LQ4 stalling/ hard starting; code P0336
03-04-2021, 06:54 PM
#1
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LQ4 stalling/ hard starting; code P0336 This is for a customer's '05 Express van, 338K miles.
He's been having a problem with the van stalling, and sometimes not starting. In addition, it is hard to start after it has been sitting for an hour or so.
It had code P0336, Crank Position Sensor circuit A performance.
A continuity and short test on the related wires showed that they were good to the PCM and not shorted out on anything.
I replaced the sensor and the connector, but the code remained.
It has not stalled on me my while I've had it, but it still remains hard to start after sitting.
When you try starting it after it's been sitting, IT WILL NOT start on the first try; it just cranks continuously, and there is an occasional backfire/pop out of the exhaust.
After the first attempt, it will start with no problem.
If I unplug the cam position sensor, it will start up every time with no problem, no matter how long it has been sitting.
The reason why I was messing with the cam sensor was because the owner told me that the problem began after he had his cam sensor changed a couple of months ago (I didn't do the diagnosis on that, so I don't know why it was changed).
At this point, the only things I can think of is either a bad PCM, or a mechanical problem like a stretched timing chain.
Any help is appreciated.
He's been having a problem with the van stalling, and sometimes not starting. In addition, it is hard to start after it has been sitting for an hour or so.
It had code P0336, Crank Position Sensor circuit A performance.
A continuity and short test on the related wires showed that they were good to the PCM and not shorted out on anything.
I replaced the sensor and the connector, but the code remained.
It has not stalled on me my while I've had it, but it still remains hard to start after sitting.
When you try starting it after it's been sitting, IT WILL NOT start on the first try; it just cranks continuously, and there is an occasional backfire/pop out of the exhaust.
After the first attempt, it will start with no problem.
If I unplug the cam position sensor, it will start up every time with no problem, no matter how long it has been sitting.
The reason why I was messing with the cam sensor was because the owner told me that the problem began after he had his cam sensor changed a couple of months ago (I didn't do the diagnosis on that, so I don't know why it was changed).
At this point, the only things I can think of is either a bad PCM, or a mechanical problem like a stretched timing chain.
Any help is appreciated.
03-04-2021, 06:58 PM
#2
TECH Senior Member
Bad cam sensor....
03-04-2021, 08:43 PM
#3
Sure sounds like the new (never even worked) cam sensor. Was a factory sensor used? You could clear PCM and do a crank relearn and see how it responds. If no good, install factory cam sensor and do relearn.
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G Atsma (03-05-2021)
03-05-2021, 09:05 AM
#5
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Thanks for the input, guys.
However, I forgot to mention that I warrantied-out the cam sensor (made by Standard Motor Products) already, and I did a signature and frequency test on it, and it's generating a signal that increases with RPM.
It appears to be performing properly.
However, I forgot to mention that I warrantied-out the cam sensor (made by Standard Motor Products) already, and I did a signature and frequency test on it, and it's generating a signal that increases with RPM.
It appears to be performing properly.
03-05-2021, 11:07 AM
#7
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Reluctor mighta walked on the crank too.
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03-05-2021, 02:45 PM
#8
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I don't think that's the issue, and here's why: I left a pressure gauge on it over the weekend, and Monday morning it had zero PSI. But I unplugged the cam sensor, and it started right away anyway.
I was thinking about that, too. When I had the crank sensor out, I inspected the reluctor as best I could with a mirror, and I looked okay, but that sort of thing would be hard to tell.
For the record, I've tried doing a cam/ crank relearn on this thing a few times, and sometimes it will relearn, sometimes it won't. I really don't like the idea of an engine this old bouncing off the rev limiter for too long.
03-05-2021, 05:48 PM
#9
TECH Senior Member
If it's starting and running at all, the reluctor is fine. If it was off kilter I doubt it would run well, if at all.
03-05-2021, 06:03 PM
#10
Scope cmp and ckp signal to make sure they are clean square waves. No slewed or low amplitude waves. Frequency changes with rpm. Google or check IATN.net if you want to see what a good trace looks like. Relearn should work with pedal to floor and count 2s (might barely hit 5K). Don't need to hit rev limiter.
03-05-2021, 06:54 PM
#11
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Scope cmp and ckp signal to make sure they are clean square waves. No slewed or low amplitude waves. Frequency changes with rpm. Google or check IATN.net if you want to see what a good trace looks like. Relearn should work with pedal to floor and count 2s (might barely hit 5K). Don't need to hit rev limiter.
I've done plenty of relearns in my day, and yeah, they usually only take a couple of seconds.
But sometimes this one won't do it at all, even after holding the pedal down for almost ten seconds (when I chicken out).
03-05-2021, 07:22 PM
#12
OK. Are you back probing PCM connector for cmp signal (i.e seeing what the PCM is seeing)? I'm assuming your scan tool live data showing cam/crank "no sync" or maybe intermittently showing "sync"? Post pic of sensor traces so we can see what's up.
Yeah, I wouldn't want to hold WOT for 10s. If not working in 2s, likely not going to work.
FYI - make sure no issue with the low ref also. I had a 2011 GM truck trans input sensor error code that turned out to be the return signal (gnd) in the TCM was bad. Using an alternate gnd allowed the TCM to handle the input speed sensor signal fine.
