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Chris (aka-Moose)
Chris (aka-Moose), Technician
Category: Chevy
Satisfied Customers: 47967
Experience:  16 years of experience
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I have a 2003 Silverado SS that has been experiencing

Customer Question

I have a 2003 Silverado SS that has been experiencing intermittent driveability issues and reduced fuel mileage for several months. The symptoms have been: Misfire when tipping into the throttle as speed, low power, both upshift and downshift points affected. Initially I had a MAF code and O2 sensor codes. Changed all sensors and MAF. While chasing this and based on various parameters at the time, the following have also been changed. MAP sensor, fuel filter, spark plugs and wires, throttle body, coils from another vehicle, injectors, fuel pressure regulator, air filter. The engine was changed prior to all this at the local chevy dealer. Original muffler and tailpipe replaced with Ultra Flo X muffer and turn downs. Cats seem to be okay, as temps are consistent across the pipe before and after and including the cat. Flow check for cats using MAF data showed cats okay (Stop to 1-2 shift showed MAF went from 8 g/sec to over 300 g/sec with spec I was given to be at least 230 g/sec). Vehicle has been tuned several times due to fear of tuning to bad components, such as the MAF sensor. Tuning has helped, but not eliminated the problem. ECM recently changed with no help. Fuel pressure early on showed 50 psi, but recently it was rechecked and i found the following and main reason for inquiring. Fuel pressure at idle - 44 to 45 psi with fpr vacuum line on, 53 psi with it off. Pressure would jump from 44 to 54 when stepping on the gas, but fall back to 40 when letting off and then stabilized at 44 psi. Unregulated pressure of 53 not affected by throttle. Drove vehicle with both configurations and experienced both the problem and good performance with no effect on fuel pressure. I suspect a bad from stock regulator, so I plan to return it and put a new one on. I still have the old one also. Local chevy dealer said the pressures were okay (he said 55-62 psi unregulated, but 53 not a cause of issues), but internet sites vary what they say is the proper spec. Current econonmy is 10 mpg average city and highway. To help describe it, I saw that after the muffler change the airflow had caused alot of lean places in the tune and thus, alot of timing being pulled. We tuned it and things got way, way better, but later returned in the very same fashion. Getting the tune and things in line helped. Found the bad MAF, so we had tuned to a marginal MAF that went further out and we then retuned after new MAF. Temporary improvement again. It feel like the cylinders are being mixed up or cut out kind of like a DOD engine when only on 4 cylinders until the others are brought online. You can hear this, feel this, and the acceleration is greatly affected when acting up. Runs strong and smooth when not. Sorry for the long story, but I thought you deserved all the data I had. My question would twofold at first. Do you agree that the fpr might be bad out of stock? Do you find the fuel pressures concerning, from either a fpr or fuel pump standpoint? This is a return style system. I have a 1975 K5 2wd Blazer with a 6.0 in it, and it is tuned by the same guy and runs great, even with a cam and headers, and gets better mileage than the truck. Thanks for any help you can offer. I will try to answer any questions you might have. I am sure I missed something in my description. Thanks again. *****
Submitted: 11 months ago.
Category: Chevy
Expert:  Chris (aka-Moose) replied 11 months ago.

Welcome, I'm Chris (aka Moose) I prefer to work when others don't, I am here around lunch and after dinner till after midnight.

  • Sorry no one has helped yet.
  • This is not an answer to your question, just an inquiry if your still needing/wanting help. Because I believe I can help.
Customer: replied 11 months ago.
Hello Chris! I rechecked the fuel pressure last evening, with it still showing the described low numbers. I subsequently removed the relatively new fuel pressure regulator and returned it to the auto parts store. They did not have a replacement, so I put the original one back on. Pressures are now 52 and 62 psi (fpr vacuum line connected and removed, respectively). The performance and driveability seem much better. I would like to continue the conversation, as I have only driven it a short time since last night and I have seen the problem get better for a day or two and then come back. The fpr might have been the culprit as bad out of stock. Jim
Expert:  Chris (aka-Moose) replied 11 months ago.

