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I have a 2001 suburban. Difficult to diagnose crank

I have a 2001...

I have a 2001 suburban. Difficult to diagnose crank intermittent no start. I need an expert in GM electrical systems rather than a general automotive tech

Mechanic's Assistant: Will the headlights turn on? Is the battery fully charged?

Of course. checked all grounds, 5v reference at mutliple sensors. Checked power and grounds to pcm. Voltage drop test at starter and fuel pump, etc

Mechanic's Assistant: Are you fixing your Suburban yourself? What have you tried so far?

Replaced ignition switch, bcm, PCM, battery, neg battery cable, fuse block, fuel pump, fuel pressure regulator, plugs and wires, MAF sensor

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

I need a mechanic who specializes in electronics

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Answered in 1 hour by:
3/14/2018
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,694
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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I am your guy.

Let's preface this by saying that our testing won't mean anything as long as the truck starts and runs. We have to get this in no-start mode before we can do any testing. I'll stick with you on this until it's resolved, even if it takes days or weeks to replicate, but until it acts up, no amount of testing will reveal the problem.

So, give me the rundown. When it doesn't start, what does it do? Cranks, won't start? Have you checked for spark, fuel pressure, etc. when it's acting up? Do you have a scan tool, and any scan data to share?

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Customer reply replied 4 months ago
Problem started 6 months ago. Intermittent. Always cranks. Starts on the first crank maybe 2 out of 3 times. When it doesn't start after 5 seconds or so, I shut it off, crank it again and it starts. I have a ses light on only in KOEO or during cranking. It shuts off as soon as he vehicle starts. If I shut the vehicle off then put the key back to run position within 5 seconds, the SES doesnt come back on and vehicle will start. Have consulted other mechanics and as a result have changed the ignition switch, BCM, battery, fuse block, starter, fuel pump and FP regulator, Plugs and wires. No improvement. The only thing that helped was putting a jumper between the ground points on the firewall.I have a tech 2 clone. in the KOEO position it shows a battery voltage of 11.4 even though its 12.6 at the battery. Also a class 2 DTC is present in PCM

What is the class 2 DTC number?

Battery voltage reading is correct, the PCM always reports lower than nominal battery voltage, I assume due to various voltage drops before this value gets read, or whether it's filtered inside the PCM, or something like that, but nonetheless, you'll find this same nuance with any GM car or truck, so don't get hung up on that.

The grounds at the back of the engine are notorious issues, have you put your eyes/hands on these to make sure they're still connected and not broken?

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Customer reply replied 4 months ago
It's an 8.L engine. there are 3 grounds on the Drivers side back of the block. A double ground(PCM?) a single ground and the body strap ground. Removed all 3, inspected wires, cleaned contact points, reattached. No improvement.
Re routed the body ground direct to the battery, no improvement.
Only improvement was seen 3 months ago when jumper was placed between 2 ground points on the firewall, trickle charger went from solid red to flashing green instantly and no start condition went from 3 of 4 times to 1 of 4 times . Since then has slowly gotten worse
When it fails to start, have you been able to determine whether there is spark, fuel injector pulse, fuel pressure, etc?
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Customer reply replied 4 months ago
Fuel pressure has always run between 46 and 50 at the rail. Thought that miht be low, but couldnt find proper specs for 8.1L engine. Put in new fuel pump and pressure reg and filter. ;No change in pressure or improvment in no start.Checked spark. No or weak spark when cranking and no start. Once vehicle starts, good spark.diagnostic circuit check reveals dtc in PCM, SIR and RSS, but doesnt give code #. I have the scan tool but not the computer software
Customer reply replied 4 months ago
Also backprobed the crank position sensor power input KOEO, was getting 11.4 volts on the power side which over several seconds slowy went up to 12.2 V

A good scan tool would go pretty far here. You have trouble codes but don't know what they are. We could verify crank sensor circuits easily with a scan tool, by simply watching cranking RPM. If it won't start, but cranking RPM is 150 or so (at least something other than zero) we know the crank position sensor circuits are working.

You seem determined to figure this out yourself, and I admire that. But there are technology limitations, if this is something you're interested in and may want to deal with in the future, investing in a good quality scan tool may be something you want to look into.

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Customer reply replied 4 months ago
I'm not clear about the cranking RPM but It cranks normally( same sound whether it starts or doesnt start. I have some wire piercing probes coming tomorrow. Will test to see whether PCM is getting the 5 V signal from the Crank sensor
and the crank sensor is getting a stable 12 volts and has a good sensor ground.Any idea why jumping ground points on the firewall would would make such a dramatic improvement?

To answer the second question first, it would seem that there's a weak ground connection somewhere, if that's what happened. Although the problem isn't resolved entirely, so I don't know how much weight to give that. It still acts up, just less than it did.

"Cranking RPM" is merely how fast the engine is spinning, but I'm talking about it through the eyes of the computer. What does it "see"? If it sees zero cranking RPM while the engine is cranking over, the engine might as well not be cranking over. If the computer doesn't see an input there, it's not going to generate spark, fuel injector pulse, fuel pump, or anything else. Since you had no spark when it failed to start, this seems like a logical place to start looking. I suspect you'll find that there is no RPM indicated on the scan tool when the engine fails to start, likely because the crank position sensor is no good. I'd verify that, of course, but that's where I'm headed with that path.

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Customer reply replied 4 months ago
I will test the crank sensor tomorrow and let you know the outcome. Thanks for your patience

Of course, I'm happy to help you along. If you can get a proper scan tool, you might find your life getting easier a lot faster on this. :-)

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Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,694
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Customer reply replied 4 months ago
Unfortunately no. Amazon sent my wire piercing probes to the wrong address. Hopefully they come next week and I can solve this problem. I'd almost rather it be the wiring than the crank sensor as that sensor is very difficult to replace on the 8.1L engine.
It's in the back of the block, covered by the fuel lines and a large loom and it often breaks off in the block when you try to remove it. In any case, appreciate your feedback. Will get back with you if I get stumped.Thanks,
Rich
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