Mystery P0300 root cause?

ikuo78 said:

It's about possibility.
The conditions that may indicate a canister failure are described above.

I think you can think about it by recording the correlation between live data of purge pressure and intake pressure while driving.
I've never done it though.
It appears that the Liberty's fuel pressure regulator does not mechanically synchronize the intake pressure.
There may be electrical synchronization, but I have never investigated it that deeply.

In Japan, a professional car mechanic tells Sunday Mechanic, "If you underestimate the canister, you'll end up with a hole in the fuel tank."

I've never heard of this actually happening, but
Canister effects often lurk behind mysterious phenomena.

Has anyone replaced it?

In Japan, it is a legal inspection item, but too many Sunday mechanics ignore it.
The 2007 model is definitely at the end of its lifespan.

Click to expand...

Yeah I'll just replace it as just about everything needs replacing on this car, I'm restoring the jeep apparently vs making car payments, a pain in the neck but still cheaper then new car and good for me to step my wrenching game up as I don't ever plan on buying new again with today's car prices and depreciation

jeepwarrior said:

Yeah I'll just replace it as just about everything needs replacing on this car, I'm restoring the jeep apparently vs making car payments, a pain in the neck but still cheaper then new car and good for me to step my wrenching game up as I don't ever plan on buying new again with today's car prices and depreciation

Click to expand...

It's a good idea to fix it and use it.
So am I.
These types of problems tend to take a long time to resolve.
It confuses people because it sometimes heals once.

I would appreciate it if you could post if there is any progress.

If you keep posting and put it in the "New Posts" section, it will be more visible to people and you should be able to get a lot of advice.

ikuo78 said:

It's a good idea to fix it and use it.
So am I.
These types of problems tend to take a long time to resolve.
It confuses people because it sometimes heals once.

I would appreciate it if you could post if there is any progress.

If you keep posting and put it in the "New Posts" section, it will be more visible to people and you should be able to get a lot of advice.

Click to expand...

In the month I've been trying to fix the P0300 code, I also developed a long hot start issue. If car is warm and I turn off for 20 minutes and restart it takes 3 seconds vs the usual <1 second to start. I'm not sure if it's related but since it's easier to reproduce problem in my driveway I figured I'd prioritize it , and it might be related to the p0300 code so I've been tackling that.

I found that yesterday and this evening I've been checking to see if crank signal or cam signal is getting to the PCM since it needs them to start.


I setup scope
channel 1 to trigger off cranking
Channel 2 cam position sensor signal
Change 3 crank position sensor signal
Channel 4 cyl 1 secondary ignition

What I'm seeing is on a long cranking start is two things
1. A 2 or 3 second delay before 1st injector pulse and or a spark issue
2. The 1st spark that normally looks real strong a quarter second later is either missing or other times it's weak looking 50-75% weaker.

Here's a trace showing the problem, it doesn't look like it's a crank sensor issue, or cam sensor. I'm going to recheck at the harness connector at pcm make sure it's not a sensor wiring issue.

FYI, light blue trace is secondary ignition on cylinder #1 for reference.
yellow trace is battery voltage relative compression waveform on crank, I actually trigger off it since I needed something that for sure will trigger starting trace since I didn't want to trigger off something that might be the problem like ignition and this way I can clearly see entire startup event, just wish I had 8 channel picoscope so I could put all startup sensor inputs in one trace.



Here's what a normal start looks like, took about 3/4 of second to start


They're aftermarket sensors so I'm suspicious of them but they look fine so far, and they correlate properly, so not a timing issue.

Next on the list to figure out why there's a delay w both injector and spark and weak 1st sparks and also jumping up and down burn line on the spark while running when looking at secondary ignition is

PCM's grounds, they look rusty so I'm going to ckean them. But doubt that's it as it runs fine expect the dowhill misfires. Maybe the pcm Grounds are bad and giving the injector and coil drivers that are ground switching a hard time?

