I have a 1992 dodge d150 with a 5.2l magnum. As I've been driving it lately it's sputtering and missing while cruising on the highway causing vibrations and shaking. The weird part about it is that it only does this when the load is off of the engine and I'm just cruising. The truck accelerates very well and doesnt miss at all while im accelerating. I've already done plugs and checked for a vacuum leak as well as a fuel filter in case it was plugged up, but nothing seems to be making a difference. Does anyone have any idea of what could be going on? Any response is greatly appreciated 👍

 

Unplug the O2 sensor, and see if the problem goes away.

 

I'm not familiar with your exact engine and what sensors you have. ..... I have been fighting something kinda sorta the same but different on a 1993 Caravan with a 3.3 V6

Going to need to do the key dance to pull the codes off of the old OBD1 computer ..... see what codes it is showing.

Giving it some more thought, I believe your engine has a distributor and not a crank sensor.
A quick down and dirty test, pull the cap off of the distributor and find some way to turn the engine over by hand ..... you only want to rotate it clockwise until the rotor button starts to move.
Then rotate the engine counter clockwise and while watching how far the crank shaft pulley moves and when the rotor button starts to move again.
The amount of play or difference between the two is showing how much slop you have in your timing chain. ..... Expect 1/2" minimum ... you start getting to 2" and your chain is stretched and needs replaced.

The reason why I suggest this, under load the engine will run fine and accelerate fine, all the slop in the chain is on the other side of the gear.
When you let off of the gas pedal the slack in the chain is transferred to the other side and this is changing the timing or when the plugs fire because now the distributor is not working in time with the engine.
Just get to cruising and barely using the gas pedal, .... all the slop in the chain is bouncing back and forth and timing is changing with it.
Nail the gas pedal to the floor and the engine is back in time and accelerates ..... does this sound familiar?

Again just a quick and dirty test to see how much play is in your chain .... it does fit your symptoms and you did not say how many miles on your engine.

 

His engine does indeed have a distributor, but, it only controls injector timing, there IS a crank sensor, that controls ignition timing. Chain stretch will reduce power, but, that's about it.

 

Just asking @Hey You because I really do not know. Does the distributor run off the crank, cam, oil pump?
The engine does have a timing chain .... The chain connects the cam to the crank ..... I say there will be some slack in the chain .... properly adjusted any chain needs some slack .... chainsaw, motorcycle .... any chain.

If the crank sensor is reading the revolutions of the crank and sending that info to the PCM to control the timing.

But the cam has a stretched sloppy timing chain controlling it ....
Maybe the timing is correct, but the lifters and valves are opening at a variable rate to match the slop in the chain controlling the cam?

So the timing is correct and firing at the right time ... but the valves are out of sync and not opening at the right time? Valves opening late will cause issues.

I had a old 1978 Toyota truck with the infamous 20R engine .... it ran great for years ..... yes it was a total junker when I bought it but fixed it up ..... they knocked the front windshield out so they could go deer hunting with it and fire the rifles out the front with no glass obstruction.

I drove that engine so far with a stretched chain in it, the timing chain cover started leaking oil because the chain would slap the passenger side of the cover when letting off of the gas.
Give it gas and it was just fine ..... It really ran good when I finally replaced the chain and cover ..... just a common issue with a timing chain.

 

Yeah this does fit my symptoms... my truck has 237,000 km or 140k miles

 

I'm gonna take a look at those O2 sensors and probably replace the distributor cap as well and see where it goes from there... The only reason I can't see it being my timing chain is because it only misses bad while going 55mph or faster. If it was the timing wouldn't it be doing that at a slower speed as well?

 

140K miles I do not think it would be the timing chain .... not impossible but probably a fluke if it was. ..... still worth checking though.

There are several sensors that either work with timing or with air/fuel ratio ..... really might help to use the key dance to pull what codes you can from the computer for a clue.

 

yeah, tried that already but didn't get any codes.

 

A stretched chain will reduce power, but, that's about it. I had almost 180K on my 96 when I replaced the timing set, that made a HUGE difference in how the engine ran. Much smoother, and far more power. Misfiring at steady-state cruise is more likely a mixture issue. Insufficient fueling due to a mis-reporting O2 sensor. Step on the gas, and the PCM goes into power-enrichment mode, and feeds the engine more gas. Step on it hard enough, and it will go into open-loop, and just ignore the O2 sensor completely.