GJR1212

GM issued a revision to the ECU software in early October that seems to be fixing this problem for most folks. They apparently are not going to notify owners about the hew software and your dealer may not be aware of it either. Tell your dealer that there is a revision. You can find more info from earlier posts.

What I read says that GM released the version to correct long crank/no starts 10/1/21 and then revised it again 10/4/21. Sounds like you still have the not had the ECU program update that addresses this problem.

One bit of misinformation that my saleman had been told by the service department, is that the auto restart system does not cause wear on the engine starter. I think this bit of 'folklore' comes from the fact that hybrid cars use the big electic motor to restart the gas engine. If you notice when riding or driving a hybrid the starting of the gas engine is almost undetectable since it is so smooth a transition. Our truck use an old fashion starter to start. It and the flywheel gear, bendix and starter gear teeth all wear incrementally with each start.

My truck has three of those same relays in the underhood fuse box. One for start, one for the headlights, and one for running lights. If someone wants to see if it the relay, just rotate it with one of the ones doing light duty. It pretty easy to tell if your lights still work. My guess is your dealer didn't know what to change so they changed the cheapest part they could find. These relays sell for about $10 from GM OEM dealers. Dealers seem lost when it comes to troubleshooting this problem. I wish GM would explain what they changed in the ECU program. My guess is they are adding some time delays to give the ECU time to see all the inputs in what is likely a long list of parameters. Or, perhaps they are just giving the fuel pump and little extra time to spool up.

Googled that part number. It is a pretty genaric relay of the type used to isolate the higher current load circuit from the lower current control circuit. It is a $10.20 part. I can't see how it could be related to the parts shortage since almost any GM suppliers of electronics could make this part.

I've read most of the post on this and I do not recall anyone having a long crank, no start from the auto start/stop. The ones that do it most offend seem to get with the remote start as well as the dash button start. If your foot is touching the gas or the floor mat is laying on the gas, you will get long crank, no start as well.

I like my 3.0 GMC 4wd truck. It gets great fuel mileage in normal rural driving. It is averaging around 25 mpg. It also tows my RV better than my old 5.3 Silverado or my 5.4 Expedition. The 10 speed transmission is very smooth. I have had one long start episode. Unless you have to have a new SUV today, I would wait and get one next spring. GM will eventually fix this problem, but right now they seem to be selling their gas trucks as fast as they can build them. They do not seem to see the hiatus in 3.0 productions a serious problem- in my opinion. My guess is the number of truck really seriously impacted with these long starts is a modest number. Albeit, it is a big issues with those of us involved. Discounts and Incentives do not seem to be what you would expect at a model year end either which is perhaps another reason to wait and get a 2022.

I think GM, by now, knows exactly the cause or causes of long starts. IMHO, their problem is finding a solution that does not involve a major recall of thousands of trucks. As I have mentioned, it could be substandard chips in the ECU. Replacing 20k or so ECU means they sell 20k fewer trucks since chips are limiting production. Why did they stop sales of 3.0's? Perhaps the long crank problem is at least part of that decision. They also figure that their gas trucks will garner any lost 3,0 sales. I do like my truck and do not want to sell it back to GM, but I'm getting a little annoyed at the lack of progress from GM on a significant number of defective trucks. I do not want to keep a truck that a dealer has taken completely apart and fully support those who have enacted the lemon laws to get their money back.

I have wondered whether the fact that GM has been short processors if they have lowered their acceptance standards for ECU chips. A substandard chip would be slower than expected. Modern chips are really faster than we are at tripping switches. The designers probably assume this. Assuming you are correct that the ECU is loosing the relay race, it should be easy for GM to add logic to wait for all required automatic actions to complete before starting the cranking. Also, they should shorten the cranking to something like 4 seconds instead of 20 or 30 as it is now. About half the time I hit the button before remembering to step on the brake. So, I have been pretty much following you method. I've have had only one long start. Hope some other folks with frequent long starts will try your method and report if they still have long starts.

You should at least get the code read at an auto parts store, if you do have a code reader, before you head out on a long trip.

My 2021 had a CEL come on so I returned home and turn the truck off while I opened the gate. When I restarted it, I got my one and only, so far, long start. Once it quite grinding the starter, I attempted a restart. The engine started normally and the CEL was off. Took to the dealer ASAP and the dealer found no explanation for the long start but wanted to order a bunch of parts to address the CEL they found in the ECU memory for control of the radiator dampener. When I got home, I torque the fuse box which was quite loose to 53 inlbs. I did not take it back to the dealer since it has not throw another CEL. It also has not had another long start in about 1000 miles of driving. The four bolts of the fuse box hold the harness plug on the back side of the box to the corresponding connects to the fuses and relays. I do think some of the problems with intermittent elecrical problems relate to these bolts working loose. I believe mine were torqued from the factory because each of them had a white grease pencil check mark net to them. For some reason they work loose. I re=torqued mine a week or two later and they were again somewhat loose. Check yours after a few weeks and see if they have loosened.

Did they say what they did to correct the regen knocking?

