NEED HELP Cranks but won't start after driving.

gmcman

Well-Known Member
One reason is I didn't want to recommend changing something that may not need to be changed. Second reason is you will need a CASE relearn performed by either the dealer or a shop with a higher end scan tool, like a Tech2.

It will run with the sensor replaced, but will throw a code.
 

Maverick6587

Well-Known Member
You can if you have the parts. You'll need to have a scan tool that can perform a crank relearn though. It would be either a professional grade scanner or an OEM scan tool like the Tech 2.

It will still run if you can't run the crank relearn. The only problem is, if the problem is the crank sensor you will still have the crank no start issue after you install the new crank sensor. I'm not sure if the problem would be more or less frequent if you install a new sensor though. Maybe there would be no change at all and it would still be just as random.

You will have to run the crank relearn either way though if you install a new crank sensor.
 

Maverick6587

Well-Known Member
@Mooseman did you ever get that list together for Tech 2 owners and location? If not, do you need any help gathering the data from those posts?
 

Mooseman

Moderator

Maverick6587

Well-Known Member
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GBOfan

Member
Hello guys

Well I have replaced the sensors that I mentioned earlier and completed the relearn process and still have the same problem. Stop by AutoZone and borrowed their fuel testing equipment again and this is what I found.

The first two times I turned the key on and cranked it the fuel pressure was at 63 lb cranked run fine. I waited about 45 minutes and repeated the process only this time the fuel pressure would only get to 40 lb of pressure. as soon as shut the engine off the fuel pressure would start dropping and would take about 15 minutes to get to zero. so with this information I would say my fuel pump is bad. the new AC Delco fuel pump that I installed 3 months ago is bad.

I am just asking if y'all agree with my opinion. I really hate having to slide my fat A$$ back under my truck And make a repair that I have already made!
 

gmcman

Well-Known Member
as soon as shut the engine off the fuel pressure would start dropping and would take about 15 minutes to get to zero.
I have not tried to duplicate this, but I don't see why this would be an issue. If I shut mine off and turn the key within a few min, the fuel pump has to prime the system again.

So you're saying it's still shutting down?

When the motor will not start, are you getting power to your injectors? Do you have a crank signal at the PCM?

This is hard to troubleshoot standing next to it, let alone not being there. If the motor doesn't run, it either is missing fuel, air, spark, or compression.

Take a DMM or 5V test light, not a 12V test light, and verify a crank signal at the PCM when the motor wont fire. Or...verify you have power to your injectors, this will at least tell us if we have a somewhat functioning ignition system.

When it doesn't run, does it not have fuel, or spark..or both....hard to say.

I would think 40 PSI would get the engine running.

Let me ask you this.....will it stay running if you don't shut it off?
 

Maverick6587

Well-Known Member
The first two times I turned the key on and cranked it the fuel pressure was at 63 lb cranked run fine. I waited about 45 minutes and repeated the process only this time the fuel pressure would only get to 40 lb of pressure. as soon as shut the engine off the fuel pressure would start dropping and would take about 15 minutes to get to zero. so with this information I would say my fuel pump is bad. the new AC Delco fuel pump that I installed 3 months ago is bad.
I don't know the exact time period it takes to get to zero but, the fuel pressure should start dropping off once you turn the vehicle off.

Earlier you said you checked and cleaned the grounds. When you did that, did you verify continuity on all of those grounds? Specifically the two grounds on the driver's side of the frame and the 4 grounds on the driver's side of the engine?
 

gmcman

Well-Known Member
Have you tried swapping fuel pump relays when you have the no-start condition?

Here's what I was getting at earlier and something to verify when it doesn't start. May not be the cause, but it's something I would want to check knowing the nature of a faulty crank sensor..even though you replaced it.

The center wire harness at the PCM, the purple wire towards the right when looking into the harness...not the purple/black on the left.

Screenshot_20190808-150229_Photo Editor.jpg


When the engine is cranking, it needs to read the reference mark in order to begin the ignition sequence.

