IronTom

Hi Guys,

I have an elusive problem with low MPG, shuddering etc. I did a propane test to exclude a vacuum leak and it tested negative.

Out of pure desperation I disconnected one of the manifold vacuum lines and checked if direct propane injection will change the RPM. It did not. What the hell? This basically means that my previous propane test is invalid. I had a deliberate leak made and it did not "register" with the propane test.

How is that possible?

cruiser54

Got me, and I drive a propane truck!!!

How about spraying suspected areas with throttle body cleaner to detect the vacuum leaks instead?

IronTom

Thanks for your answer!

I did then proceed to check the following:

- WD40
- Carb Cleaner
- Lighter fluid.

The result was the same - no result. RPMs did not change.

My truck is also propane adapted (dual fuel), but I had the same problem before the conversion. Runs the same on both gas/LPG. I love the propane conversion

cruiser54

My truck is propane only-2400 gallon tank.

IronTom

2400 gallon... http://goo.gl/q7JR1 pink like this one?

But back on topic... no flammable substance is causing an RPM change when I inject it directly into the manifold. Why?

cruiser54

I assume you can feel good vacuum at the line you're using?

IronTom

I am putting my finger to the vacuum socket at the manifold and I can feel vacuum. I am injecting propane directly into the hole. I did not measure the vacuum yet, I don't have tools here.

The only thing that comes to my mind is that I am using too little (and I think I am not) or there is a bigger hole somewhere down the line. Both seem a bit improbable. Or maybe the propane is hitting a dead cylinder? But I think I got all six running.

How else can I check the vacuum lines? I don't have a smoke machine.

cruiser54

Spray them with throttle body cleaner with the engine running.

The Blacksmith

check to make sure your exhaust is flowing properly.

Shot in the dark here but propane vehicles can create black soot and clog cats.

IronTom

Thanks,

This is correct, but I had the same problem before converting it to propane.

Exhaust is a bit irregular in sync with the vibration/shaking I think. The manifold and the throttle body were dropped and cleaned recently (problem already existed), so I don't think it has gunk/sooth in it. The shaking is very slight and barely visible on the tachometer but I can feel it when sitting in the Jeep. It sometimes gets worse when I drive more aggressively or after wheeling/pulling a trailer.

This thing got me and my mechanic stumped at this point. I don't want to randomly start throwing parts at it.

I have a leak from a side freeze plug (already fixed in the most part, but one did not take and I need to redo it). But I don't think that is connected.

IronTom

Here is a video of a start + idle
You can see the needle move at 0:55. I tried to capture the vibration by putting the phone on the dash, but no luck.

Sorry for the quality.

cruiser54

How's your tune-up hardware? I know the 496 in my propane truck is really hard on spark plugs. We found that reducing the plug gap down to .o32" helped with driveability, too.Something about propane being a dry fuel and harder to ignite?

IronTom

New plugs, wires, distributor cap and rotor. Same condition pre and post conversion. Video shows it running on gasoline.

rrich

What tool are you using to add propane? If it's just a propane torch it doesn't give enough to do much. Drill out the tiny orifice - then it'll work.

And - if it's in closed loop, the system will detect it's rich from the propane - then it'll adjust idle speed and mixture to compensate - thus no real change. You will probably feel a slight difference immediately when you add the propane, but it's quick. Same when you shut it off. Otherwise unless you really richen it, no change.

if you are looking for a vacuum leak, shoot the propane around suspected areas, listen for that slight change.

But by far the best way to look for a vacuum leak - connect a small hose to a vacuum port and blow cigarette smoke through it - engine off of course.
If needed, duct tape the top of the throttle body closed to keep the smoke in. Anywhere else it comes out is your leak.

bambam0417

What a bout your o2 sensor? the first one controls you air/fuel ratio. that is when it is closed loop tho. I see you have 209,xxx on your odometer could be time for a new sensor if that one is on its way out.