Greetings forum readers. I have a troubleshooting question.

I have a mid 90s propane dodge ram 1500 van (conversion from gas) stuck in my driveway.

It started at my buddy's house after a several turns and I drove it over here. Won't start at all now. This thing used to start without any trouble.

New air filter, battery fully charged, tanks full, starter turns engine over and over just fine, took a plug out to confirm sparking. I tied a bag on the exhaust pipe to see if any fuel was being delivered. Removed bag, smell and burn test says fuel is going through.

But it sounds like no combustion is happening at all, as far as I can tell.

Ambient air is a little below freezing.

What else should I be checking before I see a mechanic? Fuel ratio? Spark gap width? Timing of some thing or another?

I'm handy enough with electronics (having designed circuits) and so-so with a wrench (I can change my motorbike's oil.)

 

Compression test a couple of the cylinders just to make sure you're getting compression. Does it have spark?

 

I pulled a plug out to test for spark, grounded the case with an alligator clip, and it does spark when I turn the key.

I haven't tested to see if any plugs have spark while they are screwed in. I don't know how to do this, except for maybe wrapping a few turns of magnet wire around a lead wire and see if anything shows up on an oscilliscope when I turn the engine over.
I've used this magnet-wire sensing method to build a simple tachometer for a dirt bike, so the technique might help here.

I haven't tested compression as I don't have a compression gauge, but I can borrow or buy one.

 

Short version: after some fiddling I have the engine starting no problem.

Long version:
Well. I hooked up a remote start switch to do a compression test.

Found out that my compression tester does not "hold" the reading. (Needle bounces up on compression, then drops right back to 0) It looks like the proper tool, the box says compression tester, and it has a pressure release valve. Maybe it's broken or maybe I was doing something wrong.
I realized after that I hadn't opened the throttle and remove the air filter like the instruction sheet said to. Oops...

I decided to try again another day with a different tester.

Just today, for curiosity's sake, I took off the air filter to have a look inside. I unbolted a hose from the top -- this hose led to what looks like some sort of fuel regulator. Wiggled a valve that said L/R (guessing this means lean/rich) but I left it in the same position. Unplugged then plugged in the electrical connectors to a couple sensors (guessing these were the throttle position and MAP sensors, but not sure.)

After satisfying my curiosity I bolted, plugged and screwed everything back together and tried to start the engine. And what do you know, it started without any trouble, even less trouble when I first drove it over here the other week!

 

Hopefully it was just a loose connection on an electrical connector.

 

Does the fuel gauge work and what does it show? You could be out of fuel.

 

Gauge works, it reads "full", and I used a bag on the exhaust pipe to collect whatever was going through the system. The gas in bag burned so I definitely had propane going through.

Continuation from my previous post:

Anyway after a few days the engine wouldn't start again. I did some more tinkering with the propane carburetor and found a pretty consistent starting method, which was to disconnect the fuel hose+attachment, turn the engine over a few times until I heard combustion, stop, reassemble the fuel host+attachment, then start it up again.

This led me to believe there was a stuck part or a leak or something.

Anyway I disassembled the carburetor, and the inside looks like it hadn't been cleaned in 15 years, it was caked in black oily-looking dust. I cleaned everything up with kerosene, put it back togethor, sealed some obvious air leaks, and so far I haven't had any starting problems... Huzzah! Hope it stays that way.