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HELP!! 8.1 Engine Not Starting (Sometimes)

GMCbigblock

GMCbigblock

1/2 ton status
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Dear CK5ers,

Am seeking your valuable help & sharing experience.

For the past few months, I started to run into a weird-issue with my 2006 HD2500, GMC Sierra, 8.1 Engine, 6-speed ZF Manual.

It runs & drives great. However, occasionally the engine does NOT start when I drive it for a while, or sometimes for few minutes!!

For info, my ECM is Perf.-tuned. I have the MAF & O2 Sensors deactivated.

I replaced the Crank-Sensor & fitted a new Battery. I got it diagnosed at the Dealer, no clue!

When the "no start" issue happens, there is power on the Coil, but the Plugs are not receiving order to discharge sparks. The fuel-pressure is quite strong & falls within the Dealer's service-limits.

Note: the Dealer did a repair-work for the Cam-Sensors connections, saying that it had bad-connections & hence replacing the short-circuited wires.

How can I solve this problem for good & reinstate the legendary reliability of the 8.1 engine. Now, I can't trust it anymore for long-distance trips!

Guys...your assistance is truly appreciated.
 
crazy idea . try spare key to truck .

security chip keys can wear out and cause weird no starts . my old friend had this happen . towed to shop 3 times . last time it no started and security code was on . used spare key to this day no problems and fixed .

why turn off maf ?

and all or just rear o2 off ?
 
Good possibility! In this case I'll rush to make the spare one, as I don't have one for now.

All O2s are put off, for less trouble (gas is so far affordable). My 8.1 Blazer was similarly tuned since the past 5 years, with no issue so far.

And for the MAF, as I understood, the program is tuned in considering several pre-readings of MAF in correspondence to the Throttle-positions (I may not be able to explain it precisely, but the tuning-specialist did it many times earlier & I liked its performance-outcomes, especially that it's coupled with the idea of lessening the reliance on 'digital-stuff').

Thank you for responding. Looking forward to hear more possibilities, as practical as possible.

sweetk30 said:
crazy idea . try spare key to truck .

security chip keys can wear out and cause weird no starts . my old friend had this happen . towed to shop 3 times . last time it no started and security code was on . used spare key to this day no problems and fixed .

why turn off maf ?

and all or just rear o2 off ?
Click to expand...
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Further addition for better clarity, when the problem occurs (no start), "NO Code" blinks on the ECM...perfectly clean.....kind of weird!
 
sweetk30 said:
crazy idea . try spare key to truck .

security chip keys can wear out and cause weird no starts . my old friend had this happen . towed to shop 3 times . last time it no started and security code was on . used spare key to this day no problems and fixed .

why turn off maf ?

and all or just rear o2 off ?
Click to expand...

Security system problems can cause these strange issues, that'd probably be the easiest place to look. I'd also start looking at the connections; and when it's running, hook up a scanner and see if anything looks wonky on the live data from the ECM. If you have a multimeter, you can start checking the sensors. Don't assume just because a dealer fixed something that they did it right. A recent repair is one of the first places to look when you start having an issue.

Speed density tunes and deactivating the MAF is pretty common in performance tuning, although these engines are designed to run with O2 sensors. Turning off the main O2 sensors (not the ones after the cats) would most likely result in a less efficient engine, but shouldn't be the cause of your no start condition.
 
paratrooper307 said:
Security system problems can cause these strange issues, that'd probably be the easiest place to look. I'd also start looking at the connections; and when it's running, hook up a scanner and see if anything looks wonky on the live data from the ECM. If you have a multimeter, you can start checking the sensors. Don't assume just because a dealer fixed something that they did it right. A recent repair is one of the first places to look when you start having an issue.

Speed density tunes and deactivating the MAF is pretty common in performance tuning, although these engines are designed to run with O2 sensors. Turning off the main O2 sensors (not the ones after the cats) would most likely result in a less efficient engine, but shouldn't be the cause of your no start condition.
Click to expand...

