After replacing almost everything, ranger still doesn't want to go.

[Mazanger]

Well, I am a bit stumped here and would definitely appreciate any advice anyone would like to offer.

The truck: 1994 Mazda B4000 - 5speed - 4x4 - lots of miles

The symptoms:

First noticed an issue the day before everything fell apart, I was in 3rd gear getting on the freeway and had the pedal to the floor, the truck hesitated for a split second, and then kept chugging along. The next day I noticed another instance of the same, but it happened just these two times.

Then, driving down the road, the truck just bucked and lost power. Refused to rev under much of any load. I pulled over and the truck would idle perfectly, but giving it any gas would cause it to miss and hesitate. Before I get to the list of things I have fixed, I will mention that I have noticed a few different behaviors. Lightly applying the gas will usually allow me to rev the engine up to redline, but anything over about 75% throttle will cause it to stop and sit at whatever RPM it currently is at, and will not go any higher. I also have had instances where no amount of feathering the gas pedal will get it to rev past 1500. It's random. Also, a couple times while starting it cold, I have been able to floor the gas pedal a couple times and it will roar to redline just as normal, but after a few goes it will revert back to its current behavior. The only constant is it always idles perfectly.

Here is a video of the symptoms. https://www.youtube.com/watch?v=_BbPZM7x0bk. I know it is idling high in this video, I do not know why that is, as it usually idles right at 850, this is not typical of its current behavior. When I am in the truck looking at the tach and it is around 2k, that is with the pedal to floor.


I get two codes, Lean in bank 1 and 2.(172 & 176)

Things I have done so far:

Changed/cleaned MAF(junkyard special, no change between units)
Changed coil pack(same as MAF, no change)
New TPS(old one was out of spec, no change)
Found one sparkplug that looks to be running lean, replaced injector with new, changed all injector O-rings, no change)
New plugs all around
Wires are not new, but they are NGK's and about 2 years old
Checked fuel pressure at rail - 38 KOEO, 35 KOEO- Idle, 40 KOEO - WOT
Changed fuel filter and pump anyways, no change.
Cleaned IAC, looks good.
EGR appears to be opening and closing normally.
Checked around lower intake for leaks with starter fluid, no signs of leaking. I replaced the lower and upper gaskets last summer and bolts are all still torqued to spec.

Next I am going to check the trim on the O2 sensors, though unplugging them does nothing and will not put the ECU into open loop, nor throw any codes. I have checked the wiring and it appears to be fine. My next guess is that the ECU is bad. Part of my thinking with the ECU being bad is I did weld on the truck the day before it took a dump, and I never unhook the ECU/Battery because I live on the edge.

Anybody have any ideas? I have searched extensively and found a few people that have had the same probelm, but they never follow up with what the solution was. I am inclined to believe that their truck murdered them in their sleep, or their wives did after they spent stupid amounts of time and money trying to figure this out. I'd prefer a different outcome. Thanks!

 

[SenorNoob]

Considering the things you've done already, I'd say you most likely have (a) dirty injector(s).

The description of the way it runs perfectly describes a lack of fuel. since the filter and pump are new that only leaves the injector(s) or a partially clogged line.....

 

[RonD]

Classic symptoms of Cats or muffler starting to collapse internally.

As that happens exhaust can't get out easily so pressure builds, new air:fuel can't get in, power drops and RPMs are limited.

Remove both O2 sensors to give the exhaust open holes to exit, if Revs return then you have blocked exhaust system

 

[Mazanger]

Considering the things you've done already, I'd say you most likely have (a) dirty injector(s).

The description of the way it runs perfectly describes a lack of fuel. since the filter and pump are new that only leaves the injector(s) or a partially clogged line.....

I thought that too, and replaced the one. All of them seem to be firing normally, though I just hooked them up to the battery. I will pull them all again and test them attached to the rail and see what their spray patterns are like.

Classic symptoms of Cats or muffler starting to collapse internally.

As that happens exhaust can't get out easily so pressure builds, new air:fuel can't get in, power drops and RPMs are limited.

Remove both O2 sensors to give the exhaust open holes to exit, if Revs return then you have blocked exhaust system

Also something I thought, though the exhaust is about 4 months old, including the cat. I will try pulling the O2's, though I already tested vacuum at higher RPMs. My understanding is that with a blocked exhaust system, the engine will build vacuum with the throttle open, my truck isn't doing that but I will still check it to be sure.

 

[RonD]

With vacuum gauge hooked up, open throttle plate and let it snap closed.
Vacuum should do a quick drop to 0 then return to 17+
Slow drop and slow return means clogged exhaust.

If you slowly bring engine up to 2,500 RPM and then leave it there for a few seconds, vacuum should stay the same or even be higher than idle vacuum(17+), if vacuum starts to drop exhaust is plugged up.
So opposite of what you said above, if I under stood you correctly, if so then you do have a clogged exhaust.

Engine is basically an air pump, throttle plate closed causes a low pressure(vacuum) in the intake as the engine tries to pump out more air than can be pulled in, as throttle plate is opened the the vacuum level will drop but the "air pump speed"(RPMs) goes up and will maintain that low pressure when throttle plate stops at any point.
That's how those old, and new, "best economy" gauges on a dash worked, they measured vacuum, when accelerating the gauge would show lower vacuum, in the red, when cruising(throttle in fixed position) vacuum would be high again and gauge is in the green.

