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2.8 starts misfiring after 45 mins of highway driving


zachdimaio7

New Member
U.S. Military - Veteran
ASE Certified Tech
Joined
Jun 26, 2025
Messages
3
City
Greensboro
Vehicle Year
1984
Transmission
Manual
Hey guys, new member here. Have an 84 flat bed ranger with a pretty odd issue. Runs great for about 45 minutes of 65mph+ driving. After that it starts missing. If I slow down to about 40mph it will usually correct itself and run normal for a few miles but then start again. Now it will still maintain speed but it is defiently misfiring on atleast on one cylinder. I have shut it down while this has happened and then checked for spray from accelerator pump to try to rule out vapor lock. It does spray but I havent totally ruled out a vapor lock scenario but does seem unlikely. If i keep it under 60mph I can drive all day no issues. Now more about the truck. 2.8l v6 not orginal to this truck. I swapped this motor in about 4 months ago and this is when the issues started. From what I was told motor came out of a B2 and was a ford reman unit. It does have the reman sticker and before i installed the engine pulled rod cap and main cap. Both were .010 under and pistons were .040 over so seems to make sense it was a reman. I resealed the engine along with stage 2 cam,stiffer valve springs,remaned heads. Truck has been converted to duraspark 2 running a non feedback 2150 carb. New dspark box, distriubutor, cap and rotor plugs, plug wires fuel pump and fuel lines. Dropped the tank and its clean and venting. New fuel lined from tank to pump and wrapped in heat resistant sheilding where it gets close to exhaust. The only “weird” thing I have found is base timing is set at 12 btdc. Now she will run there but doesnt sound too happy. She sounds better at about 25 btdc. I have set it to 12 because with with the mechanical and vaccum full in if it was set to 25 the timing would be almost 50 which seems like alot for this motor. My old 2.8 was set at 12 btdc and seemed happy there.I have also tested this issue with diffrent base timings and with the vaccum advance plugged and did not make a diffrence, however thought it was odd enough to add it here. I have replaced almost everything related to fuel and ignition in an attempt to fix this issue but I am at a loss and running out of money and ideas. Any help would be greatlyappreciated. I apologize for the long winded first post but wanted to make sure I gave as much info as I could. Thanks again!
 
I would try setting the timing back a little bit. If you want a quick test, take the vacuum advance and unplug it off the distributor and put a screw in the hose. Then drive it like that and see if the problem goes away.

If it does go away you can set the timing back to 10 BTDC and try it. You could also research and find a cooler sparkplug and install them in the engine and see if that cures it. I have had this same problem with other engines, and setting the timing up made them run really good, but sometimes I would also get a miss-fire after doing it. I found on some of them running a step down in the heat range of the sparkplug made it go away. But each engine seems to be different.
 
I have plugged the vaccum advance and it still had the issue. I havent tried a cooler plug, but I have replaced them a few times. Would a plug running to hot show any signs on plug itself? I would suspect the insulator would be melted or discolored in some way if it was getting hot enough to cause a misfire. But I guess and easy test would be finding cooler plugs as a set of plugs isnt too much at a local parts store.
 
I'd try running with the fuel cap loose or off, might be building up to much vacuum in the tank and starving for fuel.
 
I have tried running it with the cap off and issue was still there, I am also considering adding a electric pump or a return of some kind to try to aid in vaccum lock or vapor lock situation
 

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