BTW - Where in Texas? Bee Cave here.
Yeah, I wouldn't want to hold WOT for 10s. If not working in 2s, likely not going to work.
FYI - make sure no issue with the low ref also. I had a 2011 GM truck trans input sensor error code that turned out to be the return signal (gnd) in the TCM was bad. Using an alternate gnd allowed the TCM to handle the input speed sensor signal fine.
BTW - Where in Texas? Bee Cave here.
Last edited by tblentrprz; 03-05-2021 at 07:35 PM.
03-07-2021, 12:50 PM
#13
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Originally Posted by dixiebandit69
Yeah, I did. This thing will slowly drop pressure as time goes on, but it comes right back as soon as you turn the key.
I don't think that's the issue, and here's why: I left a pressure gauge on it over the weekend, and Monday morning it had zero PSI. But I unplugged the cam sensor, and it started right away anyway.I was thinking about that, too. When I had the crank sensor out, I inspected the reluctor as best I could with a mirror, and I looked okay, but that sort of thing would be hard to tell.
For the record, I've tried doing a cam/ crank relearn on this thing a few times, and sometimes it will relearn, sometimes it won't. I really don't like the idea of an engine this old bouncing off the rev limiter for too long.
I don't think that's the issue, and here's why: I left a pressure gauge on it over the weekend, and Monday morning it had zero PSI. But I unplugged the cam sensor, and it started right away anyway.I was thinking about that, too. When I had the crank sensor out, I inspected the reluctor as best I could with a mirror, and I looked okay, but that sort of thing would be hard to tell.
For the record, I've tried doing a cam/ crank relearn on this thing a few times, and sometimes it will relearn, sometimes it won't. I really don't like the idea of an engine this old bouncing off the rev limiter for too long.
If you run out of options, look there last. We did not see it until the crank was pulled. We're just out of options it could be. I hope I am wrong tbh
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G Atsma (03-07-2021)
03-11-2021, 02:41 PM
#14
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Okay, guys, here's an update: I followed your advice and backprobed the cam and crank sensor signal wires at the PCM and ran a signature test with a scope meter....
And all week this thing has been starting up the first time you hit the key, and I had a good square-wave pattern that increases with RPM.
However, this morning it finally did it. I was viewing the cam sensor signal, and while it was failing to start, the square wave was a lot larger than normal; instead of series of 5-10 little plateaus, it was just one big one that took up half the screen.
Is this indicative of a problem, or is it just because of the low rpm during cranking? (Cranking speed is normal, for the record.)
And all week this thing has been starting up the first time you hit the key, and I had a good square-wave pattern that increases with RPM.
However, this morning it finally did it. I was viewing the cam sensor signal, and while it was failing to start, the square wave was a lot larger than normal; instead of series of 5-10 little plateaus, it was just one big one that took up half the screen.
Is this indicative of a problem, or is it just because of the low rpm during cranking? (Cranking speed is normal, for the record.)
03-11-2021, 09:43 PM
#15
Remind me - How many cam sensors have been installed since the problem first appeared? AC Delco sensor currently installed?
Slow down time base (freq) so you can see square wave while cranking. Confirm waveform amplitude is always the same (~12V) and just signal frequency increases with rpm. Have you verified 12V ref voltage and ref return are stable at cam and crank sensors? Should be stable while doing wiggle test.
Slow down time base (freq) so you can see square wave while cranking. Confirm waveform amplitude is always the same (~12V) and just signal frequency increases with rpm. Have you verified 12V ref voltage and ref return are stable at cam and crank sensors? Should be stable while doing wiggle test.
Last edited by tblentrprz; 03-11-2021 at 09:48 PM.
03-12-2021, 07:41 AM
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Okay, guys, here's an update: I followed your advice and backprobed the cam and crank sensor signal wires at the PCM and ran a signature test with a scope meter....
And all week this thing has been starting up the first time you hit the key, and I had a good square-wave pattern that increases with RPM.
However, this morning it finally did it. I was viewing the cam sensor signal, and while it was failing to start, the square wave was a lot larger than normal; instead of series of 5-10 little plateaus, it was just one big one that took up half the screen.
Is this indicative of a problem, or is it just because of the low rpm during cranking? (Cranking speed is normal, for the record.)
And all week this thing has been starting up the first time you hit the key, and I had a good square-wave pattern that increases with RPM.
However, this morning it finally did it. I was viewing the cam sensor signal, and while it was failing to start, the square wave was a lot larger than normal; instead of series of 5-10 little plateaus, it was just one big one that took up half the screen.
Is this indicative of a problem, or is it just because of the low rpm during cranking? (Cranking speed is normal, for the record.)
If you're getting the pulse, the sensor is seeing something present. Since it's an 05, it's kinda in that gray area where the cam sensor could be in the rear or the timing cover, so I'm not sure which you are looking at. If it's a gen3 with te sensor in the back, it makes me think that possibly the cam reluctor (that spiral thing on the back of the cam) has cracked and is loose. If it's a gen 4 with the sensor on the timing cover, might be time to take a look at the timing gear.
I do know that a stretched timing chain would not cause this. That square wave with a big plateau tells me that the sensor is seeing something. Was there any recent repairs or mods done, or is this just 330K showing it's ***?
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G Atsma (03-12-2021)