Intermittent misfire, stall, and no-start condition

J

jrschatz

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Apr 29, 2023
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Pennsylvania
Have been trying to diagnose this intermittent problem and avoid just throwing parts at the situation... here is what is happening--

When jeep is cold and first starts, it runs great for the first 10 minutes. It will idle in the driveway fine. After driving for 5-10mins, it will start to exhibit what feels like a misfire or stall, which gets progressively worse until the jeep shuts down and will not restart. Once it sits for 5-10mins it will re-start again, but still exhibit the stalling. This seems to happen only under load — not just idling. When it happens, the rpm's drops suddenly. During the crank/no-start I have tested for spark and it is present.

No CEL

Have checked the following:

IAC
Crankshaft sensor
IAT
Battery/Alternator Voltage
Fuel pressure at rail is good
TPS
Tried unplugged O2 sensor during crank/no start with no change


I hooked up my power probe to monitor injectors, and see video below —- the pulse width is jumping way up to 20ms when acting up and when in crank/no-start. Something is causing this excessive injector pulse width but I'm not sure where else to look other than possibly the PCM?

 
You didn't say if you had a good scan tool, if you do have it hooked up and communicating when jeep dies don't shut key off and see on scan tool if any codes are pending. Then go to data and see if cam and crank data is coming throu or synching while you try and turn it over.
I'd go through it and clean all grounds , power and connectors first then put scan tool on and record data while driving and see if anything is falling out, that's if you have a scan tool that will record data.
But try that on pending codes.
Good luck and keep me posted !
 
thanks...

I have a scan tool that I can monitor data on, but when the jeep dies all the data freezes and it loses the ability to communicate. this is with the key still one....
 
Clean all the grounds, power and check connectors. Has it ever had a ignition switch replaced ? If you're going to throw parts at it try a ignition switch.
 
I did replace the ignition switch a few years ago… albeit with a cheap one I believe.

Would that cause the high injector pulse width though? I will definitely check power and grounds.

Also going to test O2 And Map.

The annoying part is it’s so intermittent - I have to let it idle for 15 minutes in the driveway to even start replicating the problem. And eventually when it dies and won’t start, I am wondering if it’s just flooded with gas and once that dissipates it allows it to start again?

I’m certainly open to all suggestions, just trying to provide more details.
 
This isn't really "intermittent" if it happens every time it gets hot. Your suspicion is fuel. Ok, test that. You say you have a scan tool. What's your long and short term fuel trims? Graph your o2's while its running all the way up until it dies. Do they ever drive rich?

This sounds more heat related to me. I've seen cam and crank sensors fail once they get to operating range. They send a fuzzy signal to the PCM that our PCM is too stupid to interpret as implausible. When it dies and you're unable to restart, does the tach move while you're cranking?
 
P.S. (I thought of more once I hit send)

Your scan tool losing communication is normal on this old system. Cycle the key, and she'll come back up.

You "checked the crank sensor". How? 5 volt ref, good ground, and tested that the sensor pulls the 5 volt ref to ground (or viewed a 5v square wave on an oscilloscope?) when its hot and dead?

Also, your thought process is wrong on how "annoying" it is that you have to wait 15 minutes to get it to fail. A shop's or mechanic's nightmare is a truly intermittent problem. This isn't that. When you can't get it to start, that's excellent news. Something's missing in that moment.
 
@ItchyDong that is 100% correct.... intermittent is a wrong choice of words... it is very repeatable, the annoying part is waiting for it to warm up but that is only because between work & kids I only get small 1hour windows of time to head out to the garage and diagnose.

I also agree it is heat related — o2 graphing & fuel trims will be my next step. I'm assuming it is some sensor sending wrong info and the PCM increasing pulse width to "compensate"

appreciate the note about the PCM connecting being typical.... was initially thinking that was telling me the PCM was failing but that is very good to know...