Thank you for giving attention to this issue & sharing your experience. Certainly, I'll go along the lines of your suggested approach.

And yet, am hopeful to hear further possibilities on the potential causes, especially those related in particular to "8.1 common-issues"!

As well, I strongly wish seeing superior after-market products, that can help enhancing the reliability of such a great 8.1 engine (curing factory-inherited design shortfalls and/or defaults).
 
So the truck cranks but does not start?

I doubt it's a security issue, this year truck does not use a coded key and it would certainly set a code if there was a problem.

I think these motors have had issues with crank and cam position sensors. If you have access to a scan tool that can watch live data I would watch those parameters when it is acting up. I know that's easier said than done though, intermittent problems suck.
 
No spark and injector is a crank sensor/reluctor related issue. Gen 3 relies on crank position signal for that.

Now this tune sounds interesting, thats for sure. Speed density is totally fine but running no O2s is a gamble. You are relying on the base tune and baro reading. This MAF pre reading thing is interesting as well since it does nothing of the sort.

Is there a MAF sensor?
If yes, is it plugged in?
If yes, then no matter the cross over rpm setting the ECM uses the MAF instead of calculated airflow.
IF no, do you had an intake air temp sensor?
This question list can go on and on for proper speed density tuning. Without a MAF timing control becomes the next set of things, a custom operating system is needed to restore true full timing control.
 
Cili said:
So the truck cranks but does not start?

I doubt it's a security issue, this year truck does not use a coded key and it would certainly set a code if there was a problem.

I think these motors have had issues with crank and cam position sensors. If you have access to a scan tool that can watch live data I would watch those parameters when it is acting up. I know that's easier said than done though, intermittent problems suck.
Click to expand...

Good to note this piece of info about the "security coded key" as I didn't know that earlier.

Now for the C & C Sensors, the scanning seems to be a way to try. Hope we will nail down the issue sooner than I expect!

And, thank you for giving your attention & efforts to respond to this query.
 
Team208Motorsports said:
No spark and injector is a crank sensor/reluctor related issue. Gen 3 relies on crank position signal for that.

Now this tune sounds interesting, thats for sure. Speed density is totally fine but running no O2s is a gamble. You are relying on the base tune and baro reading. This MAF pre reading thing is interesting as well since it does nothing of the sort.

Is there a MAF sensor?
If yes, is it plugged in?
If yes, then no matter the cross over rpm setting the ECM uses the MAF instead of calculated airflow.
IF no, do you had an intake air temp sensor?
This question list can go on and on for proper speed density tuning. Without a MAF timing control becomes the next set of things, a custom operating system is needed to restore true full timing control.
Click to expand...


I observe responses are so far mostly leading to the same issue. I have to give it more attention & focus, though.

Regarding the tune, I re-checked with the Specialist and he confirmed that:

- MAF is connected but its value is set at 0 (Zero) & deactivated (this was also noted by the Dealer).
- Yes, the air intake temp sensor is connected.
- Correction for my previous statement on "O2 Sensors": Out of 4, only 2 are working (not those connected to the Cat. Converters).

Hope this better clarifies the previous Posts!

And, thank you for giving your attention & efforts to respond to this query.
 
Depending on what value he is talking about...... If its the air flow calc value then it is now in MAF ONLY not speed density. If its the frequency max for MAF fail then it still uses the MAF if it is hooked up. It wont dynamically calculated air with a MAF reading coming in even if its failed. By that I mean he lowered the high range test and now its throwing a P0101, 102, or 103 code. To fully ignore the MAF the frequency has to be gone.

On a stock engine, unless its all out extreme condition race, there is no huge gain from going speed density if the fuel table and the MAF are tuned properly. That involves more than tuning just the MAF or just the VE Table "Fuel Table". You have to come back after doing both and tune the blend or dynamically calculated air vs actual MAF air.
 
