5.3 hesitation, stumble, lean and random misfire codes, worse when cold.

[kleake]

This is kinda long winded, but I am at a loss as to what is the problem is...

I have a 2000 suburban with the 5.3. For the last several years it has been randomly giving bank 0 lean, bank 1 lean, and random misfire codes. It is MUCH worse when cold, especially on startup, but it is getting to the point it does it even when hot. Normally once warmed up it runs pretty good, although when pulling my camper and trying to back into a space it will stuble and cut out as I am trying to accelerate. It basically hesitates to the point it won't hardly move at times. Over the years I have changed spark plugs, fuel pressure regulator, fuel pump, MAP sensor, MAF sensor, throttle position sensor, and tested EGR by completely blocking it off. I have also checked for vacuum leaks by disconnecting hoses and capping them temporarily. I was completely at a loss, and heard about intake leaks or cracks so yesterday I put on a new intake gasket and inspected the intake pretty closely. This still did not solve, nor even change the issue. I have even sprayed carb cleaner and such all over the intake while its running, and there is no change in the way it acts while spraying it. I don't remember what the fuel pressure is at, but it was within spec when I checked it last. I can unplug the regulator to up the pressure and it helps a tiny bit at idle, but never fixes it. What else could I be missing? The IAC was also taken out, cleaned and checked to make sure it is working, and it is, but I haven't replaced it. 02 sensors are within normal reading as well. It basically has the symptoms and such of a vacuum leak, but I don't hear one, nor can I find one. No other codes have ever been set, and none ever show up. It is definitely a random misfire at idle as it sounds like it has a cam in it, but uner heavy acceleration while backing the trailer, or holding the brake, it seems to be more of a steady misfire when it does it. And, it doesn't do it all the time, but under the right conditions, and the right throttle it will. And only at low rpm, never anything above about 2500rpm. Any other idea's what this could be?

[danielsatur]

I would considerer a smoke test to pinpoint unmetered air leaks on the PCV, EGR, and Air intake systems.

[kleake]

Originally Posted by danielsatur

I would considerer a smoke test to pinpoint unmetered air leaks on the PCV, EGR, and Air intake systems.

I have inspected all of these pretty close, and tried the carb cleaner approach. I blocked off the EGR, so the only one I haven't specifically plugged directly is the PCV. I did pull a couple of the larger hoses from the intake end and plugged them, so I may have actually checked it without realizing it. I will verify again.

While reading about how a smoke test is done, I did notice it mentioned the short term and long term trim values, and how they should be pretty steady. I will check mine again as I don't remember how mine responded, but I know the o2 readings jump all over the place and don't stay steady so it's hard to really see what's happening. All sensors are acting the same so I figured that was normal,,,, but maybe not?

[danielsatur]

For DIY, see http://www.youtube.com/watch?v=FfiQ7qWiG-M

I think Meineke will do it for $50, it's so much safer than carb cleaner.

[j cAT]

with what you stated here my question is what do you consider normal fuel pressure ? how did you measure the pressure ..

engine idling pressure ? engine off pressure ? how long does it hold pressure engine off ?

what about the fuel filter ? did you cut open to see if its got debris in it from ???

[kleake]

Originally Posted by j cAT

with what you stated here my question is what do you consider normal fuel pressure ? how did you measure the pressure ..

engine idling pressure ? engine off pressure ? how long does it hold pressure engine off ?

what about the fuel filter ? did you cut open to see if its got debris in it from ???

I will verify the fuel pressure again, but if I remember right it was around 43 at idle, and jumped to 50ish when I touch the throttle. I have pulled the vacuum line off of the regulator and it jumped up and stayed, but didn't effect the way it ran. It was losing pressure after the key was off, but it was the check valve doing that. I have since changed the pump and it now holds pressure overnight. I have not changed the filter as it seems to flow plenty. With the bad check valve, setting at 0 psi on the fuel rail, turn the key on and it jumps to 50 psi within a couple of seconds. Maybe a fuel pump, and injector cleaner should be next on my list.

The smoke test might be worth it as well, if it's only $50.

