Intermittent Crank no Start
Thread Starter
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Beginning Member
Joined: Apr 2024
Posts: 12
Intermittent Crank no Start
Got a 2002 4x4 Blazer with the 4.3. Basic problem is sometimes the car will crank but not start and when it doesn't start theres no spark.
Things that have been tested/replaced:
Fuel pressure: Good
Fuel pump: can here it turn on
Distributor: Good
Coil wire: Good
ICM: Replaced, thought this was the problem due to shaking the black and white wire and the engine turning off and on but the intermittent issue still happens.
Crank Position sensor: was replaced a month before car was bought.
ECM: Didn't replace anything but no obvious damage to the board or pins. Security light not flashing when its cranking so dom't think its a passlock thing. Also no codes found or stored. Any help is appreciated
Things that have been tested/replaced:
Fuel pressure: Good
Fuel pump: can here it turn on
Distributor: Good
Coil wire: Good
ICM: Replaced, thought this was the problem due to shaking the black and white wire and the engine turning off and on but the intermittent issue still happens.
Crank Position sensor: was replaced a month before car was bought.
ECM: Didn't replace anything but no obvious damage to the board or pins. Security light not flashing when its cranking so dom't think its a passlock thing. Also no codes found or stored. Any help is appreciated
Super Member
Joined: Nov 2011
Posts: 1,139
From: Pennsylvania
One basic thing that can cause an issue is a heavy set of keys. When it's running try moving/jiggling your key(s) and also the ignition lock cylinder to see if it cuts off, indicating a bad lock cylinder/ignition switch. Intermittents are the worse to find.
You did check for spark again while cranking since you fixed connector?
What do you have to work with? A scan tool you can read cam retard with? DVOM? Incandescent test light?
What were your fuel pressure and leakdown readings? Below my signature block is a "How-To" if you need for checking pressure/leakdown.
You did check for spark again while cranking since you fixed connector?
What do you have to work with? A scan tool you can read cam retard with? DVOM? Incandescent test light?
What were your fuel pressure and leakdown readings? Below my signature block is a "How-To" if you need for checking pressure/leakdown.
Thread Starter
|
Beginning Member
Joined: Apr 2024
Posts: 12
One basic thing that can cause an issue is a heavy set of keys. When it's running try moving/jiggling your key(s) and also the ignition lock cylinder to see if it cuts off, indicating a bad lock cylinder/ignition switch. Intermittents are the worse to find.
You did check for spark again while cranking since you fixed connector?
What do you have to work with? A scan tool you can read cam retard with? DVOM? Incandescent test light?
What were your fuel pressure and leakdown readings? Below my signature block is a "How-To" if you need for checking pressure/leakdown.
You did check for spark again while cranking since you fixed connector?
What do you have to work with? A scan tool you can read cam retard with? DVOM? Incandescent test light?
What were your fuel pressure and leakdown readings? Below my signature block is a "How-To" if you need for checking pressure/leakdown.
Last edited by Kana44; Apr 17, 2024 at 10:19 PM.
Super Member
Joined: Nov 2011
Posts: 1,139
From: Pennsylvania
Am I understanding correctly that after your first post you fixed the connector and it is no longer stalling...it is fixed?
The signal wire test is done with Key On Engine Cranking (KOEC), not Key On Engine Running (KOER). I have not done it before with engine running, but at idle it is going to be somewhere around 33hz frequency and I assume a 50% duty cycle. You could check frequency with your DVOM with the truck running, but the fact is with your truck running ok, there is probably nothing you would find at issue...as you already know, why intermittent problems suck.
If you're still having the issue, below are some suggestions. If you get to point where you can't get the truck started, that's when we can really help. BTW, highly unlikely you have an issue with your ECM, they're very robust in these trucks.
You could pull the ECM1 (15A) fuse in your underhood fusebox (Underhood Bussed Electrical Center (UBEC)) and try to see if there is any green crusties in the sockets.
The ICM grounds on the back of the drive's side cylinder head after going thru a splice 13cm from that location, you could stick a mirror back there and look for breaks.
Was a crank relearn done after the new CKP sensor was installed? I don't really think this would cause intermittent stalling, but it should have been done anyway for truck to run optimally.
With engine running, wiggle the wiring harness from the ICM/COIL all the way back to the ECM, do the same from the CKP sensor to see if you stall the truck.
Inspect the wire from the coil to the distributor.
The signal wire test is done with Key On Engine Cranking (KOEC), not Key On Engine Running (KOER). I have not done it before with engine running, but at idle it is going to be somewhere around 33hz frequency and I assume a 50% duty cycle. You could check frequency with your DVOM with the truck running, but the fact is with your truck running ok, there is probably nothing you would find at issue...as you already know, why intermittent problems suck.