Sorry, I misread your earlier comments. So far I am pleased with how the 3.0 runs and how well the 10 speed compliments the engine. I hope you are wrong about there being a fundamental flaw in the design. You may recall the fiasco in the mid 80's when GM tried to make a diesel v8 out of a gas v8. That was a flawed design for sure. My point was this engine seems very similar to the TDi design that runs a great number of euro cars and still a few here. I do think carbon build up could happen quickly when running excess fuel over a hot exhaust valve. Perhaps your truck has some problem with the regen fuel enrichment controls. I did see on one of these forum someone else complaining about knocking, I wonder if this is problem is common to the design or more something specific to a few offending trucks?.

No spark plugs in Diesel engines. My guess, the cam sensor and infamous exciter wheel are used for timing of the direct injectors pulses and, mostly likely, for the control of variable cam timing. These 3.0 inline 6 cylinder engines appear to be very similar to the VW TDI 2.0 inline 4 cylinder engines which are also all Aluminum. The VW engines seem to be reliable. I have a 2015 TDI Golf wagen. I liked it so much, I opted for the 3.0 GMC to replace my old truck. Mine has done the long start once in 3000 miles. Other than the fuel mileage going to pot, I can't tell when the GMC is in regen. Regeneration seem to kill the fuel mileage to about 70% of what you normally see. Bad fuel causes Pre ignition knocking. Any chance you got some bad diesel? Carbon build up can also cause pre ignition.

My GMC 3.0 was assembled in Mexico. I suspect the engines are probably assembled in one or two plants and then shipped to the other locations to install them. Seems like most of the problem trucks like mine were made in March 21. It is pure speculation, but this is about the time all the car makers were running out of chips for their cars. I recall from the early days of PC's that, in some cases, the higher priced fastest version of the chip were identical to the slower clocked, cheaper versions. They determined which ones to clock at what speed by testing them. I wonder if GM accepted a batch of marginally performing chips to keep their lines running. The sporatic nature of most of these failures could just be a misread bit of data that reads correctly the next try. As computer guys use to say, "just a bit out of place".

Please let us know if you are able to add the shim and how you did it, ie directly removed the valve cover or pulled the cab, etc.

Thanks, I will keep that in mind. I would prefer a factory fix to keep clean with warranty service.

Thanks for the comment. Sure seems like GM has had enough time to figure out whats going on. If they would just reprogram the crank cycle to running a maximum of say three or four seconds instead of 20 would be an improvement. I would also like for the auto off/on button function to allow you to either turn on the function or turn it off and then hold that setting until you changed it. I dislike having to turn it off each time you restart the truck. When you have a truck that might not restart at a busy intersection shut itself off, you feel sort of stupid wondering what it will do when you let off the brake.

Have you or the dealer tightened the four screws that sandwitch the fuse box together? I have tightened mine to the recommended 53 in lb and then a few weeks later they were loose again and I retightened them. I think the plastic pieces are deforming, probably aided by the high heat of the nearby exhaust reactor that gets super hot during regen. So far I only had one event and none since the first time I tightened the fuse box bolts. It would be interesting to see if a truck like yours with multiple events has loose fuse box screws. I'm sure GM emphasizes the tightening of these screws because my fuse box has hand made, check marks by each screw. It is probably a final assembly QC check. They seem to work loose over time. Nearly all circuits dealing with the engine controls involve this fuse box.

I was looking at some pictures of a 2020 Silverado fusebox on ebay. The four bolts appear to screw into the wiring plugs for each of the four quadrants of the fuse box. So when you torque the screws you are sandwitching together five layers of electrical connections with the relays and fuses on top, the sandwitch layer of curcuits and then the wire harness plugs on the bottom. The screws are completing ground circuits and also all the 12v supply and feed circuits as well.

You can get a 1/4" drive, click type, torque wrench from Habor Freight. Seems like mine was $10 a few years ago. I first tightened mine after the one and only long crank event I've had. It took several rounds with the wrench to get to 40 in-lbs. Once at 40 I finished taking them to the spec of 53 in-lbs in several steps, like 42,45,48 ,51 then 53. I noticed that my fuse box was marked with a white grease pencil checks at each of the four bolts. Most likely GM has a procedure for these bolts to be checked at final QC inspection. After three weeks, I rechecked my fuse bolts and they where loose again, but nothing like the first time. They worked loose over heat cycles. The box is pretty close to that super hot exhaust reactor system which may explain why this problem seems to be more severe in the LM2 trucks than the gas trucks. GM has a design problem with the fusebox IMHO. If you read this whole thread you will see that fixing bent timing wheels does not correct this problem, changing Engine control hardware and software, does nothing for this problem. Most cases do not throw any codes.

I had one long crank episode. It happened on the same trip to town that set the CEL for a circuit fault, short or open, to the radiator dampner motor. I don't know think they are directly related, but I did decide to torque my fuse box per the service bullitin I had seen on this forum. The screws were loose. It took several turns to get to 40 in-lbs. I stepwise took the four screw to the 53 in lb spec. Since then, no CEL codes or long starts. An anadotal observation is that my charging volts indicate above 14 Volts and I recall the gauge always reading less than 14 before tightening the fuse box. GM should tell dealers to chech and torque these bolts as part of any routine service.

Is it a pretty straight forward job to pull the valve cover? My March 2021 build date GMC has only done the long start once, but my dealer over in Daphne avers that this is the first time he heard of such a problem..... No help. All they know is to read the code and then replace, on GM's nickel, the easiest to change out parts.