: Severe engine miss at idle-dirty gas in filter



Steven P Howe
06-01-08, 08:53 PM
I have a 2002 SLS w/Northstar, of course. Last week, it developed a severe engine miss at idle. But sometimes it's noticable at any speed. After a few days, the service engine light came on. I'm fortunate enough to own a Autoxray6000 diagnostic unit, so I scanned it and received the P0174 Too Lean Bank 2 code. I cleared it and filled up with 93 octane with boost and treatment. Not much change. Next day, light came back on with P0171 Too Lean Bank 1. So, I replaced the fuel filter. The gas that came out of the old filter was almost as dark as a cup of coffee - literally. I poured about a half-inch of it into a can and couldn't see the bottom of the can. So, with new filter had a great improvement for about a day and now back in the same boat. My main question is: If the MAF sensor is dirty and/or the throttle body area internally, could it make the gas dark and dirty, or is the dirty gas coming from the gas tank? Any advice will be appreciated.
Thanks,
Steve

Ranger
06-01-08, 09:19 PM
First off, you don't need Autoxray. See the sticky at the top of this page.

Check for a vacuum leak at the plenum. The plenum on the Northstar is the rubber coupler between the TB and the manifold. It usually cracks on the bottoms side where it is not visible, but a shot of Brakleen will give it away. I'll bet that is your problem.

Steven P Howe
06-01-08, 09:43 PM
Well, I don't see a "sticky" regarding trouble codes, but I'm new to the site, as you can see and I also didn't know that I couldn't post in 2 sections.
I will definately check out the air leak idea. Would that make the fuel in the fuel filter dark?
Thank you for the help. This site is excellent.
Steve

Ranger
06-01-08, 09:51 PM
Sorry. I should have said the sticky at the top of the Northstar page where you posted this thread.

The problem with double posts is that people have a hard time following what has been said and done and thus you don't get the proper help.

Steven P Howe
06-01-08, 10:35 PM
:bonkers: OH, I see the sticky now. Where to check the codes. Pretty handy.
Thank you.
I will look for the vacuum leaks in the morning.
I'm just not sure if I need to have the fuel lines flushed out somehow because the gas from the old filter was really dark.
I will post results tomorrow.
Steve

Submariner409
06-02-08, 11:12 AM
Steven, You asked a question about the MAF sensor and the TB affecting gasoline quality. No way. Gasoline never comes in contact with either device. They are strictly air flow monitoring and control mechanisms used to send signals to the PCM in order to maintain correct injector rates and therefore fuel mixture. (The closest fuel injector is 10" downstream of the TB).

If your fuel filter was as dirty as indicated, then the new one has solved that problem.

As suggested by Ranger, use a stethoscope and start probing for vacuum leaks around the TB-to-intake manifold plenum and around the base of the intake manifold itself. (The intake manifold bolts should be torqued to only 8 or 9 ft/lb, so they won't feel "tight".)

Search back in here 6 months or so and read up on throttlebody cleaning. Read up first, then clean...........

Practice pulling the codes, but don't get paranoid over voltage or serial codes which crop up here and there. If a code doesn't set for 3 ignition cycles it goes to H (history) and if no reset for 40 cycles, it clears itself. Just adapt yourself to the diagnostic functions of the INFO, RESET, and ON/OFF buttons and you'll be able to look at your car's trends any time. No scanners needed.

Steven P Howe
06-02-08, 12:49 PM
Well, I spent this mornng looking for vacuum leaks and finally found it coming from the large rubber connection between the intake plenum and the throttle body. I can't see a break, but when I spray air and/or brake cleaner there, I hear the leak change and the engine revs a little. Looks like a moderately involved job, though. You will probably ask, "Am I sure it's coming from there"? But I spent 1 1/2 hours chasing the leak including taking off most vacuum hoses (one at a time) and inspecting under magnification. It sure doesn't seem likely that this part would go bad, unless it's a defect?
I'll check back.
Thanks again for your help.
Steve

Ranger
06-02-08, 12:58 PM
Steve, that is actually a common failure on the 2000+ Northstars. Do a search for plenum. It has been discussed several times and there might be instructions on replacing it. Not terribly difficult. I am sure you found the problem and replacing it will solve it. Remember, on most engines the intake manifold is called the plenum. Not so on the Northstar. The "plenum" is the rubber coupler. Just to be sure you get the correct part.

