For about a month now, my ’93 Ranger has been running rough. At first the problem was intermittent, but it seems to be full time now. When it is running rough, the check engine light comes on, the idle is uneven (though seemingly not with regular misses like a missing cylinder), the exhaust smells really bad, and it doesn’t have much power beyond ~1/3 throttle. I’ve checked engine codes many times. I generally don’t get any code unless I take actions that clear the long term memory, in which case I get the Keep Alive Memory code. While I certainly am not an expert, the lack of power while driving reminded me of old cars that had lost their vacuum advance and also reminded me of the last time I had a bad fuel filter. I replaced the fuel filter and nothing changed. Next, I monitored the fuel pressure, which was within spec (38-40psi) at KOEO (Key On Engine Off) and within spec (~30psi) when the engine was running well, and jumped out of spec to 40psi when the engine started running poorly. I figured that was a smoking gun and I replaced the fuel injection pressure regulator (actually twice since the first replacement was controlling at 35psi KOEO instead of 40psi). During the fuel injection pressure regulator replacement process, I noticed the fuel pump was no longer shutting off at KOEO, which it had been doing at the beginning of this troubleshooting process. I monitored the control signals to the fuel pump relay and discovered that the PCM (Powertrain Control Module) was leaving the fuel pump on at KOEO instead of shutting it off after 2s. At this time (and possibly related to the fuel pump control issue), self-test would also no longer run. Since the PCM controls both self-test and the fuel pump, I replaced the PCM. I also checked the resistance of the TPS (Throttle Position Sensor) and found it to be within spec with no spikes in resistance during a couple of long slow strokes from end to end. After I replaced the PCM, I found that the fuel pump was controlled properly in KOEO (shutting off after 2s). Also, self-test worked properly and I didn’t have any codes other than the expected Keep Alive Memory code. I was really happy after the engine started, ran smoothly, and the check engine light didn’t come on. That happiness was very short lived as the truck returned to its original condition (check engine light on, running rough, and no ability to run self-test) within about 2-4 minutes after I started it. I figure I have killed the new PCM but have no idea how. I don’t want to just start changing parts or pulling apart all the connectors looking for a short or open circuit. Anybody see this situation before or have insight as to how I can identify and eliminate the problem?

 

The Haynes manual talked about self-test sensitivity to battery voltage and they weren't kidding. I put the battery on the trickle charger even though it is only a year old and slightly oversized. After charging, the voltage started in the 13.0 - 13.1V range and the fuel pump shut off in KOEO mode after 2s and self-test successfully ran with no codes. So the PCM seems to be OK (and probably was all along). Self-test seemed to use a lot of power because battery voltage dropped fairly fast and the fuel pump turned back on in the 12.4 - 12.5V range and self-test would no longer run once voltage got to that range. Is this normal?

I also checked voltage when the Ranger was running and the maximum voltage was 14.81V, which seemed pretty reasonable to me.

At this juncture, I'm not sure what to look at next since there are no codes.

 

Welcome to the forum

One LONG SENTENCE is hard to read

Open the old PCM and look at the 3 blue capacitors, probably need to be changed and it will be as good as new, under $5 to fix

Is the other PCM new or used, if used it will have the same issue with capacitors

You didn't hurt the new PCM unless you plugged it in to 110volts AC, lol

Computer runs different software when engine is cold and then changes software after engine warms up, 3 to 8 minutes depending on outside temp
Computer only uses O2 sensors after engine is warmed up
O2 sensors last 100k miles or 12 years which ever comes first, they run out of the chemical that detects Oxygen

Fuel pressure looks fine-30-40psi is spec

Check exhaust system for blockage, use vacuum gauge and raise RPMs to 2,000 or so and hold steady, if vacuum is slowly dropping then you have a partially blocked exhaust, no code for that

The TPS(throttle sensor) is, in essence, the "vacuum spark advance", it gives the computer a heads up to change spark timing for acceleration, but that sensor rarely fails

All the sensors except the O2s, are monitored so should set a code if they are not "in range"
So still a computer issue or clogged exhaust in my opinion

 

Thanks for your insights and suggestions. They make a lot of sense.

I'll start with the O2 sensors. The truck has 180k+ miles on it with the original sensors so it is definitely past time to change them. Plus, I think it always runs good right after the initial startup for the day and then deteriorates quickly after 2-4 minutes. It is hot here so a short warm up and switch over to the O2 sensors makes sense.

The PCM is remanufactured, so they should have checked/changed the capacitors but they may not have. My brother and I were also wondering about voltage regulation on the PCM. I'm guessing the blue capacitors are what does it.