For the crankshaft sensor, I did monitor it via a digital oscilloscope and it was providing the 5v square wave during the crank/no start condition

I have not check camshaft yet...


again thank you all for the suggestions, I will try them all this week and report back...
 
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Well I had a few hours last night to investigate some more.... and I'm leaning more towards PCM.


The tach is all over the place.... drops suddenly to zero and bounces all around. Does not move while cranking. I checked crank sensor signal again and it looked fine via labscope, although I only checked it at the connector and not at the PCM-- did not think to do that until this morning.

When it dies and struggles to re-start, you pretty much have to hold the throttle wide open to get it to fire and when it does it pops & backfires.
 
The filter caps for the power supply in the pcm are likely leaking and are no longer able to maintain sufficient operating voltage. A very common failure with age, your symptoms are classic indicators.
 
Painfultruth said:
The filter caps for the power supply in the pcm are likely leaking and are no longer able to maintain sufficient operating voltage. A very common failure with age, your symptoms are classic indicators.
Click to expand...

thanks... I just got off the phone with Mark from Wranglerfix. Awesome guy, will be sending it off for him to test this week.
 
Update:

I received the PCM back from Wranglerfix and installed it tonight.... the Jeep immediately started idling high and set a code for the IAC. After a minute or two, it stalled out and died....



Any recommendations on what IAC to replace with? I am only seeing a bunch of no-names or car quest. At least I have a code to address....
 
You do not mention the year or engine size of your Jeep.
But, try RockAuto.com
Look for Standard Motor Products in the daily driver section.

The Standard Motor Products IAC worked well for me. Rock Auto may also show a Mopar IAC - just in case you are into drinking from that Kool-Aid glass.
 
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macleanflood said:
What did Mark @Wranglerfix find wrong with the PCM?

If I can't find a Mopar I've had a lot of luck with Napa Echlin
Click to expand...

no info as to what they repaired... they had it for a day and sent it back... I'll give them a call tomorrow


in browsing the forum here it seems like I need to verify my connections & wiring to the IAC, vac leaks etc....

any other suggestions welcome
 
Crom said:
You do not mention the year or engine size of your Jeep.
But, try RockAuto.com
Look for Standard Motor Products in the daily driver section.

The Standard Motor Products IAC worked well for me. Rock Auto may also show a Mopar IAC - just in case you are into drinking from that Kool-Aid glass.
Click to expand...

so back when this problem first presented itself, my first troubleshooting was the IAC. I took the throttle body apart and cleaned out all the passages. I also picked up a SMP iac from amazon to try it, but my problems still existed.


Now, with the repaired PCM, I am getting the high idle/stall/CEL with the SMP iac. I tried replacing it with the original one, and the jeep runs even worse and I can hardly get it to start.
 
update: I am running out of possible solutions....


I spoke with wranglerfix about the new IAC check engine light, and they told me to send it back to them.... did that, received back, CEL for IAC is gone, but symptoms still present.

So, I replaced (in order) the following:

-Map Sensor
-O2 sensor (NTK)
-Crank Sensor


checked throttle position sensor again and it is showing good readings.

Running out of possible next steps here, any thoughts appreciated —— could it be a general wiring and/or ground issue? Alternator is putting out 13+ volts.
 
A few troubleshooting steps.

- What year is this?

- Have you thrown a scope on the cam sensor? What brand is it? I've seen both cam and crank sensors produce a fuzzy/dirty signal when they get hot. When I say dirty, it will go from a square wave to something that looks like a duty cycle.

- If you force it in open loop, will it still die? Unplug an o2 when it's starting and see if it still dies. Don't wait until it dies to try unplugging it.

- During the no start, run an injector balance test. I've also seen injectors get hot and get mechanically stuck.

- Have you checked anything spark related? Plugs? Pulled rail off during a no start and verified its throwing spark?

- During a crank no start, are you still getting injector pulse? Check with a noid light instead of the power probe.
 
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