Weird Electrical Issues Seemingly Unfixable

I own a 1986 Ford Ranger 2.9L V6 and A4LD auto trans that has been running rough and intermittently missing for the past almost 2 years now. I’ve talked to tons of other people who told me several different sensors/modules could be acting up. Replaced a bunch of modules and checked a bunch of modules to verify that they’re good and it seems like every single one I’ve tested (or had tested by an actual electrical shop) has checked out. I decided to just replace the ECU and see what happens, then weirdly, it ran great for about a day. It ran the best I’ve ever heard it, super smooth and no misses. I did notice however that during that single day it ran good, it was rough for like the first 30 seconds of startup, almost like the ECU was resetting every time I turned the truck off. After that day, it went right back to running like garbage and swapping with the old ECU changed nothing about how it ran. I’m confused how it could just run great like that and then never again, almost like something damaged the ECU. I also noticed that my gauge light fuse just blew out randomly recently, after replacing it with a good brand fuse less than 2 years ago. I’m not sure if maybe something is up with the wiring (previous owner did some hacky stuff with the radio wiring so not sure if the dude screwed up something else), or if something is sending too much power to the electrical components. Does anyone have any slight idea what could be the cause of these symptoms? I have absolutely no clue and am stumped.

For the rough running, has anyone checked if the timing chain/guides are worn?

710_S.A. GEAR_76055_1


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On a truck this old you can assume they every ground connection is corroded. Some a little many a lot. There should be very low resistance from the negative battery terminal and the engine block, alternator, body and frame.

Start there. Post back with what you find and we’ll help from there.


I had a 1990 Ford Aerostar with a 4.0 liter V6. It would run roughly for about 30 seconds after startup after it sat overnight and then smooth out. It then started running roughly all the time. The problem was traced to a defective spark plug. The ceramic around the center electrode had cracked and a piece broke off. The spark plug was replaced, but 6 months later, the problem reoccurred, although the engine still ran rough for 30 seconds after startup.
It turned out that a cylinder head had a hairline crack and coolant was leaking into a cylinder. The Aerostar was still on warranty. The service tech said that the cylinder wall was scored and the engine was replaced.


That’s possible. Another possibility is a bad connection or wire crimp on the harness feeding the ECU. Disturbing the connector/harness during the swap made good connection temporarily.

There’s more than one way to skin this cat. One would be to start diagnosing any circuit that can cause the drivability issue. This would eventually lead you to the offending circuit/component. Another option would be to dismount the ECU but leave it electrically connected. Then wiggle/push/pull each individual wire to see if it has an effect- positive or negative.

Grounds can be checked with a DVM. Find a solid metal ground and then measure each ground wire looking for voltage above some small amount (a few millivolts may just be noise or meter offset).


I own a 302 equipped Ford truck w/automatic trans, 10+ years older than yours. I presume the symptoms occur at most all speeds and driving conditions, not just at idle or going slow. If so the symptoms seem consistent w/some sort of ignition system problem. Very unlikely to be caused by the powertrain computer, ECU, but could be a problematic ignition module. When diagnosing these sorts of symptoms, best to start with

  • New spark plugs
  • New distributor cap, ignition rotor, and spark plug wires (if so equipped)
  • New or recently replaced fuel filter
  • Cylinder compression verified ok
  • EGR system working correctly (not sticking even slightly open)
  • PCV system working correctly
  • Replace all vacuum system hoses over 10-15 years old

If all that stuff is in good order and still having these symptoms, might just want to replace the ignition module. Symptoms remain? Your best bet is to hire a Ford experienced shop to diagnose the ignition system. they’ll have special test equipment you don’t have access to. Doesn’t mean you can’t fix it yourself, but let them do the diagnosis & tell you what’s wrong.

Is your truck multi-port fuel injected, throttle body fuel injected, or carbureted? Your symptoms could also be a problem with the air to fuel ratio.

