Severe Misfire Under Load and Intermittent Misfire at Idle

Hello all. It's been a long time since I've posted. I sold my old 86 BII several years ago to a forum member here but I've, in the past year, bought another 86 AND an 89.

My issue is with the 86. Base model, manual everything. TK5 transmission with 3.73 limited slip rear. Running 235/75R15 Treadwright ATGs.

What's it doing? Tremendously bad performance. I can't maintain speed or accelerate on any incline. The guts fall right out of it. It feels like a very, very bad misfire. The more you actuate the throttle, the worse the condition becomes. It will do it on flat roads, too, but I can feather the throttle and very slowly accelerate then. It will also exhibit this problem if I'm trying to delay shifting on flat ground as the vehicle eventually requires more throttle input to accelerate further in the current gear.

The vehicle has exhibited this for a couple of months but it's been very mild. All I had to do was drop a gear and I could climb hills with no issue. Since about two weeks ago, it's almost impossible to justify driving anywhere with it.

I bought her in June and, since then, here's what I've done.
-Dropped the tank, replaced in-tank pump/sender assembly.
-Cleaned the tank with tap water, let it sit overnight with a couple of gallons of vinegar inside, then rinsed thoroughly the next morning.
-Replaced the frame-mounted pump.
-Replaced the fuel filter after the frame pump
-New plugs (Autolite, yes I know, horrible stuff. Will be replacing ASAP)
-New plug wires
-New distributor cap and rotor button
-Set timing to 10 degrees BTDC with SPOUT jumper removed
-New fuel pump relay
-New EEC relay
-New oxygen sensor
-Cleaned IAC
-New TPS
-Swapped ignition coils between the 89 and the 86.

The power steering pump pukes EVERYWHERE. I'm not kidding: if full, it will somehow throw fluid all the way onto the driver's side fender. This, of course, soaked the ignition coil but I did swap them to see if the condition improved. It did not.

I got into it last night with the timing light to double-check base timing. It's at 10 degrees still but I did notice that there is a random loss of spark at idle on cylinder (1). It was cold and I'm a pansy so I didn't take the time to see if this occurred on other cylinders.

Also, of note - a previous owner had completely disconnected the MAP sensor by cutting the harness. I repaired this. The truck ran OK after that, several months ago.

I'm not at the end of my rope but I sure am frustrated. I'm wondering if the TFI could be causing this. I tried swapping it from the 89 to the 86 but that was several months ago while I was chasing the less serious malfunction.

Sorry for the novel. Thanks in advance for the help.

Does it change severity with temperature or any other variable?

No. Engine cold or after running for an hour - same symptoms.

Something new today: after getting home, i shut it off and popped the hood. I heard, but cannot locate yet, a relatively quiet buzzing sound. It seems to be close to the alternator.

I think the next steps would be to check the basics. Do a compression test and check fuel pressure. Just because it's a new pump doesn't mean it's good. Also, while your doing the compression check, take a good look at the plugs. Are they fouled, clean (no fuel) or gapped improperly?

Agreed, all those are on my list after it warms up a bit.

I think you should track down that spark issue since you know that is bad. With the parts you replaced I'd start checking connections and then maybe try a distributor.

The way you describe the problem reminds me of when my BII had a bad upper intake gasket giving it big vacuum leak.

I changed the ignition control module as I had 5 minutes of non-rainy weather. I got the ICM in exchange for repairing a Pc

I haven't gotten it out on the road but it does stumble very badly if you are blip the throttle while idling in neutral. You can hear the misfire in the exhaust pulses.

I'm grabbing some motorcraft plugs tomorrow.

did you ever get this one figured out because i have the same exact problem with my bronco 2. random loss of spark on cylinder one , ive installed new ignition coil, 2 new distributors (on the whim of a defective part), a new pcm and a new tfi module came with new distributors so any information would be great. it i broke down out of no where going up a hill no signs or indications of breaking down it was running strong. popped a new distributor in it and it would run but terrible . when you accelerate it loads up almost like it has a 2 step and will backfire sometimes. (yes the dist. was installed correctly engine tdc with rotor before #1 cylinder). it will start with ease and try to idle .

I also have the exact same issue ive replaced everything and checked everything from compression to fuel pressure and have ripped all my hair out and given up and the only thing I can find online is alot of people have the same issue but no one has any answers

This maybe exhaust/intake valves issue. You may have to open up the top cylinder (s) for inspection.

voodoochylde said:

I changed the ignition control module as I had 5 minutes of non-rainy weather. I got the ICM in exchange for repairing a Pc

I haven't gotten it out on the road but it does stumble very badly if you are blip the throttle while idling in neutral. You can hear the misfire in the exhaust pulses.

I'm grabbing some motorcraft plugs tomorrow.

Click to expand...

Had similar issues that good plugs made better. Fingers crossesed. Turned out to be only half the problem though. New injectors were the final cure (expensive ones)