2003 Tahoe, 5.3 liter vortec flex fuel. Engine replaced at 60k miles due to low end knock (typical late model GM). Scanner shows mutiple misfires on all 8 cylinders and erratic timing. Rough idle. Everything replaced, knock sensors, ckp sensor, even the computer. Everything else tested, checks out fine, fuel pressure, oil pressure, compression all good, even the Chevy house can't figure it out. Any suggestions?

Your question would be better with 'year make model'.

Help and videos in the link

https://www.google.com/search?q=4.3+liter+vortec+replace+oil+pump

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year make model 4.3 oil pump replacement

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aLL FUSES GOOD.
all cold engines race high ,engine cold and the more cold the more high RPM. 1500 is PAR for RPM cold, is yours higher?
black SOOT is a huge bad RICH sign. of greatly needed engine service.
no code told P03xx what is XX.? (sad face)
we can';t guess codes we need to see them. sorry and all off them.
I thank also your scan tool is not certified for GM OEM scan codes.
P1xx up.p2XX ?
or is super old scanner useless
or just cheap $17 code reader ? also useless in the real world.

also did you know P0300 to P0306 means misfire. (weak combustion) lots of clauses.
ever do full tune up, in 20 years, spark plugs , spark wires
distrib. cap and rotor? ICM bad,?
?????
Map sensor clogged
IAC valve stuck open , RPM not regulated.

Fuel pressure was low, how low?, not told PSI
low does not cause rich burn but sure lean burn and misfire sure.
spec is.fuel real PSI test is 60-66?psi. RUNNING engine.
if not pump or FPR are bad. yes pumps can fail weak easy.
the test is called the Injector fuel pressure balance test in the SM, service manual.
the rules are.(gas)_
1: good engine, (compression at spec) EGR not stuck open and CATS not melted closed.
2: spark good, we can not burn fuel with dead or weak spark.
we call this a tune up,. (or doing the service points (today)
3: bad fueling, last fuel is last. in the due to Physics.
bad fueling can be caused by (short list)
bad MAF , or MAP, FPR, PUMP, or injectors leaking or clogged.
IAC stuck makes idle controls dead.
to find it take real tools , and gauges, a real scan tool , 2008 or newer OBD2 scan tool is best, for sure GM rated
GM has its own OEM factory codes. (vast)
or get the car serviced in a shop.,,, Bingo they have the tools!

here is your spark schematic
all these can produce P03xx codes too, but was never told.

https://imgur.com/a/gw0cHYR

2002 Chevy Tahoe + P0171/P0174 - Chevrolet Forum - Chevy ...

chevroletforum.com > ... > General Discussion Forums > General Tech
Jul 8, 2007 - I have cleaned the MAF sensor, changed the fuel filter, ... I also have a 2002 Tahoe that is posting the p171 and P174 codes and .... I have a 2000 ChevyTahoe with a 5.3L V8. ... That seems normal for the light to go off after running, the IAC motor should have its own fault code, so it may not be that, but not ...Missing: flex

SilveradoSierra.com • P 171, P 174 error code : Vortec 5300 ...

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running, not reading; what engine option? Cyl count and displacement?
Is the CEL glowing key on, then turns off running> (check engine lamp)?
the engine RPM is too low. ok that is bad.
this is caused by up to 50 reasons. (flooding and misfire huge list )
ill just post the basic physics reasons.
(Classic, good compression, spark and fueling)
no fuses blown?. and 12vdc reaches all EFI parts. (weak battery is no go)
In generic form as there many engines used on corolla world wide.
1: low compression , less than 150psi on all 4. (if not V6)
if engine has cam belt? it slipped.
2; weak spark causing misfire, tune ups never done each 60k miles.
3: bad fueling. (too much is flooding misfire, if too little it starves)
4; EGR stuck open , we clean these ever 60k mile.
if it sticks open, idle will fail. (goes low) for sure.....!
5: the ISC sticks closed, same deal; so it gets cleaned.

The first thing to do is a tuneup per the FSM and operators guide.
the 60k it's called, do that and spark is good and fuel pressure returns.

this post assumes engine has full power at all times.
if not , better explain that, in details.

there are simple tests for all the above, $100 max i tools.

misfire, cyl 1 and 2. is this the 4.0 liter engine, a KEY FACT.

ever do a tune up this decade or at 60k miles.?
50ways to misfire.
but are all under the 3 basics and lost DC power to EFI.
bad compression
bad spark
bad fueling.

I had a chevy 2002 avalanche,2004 chevy tahoe both after 20,000 miles developed a mild lifter or slight bearing knock.When I confronted chevy with this problem they stated this was normal for the 5.3 liter v8.Being a mechanic myself didnt think this sounded right.My friend has a 2003 gmc yukon and motor sounded same way.I no longer own my chevy,But my friend still has yukon with 150,000 plus miles and yet it still knocks when you start it.this is just a problem with chevys and other gm products.

Low fuel pressure will cause many of those codes to set.

Should have around 3.5bar pressure at idle and full throttle.

Check to see if VW has a recall on the ignition coils. They are cheap to replace however.....you will need your VIN# when you call.

Alternator problems can also trip codes . Check for a/c ripple at BATT + on Alternator...no more the 0.05vac allowed.

Valve train noise common complaint. ...often due to lack of oil services and/or incorrect oil type.

All 1.8T engine will clatter on start up one time or another and is considered normal as long as it goes quiet after 2 to 5 seconds at idle.

You should have synthetic oil in the engine.

P0012 is often timing incorrect. ...check timing belt at tdc.

These engines need a timing belt every 50,000 miles not the 100,000 miles VW suggests. Belt failure is catastrophic.

Transmission codes often corrosion in loom connections under battery. These need checked and cleaned with a solution of STABILANT22A applied to the pins.


If a scanner is available clear the codes and drive for 40 miles or so and recheck for codes. Transmission codes are not always due to transmission faults. They need 200 miles to re adapt after batter replacement or power off.

ROSS TECH site has lots of info' and a scanner program for your lap top.

Just went through this with my 2002 Buick Lesabre. It ended up being the Ignition Control Module. On the 3.8 l engine it is under the 3 coil pack. Probably a similar location on your silverado. Mine cost $140 but was easy to replace.

I would change the fuel pressure regulator first, if no change in pressure the fuel pump is on the way out.I have a 01 silverado 4.8 and my fuel pressure is between 55and60 psi.

replace your MAP sensor(manifold absolute pressure) the map sensor is the computer for your fuel pressure regulator telling how much pressure. Unless your tahoe runs like **** going up hill than it inevitably your fuel pump