 

I replaced both oxygen sensors and the symptoms are the same. When the truck hasn't been run for a while and is completely "cold", it starts fine and runs good initially. After about 2 minutes or so, it starts running rough and turns on the check engine light (but no codes). Recently, it does this every time, even when the truck isn't moved, the throttle isn't touched, etc. It seems like this behavior is pointing to something, but I can't figure out what it is. That is why I was so hopeful with the oxygen sensors. The computer ignores them until after warmup. Is there another sensor that is ignored for the first couple of minutes?

 

You needed the O2s regardless

Here's the thing, CEL won't come on unless there is a code or a computer issue
So unplug the MAF sensor and start and drive the truck to get CEL to come on, should come on fast with disconnected MAF sensor
Sanity test for computer
Then check codes should have MAF code at least

And how are you retrieving the codes, OBD1 reader or Flashing CEL?
In my opinion Flashing CEL is more reliable, lol

3-digit Ford code list here: https://www.therangerstation.com/tec...gitcodes.shtml

 

Yeah, I agree I needed the O2s anyway. Fuel filter too. I'm using an OBD1 reader.

Disconnected the MAF, drove the truck, and read codes (after trickle charging the battery). As usual, the truck seemed to run good immediately after starting and then started running rough after a minute or two. The KOEO test passed but the CM (Continuous Memory) failed with a 157 - Mass Air Flow Sensor Fault, low voltage. I couldn't tell if the self-test hung up due to low voltage or finished. Is that the only code you would have expected? I'll trickle charge overnight and try again.

I've noticed that KOEO test won't run (or will hang up) anytime the battery voltage gets down to ~12.44V. At that voltage, the fuel pump kicks on continuously and the battery voltage drops further. Is this normal? My battery is a year old and slightly oversized and battery voltage is ~12.6V after the short test drives. The test drives aren't long enough to fully charge the battery. I don't want to go longer because I don't want to mess up my catalytic converter with all the bad gasses. I have to put a trickle charger on the battery to make sure the voltage starts above 12.75V to run KOEO. As soon as the testing starts, the battery voltage starts dropping about 0.01V per second or two. Is all this normal?

 

After sitting overnight a good car battery will test at 12.3v to 12.8v, MUST sit for over 6 hours before testing, the longer the better
Testing a battery after charging or shutting off engine will give you a false high voltage reading, you need an "at rest" reading

No, the voltage shouldn't be steadily dropping just with key on, or OBD testing, it will drop maybe 0.1v with initial load but then stay

At rest car battery voltage
12.8v is a new battery
12.5v is a 3 year old battery
12.3v is a 5/6 year old battery and time to shop for battery sales

12.2v or lower is a done battery, or drained battery, i.e. left head light on
You can re-charge and then test at rest voltage, but once drained a car battery can never be fully charged again their design is to provide high amps for a short time(starter motor) then to be re-charged quickly, the alternator
They do not last with a steady drain and then re-charge, batteries that do are called Deep Cycle batteries, specifically made for that use, but are not great starting engines, they can, just very hard on them, like draining is on Car Batterys

 

I think my battery is OK. At rest with no trickle charging, it is 12.6V - 12.8V. After trickle charging, it is over 13V. Any idea why self-test would be causing it to drop so much? I don't think that was an issue when I started troubleshooting?

I haven't done the vacuum testing looking for plugged exhaust because I haven't had a second person to help. I do plan to do that testing when I can. Given the fact it always runs good after startup and then transitions to the rough running in a 2-4s window, I'm not sure a plugged exhaust would act like that but I really don't have much experience with that. What do you (or anyone else) think?

I haven't done the standard stuff like plugs and wires, but they were changed about 15k miles ago so should be good. Absent any better idea, I may check out that stuff but I don't think it will help.

Do you or anyone else have ideas as to what to look at next?

 

A guess: Could the spark coil pack react to heat? (hmmm...then go back to normal after cooling....repeatedly?).

 

I've been out of town without computer access. Sorry for the delayed response.

On the coil pack, are you familiar with their failure modes? Are you thinking that it is failing a pair of cylinders while the other two pairs of cylinders are OK? Is there a different failure mode you are familiar with? I was planning to look at all 6 sparkplugs to see if they all look the same. I figure that at least some of them have to look bad by now given how bad it has been running. Fortunately, I haven't had to drive it anywhere other than for troubleshooting.