Another vote for bad ground somewhere… I have seen Ford run a under the dash ECM ground out on a fender close to the battery… So don’t take any ground for granted, check them all for corrosion and or being frayed…


Sorry I forgot to mention, it runs rough at idle, but pretty decent at normal driving speeds. I can hear it missing intermittently sometimes still but it’s much smoother than at idle. At a high load (aka near flooring it), the random missing is more apparent. Spark plugs are new (tried two different sets to be sure), new distributor cap, new rotor, new wires, I should definitely check fuel filter but I’ve been putting it off because the fuel pressure tested fine, Compression is 137.5 to 147.5 psi on all cylinders with no big outliers, I’m getting EGR pressure feedback codes but all EGR related parts are brand new, PCV valve works fine, and I replaced any/all hoses that were cracking/hard or had any leaks showing during a vacuum leak test. Ignition module has been changed with no change in the idle. I took the truck to a guy who worked on the trucks when new and he gave up after having it at his shop for a few weeks, every other place refused to work on it because of its age. It’s multi-port fuel injected, pretty sure its an air to fuel ratio issue because it smells like raw gas from the tailpipe, still need to check MPG though to confirm how rich its running.

Do you mean any of the wires or just strictly power/ground wires? Also swapping back to the old ecu then back to the new one made no difference in how it ran, so I feel like it would be a pretty big coincidence for the wires to connect good at the first time the replacement ECU went in then never again. Not discounting that theory but I feel like its unlikely. Messing with the wires to see if anything affects it is still a good idea to try though. What I’m gathering from a lot of these replies is to check any/all grounds, especially ones relating to the ECU.

@Mustangman @davesmopar I did check one ground that’s for the cab electronics and that tested good, but I’ll test the other grounds you mentioned too and ground wires relating to the ECU.

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Did you make sure the EGR ports are not carboned up, I start the engine with the EGR unbolted to make sure flow is there… Carbon can also break off and stick in the EGR pintle keeping it from closing all the way…

As mentioned already, check for timing chain slop/slack…
Check for dist shaft movement (side to side)…

Possible that the ECM has a floating ground caused by a bad or corroded ground wire, could have fried the new ECM…

Also if using AZ parts or any cheap line (house brand), I have installed new AZ wires and the vehicle ran worse, missed worse… Looked under the truck at night and it looked like a lighten show from all the arcing…lol
So quality of parts (or lack of) can be a factor as well…

I’ll check for timing chain slack and grounds over this weekend when I find some time cause I’m a bit busy at the moment. The EGR port had a very thin layer of carbon built up but was still very open. The actual EGR valve is pretty clean because it was replaced less than two years ago with about 2k miles put on the truck since. There was no difference in performance from the old spark wires from the new ones, I also tried to see if there were any sparks visible at night and I couldn’t see any. I’m wondering if the cheap no name DPFE (EGR pressure feedback sensor) I put in is causing the code, so I got an OEM one from the junkyard about a week ago because it was a couple bucks so why not. Haven’t tried putting it in yet though


I wouldn’t rule out the possibility of new oxygen sensors that are not up to OEM specification. The MIL will not be on but the sensor does not perform properly. I am still diagnosing this issue on a 90s Ford where I bought two different brands of sensors for bank 1 and 2 and they do not perform the same way. A marginal sensor could contribute to poor running and make another problem harder to track down.

You can’t get any kind of live data read out from the ECU on that can you? I think the early ones don’t support this at all.

I vote for poor grounding as well. I once dealt with a similar issue. The engine would miss and stumble terribly on cold start. The exhaust would reek like an old two stroke boat motor after being started and then all this would go away once the car had been running a short while. One thing I noticed was this was WAY WORSE on cold damp mornings. It would be fine the rest of the day unless it sat for like 8 hours, then I would have it again, especially if it was cool, overcast, and damp outside. I never got a check engine light or any codes.

Then someone suggested I clean the grounds for the sensors on the engine. I didn’t do this for a bit because they looked fine. I couldn’t see anything wrong and there was no visible corrosion. I got desperate and decided to try as it was so easy and cheap. Basically the old eyelets were dull brass. I used fine sandpaper and made them shine. I did the same thing where they mated to the intake area of the engine. Then I wiped this all down with a rag soaked in carb cleaner. After that I coated everything in silicone grease and put it all back together. The difference was amazing and the car ran so much better. There was no more stumbling on cold damp mornings.

Definitely check and clean grounds, even if you think they look just fine. Mine didn’t look all that bad but obviously it was enough to cause big problems. I was being told that my valves were likely sticking open due to grime or thick oil when cold. I was using the thinnest oil I felt I could run in this engine as well.

