Silverado, low fuel pressure?

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Truckedup
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Silverado, low fuel pressure?

Post by Truckedup » Wed May 02, 2018 6:37 pm

2001 4.8....Truck starts up quick, idles well, runs smooth, good acceleration but check engine light flashes sometimes during cruising, po300, random cylinder misfire....Fuel pressure with engine off is 49, disconnect regulator and it's 55....Running is 45 psi with regulator hooked up....Is this low enough to cause a misfire when driving at a steady speed on level ground at 50 mph?
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S10LS2
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Re: Silverado, low fuel pressure?

Post by S10LS2 » Wed May 02, 2018 9:35 pm

Running is 45 psi?? You mean at idle ? Or driving? Does it stay 45 psi under load and wot? What is the fuel pressure going down the road normal driving with medium load on the engine? Why keep sending unburnt fuel out of the engine thru the cat and out the tailpipe? The flashing check engine light means you have been warned.. I guess I could go look up the fuel pressure for you.. But sounds like you already know what the correct pressure is and you are asking what you can get away with? No need to disconnect regulator at idle and other bs. fuel pressure under load is what you are concerned with. What is it ? The light aint flashing at idle lol. P0300 is multiple cylinder misfire, means incomplete combustion and unburnt fuel going thru cat converter. Give her a pump and wires and possibly a regulator depending on what the diagnostic results show.. What are the o2 voltages goin down the road while the light is goin nuts on the dash board? 8)

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Re: Silverado, low fuel pressure?

Post by dirtracr5 » Wed May 02, 2018 10:25 pm

No thats not low enough to cause a misfire. Use a scan tool capable of telling you which cylinder is misfiring as its happening then go from there.

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Re: Silverado, low fuel pressure?

Post by S10LS2 » Wed May 02, 2018 10:44 pm

dirtracr5 wrote:
Wed May 02, 2018 10:25 pm
No thats not low enough to cause a misfire. Use a scan tool capable of telling you which cylinder is misfiring as its happening then go from there.
Your way off man it is p0300 meaning multiple, not a p0302,304 etc etc. pressure is low enough for sure to cause"multiple" What do you think 45 psi drops to under load? It certainly does not go higher. Factory spec is 52-58 psi with 55 psi being about normal.. Im not saying its the pump or not but have to be able to read the live data the scan tool and gauge are telling you as long as what the o2 voltages are? O/p did you read the voltages of the 02 sensor while driving? Is this still the original or second pump replaced in the truck?

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Re: Silverado, low fuel pressure?

Post by peejay » Wed May 02, 2018 10:45 pm

How old are the spark plugs?

Low fuel pressure will set lean codes LONG before it misfires. I've seen trucks running and driving at 9psi fuel pressure. (Okay, not running WELL, mind you...) If there are no lean codes present, and you don't have a scan tool to be able to watch live data while the problem happens, I'd first look at the spark plugs. 100k isn't a guideline, it's a "you mean you didn't have to replace them 40k ago? Wow, get those suckers out of there!" maximum.

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Re: Silverado, low fuel pressure?

Post by peejay » Wed May 02, 2018 10:49 pm

S10LS2 wrote:
Wed May 02, 2018 10:44 pm
What do you think 45 psi drops to under load? It certainly does not go higher.

Yes it does... that is the whole point of a vacuum reference fuel pressure regulator. Fuel pressure is always base pressure minus manifold vacuum.

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Re: Silverado, low fuel pressure?

Post by S10LS2 » Wed May 02, 2018 10:52 pm

peejay wrote:
Wed May 02, 2018 10:45 pm
How old are the spark plugs?

Low fuel pressure will set lean codes LONG before it misfires. I've seen trucks running and driving at 9psi fuel pressure. (Okay, not running WELL, mind you...) If there are no lean codes present, and you don't have a scan tool to be able to watch live data while the problem happens, I'd first look at the spark plugs. 100k isn't a guideline, it's a "you mean you didn't have to replace them 40k ago? Wow, get those suckers out of there!" maximum.
Wow :roll: total bullshit, except for putting plugs and wires, its all a guessing game as the guy posts limited information. you can buy a scan tool for less then 100 bucks that will read live data.. Dont throw parts at it fix it. LOL 9 psi

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Re: Silverado, low fuel pressure?

Post by S10LS2 » Wed May 02, 2018 10:54 pm

peejay wrote:
Wed May 02, 2018 10:49 pm
S10LS2 wrote:
Wed May 02, 2018 10:44 pm
What do you think 45 psi drops to under load? It certainly does not go higher.

Yes it does... that is the whole point of a vacuum reference fuel pressure regulator. Fuel pressure is always base pressure minus manifold vacuum.
LOL go away guy. jesus.. Put a gauge n it while driving and see what the pressure is.. That is what im saying.. Another speedtalk tech here..