Using a piercing probe, a straight pin could work in a pinch, but connect this to your DMM or analog meter.

Screenshot_20190808-150325_Gallery.jpg


Ground your negative lead of your meter and observe the meter when cranking.

I removed the fuel pump relay #41 so it wouldn't start.

Screenshot_20190808-152237_Gallery.jpg

It's harder to see with a DMM, unless you have a 5V reference display.

The bar graph on the bottom of the meter is the 5V reference. Watch the left side as it shows a few bars every second or so, that's the crank reference pulse.

20190808_165541.jpg




If you dont have this when it's not starting, then it won't start. If you do have this, then you can verify injector pulse from one of the injectors, then we can narrow down a possible fuel issue or ignition switch.
 

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GBOfan

Member
GMCman:

I have changed the fuel relay before but I will certainly try it again. I will check and see if my vehicle has a pulse tonight when my mechanic friend gets here

Maverick:

I did clean all of the grounds that you mentioned but, did not check the continuity will also do that tonight as well

Thanks again talk to you soon
 

MRRSM

Lifetime VIP Supporter
Regarding an answer to the Question you posed in Post#40:

CASE stands for "Crankshaft Angle Sensor Error" and a Re-Learn Procedure is required whenever a NEW CKP (Crankshaft Sensor) gets replaced or you'll get an SES Light indicating a problem that ordinarily requires the use of an "Official" GM Tech 2 Scanner to perform the CASE Re-Learn Procedure. This information is an excerpt from TV that cover this topic quite nicely:


Quote:


Originally Posted by crazytb View Post

If you have access to a tech 2 scan tool here is the case relearn procedure. i am not responsible as i have never done it i just found this on the web and want someone to verify it before i attempt it in my friends shop.seems easy enough but i want some professional advice..i am only looking for this because i can't find a place around me to do it. i took it from this site

http://www.fullsizechevy.com/forums/...d.php?t=164702

To perform the relearn, proceed as follows:

Connect a scan tool to make sure there are no trouble codes stored in the computer’s memory. If there is any power train trouble code other than P1336 (Crankshaft Position Variation not learned), the computer will disable the relearn function until the problem that aused the code has been eliminated. Also, make sure that the engine coolant (check it when the engine is cold) and oil levels are at an acceptable level.

1. Set the parking brake and block the drive wheels. Make sure that the hood is closed.
2. Start the engine and make sure that the engine coolant temperature is at least 158 degrees F. (70 degrees C.)
3. Turn the engine off for at least 10 seconds.
4. Select the crankshaft position variation learn procedure (CASE Learn)on your scan tool.
5. Make sure that the transmission is in Park. Start the engine.
6. Apply the brakes and hold the pedal firmly.
7. Follow the scan tool instructions.

Remember: That you are going to increase the engine speed to approximately 3000 RPM, 4000 RPM, or 5150 RPM. That’s the variation learn fuel cutoff RPM (depending upon the engine), and that it’s important to release the throttle when the engine RPM starts to decrease as a result of the fuel cutoff going into effect. Failure to do such will result in over revving of the engine, causing possible engine damage.

8. Once the engine has returned to idle, check the status of Diagnostic trouble code P1336. If the scan tool indicates that the CASE has been learned, the relearn procedure is now complete. If CASE has not been learned, check for the presence of other power train codes. If any exist, correct the problem, then repeat this procedure.

The suggestion however is to always "avoid any entanglements" by Unnecessarily Changing the CKP Sensor...if possible. So if it is not needed... just tuck that New One aside on the shelf for a 'Rainy Day'. :>)
 
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GBOfan

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Hello guys I did check last night and I am getting signal from the ckp.