Steven P Howe
06-02-08, 02:00 PM
Oh, so the plenum is the rubber part I have identified?
Upon further leak chasing, I find that when I unhook the small vacuum line from the Air Control Valve, it makes no difference; also no air coming or going from the valve or the hose. Whatcha think about that?
Thanks,
Steve

Submariner409
06-02-08, 02:25 PM
The A.I.R. (and EGR if vacuum operated) signals are not manifold vacuum: they are "ported", I.E. dependent on throttle opening and engine load.

Yes, the "plenum" is the rubber bellows, held by a hose clamp and plate, between the throttlebody flange and intake manifold spigot. $35 + or -, dealer only. $200 or so, dealer installed. Look for the procedure in here some weeks or months ago. Not difficult with hand tools.

eldorado1
06-02-08, 05:42 PM
There is no reason for your gasoline to be coffee colored. Since you have a nylon tank, there isn't anything to rust in it either. It is hard to say what would cause that. It could be your fuel pump disintegrating, or it could be some bad gas after a fillup. If the gas has too much ethanol in it, it can become hygroscopic and cause any steel in your fuel system to rust. There was some testing done of local gas stations with ethanol in the 12-15% content, despite the national limit of 10%.

I would be tempted to drain the tank (through the fuel rail schraeder valve), put that gas in the lawn mower, and change the fuel filter once more.

Before that though, check the fuel pressure.

Steven P Howe
06-02-08, 06:14 PM
Well, I will obtain the parts tomorrow and tackle the job in the afternoon. I will post the results.
You ALL have been a great asset to this forum and for me. Thank you.
Steve

Steven P Howe
06-02-08, 08:57 PM
Ahah: To Eldorado1: (I've wanted an Eldo all my life, but settled for Seville)
So how can I check the fuel pressure? I guess there is some sort of guage that I would have to purchase? And where would it be connected to the vehicle?
Regarding the dark gas: I first thought this would solve my problem, but I guess there is a vacuum leak that I'm going to tackle tomorrow afternoon. But I really feel like the whole system needs to be flushed. I've changed fuel filters on most vehicles that I have owned (probably 15) in the past and never, never saw anything like this one when I changed it. What I poured out of the old filter had the viscosity of gasoline but the color of oil - black! The fuel that came from the tank side fuel line was clear, but from the engine side was somewhat dark.
Thanks for the help.
Steve:worship:

Ranger
06-02-08, 09:42 PM
If you look at the fuel rail, you'll see a schreader valve on it. Get a fuel pressure gauge, remove the cap (just like a tire valve) and screw the fitting on to it. You should be able to get a fuel pressure gauge for about $40.

Don't know what to tell you about the dark colored fuel. That's very strange.

eldorado1
06-02-08, 11:44 PM
Ahah: To Eldorado1: (I've wanted an Eldo all my life, but settled for Seville)
So how can I check the fuel pressure? I guess there is some sort of guage that I would have to purchase? And where would it be connected to the vehicle?
Regarding the dark gas: I first thought this would solve my problem, but I guess there is a vacuum leak that I'm going to tackle tomorrow afternoon. But I really feel like the whole system needs to be flushed. I've changed fuel filters on most vehicles that I have owned (probably 15) in the past and never, never saw anything like this one when I changed it. What I poured out of the old filter had the viscosity of gasoline but the color of oil - black! The fuel that came from the tank side fuel line was clear, but from the engine side was somewhat dark.
Thanks for the help.
Steve:worship:

I think most places now rent fuel gauges for a "deposit" (equal to the cost of the gauge in case you keep it). Remember that the fuel lines can hold a pressure for quite some time, so a rag comes in handy when you connect or disconnect the gauge.

Most gauges have a purge valve that would let you remove fuel from the fuel rail. I'd probably grab a glass cup and purge a little fuel to see what it looked like.

Steven P Howe
06-03-08, 08:16 PM
UPDATE: Vacuum leak solved thanks to you guys. It was the plenum boot, as Cadillac calls it, and I went ahead and got the injector o-rings and intake gaskets and did the procedure with 100% success. I can't thank you enough. The dealer quoted me $400 to replace the boot and I spent roughly $100 on the boot, gaskets, and o-rings.
I will address fuel pressure next just for kicks, but the vehicle running like a swiss watch now. No miss or hesitation at all.
Thank you very, very muc.
Steve

jeffrsmith
06-03-08, 09:38 PM
Seek knowledge, perform task, success!!!!

That's what these forums are all about, glad to hear that you had a successful repair with minimal expense (well, somewhat minimal).

Jeff

Ranger
06-03-08, 09:43 PM
Another happy costumer. :thumbsup: