I have a 1990 D150 with a 318 V8 that's giving me 2 problems.

#1 It has a misfire on Cylinder #7.

I have changed coil, dis cap, rotor button, crank position sensor (plate under rotor button), wires, plugs. (side note that all this stuff was changed about 6 months ago before I bought the truck). Fires up fine but you can hear a very slight misfire. So I use a plug tester (the one that plugs into the plug wire then onto the spark plug that has the light), go from cylinder to cylinder. All cylinders show to be firing good with the exception of #7. The light on the tester is very weak, will go out for a few seconds then comes back very weak. Swap plug wires on that cylinder, no help. Used tester between dis cap and plug wire, same thing. All the old parts looked ok, checked the dis cap, its on tight, rotor button doesn't seem to have any issues. I don't have any clue what to try from here. Everything from the coil to plugs is new. Battery is good.

#2 It starts bucking and poping bad from 40 to 60 mph. As long as its not under a load it doesn't do it. if you start pulling a hill, or pulling a trailer it does it bad. Sounds like someone firing a gun in the cab. When this happens if I floor it the truck picks up and takes off with no issues what so ever. Thought it might have been bad gas, So I filled it with premium and added a can of seafoam. Ran that tank and filled a 2ed tank with premium again. no change.


I put it in a local shop for these issues after I ran out of time to work on it. They have changed TPS, MAP, and the throttle body gasket. They have checked over everything I did and said that all looks ok. Checked the timing of the truck and timing is spot on. The shop will be closed all next week for the holidays so im picking it up tomorrow. I'm not sure where to go from here. My next thought was to drop the fuel tank and clean it out, maybe replace the fuel pump, but when I changed the fuel filter I had someone turn over the truck and the flow seem good from the fuel line.


I would be grateful for any help.

 

Your truck doesn't have a crank sensor that's the distributor pick up coil. Either the pick up is not sending a signal to the pcm to fire the coil or there is an issue with getting the spark to #7. Try putting a timing light on the coil wire to see if it is firing on all cylinders, If so then the issue is the spark path to #7.

 

Thank you Moparite for the reply.

Sorry, couldn't remember the name of the sensor. On my Dakota it was a camshaft position sensor. shop told me the timing is good, and I know the path is good. once I get it back home today I'm going to check the plate under the pickup coil to make sure none of the tabs that go threw that sensor aren't jacked up. I'm almost thinking of putting a new distributor in the truck anyway to eliminate that part altogether. plus getting my big add under the hood to get to that screw at the back against the firewall is a huge pain!

I will double check the path to make sure it's good, I'm not above checking over myself.

Thanks again