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Ford F-250 & Ford F-350The heavier duty full sized trucks from Ford, offering bigger, more powerful engines and drivetrains for the abuse they may go through in the workplace.
I just picked up a real clean 1995 F-250 Supercab for $1k (167k on the odometer). The engine has a misfire issue.
I pulled the plug wires while running, only 1,4,5,8 (when pulled) made the idle worse, but when pulling 2,3,6,7, there really was no apparent change to the already really rough idle.
The truck just had a head gasket repair done. The former owner drove it fine for two months, then this misfire issue started up, so he parked it and bought a different truck.
Plug wires are correctly installed, going to the correct cylinders, but the distributor cap is missing the rear clip. I'd think that this could cause the issue, but 1 and 8 seem to be firing properly and they're right next to where that rear clip would be. I'm ordering a new adapter to be sure, but wanted to check in with those who know more than me (you reading this) to see if there are any obvious things I should be checking. Thanks for any tips that you can offer. I'm eager to become part of this group.
It is always best to correct any obvious problems before moving on to other avenues. That way you know it can't/shouldn't be the cause of the problem. It's kinda like keep on pouring water in a bucket with a large hole hoping there is some other reason for it losing water.
Something I would like to bring to your attention and that is NEVER pull spark plug wires off engines with electronic ignition systems, it can wipe out the ignition module and give you something else that will need to be replaced. When you do that the energy has no place to go and spikes the ign system and can ruin the module. The better way would be to provide a path for that spark energy to get to ground instead of through the spark plug. That will still provide the same result as removing the plug wire.
Let's see what you get after you correct what you see that is not right.
Are there any scratches or cracks in the dist cap?
A scratch inside the cap can give the electricity a path to follow and contribute to a misfire. Even the tinest crack can be an issue.
Eric
@hanky , and @eric hoffmeyer - thank you both for taking time to send replies. Very much appreciated!!
Ok, I've put a new distributor cap adapter, distributor cap, and rotor on. Double-checked all the plug wiring to be sure they're going to the right places. 1-3-7-2-6-5-4-8 has been checked and confirmed. Still runs exactly the same.
I noticed the distributor bolt wasn't completely tight while I was marking the distributor position on the block with a Sharpie pen, so I started the motor and played with the adjustment a little to see if that made a notable improvement. It did not, so I set it back to the marked position and tightened up the bolt to secure the distributor position.
Next steps I can think of are to replace the plugs and wires, and/or pull the distributor and put a new pickup coil in.
I'm trying to think of what could've gone wrong to make this misfire occur. PO said he ran the truck just fine for a couple of months and then this just started out of nowhere. Could it be a vacuum leak that would cause the issue?
If anyone wants to listen to it run (this is the day I bought it - not after the work I just did with the distributor mentioned at the start of this reply), you can hear it in this file here: https://www.dropbox.com/s/d8lvbuifxp..._6653.MOV?dl=0
Since the heads were done, was the comp checked since you have been trying to diagnose?.
I’d want to make certain there were no base engine issues before moving forward.
CEL on?
Last edited by Hayapower; Dec 29, 2018 at 11:20 AM.
@Hayapower , I'll ask PO about having the computer checked, but I'd doubt he did anything to look into it. When he had the engine back together, everything worked fine and he drove it around for a few months just fine. This issue is just a phantom that apparently appeared out of nowhere. PO did put a new EGR and MAP sensor in, hoping he'd be able to fix it, but no luck.
The CEL is not on, nor have I seen it on since I bought the truck a couple of days ago. I've seen the indicator light up as it should when you first stick the key in/fire it up. Nothing else.
There’s certainly many things that can cause misfires, more so showing at idle.
I saw the new EGR, and vacuum leaks can be problematic, but at steeper throttle angles, WOT, generally a hard misfire boils down to one or more of the big three missing. Spark, Fuel, and/or Compression. If your pulling the plugs anyway, check the compression. If the heads had to be done prior, why? At 167k, I’d want to know. This is my point...
Even though the heads were done, valve train failures can cause misfires. With the comps known for certain, move on, but not a bad idea anyway so you know base engine health. Better yet since your the new owner, without any record of prior tests..
You state 2,3,6,7 have no running contributions. So,, looking at the cylinder arrangement #’s, these numbers have one thing in common, they are side by side with one another. More the reason to double check comps.
Could be ignition, injectors etc,, but their proximity to each other is suspect to me, and I’d rule it out..
Last edited by Hayapower; Dec 29, 2018 at 12:00 PM.
I tested cylinder compression 5-8 first (starting with 5) and pulled them one by one (test order, in case it matters, 5/6/7/8/4/3/2/1). For 1-4, I pulled them all and tested compression with all 8 out.
If you wire brush a plug you can leave scratch (track) marks in it. This will cause the spark to follow the track marks and not jump the gap as efficiently. Spark plugs cleaners used to use a light sand blasting cabinet arrangement.
I wanted to mention running the truck in complete darkness and look inside the engine compartment for a "fire works" show. Did this for a guy many years ago and you could see the spark "leaking" out of the spark plug wires.
New wires and plugs and he was all set.
The canister looking item you circled is called the Thermactor Dump Silencer. (Some call them mufflers) It’s part of the fresh air/smog injection system called Thermactor, including the air pump, air bypass/diverter valve etc. The nipple side remains open.
Reading your comps,,, could be better and more balanced, but given a few haven’t or aren’t running, pressures may improve when they do. When running comp tests always pull all the plugs. Cranking speeds tend to be increased for better readings, and, If say a head gasket issue we’re to exist it may be masked if a companion cylinders plug was still installed..
The pickup coil wouldn’t cause misfires like your experiencing.
The small vacuum lines if leakers, even larger leaks, and/or including EGR bleeds, may contribute to an unstable idle but not hard misfires, it would be more general random and rough idle that greatly improves off idle and into deeper throttle angles.
Do these non contributing cylinders improve or fire in mid/higher RPM ranges?
Last edited by Hayapower; Dec 30, 2018 at 10:40 AM.
95 F-250 w/351 - Misfire issue
