if not longer.

Though this isnt your whole problem, but it's part of it.

 

The sudden drop in pressure could also be caused by a bad anti drain back check valve at the fuel pump.

When I did my fuel filter there was no pressure when I cracked the lines.

 

I would check the catalytic converters to see if they are clogged.

replace the engine temp sensor. could be causing it to run rich

 

A 99 is a return style gas system. So unless you pinch off the return line, its going to bleed off the pressure.

Your fuel pressure is to low. You need to replace the pump.

I believe it Should be around 48-50 running and go up about 8-10 PSI when you pull the FPR vacuum hose

I believe Key On Engine Off test you should see about 60 PSI
This Is causing all 3 of your issues described.

When the fuel pressure is low, The engine tries to compensate by opening the injectors to dump more fuel.

BTW the knock senors can pull timing if they are not functioning correctly. You may want to fix that as well

 

Thanks bud. I'm going to replace the fuel pump this weekend when I have time off. I plan to do the same with the knock sensor.

 

For the love of God, please do yourself a favor and get a AC Delco pump.

The knock sensor code is compounding the issue with the fuel pressure. When it's running, it's pulling timing because of the knock sensor code, thus causing it to run even more like crap.

 

Certianly B4 anything else knock sensors are in order............