I need to test also all the sensors the pcm inputs for startup mode. Which are as per 2006 service manual, can't find 2007 factory service manual and that uses jtec pcm but hopefully the inputs are pretty much the same.

1. Battery voltage which I've checked, but not at the pcm yet.
2. Intake air temperature
3. MAP sensor, I've checked it and looks to function properly but need to test at pcm it's getting good signal, live data shows its acting properly
4. Crank, which I've tested
5. Cam which seems to test good
6. ECT Engine coolant temperature, which always looks right in the scan tool. But I'll test to confirm directly at pcm
7. Throttle position sensor, never seen it reading wrong but I'll test it fully
8. ? Hopefully I'm not missing anything as that is all the service manual references.

I'm starting to lean towards a bad PCM at this point but am going to double check all the pcm inputs and outputs of course first before I condemn the PCM as bad. Need to make sure its not bad ground or wiring or eketrical noise from alternator etc first

I suspect the PCM as
1. I have unexplained downhill misfires, the only thing i can think of is its not entering deceleration mode like it should which is causing the misfires or the strange ignition problems are causing it and the idling high vaccum downhill condition is bringing out the ignition issue
2. It doesn't go consistently into deceleration mode unless overdrive is off
3. Now I see it delaying switching on the injectors abd coils for no reason, and the spark is missing or weak on the first spark, bad drivers etc?
4. The secondary ignition burn line bouncing up and down 10kv every few seconds makes me think might also be coming from bad coil drivers in the pcm. But I'm going to clean up the rusty looking pcm grounds G100 and G110 now and see if that fixes anything. If that doesn't work I'll check everything coming in out the pcm and wiring and splices and connectors to see if anything is bad.

Need to check for AC ripple ftom alternator or other noise too with scope as my cheap Autel scanner reported alternator ripple fail, but it's a new denso alternator and the oem which was working but old and aftermarket one I returned I tested all reported ripple test failure, so probably just the scan tool error.

ikuo78 said:

It's a good idea to fix it and use it.
So am I.
These types of problems tend to take a long time to resolve.
It confuses people because it sometimes heals once.

I would appreciate it if you could post if there is any progress.

If you keep posting and put it in the "New Posts" section, it will be more visible to people and you should be able to get a lot of advice.

Click to expand...

In the month I've been trying to fix the P0300 code, I also developed a long hot start issue. If car is warm and I turn off for 20 minutes and restart it takes 3 seconds vs the usual <1 second to start. I'm not sure if it's related but since it's easier to reproduce problem in my driveway I figured I'd prioritize it , and it might be related to the p0300 code so I've been tackling that.

I found that yesterday and this evening I've been checking to see if crank signal or cam signal is getting to the PCM since it needs them to start.


I setup scope
channel 1 to trigger off cranking
Channel 2 cam position sensor signal
Change 3 crank position sensor signal
Channel 4 cyl 1 secondary ignition

What I'm seeing is on a long cranking start is two things
1. A 2 or 3 second delay before 1st injector pulse and or a spark issue
2. The 1st spark that normally looks real strong a quarter second later is either missing or other times it's weak looking 50-75% weaker.

Here's a trace showing the problem, it doesn't look like it's a crank sensor issue, or cam sensor. I'm going to recheck at the harness connector at pcm make sure it's not a sensor wiring issue.


Here's what

ikuo78 said:

It's a good idea to fix it and use it.
So am I.
These types of problems tend to take a long time to resolve.
It confuses people because it sometimes heals once.

I would appreciate it if you could post if there is any progress.

If you keep posting and put it in the "New Posts" section, it will be more visible to people and you should be able to get a lot of advice.

Click to expand...

You mean keep replying to this post or make a new one?

Bump...

I think the coil, injector, and O2 sensor heater were powered by the ASD relay.
Has the relay deteriorated?

Usually, if a thread is left up for a week, no one will see it.

I'm sure fuel pressure may have been the trigger for activating ASD.

Fuel pressure may be rising slowly.

