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2008 GMC Sierra 2500HD SLE Crew Cab with 6.6L Diesel
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Discussion Starter · #1 ·
I have an odd "surging" when I have the cruise control set to between 45-50 mph. The RPMs jump up to about 2500, and then drop down to about 1200, and then jump back up again, about every 5 seconds, as though the transmission is changing between gears. This happens even on level ground, but it's worse on hilly roads. I thought this was fixed when I got new tires and wheels, but it came back within a couple weeks. Any idea what could cause this?
 

· FoTfive'dpokinoutdaside
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May air flow meter had trash in it and was doing this. My 1500 would also do it unless I put the truck in tow haul to hold the shift longer. Is the truck throwing any codes?
 

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You could have a number of problems. I've seen this happen with a MAF like Hunter mentioned. I had it happen myself with a crappy connection to the rear VSS on the transfer case, but metal particles on the inside of the sender can give erratic readings. It caused my Torque converter to lock and unlock constantly. It also caused my speedometer to fluctuate, but I thought that was my cluster.

Codes would be helpful. Live data would be better if you have a scanner.
 

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2008 GMC Sierra 2500HD SLE Crew Cab with 6.6L Diesel
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Discussion Starter · #4 ·
May air flow meter had trash in it and was doing this. My 1500 would also do it unless I put the truck in tow haul to hold the shift longer. Is the truck throwing any codes?
Not that I can tell.

You could have a number of problems. I've seen this happen with a MAF like Hunter mentioned. I had it happen myself with a crappy connection to the rear VSS on the transfer case, but metal particles on the inside of the sender can give erratic readings. It caused my Torque converter to lock and unlock constantly. It also caused my speedometer to fluctuate, but I thought that was my cluster.

Codes would be helpful. Live data would be better if you have a scanner.
I don't have a scanner, though. Considering getting one, though.
 

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For a cheap scanner you can use a ELM-327 unit for around $10 and a smartphone to give you basic engine and transmission codes. You can spend a little more for $100 and get Bluedriver and a smartphone and get Engine, Transmission, 4X4, SRS, and ABS codes. All of them can give you live data.

There are a ton of options and if you need more then you can upgrade. Most people don't need the $1100 scanner with bi-directional controls etc.
 

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2008 GMC Sierra 2500HD SLE Crew Cab with 6.6L Diesel
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Discussion Starter · #6 ·
For a cheap scanner you can use a ELM-327 unit for around $10 and a smartphone to give you basic engine and transmission codes. You can spend a little more for $100 and get Bluedriver and a smartphone and get Engine, Transmission, 4X4, SRS, and ABS codes. All of them can give you live data.
I found a cheap scanner for about $28, and I asked the seller if it shows the body codes, and he said it does, along with all the other "regular" codes. Was considering getting it.
 

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Codes are good, but does it show live data? If you have a scanner that can graph live data it will help you diagnose issues that aren't big enough to produce a code. For example I had a weird power drop in my truck while cruising only. You would be driving along just fine and then it would stumble and then pick right back up again. No codes. I was using a cheap ELM scanner and the Torque app on my phone. So I set up a graph of the MAF and went for a drive. It turned out the stumble would happen at the same time as my MAF would go to zero and then immediately return. Turned out to be a bad ground wire to the MAF.

$15 ELM device and a $5 app that is actually free if you don't get the Pro version. I only bought the Pro version because I figure I owe the guy who made it, as it saved me a lot more money than that.
Just remember You need a WiFi adapter if you have an iPhone and Bluetooth like the one below if you have an Android.

 
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2008 GMC Sierra 2500HD SLE Crew Cab with 6.6L Diesel
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Discussion Starter · #8 ·
Codes are good, but does it show live data? If you have a scanner that can graph live data it will help you diagnose issues that aren't big enough to produce a code. For example I had a weird power drop in my truck while cruising only. You would be driving along just fine and then it would stumble and then pick right back up again. No codes. I was using a cheap ELM scanner and the Torque app on my phone. So I set up a graph of the MAF and went for a drive. It turned out the stumble would happen at the same time as my MAF would go to zero and then immediately return. Turned out to be a bad ground wire to the MAF.

$15 ELM device and a $5 app that is actually free if you don't get the Pro version. I only bought the Pro version because I figure I owe the guy who made it, as it saved me a lot more money than that.
Just remember You need a WiFi adapter if you have an iPhone and Bluetooth like the one below if you have an Android.

Thanks. The one you linked to, though, doesn't look like it will show the body codes.
The one I was looking at was this one:
I checked with the seller, and he said it not only has the "normal" codes, but also the body codes and others specific to my truck. Maybe I can find this one on Amazon, since I just got a gift card for $20 and another for $25, which should more than cover this. If Amazon doesn't have this one, then maybe I'll get the newer model of the one you linked (the blue one), which is $3 more, so $16 for it.
 

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What you listed will give you codes, but not data. Codes aren't everything.

Honestly I would get the bluedriver unit. It has all of the data logging and all the codes you would need. It works for iPhone and Android. I can confirm that it does the body codes as well as 4x4, ABS, Airbag, engine and trans codes.

at $119 it is well worth it.

 

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2008 GMC Sierra 2500HD SLE Crew Cab with 6.6L Diesel
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Discussion Starter · #10 ·
What you listed will give you codes, but not data. Codes aren't everything.

Honestly I would get the bluedriver unit. It has all of the data logging and all the codes you would need. It works for iPhone and Android. I can confirm that it does the body codes as well as 4x4, ABS, Airbag, engine and trans codes.

at $119 it is well worth it.

That's a bit out of my price range. I opted for a different one from Amazon.
It does the live data, too. Hopefully, it also does all the other you mentioned as well. I think it might not do the ABS, though. The price was what I could afford, though, as I used a gift card I got for Christmas to get it.
 
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