[old school guy]

I have a 1989 Dodge D250 with 140,000 miles, 5.9 L 360 CID. It has an emission system on it that looks like a California truck. About 6 weeks ago it started running very badly, it had a stumble/hesitation like a backfire with no noise. It caused the truck to jerk and lurch very hard. I replaced the fuel filter which was very badly clogged. The timing mark would fluctuate in about a 10 degree space. I replaced the distributor ( electric, not vacuum ) and the timing mark was steady at 10 degrees BTDC. This helped but did not eliminate the problem. I then replaced the spark plugs & the wires which helped a little more but still did not stop the problem completely. I then took the truck for inspection and emission for licensing where it failed the HC(ppm) readings. The standard for both idle and high speed is 220, The truck idle reading was 274 and the high speed was 492. I took the truck home and replaced the oxygen sensor because it was running so rich. I then drove the truck to see how it was and I could still notice a small jerk/lurch and the idle is not as smooth as it used to be. I have 9 more days to repeat the emission test but would like to make sure it will pass. Is this as simple as replacing vacuum lines or is something wrong that I have not completely eliminated ?

 

[68RT]

Check for timing chain slop.

 

[ImperialCrown]

Are there any or have there been any fault codes? How were the CO and NOx numbers at the emission test?
If the fuel filter was badly clogged, that makes me wonder about the conditions inside the tank and the rest of the 24 year old fuel system. The fuel pump also has a filter sock on it to catch larger particles. The injectors should have a nice conical spray pattern and no trickles or drips. An advance timing light can be used as a strobe to catch the spray pattern shape that may be hard to see otherwise.
If debris has entered the fuel pressure regulator, it could disable or block that and you could be running a too high of a fuel pressure. This could cause a rich condition and the fuel pressure should be checked. A pinched or blocked fuel return line will also elevate fuel pressure.
The correct Champion RN12YC spark plugs and an OEM (NGK/NTK) style O2 sensor may be important. Bosch can be trouble.
The 2-wire PCM engine coolant temperature sensor (not the single wire dash gauge sensor) needs to be disconnected before setting ignition timing. This will turn on the 'ck eng' light, but the light can be reset after setting timing.

 

[dana44]

If the other numbers were within limits, the high HC number can be the EGR valve not working or believe it or not, leaking exhaust. Too much advance can also do it, verify your 10 degrees advance is correct, I think it should be something like 3 or 4 degrees BTDC. I have had all three of these things give me high HC numbers in the past (but not all at the same time).

 

[Bob Lincoln]

These are also susceptible to harmonic dampener failure. That can cause the ignition timing to be set incorrectly.

 

[old school guy]

Are there any or have there been any fault codes? How were the CO and NOx numbers at the emission test?
If the fuel filter was badly clogged, that makes me wonder about the conditions inside the tank and the rest of the 24 year old fuel system. The fuel pump also has a filter sock on it to catch larger particles. The injectors should have a nice conical spray pattern and no trickles or drips. An advance timing light can be used as a strobe to catch the spray pattern shape that may be hard to see otherwise.
If debris has entered the fuel pressure regulator, it could disable or block that and you could be running a too high of a fuel pressure. This could cause a rich condition and the fuel pressure should be checked. A pinched or blocked fuel return line will also elevate fuel pressure.
The correct Champion RN12YC spark plugs and an OEM (NGK/NTK) style O2 sensor may be important. Bosch can be trouble.
The 2-wire PCM engine coolant temperature sensor (not the single wire dash gauge sensor) needs to be disconnected before setting ignition timing. This will turn on the 'ck eng' light, but the light can be reset after setting timing.

The message center has shown "brakes" and "anti-lock brakes" for about 6 months now, but that is because the rear drum brakes are anti-lock and passed inspection but are due to be replaced. Check engine light has not been on. Idle at 740 RPM for CO was 1.18 ( 1.20 standard ), high speed at 2375 RPM for CO was 0.67 ( 1.20 standard ). Idle at 740 RPM for CO2 was 12.5, high speed at 2375 RPM for CO2 was 11.4. I have not checked the injectors because if I hit the gas like passing a vehicle it smoothes out. And yes this could be an erroneous conclusion on my part. I was not aware of the procedure for timing this engine and thank you for the info on the temperature sensor, it is not mentioned in the Haynes manual and I would have never considered it. The only vacuum line I disconnect when I time the truck is the line that connects from the throttle body to the air cleaner, I don't know if this line should be plugged or not. Since my first post I have replaced all of the rubber vacuum lines on the engine, and it was about time. I really thank you for your reply and I am going to re-check all of your suggestions. I don't know how much it will change the timing with the coolant temperature sensor disconnected but I think I will set it around 6 or 7 degrees BTDC just to hedge my bet a little. The jerking problem does feel like a timing thing, if this has been it all along- boy will I feel silly. I will let you know.