I think you should check in this order: Pump-Relay → Pump-ASSY → ORVR.

ikuo78 said:

I think the coil, injector, and O2 sensor heater were powered by the ASD relay.
Has the relay deteriorated?

Usually, if a thread is left up for a week, no one will see it.

Click to expand...

Replaced ASD relay early on in this adventure. Today I tested control from pcm to asd relay, that's good

Checked that in fact there is power going from ASD to injectors and coils at startup abd.

My issue is the pcm isn't ground switching the injectors and coils as expected

jeepwarrior said:

Hi, mine shows fine in scan tool live data and freeze frame and in recorded data

Click to expand...

Understood. My scanner showed "normal" in freeze frame and live data too... Did you check to make sure the connector housing is actually locked onto the IAT? If no, it takes about 30 seconds to pop the hood and gently pull on the connector housing. If it slides off without depressing the locking tang on the housing, the connector is broken and possibly causing PCM communication faults that are not detectable by the scanner (apparently what I had going on), in which case the recommendation is to splice on a replacement connector that does lock. I know, nitpicky little things... but I gotta tell ya these things can have major systemic impact on the PCM and how it instructs the engine/trans.

It is a sobering fact to know that there is the possibility that the unavailability of a $10 plastic part (whether it is obsolete and unavailable, or simply chinese garbage that is not spec compliant) can turn your Liberty into a 3000# paperweight.

Clarification questions:
1. Are you the original owner?
2. If #1 is "no", are you sure that the Lib has the original PCM?

The point is that there is a distinct VIN/PCM interdependancy. You must have a PCM that is programmed according to VIN, not just year/engine. Mismatched VIN and PCM is a problem.

The easiest way to tell that the PCM is failing is when the condenser is leaking.
If this is not the case, the only option is to perform a wiTECH2 diagnosis, but the price may be similar to that of a rebuilt product.

*** solved***
The drive belt tensioner I replaced 4x months ago already failed and was causing belt slap or slip catch seneraio to trigger crankshaft sensor to report false misfires!

While I was waiting on salvage test pcm I thought I'd address the accessory noise that started recently and after my p0300 saga started. I found one of the tensioner pulleys was already worn out!? So I removed and noticed the tensioner was really weak. I noted that and put in original that was worn but better condition. I thought nothing more of it. Put in new pcm and everything was fixed so I thought. I confirmed fix by putting old one back in, test drove and couldn't reproduce issue!??? So I thought about it and was like I must have either bumbed a bad wire or something else going on. So I thought what is different, and I remembered changing tensioner and thought no can't be that. But after many attempts to recreate issue I couldn't get it to come back.

Also my random delayed start issue i first thought was pcm related was again a garbage aftermarket cam sensor that was replaced.

I will never ever use aftermarket parts, their failure rate is above 50% right out of the box, or within a few month 80% failure rate.

jeepwarrior said:

*** solved***
The drive belt tensioner I replaced 4x months ago already failed and was causing belt slap or slip catch seneraio to trigger crankshaft sensor to report false misfires!

While I was waiting on salvage test pcm I thought I'd address the accessory noise that started recently and after my p0300 saga started. I found one of the tensioner pulleys was already worn out!? So I removed and noticed the tensioner was really weak. I noted that and put in original that was worn but better condition. I thought nothing more of it. Put in new pcm and everything was fixed so I thought. I confirmed fix by putting old one back in, test drove and couldn't reproduce issue!??? So I thought about it and was like I must have either bumbed a bad wire or something else going on. So I thought what is different, and I remembered changing tensioner and thought no can't be that. But after many attempts to recreate issue I couldn't get it to come back.

Also my random delayed start issue i first thought was pcm related was again a garbage aftermarket cam sensor that was replaced.

I will never ever use aftermarket parts, their failure rate is above 50% right out of the box, or within a few month 80% failure rate.

Click to expand...

Interesting solution!

I will have to check out my belt tensioner. I have been getting P0300 periodically but none since my AC compressor crapped out and the AC clutch broke. I was following with interest your "AC ripple" comment.

Glad you solved it.