I was reading last night In the service manual and you can see in the pic above that my fuel system should have a reverse check valve in it to maintain fuel pressure. as I was describing yesterday my fuel system was very erratic. but today I have cranked it twice this morning the fuel pressure was at 0 then I turn the key to run and the fuel pressure went to about 5 and that's it. I cranked the TB and it started fine. after cranking the fuel pressure went to 38 lb where it remained. when I shut my TB down the fuel pressure stayed at 38 lb. And went down very slowly.the article also stated that the fuel pressure is supposed to run between 50 and 57 psi. I let the vehicle run today till it got to temperature and reved the motor to 2000 rpm and still 38 psi

And to answer your question yes I turned the key to run and heard the fuel pump come on but only got two 5 psi before I cranked it.

As always thanks very much for your help.

 

Maverick6587

Well-Known Member
At this point, I think I would just change the fuel pump again with as much time as you have into the vehicle already. It's only 3 months old and should still be covered under warranty.

You may want to try using a different fuel pressure gauge first, just to be sure.
 

MRRSM

Lifetime VIP Supporter
If a Failure in the ACDelco Fuel Pump proves out... I know this Damned Thing costs a LOT of Bread... But THIS would be my only choice for a replacement Fuel Pump:

DELPHILOGO.jpg



ROCKAUTODELPHIFUELPUMP.jpgROCKAUTODELPHIFUELPUMPA.jpg
 

Maverick6587

Well-Known Member
If a Failure in the ACDelco Fuel Pump proves out... I know this Damned Thing costs a LOT of Bread... But THIS would be my only choice for a replacement Fuel Pump:

View attachment 90396
I would try to find a Bosch if you can... I have a Delphi pump I bought last December and the Fuel Pressure Gauge went bad about 3 weeks ago. I swapped it out with a new fuel pump last week. It seems to be fine so far.

If I have to do that again, I'm switching to ACDelco or Bosch.
It might have just been my bad luck. 🤷‍♂️
 
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GBOfan

Member
Well it turns out that I need a new PCM. I finally got so upset that I went and seen a mechanic that has a reader that will tell you what's wrong. anyway my PCM is not telling the injector circuit to open after the PCM gets hot. then will not crank until the PCM cools off and then starts to working normal again

Anyone have any suggestions about getting a new PCM or how I should handle this problem as always thanks for your help.
 

Maverick6587

Well-Known Member
To get you by for now, you could just grab one from a local junkyard. Should be about $20-$50 and you can just swap them out. I think the only thing you would need to do is a security relearn (30-minute process). It should work after that without any issues.

Make sure you get a PCM from a vehicle with the same gearing as your vehicle. I can't find from searching in gmtnation but, we used to have a thread that listed off all of the RPO codes, so you can find out what gears you have, if you don't already. Here's the best I could find.

At some point, you will need a Tech 2 or high-end scan tool to reprogram the VIN stored in your "new" PCM. Shops in my area usually charge $50-$100 for their scanners touch a vehicle.

You can get a decent PCM online and they will program it to your vehicle's VIN before they mail it to you, for about $100-$120.

EDIT: Looks like you can use the PCM from an 07',08', and 09'.
 

gmcman

Well-Known Member
I'm not doubting your mechanic if he has it hooked up to a scan tool. I just like to check everything first before I fully commit to a shop.
When your truck shuts down and won't start, have you tried swapping relays or trying a new relay for the fuel pump?

You say you have 38 PSI, and in another thread, member @2001ZR2 just had driveability issues with that pressure. You already discussed this but are you absolutely certain you have fuel pressure with the key on when you have the engine shut down on you? Have you verified with a gauge there is pressure? Safe bet if you hear the fuel pump whine for a few seconds then it stops when you turn the key on, that you should have pressure.

Otherwise as far as a new PCM, not sure if only 06+ works or just 08-09.
 
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GBOfan

Member
GNC man, Maverick6587


My mechanic not only checked with his scan tool but also check the circuit directly with a light. The circuit that is supposed to energize the injectors is not getting a signal from the PCM when the PCM gets Hot. Let it sit for 30 minutes and all is working properly. We did do test before and after to confirm.

I put a different fuel pressure gauge on the truck about two weeks ago and the fuel pressure now is running at 60 lb all the time.

Thank’s y’all will let you know how it turns out.
 

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