Team208Motorsports said:
Depending on what value he is talking about...... If its the air flow calc value then it is now in MAF ONLY not speed density. If its the frequency max for MAF fail then it still uses the MAF if it is hooked up. It wont dynamically calculated air with a MAF reading coming in even if its failed. By that I mean he lowered the high range test and now its throwing a P0101, 102, or 103 code. To fully ignore the MAF the frequency has to be gone.

On a stock engine, unless its all out extreme condition race, there is no huge gain from going speed density if the fuel table and the MAF are tuned properly. That involves more than tuning just the MAF or just the VE Table "Fuel Table". You have to come back after doing both and tune the blend or dynamically calculated air vs actual MAF air.
Click to expand...

I tend to agree with your explanation that rightly describes my engine-tuning.

Your interaction here is appreciated.

Am still hoping for the annoying "no-start engine" phenomena to be cured.
 
Russell said:
The 8.1L is known for crank sensor issues. I'd start there.
Click to expand...

Noted your suggestion, with thanks.

To clarify it, I already changed the Crank Sensor with a new one from the Dealer. Yet, the issue exists (occasional no-start)!!

Further thoughts will be appreciated.
 
Hello CK5 Contributors,

Update!

Firstly, I hope all have recently enjoyed their holidays & are still having good warm times with their families & beloved ones.

This time, am forced to raise the white-flag, as I did run out of workable-resolutions :dunno:

Over the past period, I tried all the good-suggestions by the Members, but I regret to state that the issue is still persistent & not showing any progress in response to my attempts & your suggestions.

Having said that, the only way up to me is either to:

1) Replace the whole wiring along with the ECM, with after-market plug-n-play simple harness (i.e. for engine operations only, with no gauges & no nothing)....

Or sadly,

2) Swap the existing 8.1 for a 6.0 or 6.2 Engines, complete & fitted with "reduced-wiring kit".

I call for your votes, on which way to take, pleeeeeeze.

Note: So far, I consider the truck's reliability worth ZERO (since failing to start in long-distance trips is seriously worrying, especially in remote desert trips).

Awaiting, favorably.
 
Talk about throwing the baby out with the bath water. You have an electrical issue, don't matter what engine is in there. Are we dealing with a crank no start or a no crank issue?

Despite the dealer saying they addressed the crank sensor issue and connector, I'd check it again.


Keys on a 2006 truck are not chipped. Changing keys probably won't make a difference. You could have an ignition switch problem though. It will sometimes manifest as a crank no start or no crank. They can fail by only powering up only a couple of key on circuits that can let some items get powered and others not. We've had them get towed in since they wouldn't start and by the time they got to us it cranks and runs. This will not set codes either. The only way we can prove it normally is to watch the power mode pids on the tech two while turning the key on. We will see one or two pids without power when they should. It is hard to catch for sure.

The other one hasn't been talked about. Clutch switch. If the clutch switch shows open, the ECM won't allow the crank to happen. If the engine is cranking this is not the issue. If it isn't, I'd check it to rule it out.
 
How is the harness wired? Does it kill all power to the ecm with key off?

If it is speed density for it to properly run it has to fault the MAF out first. If the keep alive is not wired to constant 12v it has to set the maf fault before it will run. Depending on how it's set to fault it it can take a couple cranks and even run time.

Until it faults it's trying the use the maf signal and the calculated airflow signal to calculate air flow for fuel delivery. If one of the values is zero, as in no maf or maf fault, it's simple math.
 
Gent's,

Thank you all for your attention & interaction, which helps pumping some hope in resolving the issue, I still hope.

I'll keep the votes under the radar.

Meanwhile, I managed to upload the Vid on the U.Tube, for better demonstration. I hope this helps.

 
And to clarify, this "no start" happened yesterday. Luckily & as normally experienced, the Engine started up after cranking for 7-8 times, in trials spread over 45 minutes (waiting about 15 minutes in between).

The No-Start occurred after driving about 10 miles, whereas it used to crank normally while shifting the Truck in spots within short distances.
 
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