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Old September 7th, 2009, 12:02 PM   #1
clint357
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Default Sonoma running rough

My 1998 Sonoma is running rough so I had it scanned. It said I needed a new MAS and cam shaft position sensor. I put both in and it's still running rough. At idle it just seems to shake a little, almost like a misfire but I don't think it's a misfire because it would show up on the scan. Is there some kind of electrical problem that would make both of these sensors not work? If so, is there a way to test and make sure the sensors are getting power?
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Old September 7th, 2009, 12:11 PM   #2
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bad spark plug maybe check out your plug wires make sure they are off the manifold
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Old September 7th, 2009, 12:35 PM   #3
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Clear the codes and drive it for a while to give computer time to adjust. Then scan it again and see if it shows anything. Your timing chain could also be stretched. You also have the famous intake manifold gasket leaking motors and that will also cause it to run rough.

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Old September 7th, 2009, 01:51 PM   #4
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Scanner says that there is some sort of short for both sensors. I then found that part of the wiring has been melted by the exhaust manifold. I'll fix the wires and see if that does the trick.
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Old September 7th, 2009, 07:26 PM   #5
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Damn! not it. Time to take it to somebody who knows more about electrical than I do.
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Old September 7th, 2009, 10:05 PM   #6
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My 1998 Sonoma is running rough so I had it scanned. It said I needed a new MAS and cam shaft position sensor. I put both in and it's still running rough. At idle it just seems to shake a little, almost like a misfire but I don't think it's a misfire because it would show up on the scan. Is there some kind of electrical problem that would make both of these sensors not work? If so, is there a way to test and make sure the sensors are getting power?
the ECM will never tell you what part to replace, it can only tell you what it sees at the end of the wire.

I hate it when people say that "the computer said this part is bad". No, it didnt, it is not possible. The computer cannot test the input to any sensor, nor can it tell if there is a problem in the harness between itself and any sensor.


You said you found some melted wires (ding ding ding, red lights flashing and all that) So exactly how did you fix them? did you use some butt connectors and just splice in some wire, or did you actually pay attention to the guage of the wire and do a good job of soldering them in? Personally, I prefer not to have any splices if I can avoid it, and I remove the terminals from the connectors at each end of the wire and install a single new wire with no splices.
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Old September 7th, 2009, 10:11 PM   #7
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check and clean out the EGR. they are famous for that. its a start for ya.
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Old September 22nd, 2009, 08:14 PM   #8
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OK thanks for the help. I did a little research on the egr valve and it seems like a likely suspect. I also noticed that it seems to idle better on a full tank? When it gets down to about 2 gallons it REALLY runs rough....any ideas on that?
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Old September 22nd, 2009, 08:32 PM   #9
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I also noticed that it seems to idle better on a full tank? When it gets down to about 2 gallons it REALLY runs rough....any ideas on that?

Water in fuel.
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Old September 22nd, 2009, 08:37 PM   #10
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Water in fuel.
I've ran a couple of tanks through it since I bought it though.
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Old September 22nd, 2009, 09:30 PM   #11
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4.3 liter? If so check fuel pressure they are real finicky, needs to be between 60 and 66psi. Schrader valve right by throttle body and Autozone has a nice gauge in their loan a tool program.
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Old September 23rd, 2009, 06:37 AM   #12
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If it is the 4.3L check the EGR
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Old September 23rd, 2009, 08:00 AM   #13
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4.3 liter? If so check fuel pressure they are real finicky, needs to be between 60 and 66psi. Schrader valve right by throttle body and Autozone has a nice gauge in their loan a tool program.
I'm thinking this might be it. It would explain why it would be making more pressure on a full tank. I'll get a gauge and give it a checkup. Thanks.....I was loading some brush in it last night and noticed a quick gasoline smell when I walked up to the truck.
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Old September 23rd, 2009, 08:01 AM   #14
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If it is the 4.3L check the EGR
Yeah, I'll play with that a little bit after I check the fuel pressure.
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Old September 23rd, 2009, 01:23 PM   #15
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Yeah, I'm parked on an incline at work with the rear up higher and there is a drop of gas coming out about every 3 seconds between the tank and the filler tube. Could this be the problem?
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Old September 23rd, 2009, 03:23 PM   #16
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Yeah, I'm parked on an incline at work with the rear up higher and there is a drop of gas coming out about every 3 seconds between the tank and the filler tube. Could this be the problem?
Most definitely that is a problem...if you got a leak you will not have proper fuel pressure. Fix that leak and change the filter while you are under there, they are $12 at autozone.
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Old September 25th, 2009, 07:01 PM   #17
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What's the rubber piece that connects the tank to the filler tube? Having trouble using the interwebz to find it.
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