Then it sounds more like a vacuum leak of some sort, or an EGR problem. That DPFE gadget is supposed to modulate the vacuum signal to the EGR so the EGR only opens as far as it should for the driving condition and not at all at idle. That Ford part ( in that era, not so much currently) seemed to be the topic of quite a few posts about drivability problems by reports here, the forum search feature will find them for you, link upper right this page. Suggest to be suspicious of a DPFE problem. Your shop might be able to completely disable the EGR valve as a test. If all the symptoms immediately disappear, you know you are on the right track. (Don’t drive with a disabled EGR except for testing purposes, b/c one of the EGRs function is to limit the temperature of costly-if-damaged internal engine parts. )

Vacuum leaks have been a pretty common problem affecting idle quality on my Ford truck. Vacuum hoses not seating tightly on the fittings, cracks in the vacuum line T’s, and vacuum devices failing b/c of split internal diaphragms. I had that problem with both the advance & retard gadgets for the distributor timing, and the actuators in the air cleaner housing that control the intake air temperature. With poor idle quality it is always a good idea to verify there are no leaks in the vacuum system. You’ll (or your shop will) need a hand-actuated vacuum pump w/gauge to do it. The vacuum source begins at the intake manifold, so that’s where I start testing. Any vacuum line connected to an intake manifold port should hold vacuum to 20 mmHg. Follow the directions that come with the hand-held vacuum pump.

My guess however, problematic DPFE. When that part fails it produces very difficult to diagnose drivability problems.

I don’t think live data is possible from this, even if it is, I don’t have a scanner that could do that. I tried two different oxygen sensors (Bosch and Denso) and it ran the same with both of them. There’s only one sensor before the cat. Also, I’ve heard that oxygen sensors don’t work until they’re warmed up, and the truck runs bad the second it starts.

@Mustangman @davesmopar @cwatkin @TwinTurbo I just went out today and checked a bunch of grounds. There’s a ground strap to the hood that’s partly frayed and I could barely get a connection and when I did, it was all over the place. The other questionable ground strap was from the body to the exhaust which looked pretty crusty/corroded and was a bit tricky to get a resistance reading from.

Hood to Body Strap: Anywhere from 0.5 to 15ish ohms, very jumpy and could barely get a reading, also frayed.
Head to Firewall Strap: 0.4 Ohms (This one is supposedly the cab electronics ground strap.)
Exhaust to Body Strap: 0.9ish ohms, crusty and hard to get a reading off of
Engine Block: 0.4 Ohms
Alternator Housing: 0.3 Ohms
Frame: 0.3 Ohms
Body: 0.4 Ohms
Rad Support: 0.5 Ohms
ECU Ground Wire I think…???: 0.4 Ohms
ECU Pin 16 (Ground): 0.5 Ohms
ECU Pin 20 (Case Ground): 0.4 Ohms
ECU Pin 40 (Ground): 0.4 Ohms
ECU Pin 60 (Ground): 0.4 Ohms
ECU Pin 46 (Signal Ground): Not connected to ground. Pretty sure this is supposed to be connected to ground if I’m reading this diagram right. Read some stupidly high ohm number and kept increasing

Have to send imgur link for second pic because new users aren’t allowed to have two embed images.

Just saw this article here mentioning issues with that signal ground wire that doesn’t seem connected to ground.

Is there a service lamp on the inside of the hood? I see no other reason to ground the hood.
Not too familiar with Fords but is that a ground on the exhaust pipe?? Does that save a wire from the O2 sensors?

Anyway, the braids are good enough for the amount of current going through them. The attachment points are where some attention may be warranted.

It only takes a few minutes and it’s free :grinning: I have been subjected to many weird circumstances I thought were unlikely in my years diagnosing problems.

Any wires. Some circuits have companion/dedicated grounds but it’s not just grounds but the signal wires as well. If the ECU cannot reliably read a signal wire that is intermittent, it’s probably going to result in some abnormal behavior…

Pin 46 seems to be open… You might want to check resistance from the ECU ground to pin 46. You might also jumper pin 46 to ground and start the truck.

The others look fine. The hood strap, I think, is to prevent static build up on the paint or maybe it helps the radio antenna performance. My Chevy has one but no hood light.