[procaddytech]

With key on, fuel pump running, pressure should be 55-62psi. I'm confused as to what bank 0 is. Do you mean bank 1 and bank 2? Dtc's P0171 and P0174? Fuel pressure would be common to both banks causing lean codes. With the regulator hose off the pressure should jump back up into spec. Make sure the air duct between the MAF and throttle body is tight, don't want any leaks around the MAF. The throttle blade should be clean. If you are using an aftermarket air filter it can cause lean codes. Make sure the oil fill cap seals and the dip stick seals. Make sure the pcv is correct for the vehicle. You need to look for things that affect both banks. Seems like fuel pressure is low and if you have not changed the filter in a while, it would be a good idea. Vacuum leaks and fuel presssure/volume are the most common causes for both banks running lean.

[j cAT]

what your problem is you got the wrong fuel pressure spec's..

THE SHOP GM MANUAL STATES FUEL PRESSURE MUST BE 53-62 PSI. !

ON THESE FORUMS it is a common fact that members have what you have with 50 psi. below 50 psi the engine should not run or start.

the fuel regulator does not create any pressure... the fuel regulator drops pressure when vacuum is high [like idle or when down shifting ]


no/low vacuum [like starting engine] you should have 60-62 psi.

check for fuel in the regulator vacuum line if so its bad...

when checking fuel pressure see how long it holds pressure..if it drops off quickly and the injectors are not leaking the fuewl pump is bad ..low pressure at the fuel rail is 90 % fuel pump/fuel pump relay or bad grounds/wiring at the fuel pump..


purchase a delphi fuel pump..do not get the airtex asa they fail in a few months..

It always helps/makes this easier, and most times costs much less when you know what your doing with the shop manual...

[procaddytech]

I don't know where I got 51psi from. (edited)
Copied from GM Service information:
The numbers below refer to the step numbers on the diagnostic table.

2. When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 379-427 kPa (55-62 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.

15. A lean condition may result from the fuel pressure being below 379 kPa (55 psi). A lean condition may cause DTC P0131, DTC P0151, DTC P0171, or DTC P0174 to set. Driveability conditions associated with lean conditions can include hard starting, hesitation, poor driveability, lack of power, surging, and misfiring.

Yours is below 55psi. I would go with a fuel pump also as j cAT recommended. Delphi

[kleake]

I guess I have been in IT too long, I was thinking it was bank 0 and bank 1 when really it is bank 1 and bank 2. Those codes were set back in the winter as it normally isn't bad enough to trigger a code in the summer, although the problem is still there.

I double checked the fuel pressure last night. 50-52 at idle, jumps to 60 at WOT or by unplugging the regulator. This is the 3rd fuel pump I have had on here. The first one was changed as a possible source of this problem, but I did just the pump, not the assembly. After a while the check valve wasn't holding the pressure with the key off, so I just installed a new complete assembly. The problem existed through all 3 pumps. Also, the rough idle continues even if I unplug the regulator and fuel pressure is at 60psi, so I don't think pressure is the issue. I bought some concentrated injector cleaner last night, but am going to wait till I am below 1/4tank of fuel so I can run it pretty concentrated. I was hoping to find the type that hooks to the fuel rail directly, but I couldn't find any that did.

I also have a handheld scanner that I can monitor parameters while it is running. Does anyone have a good running 5.3 of the similar vintage that I could compare specs to such as o2 readings, and short/long term fuel trim settings?

[j cAT]

Originally Posted by kleake

I guess I have been in IT too long, I was thinking it was bank 0 and bank 1 when really it is bank 1 and bank 2. Those codes were set back in the winter as it normally isn't bad enough to trigger a code in the summer, although the problem is still there.

I double checked the fuel pressure last night. 50-52 at idle, jumps to 60 at WOT or by unplugging the regulator. This is the 3rd fuel pump I have had on here. The first one was changed as a possible source of this problem, but I did just the pump, not the assembly. After a while the check valve wasn't holding the pressure with the key off, so I just installed a new complete assembly. The problem existed through all 3 pumps. Also, the rough idle continues even if I unplug the regulator and fuel pressure is at 60psi, so I don't think pressure is the issue. I bought some concentrated injector cleaner last night, but am going to wait till I am below 1/4tank of fuel so I can run it pretty concentrated. I was hoping to find the type that hooks to the fuel rail directly, but I couldn't find any that did.

