8.1L Sub 2500 has phantom stalling

concours · December 3, 2006

Hi guys, My 2001 Suburban 2500 8.1 liter has some issues that are wearing me out. About a year ago, the fuel gage began behaving erratically... driving along with plenty of fuel and suddenly BAM, pegged on empty, Low Fuel Light on. Then a few miles later, it goes back to reading the proper level. No real problem, just an annoyance.

A few months after that, the "Service Ride Control" (truck has "Autoride")message would come on in the same fashion, intermittantly and for a short time, then go away.

Now, for the last few months, the engine will just shut off, without any warning. None of the other symptoms (fuel gage, ride control message) are present when it happens. It USUALLY restarts on the first try. Sometimes, a short wait (1-2 minutes) is required before a re-start attempt is successful. A couple times it has failed to fire when first starting out on a trip. It fired on the second attempt. This stalling problem happens rather infrequently (1-2 times a week), but it is a significant safety hazard, and my wife is scared to drive it. I'm a bit concerned the truck will poop on me when it's -20F and I have several passengers going snowmobiling in remote areas.

I have the factory service manual, and consider myself an excellent mechanic. I service and repair CNC machine tools for a living. I have diagnostic software for my other car (VW TDI, VAG-COM) and it's great. I've priced some GM stuff and might spring for it if I was sure I could "trap" the bad signals.

I'm hoping to hear from you professionals who might have some pattern failure info.... or any other help you may have to offer. Thanks in advance, Bob

GM-Tech · December 3, 2006

Crank sensors are a pretty common problem on the 8.1 and causes the exact symptoms you're experiencing. I'm willing to bet this is your problem.

Replacing it is kind of a pain. It's in the block behind the left cylinder head where access is practically non-existant. To top it off, they often break off when you try to remove them and since there's little access, it's extremely difficult to get out the broken section. Try to be very careful removing it so this doesn't happen.

Trapping the bad signal, if you can get it to occur while testing, would probably best be accomplished with an oscilloscope. Using the plot feature of the Tech 2 *may* pick it up, but since a Tech 2 costs around $2400.00, it's an awful expensive piece of equipment just for this problem. It'd be much cheaper to just change the crank sensor and hope for the best.

concours · December 4, 2006

Crank sensors are a pretty common problem on the 8.1 and causes the exact symptoms you're experiencing. I'm willing to bet this is your problem.

Replacing it is kind of a pain. It's in the block behind the left cylinder head where access is practically non-existant. To top it off, they often break off when you try to remove them and since there's little access, it's extremely difficult to get out the broken section. Try to be very careful removing it so this doesn't happen.

Trapping the bad signal, if you can get it to occur while testing, would probably best be accomplished with an oscilloscope. Using the plot feature of the Tech 2 *may* pick it up, but since a Tech 2 costs around $2400.00, it's an awful expensive piece of equipment just for this problem. It'd be much cheaper to just change the crank sensor and hope for the best.

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concours · December 4, 2006

Crank sensors are a pretty common problem on the 8.1 and causes the exact symptoms you're experiencing. I'm willing to bet this is your problem.

Replacing it is kind of a pain. It's in the block behind the left cylinder head where access is practically non-existant. To top it off, they often break off when you try to remove them and since there's little access, it's extremely difficult to get out the broken section. Try to be very careful removing it so this doesn't happen.

Trapping the bad signal, if you can get it to occur while testing, would probably best be accomplished with an oscilloscope. Using the plot feature of the Tech 2 *may* pick it up, but since a Tech 2 costs around $2400.00, it's an awful expensive piece of equipment just for this problem. It'd be much cheaper to just change the crank sensor and hope for the best.

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EXCELLENT!!! I love non accessable, fragile parts that are expensive, LOL. But seriously, sounds like a plan. I had a 1990 Jeep 4.0 L Cherokee with intermittant no starts. Called a firend to borrow his service manual, he said "I'll do one better. I'm drinking beer with a Jeep tech right now. Ask away..." Turns out the one model year they used a lame azzed system with a distributer AND a crank position sensor. Changed it out and ran.

Thanks again for your help, I'll give it a try.

rockertype · December 14, 2006

It's the crank position sensor - I'm installing my new one tomorrow! I'm tired of walking everywhere I have to go while my 40 thousand dollar truck sits on the side of the road with it's hazzard lights flashing!

Chevy built: "Like A Rock", a rock I'd like to throw off a bridge! Way to go GM!

pm26 · December 14, 2006

Chevy built: "Like A Rock", a rock I'd like to throw off a bridge! Way to go GM!