Do you want to drive a couple of days then report back on what symptoms remain and we can go from there?

Customer: replied 11 months ago.
please. I have not had the problem as of yet, except for a possible episode this morning, but it was so subtle and then went away rather quickly. I am watching my mileage and it appears to be up 3 mpg both city and highway. I will come back with more info in a couple more days, unless it has a definite flare up. Thanks.
Expert:  Chris (aka-Moose) replied 11 months ago.


Expert:  Chris (aka-Moose) replied 10 months ago.

Hello again Chris here,

  • I am following up with you to see if you got everything you needed from my answer.
  • I have not heard back from you, so I assume you were not pleased. We're you pleased with my help?
  • If you were not pleased with my help, please let me know by replying so I can help more.
  • This post is coming up on the 7 day mark where it times out. When a post times out, that means it's no longer on my active list (you reply and I may never know it.)

Thanks Chris

Customer: replied 10 months ago.
Hi Chris,Sorry, I got tied up in an alternator change on out old minivan getting it ready to sell. 2002 Venture van is a complete nightmare for an alternator change. The truck seemed to respond well to the pressure regulator change, but after a few good days, it was back today on the way to work. I have to run an errand at lunch and will get long some more miles. Today was a run with the fpr vacuum line off, which would raise the fuel pressure as discussed, so I was dissappointed with it. Not sure if I have a weak and/or intermittent fuel pump or something else. It would seem that a fuel pump would be more consistent, but I guess it could be intermittent, especially if electrical in nature. I am going to look at the STFT's while out too. Just to be sure I have things correct, can you confirm what I should be seeing for fuel pressure on this return style system, both in the regulated and unregulated state? Thanks!
Expert:  Chris (aka-Moose) replied 10 months ago.

53-62 is the fuel pressure spec

Customer: replied 10 months ago.
I am typically seeing 50 and 60 psi. Do you think that this amount under spec could cause issues such as mine? I hate to change a fuel pump, but other than the throttle pedal, I think I have changed about everything else. On the phone, the chevy dealer said I was in spec and these numbers should not be causing the problem, but some internet forums, tech sites, etc., would say yes. Even an intermittent one like mine. I could see the injector spray pattern affected by pressure, but where is the line of what is too low? That is my current dilema. You would think that with the vacuum line off and running 60 psi would solve the problem seen at 50 psi regulated, unless there is something related to not being regulated that causes a problem. I am off on an errand. I will get some STFT data. The only other thing I could think of is that we once again tuned it with a bad component (bad out of stock fpr this time) and we need to do another tune. I just had an idea. Could my exhaust turndowns be causing heat buildup under the truck that is affecting the gas tank and fuel pump temperature and performance? It is stock exhaust manifolds, cats, and mid-pipes that feed into the 3 inch duel inlet and outlet Dynomax Ultra Flo X muffler into two 3 inch turndowns just before the axle? That is a real out there hypothesis, but something to consider. The problems did seem to arrive after the muffler change. Tune was way off due to the higher flow, but we brought everything back in with a tune, but only temporarily. The details are part of the original post I believe, so I won't go through the whole timeline here again.
Expert:  Chris (aka-Moose) replied 10 months ago.

I see no problem with 50 when 53 is the minimum. I do not believe the exhaust is to blame. Though fuel and pump temp can effect a weak pump. Your pressure is close enough that I believe we can rule that out.

Customer: replied 10 months ago.
The only things I have not changed are the cam position sensor, crank angle/position sensor, and the accelerator pedal. Not sure if either o the sensors were part of the GM long block, but I doubt it. Someone on the Silverado SS forum mentioned both a crank and cam position relearn due to the engine change. I have no codes for either of those and have not the entire time I have had the issue. Would the relearns be part of the engine change procedure at the dealer? I have started to do a bit of research and see alot of info on the crank relearn, but not anything on a cam relearn. I will look some more. Is that a possible source of my problems?
Expert:  Chris (aka-Moose) replied 10 months ago.

If the engine was swapped at the dealer they would of done the relearn procedures or it would not have ran to leave the dealer

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