If you're still having the issue, below are some suggestions. If you get to point where you can't get the truck started, that's when we can really help. BTW, highly unlikely you have an issue with your ECM, they're very robust in these trucks.
You could pull the ECM1 (15A) fuse in your underhood fusebox (Underhood Bussed Electrical Center (UBEC)) and try to see if there is any green crusties in the sockets.
The ICM grounds on the back of the drive's side cylinder head after going thru a splice 13cm from that location, you could stick a mirror back there and look for breaks.
Was a crank relearn done after the new CKP sensor was installed? I don't really think this would cause intermittent stalling, but it should have been done anyway for truck to run optimally.
With engine running, wiggle the wiring harness from the ICM/COIL all the way back to the ECM, do the same from the CKP sensor to see if you stall the truck.
Inspect the wire from the coil to the distributor.
Thread Starter
|
Beginning Member
Joined: Apr 2024
Posts: 12
Am I understanding correctly that after your first post you fixed the connector and it is no longer stalling...it is fixed?
The signal wire test is done with Key On Engine Cranking (KOEC), not Key On Engine Running (KOER). I have not done it before with engine running, but at idle it is going to be somewhere around 33hz frequency and I assume a 50% duty cycle. You could check frequency with your DVOM with the truck running, but the fact is with your truck running ok, there is probably nothing you would find at issue...as you already know, why intermittent problems suck.
If you're still having the issue, below are some suggestions. If you get to point where you can't get the truck started, that's when we can really help. BTW, highly unlikely you have an issue with your ECM, they're very robust in these trucks.
You could pull the ECM1 (15A) fuse in your underhood fusebox (Underhood Bussed Electrical Center (UBEC)) and try to see if there is any green crusties in the sockets.
The ICM grounds on the back of the drive's side cylinder head after going thru a splice 13cm from that location, you could stick a mirror back there and look for breaks.
Was a crank relearn done after the new CKP sensor was installed? I don't really think this would cause intermittent stalling, but it should have been done anyway for truck to run optimally.
With engine running, wiggle the wiring harness from the ICM/COIL all the way back to the ECM, do the same from the CKP sensor to see if you stall the truck.
Inspect the wire from the coil to the distributor.
The signal wire test is done with Key On Engine Cranking (KOEC), not Key On Engine Running (KOER). I have not done it before with engine running, but at idle it is going to be somewhere around 33hz frequency and I assume a 50% duty cycle. You could check frequency with your DVOM with the truck running, but the fact is with your truck running ok, there is probably nothing you would find at issue...as you already know, why intermittent problems suck.
If you're still having the issue, below are some suggestions. If you get to point where you can't get the truck started, that's when we can really help. BTW, highly unlikely you have an issue with your ECM, they're very robust in these trucks.
You could pull the ECM1 (15A) fuse in your underhood fusebox (Underhood Bussed Electrical Center (UBEC)) and try to see if there is any green crusties in the sockets.
The ICM grounds on the back of the drive's side cylinder head after going thru a splice 13cm from that location, you could stick a mirror back there and look for breaks.
Was a crank relearn done after the new CKP sensor was installed? I don't really think this would cause intermittent stalling, but it should have been done anyway for truck to run optimally.
With engine running, wiggle the wiring harness from the ICM/COIL all the way back to the ECM, do the same from the CKP sensor to see if you stall the truck.
Inspect the wire from the coil to the distributor.
Super Member
Joined: Nov 2011
Posts: 1,139
From: Pennsylvania
Just to be clear, I was more referring to wiggling the wiring harness from the ICM and CKP as they head back towards the ECM...not so much wiggling at the ECM to get it to stall. Maybe should have been more clear on that
Currently I am leaning towards the ignition switch/circuit or the CKP sensor/circuit. It's kind of good thing (for diagnostic purposes) that you are getting crank no start. Next time it has crank no start, watch your tach, the needle should flutter a tiny bit while cranking. This would indicate that there is a crank signal which should rule out a CKP/circuit problem.
There is a good video I'll find that can help diagnose if there is an ignition switch problem. Of course you're dealing with an issue that while your in the midst of testing, the truck may be ready to start and not be in the no crank situation so the test won't be helpful at that specific moment. I'll find that video and will post. It's faster and more visual than me typing it out.
Currently I am leaning towards the ignition switch/circuit or the CKP sensor/circuit. It's kind of good thing (for diagnostic purposes) that you are getting crank no start. Next time it has crank no start, watch your tach, the needle should flutter a tiny bit while cranking. This would indicate that there is a crank signal which should rule out a CKP/circuit problem.
There is a good video I'll find that can help diagnose if there is an ignition switch problem. Of course you're dealing with an issue that while your in the midst of testing, the truck may be ready to start and not be in the no crank situation so the test won't be helpful at that specific moment. I'll find that video and will post. It's faster and more visual than me typing it out.
Last edited by rockp2; Apr 19, 2024 at 02:43 PM.