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Or, Chevy built "like a Knock" if you have a piston slapper.

boomer 33 · December 14, 2006

Crank Sensor, like they say, I fixed my buddies the same way. $126 for the part. No sweet.

TEE · December 14, 2006

Now now....we don't wan to pee off the Tech's

Even thought they definitely know of all the GM crap that's been going on for years now.

trooper04 · December 20, 2006

Well my truck is doing the exact same thing. Im going to take the Tech 2 home tonight and try and capture the random misfires. I tried (unsuccessfully) last weekend to change the sensor. I was wondering if you guys have any tips or tricks on changing that dam thing out? I was having a hard time acknowledging the fact that one bolt and one connector were kicking my ass. Thank you in advance !!

Flash2004 · January 4, 2007

I'm really keen to discover if this works for you. My 01 Sierra 8.1 has been doing this since 03 and I've had the sensor changed with no effect. Its refusing to start for over an hour when it stops now! Please keep your thread posted on any progress. I'm 10,000 miles or more across the ocean from any GM dealer so need help with this real bad. I also posted my problem before I found yours. cheers

concours · January 8, 2007

It's getting worse now... three no-starts for me in the past week, after it had been run a while. I looked at that crank sensor, gotta laugh at Trooper04's post. I like to think I can get confined stuff done too.... That CKP looks like a beotch. You guys going in from the top? Bottom? Is that a 10mm (a guess, haven't actually SEEN it yet) socket size on the retaining bolt?

As for Seger's "Like a Rock" used in the Chevy commercial, I once towed a 2,000 lb. piece of non wheeled machinery up and out of a ditch with my '89 2500HD C6P 5.7L after my friend's 400M powered Ford would just sit and slip the converter. I casually began singing the song nice and low as I unhooked the chain and stowed it.... he was FIGHTING mad, LOL.

bert55441 · January 8, 2007

My truck did the something last year. I have a 2002 2500HD with the 8.1 and about 43000 miles on it.

To do a Crank sensor if I can remember right a 10mm gear wrench, my dad got it in place from under the truck on the drivers side. I was on top of the engine with my arm wedge down turning the wrench. I long screw driver and some gentle taping on it, it came out. There are one or two o-rings holding on to it. The check engine light never came on after we did this so I never did the crank re-learn. Some people say you should do it though. ( Have to have a Tech 2 to do this I believe)

Like everyone else said, be careful not to break it or you’ll spending $$$ to get it fixed.

Good luck

concours · January 8, 2007

My truck did the something last year. I have a 2002 2500HD with the 8.1 and about 43000 miles on it.
To do a Crank sensor if I can remember right a 10mm gear wrench, my dad got it in place from under the truck on the drivers side. I was on top of the engine with my arm wedge down turning the wrench. I long screw driver and some gentle taping on it, it came out. There are one or two o-rings holding on to it. The check engine light never came on after we did this so I never did the crank re-learn. Some people say you should do it though. ( Have to have a Tech 2 to do this I believe)

Like everyone else said, be careful not to break it or you’ll spending $$$ to get it fixed.

Good luck

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I'm guessing there is a nice berm of rust in that bore that will try and hold the o-ring festooned sensor from sliding out, New england salt and all...

concours · January 12, 2007

Checking in... I replaced the crank sensor today, $72 for the part. It'll be a while before I know if the problem is solved, being an intermittant problem and all.

A few observations: I tried the Gearwrench advice, couldn't come close. I ended up with a 1/4" drive Snappy 6-point socket, u-joint (electrical taped into non-floppiness) and two 14" extension bars. I came up along side the trans, over the small sheet metal bracket right by the sensor and onto the bolt, spun it right out. That was the easy part.

I used various size screw drivers and prybars to get under the lip and begin prying up. It was awfully stiff going, course the thought of trying to remove a broken plastic stump was in my mind. A liberal hosing with "PB Blaster"

weasel-piss penetrating oil helped lube things up and it moved easier. (ATF would prolly be better) I got it about 1" out and could see the O-ring but it still was fighting. Long story short, the new one has one O-ring while my original was sporting TWO O-rings. The one near the bottom resisted all the way.

I greased the new one liberally both below the O-ring for slide-in-ability and above the O-ring for combat-the-New-England-road-salt-rust-the-bore-to-hell issue.

Hope my observations can help someone else...

Sherlock · January 13, 2007

I had the same problem in my 02/8.1 There were no codes coming up, but based on the info from this forum, I told them to replace the crank sensor. It has not stalled since.