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Discussion Starter · #1 ·
Guys, I need some help here. I have a 2004 Silverado, 2500HD 4wd. The knock sensors were bad so I replaced them. I got a new intake gasket when I put it back together and ever since I've been blowing these two codes. When I keep the engine under 2K RPMs it is fine, but under a load it loses power and starts to backfire. Fuel injector pressure is between 50-60. I'm at the end of my rope I've replaced the following: New intake gasket, new intake, MAP sensor, MAS airflow sensor. I can't figure out where the problem is. Also, another issue, may or may not be related, I'm having trouble at times with my brakes working...Any suggestions would sure be appreciated! Thanks in advance!
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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did you also put on a new wiring harness for the two new knock sensors (it's just from the knock sensors to just outside the intake manifold, where it connectors to the main wiring harness)? maybe unplug it and check the resistance for both knock sensors (at the connector, not taking off the intake manifold).
 

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After you verify the wiring, I would check for a vacuum leak next. Your truck is running lean which means too much air is getting into the system compared to the fuel. This usually happens due to a leak in the intake (after the MAF). Provided the MAF is functioning correctly and telling the truck the correct amount of air going in, then there is likely a leak after the MAF letting in extra air between the MAF and before it gets to the cylinders and mixed with gas. A vacuum leak will let in that extra air that the truck computer is not expecting causing the Lean Condition.
Also, a faulty O2 sensor could be causing the codes as well but generally, when they both (right and left banks) are set, it is not an O2 sensor problem.
 

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Usually the leak will be along the intake manifold gasket or one of the attached vacuum lines leaking. I don't think the intake would be an issue as it is prior to the MAF.
There are several ways to check for a vacuum leak. Just search on youtube. There are a bunch of how to's showing different methods.
 

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It's easy to get a manifold leak once you remove or replace it.
Did you tighten the manifold bolts in sequence with the right torque specs ?
You can try and spray around it to find a leak. There are many different ways to spray it and that's a topic all in itself.
You can try soap, propane, wd40, and many others. Some like soap you look for bubbles. Some like propane you listen for engine changes.
 

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Discussion Starter · #8 ·
Guys, I checked the resistance on both knock sensors and they both were 100 K ohms.
I had used water with the old intake and couldn't find any leaks. I'll do it again with this. I did tighten the bolts to the specified torque in 2 round, and according to the right sequence.
I used a smoke test too, and couldn't find anything.
Any other ideas?
I read somewhere to disconnect the o2 sensors and run the engine for a bit and see if that makes a difference? Is this a good idea?

thanks again, I so appreciate all your time.
 

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Discussion Starter · #9 ·
One bit of info, not sure how it relates, but I just water tested the intake and couldn't find any leaks. I unplugged the negative terminal on the batt too. I started it, and let it run for a while. It idles perfectly. Then I slammed the hood shut (was going to test drive it), and just when I did that, the engine stumbled and threw code P0102????

Any ideas?
 

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Not sure if the disconnected battery has anything to do with it, but P0102 points to an issue with your MAF. If so, that could be directly related to the rich fuel mixture problem.

Make sure the wiring is all good and the electrical connector is connected fully. Otherwise, I would try cleaning the MAF especially if you have an aftermarket air cleaner and reusable filter. Then, I would consider trying a different MAF (Mass Airflow Sensor). I know you already swapped it out once, but maybe you got a bad one.

"A code P0102 may mean that one or more of the following has happened: The MAF may be disconnected, or a wiring connection may be bad The MAF may be dirty or otherwise contaminated (Note: if you use a reusable oiled air filter, be careful not to apply too much oil or that can contaminate the MAF). The MAF sensor may be faulty The vehicle computer may be faulty (very rare)"

If you have access to a code reader, many will have the ability to read real time fuel mixture ratios and you might get a better idea of what is going on.
 

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Discussion Starter · #12 ·
esox07 thanks so much for your help. Yes, I switched out the old MAF and it still does the same thing. It idles great, but as soon as you try to drive it and the RPM's exceed 2500 it dies out. This also seems to be getting worse. How could I check if the vehicle computer is bad? I don't know what else to do, and right now this is our only vehicle.
Maybe I'll get a better code reader and see what other data I can gather.
Thanks again!
 

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Thanks Gozzie.
One other thing I would like to add is to be extremely careful with the MAF. You don't want to touch the sensor part of the MAF. It is extremely sensitive. So if you clean it or replace it, be very careful when handling it.

As far as the vehicle computer goes, I doubt that is the problem....you would probably have a lot of other issues going on if that was bad, but it is possible. Other than taking it to a shop or the dealer with proper equipment, I don't know how to check the ECM (Engine Control Module).
 

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Discussion Starter · #14 ·
Thanks. There are other issues going too...the language on the drivers display changes randomly, and often none of the steering wheel display controls work. Also before this latest problem, while getting new ball joints, the garage damaged one of the ABS sensors. They replaced it, only to find out that there is some electrical problem in the circuit and now the light won't go off.
I think I'll have to breakdown and take it to the dealership.
 

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For the cluster problem, read this.... Instrument Panel Cluster

Got freeze frame data on your scanner? See where the codes set, at or near idle is a vacuum leak, under load will be fuel delivery or MAF problem. From your slamming the hood and getting a code description, it's possible you have a broken wire on the MAF connector, tug on each one and see if one separates.

For the ABS, get it scanned. Some parts stores will loan you a scanner that will do ABS codes.
 

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Try this for the language problem on the instrument cluster. While you are disconnecting the battery and forcing the truck computers to reboot, it might just solve one or more of the other issues as well.

Odometer Issue
 

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Discussion Starter · #17 ·
Thanks for all the input. Should I replace the o2 sensors? Here's a little more history on what happened: I replaced the knock sensors, put on a new felpro gasket, it didn't fit really well, so I really had to wrestle it in there. Then I test drove it for a maybe 20 miles, and everything seemed fine. So I left on a 12 hour trip (pulling a small trailer) and 1 hour into the trip the left bank code came on. (I stopped to check the code at a parts store). I had no choice but to keep driving. By the time I got home both codes had blown. So I replaced the gasket with the OEM plastic one instead of the metal Felpro. Could it be over the course of the trip I ruined both sensors? Or could I have destroyed some of the injectors? Thanks again!
 

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The O2 sensors just measure the air/fuel mixtures before and after the catalytic converters and determines if there is a problem with the cats or fuel mixture. While it could be that one wen't bad, the fact both are throwing codes, leads me to believe that the problem is elsewhere.

Here is a good explaination of the codes, possible causes, common fixes and also shows that incorrectly blaming the Oxygen (O2) Sensors is common as well.

http://repairpal.com/OBD-II-Code-P0171-and-P0174
 

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Not to hi jack thread because checking to see if it gets resolved to expand my repair knowledge ;)
But esox07 do you subscribe to this repair pal thing it looks like it can come in real handy. I will look into it thanks. And glstevens let us know what you find?
 

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No, I dont subscribe. I just got to it through a google search on the codes. I am very skeptical of the online "for pay" help sites. I think that most of them are less reliable than sites such as this one.
 
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