Checked vacuum at the source (just downstream of the air cleaner) and it was very low. It was approximately 2-3 in Hg if I believe the two cheap gauges I have. I am concerned that the hose might not have been quite small enough to make a completely tight connection. I plan to retest with smaller hose and when the engine is completely cold (since it runs better then). If the pressure is higher when cold and drops off when it warms up, I think RonD might be right with his plugged exhaust idea.

I still don't understand how the CEL comes on without setting a code. Anyone have any experience with this happening? If so, what ended up fixing the problem? Given the PCM only looks at a very limited number of inputs on a OBD1 engine like mine, I'm thinking of replacing the MAF and IAT (Intake Air Temperature) sensors.

 

Don't use OBD1 reader, use the jumper wire method and count the CEL flashes on dash to see if codes come up and also see if you get the same voltage drop WITHOUT the reader connected

Jumper wire method, Google: How To Run A Self-Check On Fords 1987-1995
3:48 video

Also pull out computer and have a look at the circuit board

 

Thanks for the suggestions. Before RonD sent his suggestions yesterday, I did some more troubleshooting.

Inspected the PCV system (hoses and valve). No vacuum leaks and the PCV valve works as it should passing air from valve cover to intake manifold, but not in the opposite direction.

Measured vacuum again, correctly this time at the manifold. Got 20 in Hg at idle when the truck was running well and it dropped to 16 in Hg when the truck started running poorly after about 2 minutes.

Wondered if the PCM might be turning on the fuel vapor canister purge system after 2 minutes of run time. Disconnected the control to the canister purge solenoid and retested. Same results - ran good for ~2 minutes and then started running rough.

 

I used the jumper wire and got the similar results as with the OBD-1 reader. I had driven the truck to warm it up, hoping to run all 3 tests. However, self test would start to run but kept getting hung up and wouldn't finish. When it hangs up, the CEL light just stays lit indefinitely. One time it hung up immediately after it did the 3 flashes for 6 cylinders. It always hangs up when the fuel pump kicks back on, which shouldn't even happen in KOEO. Battery voltage is at least 12.5-12.6V when it hangs. I've also seen it hang when the battery voltage is over 13V. I've checked the battery voltage using the engine ground, the body ground, and the negative terminal on the battery. The voltages are the same within 0.01V, so grounding doesn't seem to be an issue.

I had already replaced the PCM with a remanufactured one and the symptoms were exactly the same with both of them. Originally, I thought that meant my original unit was good. RonD got me thinking otherwise. After watching Steve Stolz's excellent YouTube video called Ford OBD-1 ECM repair, I pulled the cover off my original PCM and found that two of the three blue capacitors had definitely failed. He said those capacitors provided reference voltages for the sensors, so are critical to the PCM working correctly. I ordered all the parts from Digikey to do the repairs. I pulled the remanufactured PCM back out of the truck and looked at the capacitors. They had been replaced but the person doing the job wasn't careful and the board was pretty badly burned. Since that board is having the same symptoms as my known bad board, it is going back to the store. However, I won't be able to work on my original PCM for at least a week so this ordeal continues on.

 

Success at last...at least I think and hope! After I installed my repaired PCM, the truck started and ran normally, even after a 20 mile drive. Hopefully that is the last of this problem. Thanks so much to RonD for all his expertise. His judgement that I had a PCM issue, even though I had already replaced it, was spot on.

Post Mortem
I repaired my original PCM by replacing all three blue capacitors (2@ 47uf and 1@10uf). It was kind of a pain since it has conformal coating on both sides that you had to remove. I found that heating the conformal coating up at the solder connections using the soldering iron loosened it from the board at the pad and allowed me to remove it. Without removing it, I couldn't get the old solder away from the capacitor legs. Then I used a solder sucker and wick. Removing the old solder was the worst part of the job.

Near the beginning of this ordeal, I purchased a Cardone Industries remanufactured PCM from Advance Auto Parts and installed it. Clearly it was a piece of crap! The Ranger had the exact same symptoms after installing the Cardone Industries remanufactured PCM as it did with my old (original) PCM. Naturally, I assumed my troubleshooting was faulty and that I had replaced a good PCM. That sent me off chasing all sorts of ghosts for 3+ weeks - hours and hours of wasted time and a lot of effort. Plus, every time I test ran the truck, it was running terribly and throwing all sorts of terrible gasses and unburned hydrocabons through the exhaust system. I'll be lucky if I don't end up with a plugged catalytic converter. I will never buy anything from Cardone Industries again. I'm also going to be more careful about buying parts from Advance Auto Parts, at least anything that isn't from an OEM-quality manufacturer.

 

Good work

Glad it worked out, and thanks for posting the update, it WILL help some one else down the road