I also have a handheld scanner that I can monitor parameters while it is running. Does anyone have a good running 5.3 of the similar vintage that I could compare specs to such as o2 readings, and short/long term fuel trim settings?

when replacing the fuel pump with older higher mile vehicles you need to replace the fuel pump relay and make sure that the wiring is good..no voltage drop at the fuel pump...too much injector cleaner can damage the fuel system.

you do have a pressure issue and your pressures /issues say that.

explain what manufacturer fuel pump you used and the part number ...this will be helpful to others to avoid that manufacturers part............

after the fuel pressures are normal , then the other problems can be looked at...

[kleake]

Fuel pressures are normal as this is the same pressures I have had with 3 different pumps including the stock original. Also, it is the same pressure as before and after a regulater change. The pump doesn't have any problem building up to any pressure needed, the regulator is what is setting it.

If it was a pressure issue, why wouldn't unplugging the regulator so that pressure goes up to 60psi solve the problem temporarily? If spec shows 62 psi, and I am at 60, that shouldn't be that much of a difference,,, and my guage could be off by that much. There is more tolerance in the systems then that I would think.

[j cAT]

Originally Posted by kleake

Fuel pressures are normal as this is the same pressures I have had with 3 different pumps including the stock original. Also, it is the same pressure as before and after a regulater change. The pump doesn't have any problem building up to any pressure needed, the regulator is what is setting it.

If it was a pressure issue, why wouldn't unplugging the regulator so that pressure goes up to 60psi solve the problem temporarily? If spec shows 62 psi, and I am at 60, that shouldn't be that much of a difference,,, and my guage could be off by that much. There is more tolerance in the systems then that I would think.

I have not problem with you disagreeing with me on this pressure.

It is not my ride. you are the person effected.

I asked for so info in the other posts to narrow down this for you , still only your statement of can't be this or that...

do some of those tests and checks and post back your results, including what manufacturer made the fuel pump in it now , and the part number...

[kleake]

Originally Posted by j cAT

I have not problem with you disagreeing with me on this pressure.

It is not my ride. you are the person effected.

I asked for so info in the other posts to narrow down this for you , still only your statement of can't be this or that...

do some of those tests and checks and post back your results, including what manufacturer made the fuel pump in it now , and the part number...

I haven't had a chance to look at it any more in the last few days.

Original pump started getting noisy, and this issue was starting to appear, so I replaced the pump with a new pump only, not the assembly. I don't remember the brand as it has been almost 5 years ago. This pump has been holding 50psi at idle, and 60psi with regulator unplugged. Over time the check valve started bleeding down pressure badly, so startup was diffiucult unless you cycled the key a time or two first. Since the check valve was bad, I replaced the assembly with a generic one (Airtex E3509M). I realize this is a cheap pump, but I was really only needing the check valve in the top and couldn't justify $300 for that. I figure if the pump goes bad, I will put my original, or my second pump in that assembly so I have a good pump and check valve. This pump is brand new, and is holding 50psi at idle, and 60psi if I unplug the regulator. It pressures up the system instantly, and the startup issue is completely resolved, however the rough idle when cold is still there. The system does hold pressure with the key off for quite some time, exactly how long I am not sure, but I left it for an hour and didn't notice any drop in the guage.

Again, I realize the Airtex pump is cheap and may not last. I rebuild automatic transmissions on the side, as well as do all sorts of automotive, ATV, motorcycle, boat, and almost anything else repairs, so I understand the way these things work. I just have a hard time paying the price they want for some of these things when I know what is involved in them.

I am leaning towards injector issues since it is slowly progressing with age, and the poppet style injector is likely getting worn.

[mogyver69]

Hey guys, the problem this guy is having is a vacumn leak from his upper intake manifold gasket. Theres a bulliton for this issue but its probly out of warranty... still the bulliton helps the dealer mechanics with what to look for....Nothing bothers me more then someone telling this poor guy to go to the dealer and spend a few bucks.. I have been a dealer mechanic for yrs. 1 out of 10 mechanics actually knows what there doing.. Half the time the other mechanics were in my stall asking me to help them rather then opening a book and reading the scan tools... I commend people for trying to learn on there own.. I own a 99 silverado with the 5.3L with the same issue and I am getting ready to take a look at mine for the same issue. Also check your pcv system the plastic vacumn lines break and leak often. Hopes this helps!