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Re: Silverado, low fuel pressure?

Post by S10LS2 » Wed May 02, 2018 10:54 pm

peejay wrote:
Wed May 02, 2018 10:49 pm
S10LS2 wrote:
Wed May 02, 2018 10:44 pm
What do you think 45 psi drops to under load? It certainly does not go higher.

Yes it does... that is the whole point of a vacuum reference fuel pressure regulator. Fuel pressure is always base pressure minus manifold vacuum.
No shit if the pump is going bad.
First test should be a pressure test, o/p did it just did not do it correctly. Under load is where it needs to be checked.. Not here to argue with you master tech ..

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Re: Silverado, low fuel pressure?

Post by joe 90 » Thu May 03, 2018 3:36 am

Is the base pressure supposed to be 3, 3.5 or 4 bar?
45 PSI doesn't sound right.....43.5 ish or 50 ish. Or 58 ish.
But 55 dropping to 45 sounds about right for a 4 bar system, given gauge error.

Easiest test is to data log oxygen sensors, make sure they're cycling up and down.
That means it's in closed loop and the AFR is averaging lambda 1.
Try to read the fuel trims off the logger.

Better still take a look at the sparkplugs or do both.

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Re: Silverado, low fuel pressure?

Post by Truckedup » Thu May 03, 2018 6:07 am

Thanks...The fuel pressure under load jumps up slightly to about 47 then settles at 43-45 psi again...I can't feel or hear any misfires, no hesitation, normal idle, but the CEL says PO300.....I'll get a scanner on it...So...just wanted to know if 45 psi fuel pressure by itself can trigger a PO300
The plugs look new, dull slightly off white ...NGK TR5 non platinum, .040 gap
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Re: Silverado, low fuel pressure?

Post by peejay » Thu May 03, 2018 6:21 am

S10LS2 wrote:
Wed May 02, 2018 10:52 pm
peejay wrote:
Wed May 02, 2018 10:45 pm
How old are the spark plugs?

Low fuel pressure will set lean codes LONG before it misfires. I've seen trucks running and driving at 9psi fuel pressure. (Okay, not running WELL, mind you...) If there are no lean codes present, and you don't have a scan tool to be able to watch live data while the problem happens, I'd first look at the spark plugs. 100k isn't a guideline, it's a "you mean you didn't have to replace them 40k ago? Wow, get those suckers out of there!" maximum.
Wow :roll: total bullshit, except for putting plugs and wires, its all a guessing game as the guy posts limited information. you can buy a scan tool for less then 100 bucks that will read live data.. Dont throw parts at it fix it. LOL 9 psi
Furthermore. If the PCM is outside of fuel control (fuel trims outside of acceptable) it won't monitor for misfire. You can run so lean that the engine is choking and surging and it won't set misfire codes because it won't be looking. You'll have yourself a fine 171 and 174, though.

But go on, explain how this is bullshit.

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Re: Silverado, low fuel pressure?

Post by peejay » Thu May 03, 2018 6:25 am

Truckedup wrote:
Thu May 03, 2018 6:07 am
Thanks...The fuel pressure under load jumps up slightly to about 47 then settles at 43-45 psi again...I can't feel or hear any misfires, no hesitation, normal idle, but the CEL says PO300.....I'll get a scanner on it...So...just wanted to know if 45 psi fuel pressure by itself can trigger a PO300
The plugs look new, dull slightly off white ...NGK TR5 non platinum, .040 gap
What are fuel trims under heavy load (but not heavy enough to go into open loop)? That is the only thing that matters. If fuel trims combined long and short term are under 15-20% or so then look elsewhere. Most/all of these older trucks are calibrated for E0 so you will probably never see fuel trims under 10% combined. Only recently did they start calibrating for a stoich of 14.2 instead of 14.7.

You will generally see fuel trims climb up into the 30-50% combined range before you, the driver, notice anything is wrong.

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Re: Silverado, low fuel pressure?

Post by gnicholson » Thu May 03, 2018 8:53 am

The guy above is correct that it will set lean codes long before it will misfire from being to lean.

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Re: Silverado, low fuel pressure?

Post by Truckedup » Thu May 03, 2018 9:08 am

gnicholson wrote:
Thu May 03, 2018 8:53 am
The guy above is correct that it will set lean codes long before it will misfire from being to lean.
So what you are saying is PO300 is related to ignition issues or a mechanical problem more than actual fuel issues? I know how modern engine management works..but I'm from the thinking that if the engine pulls well under load and idles smooth, the ignition is doing it's job and the mechanical stuff must be ok.......
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