Cheers everyone,

I hope you guys can help me with a little problem I recently encountered.
My truck is a 2004 Dodge RAM with the 5,7 HEMI and 4x4. It is a canadian model that was imported to Europe and has a LPG system installed (it is quite a lot cheaper to run these cars on LPG in Europe).
The LPG system had a small problem and I found out that one wire was connected wrong. So I located that wire and connected it to the correct PIN on the ECU (PCM), which was the ASD relays output (switched +12V).

I had the battery disconnected the whole time.

After re-installing everything I went for a drive and encountered a problem: when on a stop and getting off the brake pedal the car will start to "creep", but not smooth. It is more like a slow pulsating vibration, like 1-2 pulses per second.
When stepping on the accelerator it will go forward no problem, but when on the Autobahn at 1500rpm and pressing the accelerator just slightly to maintain the speed, it will also pulsate/vibrate.

Now to track the problem down I switched on the LPG system (which will switch off the fuel injectors and switch on the seperate LPG injectors) and immediately the pulsating is gone.

So the ignition system can't be the culprit here, as it is just the fuel type that changed.

But what might be the problem here and how could I test it best? I have an OBD code reader, but there are no codes stored.

I did several tests yesterday, so when accelerating full throttle it will pull hard and seems to have full power.
And the problem with the vibrations/pulsations are not there all the time.
Can it be a problem with the wiring somewhere? Or a fuel injector not working properly?

Thanks for any help and input!

 

Sounds like you are getting a misfire, but, I would expect that to set a code..... Might try disconnecting the front O2 sensors, and running it on gasoline for a bit, see if the problem goes away....

What spark plugs do you have in there, and how long have they been in there??

 

But, as I said, spark plugs are the same when on LPG, and there is no problem on LPG. So I can't see why the spark might be a problem. Normally a problem with the spark plugs would show on LPG first, not on gasoline though.

I have no idea what spark plugs are in there, bought the car 2 months ago like that. Never had a problem, until I touched the wiring there. Very strange.

I checked O2-sensors via OBD and it seems like they are doing their job (values going up and down all the time so "normal" behaviour I'd say).

So, what do you think would disconnecting the O2 sensors do? Making the ECU run in open-loop mode?

 

O2 sensors can 'look' like they are behaving properly, but, in reality, aren't. As they age, they can become slow to react, or just not respond properly. (reading wrong.) Disconnecting them does force the PCM into open loop, such that it runs from pre-programmed tables (which tend to be a bit rich), if the problem disappears, likely it's just time for new O2 sensors. (NTK or Denso sensors only please.)

Gasoline, and LPG have dramatically different ignition properties. A spark hot enough to ignite one, may not be sufficient to ignite the other.... and it has become apparent that these engines just don't like the rare-earth plugs. (platinum, iridium, et al.) At least, not when running on gasoline. You get mystery misfires. Since you haven't had the truck very long, it sure wouldn't hurt to pull a couple plugs, and see what they look like... they may just be old and worn.

ASD relay provides power to the injectors, and coils. If it ran before moving that wire...... then the ASD was getting triggered elsewhere, either that, or the coils were being powered from somewhere else. Would need to see wiring diagrams, and exactly what you changed to make any more guesses there.

 

Thanks for your replies, will pull the O2 sensors and check if it is gone. Easy to check.

Yeah, but normally LPG needs a "better" spark to ignite though. Had no problems before fettling on the wires, so that would be quite a coincidence, but pulling a few spark plugs is always a good idea, that is very true. Will do that, too and check it.

I did not change anything on the "stock" wiring, just grabbed that signal from the ASD to switch the LPG system on and off. This is what is recommended by the manufacturer, too.
But I also removed that wire to test if it was the culprit, but it wasn't. Even with the wire disconnected the problem remained though.

Thank you very much for your help!

 

So the timing was just a coincidence. I've been down many a rabbit hole due to that.

 

Maybe, I really don't know. At first I thought it was because of the PCM that was reset due to the battery beeing disconnected for a few hours.
But will check on Tuesday if it is due to a failing o2 sensor.

 

Changing hemi plugs takes a bit to do, its not that easy and there is 16 of them. First mistake people make is they think, oh hell I'm putting long life plugs in this thing so I don't have to do this so often. So in go the iridium plugs, and some time goes by, and misfire.... Make sure you're running copper plugs. I always use the oem champions, but others make them in copper.

 

Cheers everyone, I have to admit that you were right. Today, while checking and driving and trying to re-produce the error, the MIL went on. Did a quick scan with the Torque Pro App and there were two errors - P0300 and P0307. Misfire on cylinder nr. 7.
I have a spare coil that came with the car and will change the plugs over now. So hopefully the error will be gone.
And just to be sure, the Champion RE14MCC4 Copper Plus Spark Plugs 570 are the correct ones, right?

Thank you again for your help!!!

 

Those would be the ones.

They say they are pre-gapped, but, I check 'em anyway..... as ya just don't know how many times they have been bounced off the electrode in shipping and handling.