Intake Manifold Problems

[halflenny]

Guys I'm kinda new here, and am about to lose my mind

I have the 4.8L Vortec in my 2002 silverado 2WD. It is idling very rough and very low, and is storing codes p0171, p0174, and p0300 (Bank 1+2 too lean, random misfire). I just replaced the MAF sensor, because that seemed the logical option. Fuel pressure is fine, and just replaced the fuel filter. When the brand new MAF is unplugged, the rough idle goes away. Can someone explain to me why unplugging the MAF would smooth out the idle if this is an intake gasket problem? I'm having trouble seeing how they are connected.

 

If anyone can explain this it would really save me a lot of frustration.

 

Also, there seems to be a lot of play in the Evap canister purge valve fitting... is that normal?

 

Thanks -- Lenny

[txab]

What fuel pressure reading are you getting? Spec is 55-62 with key on, engine off. Have you checked the fuel pressure regulator? Check for intake leaks, quite common.

[GMstrong]

I would check the ECM ground,But would also get some carb cleaner and spray around the intake manifold.Hows the wiring to the MAF?

[halflenny]

Actually, the fuel pressure reading we got was between 48 and about 54, depending on when we checked it. I didn't have the specs for the '02, but the (I think) '00 or '01 specs were 60-65. I called my mechanic and he said it should be 48-52, so I didn't give it more thought. Not to be insulting, but how sure are you about that number, txab? I don't have the book.

 

GM: I sprayed carb cleaner all around, didn't seem to change much. When I sprayed the EVAP canister purge valve, I can hear it suck it in, but it doesn't do much, if anything, to the idle. Also, every time I kill the engine now, I get a clicking/gurgling sound, sounds almost like the line is depressurizing somewhere up near the throttle body.

 

Currently heading to AutoZone to make sure they didn't give me a bad MAF. I'll check the wires and get back to you.

 

Thanks

[txab]

From the GM service manual.

2002 4.8 VIN: V

 

(55-62 psi) VIN: (V, T,U)

 

Low fuel pressure can cause lean conditions and misfire.

[halflenny]

Ok: went down to AutoZone, picked up a new MAF thinking the one I just got was faulty. Truck ran great, got all the codes cleared, and drove it around without any problems. After the ECU relearned everything, it went right back to acting up. Unplug the MAF, and it runs fine. Plug it back in, tries to stall. I plugged into the computer and watched the fuel trim while spraying carb cleaner around the intake gaskets, and that was definitely a problem. I couldn't hear it very well in the idle, but the computer can't lie. I'm going to be tearing into these gaskets here shortly; hopefully that will solve the problems. I will post the results when I get done. I just can't figure out what unplugging the sensor has to do with correcting the bad idle if it hasn't gone bad.

 

Fuel pressure read between 52-54 this morning, so not perfect, but I'm thinking the gaskets are the underlying problem.

Thanks for the help guys.

Lenny

[txab]

These fuel systems can be pretty picky. Even a few pounds off and you can have problems

[Doug_Scott]

Ok: went down to AutoZone, picked up a new MAF thinking the one I just got was faulty. Truck ran great, got all the codes cleared, and drove it around without any problems. After the ECU relearned everything, it went right back to acting up. Unplug the MAF, and it runs fine. Plug it back in, tries to stall. I plugged into the computer and watched the fuel trim while spraying carb cleaner around the intake gaskets, and that was definitely a problem. I couldn't hear it very well in the idle, but the computer can't lie. I'm going to be tearing into these gaskets here shortly; hopefully that will solve the problems. I will post the results when I get done. I just can't figure out what unplugging the sensor has to do with correcting the bad idle if it hasn't gone bad.

 

Fuel pressure read between 52-54 this morning, so not perfect, but I'm thinking the gaskets are the underlying problem.

Thanks for the help guys.

Lenny

 

 

It is pretty simple actually. When the MAF sensor is connected, it measures the amount of air going into the engine. If you have a vacuum leak, that will introduce air into the engine that the MAF did not see. It basically confuses the MAF sensor.

When you disconnect it, the engine goes into a kind of failsafe mode (some call it limp mode). It uses a predefined set of parameters that will allow the engine to run, like open loop mode.

[halflenny]

Hey Guys,

 

I still want to run a few more tests on the fuel system, since the number is a bit low. However, I did the intake gaskets on Wednesday and the truck has run without a hiccup since then. Even got a cold start this morning (~20 degrees F) and it didn't even think about missing. I think the problem has been solved. Since I had so much trouble finding this thing, I will post how I finally diagnosed it.

 

I hooked the car up to a diagnostic computer, and set it to give me a real time fuel trim reading. I then put the monitor up under the hood where I could see it, and soaked the entire length of the gasket with carb cleaner. Watching the meter, the fuel trim ratios for the bank that I was spraying would decrease significantly. I would play with the throttle controller to burn off the excess carb cleaner, and the number would go right back up. When I sprayed the other side, I saw similar results. The compensating feature of the ECU was good enough that I couldn't hear the change, but the numbers were pretty clear. For anyone seeing these problems, I would recommend checking it like this early, rather than checking and replacing other parts to rule them out.

 

Doug: thanks for explaining! I was beating my head against the wall because to me, that signaled a bad sensor. Your explanation cleared that up for me.

 

 

Thing's running like a new machine, hopefully it stays like that. Thanks for all the help.

[esthrlnd]

The stock intake manifold is faulty. You're going to run into this issue again and again unless you replace the manifold altogether. There's a plate you can use to help protect the gasket but you'll still have to deal with it. Google search 02 GMC 4.8L intake manifold.