 

[Mazanger]

I took the exhaust off at the cat. No change. Swapped ECUs, no change, swapped ignition control unit, no change, swapped crank position sensor. No change. I'm running out of things to waste money on.

 

[RonD]

You aren't helping yourself by swapping out electronics, they rarely cause problems like you have reported.

So without exhaust system connected the engine was still slow to REV or didn't REV as before, no change in symptom.
Exhaust being blocked is off the table.

Fuel pressure stays above 35psi even at higher RPMs
Thats off the table

With MAF sensor unplugged engine is still slow to REV?

Pop open the PCM(ECU) and have a look inside it for damaged areas
What it should look like is seen here: http://www.auto-diagnostics.info/ford_eec_iv

The EDIS-6(ICM) for the 4.0l can run the engine spark without computer input, so shouldn't cause an issue like yours.
And engine can't start unless CKP(crank position) sensor is working.


You reported you have a Vacuum gauge.
Good read here on how to test and what results mean: http://www.gregsengine.com/using-a-vacuum-gauge.html
What vacuum do you have at idle?
17-21 is expected

If lower I would remove all spark plugs and do a compression test, my '94 4.0l has 300k+ and is still at 165psi average.

An engine is an air pump, so limited or slow REVing is caused by a problem in the flow of air in or out.
You have confirmed the air can get out by removing exhaust pipes.
Incoming air is a little more tricky since even if it's flow is unrestricted there is the issue of too much air when adding fuel to "keep pump running", too much air will cause lean condition and slow REVs.
Unplug IAC Valve, it will close and there will be less air coming in, engine should still idle but at 500rpms.
Use your hand over air intake while trying to rev engine, if it gets more responsive in RPMs then you have too much air or not enough fuel.
Other issue is compression, compression is like winding a rubber band on a model airplane, then more you wind it the more energy is stored so the more energy you get when you release it.
Lower compression means lower power.

 

[Mazanger]

You aren't helping yourself by swapping out electronics, they rarely cause problems like you have reported.

So without exhaust system connected the engine was still slow to REV or didn't REV as before, no change in symptom.
Exhaust being blocked is off the table.

Fuel pressure stays above 35psi even at higher RPMs
Thats off the table

With MAF sensor unplugged engine is still slow to REV?

Pop open the PCM(ECU) and have a look inside it for damaged areas
What it should look like is seen here: http://www.auto-diagnostics.info/ford_eec_iv

The EDIS-6(ICM) for the 4.0l can run the engine spark without computer input, so shouldn't cause an issue like yours.
And engine can't start unless CKP(crank position) sensor is working.


You reported you have a Vacuum gauge.
Good read here on how to test and what results mean: http://www.gregsengine.com/using-a-vacuum-gauge.html
What vacuum do you have at idle?
17-21 is expected

If lower I would remove all spark plugs and do a compression test, my '94 4.0l has 300k+ and is still at 165psi average.

An engine is an air pump, so limited or slow REVing is caused by a problem in the flow of air in or out.
You have confirmed the air can get out by removing exhaust pipes.
Incoming air is a little more tricky since even if it's flow is unrestricted there is the issue of too much air when adding fuel to "keep pump running", too much air will cause lean condition and slow REVs.
Unplug IAC Valve, it will close and there will be less air coming in, engine should still idle but at 500rpms.
Use your hand over air intake while trying to rev engine, if it gets more responsive in RPMs then you have too much air or not enough fuel.
Other issue is compression, compression is like winding a rubber band on a model airplane, then more you wind it the more energy is stored so the more energy you get when you release it.
Lower compression means lower power.

Thanks for your detailed reply, I definitely appreciate it.

If the MAF is unplugged it will start, but die quickly and not want to rev up.

I don't actually have a vacuum gauge, I tested it by feel. I realize that isn't very accurate but it appears to be creating vacuum through the RPMs normally. I will look into buying or renting one and will report back.

I am relatively convinced that the ECU is toast. The ECU that I swapped in was from a 93 navajo. It was a manual, but did not have EGR and who knows what else is different. It started, but did not want to idle and while revving it it would lose power around 3-4k and die. This is different behavior from my ECU, which has very erratic and inconsistent symptoms, but typically will rev to 3k and hit a wall and stay there. I tested the impendance of the wires between the MAF and TPS and the ECU, and they seem good with no shorts. I tested the voltage at the MAF and TPS, and both seem to be operating normally.

I realize I'm being a "parts replacer" and not a mechanic, but this has been relatively frustrating. I've worked on cars for almost 2 decades and have never come across such bizarre behavior. What really sold me today on the ECU is I swapped out the MAF with the engine warm, started it back up immediately and I got a good 15-20 seconds of free-revving normal behavior. I could floor it and the tach would go to redline like a raped ape. After those 20 or so seconds, it went back to it's usual behavior. To me, that completely rules out any mechanical issue(vacuum, exhaust, blockage, compression, etc) It also mostly rules out any sensor issue, partially because they've all been replaced. It still could be a wiring issue, and I just got lucky that whatever short might be there wasn't shorting out when I started it that time. But to me it seems more like a computer issue - I start the truck warm, the ECU sees that the MAF has been changed(one is original, the other is out of an X), is confused for a second and lets everything go, learns the new sensor and switches to closed loop, or whatever program that's corrupted. I was not able to recreate this, but nothing is consistent with this issue. In part, that is why it makes the most sense to me.

I will pull the ECU and look for signs that it has been compromised, and report back on the other things you've suggested, thanks again for yours and everyone else's help!

Edit: There is also a chance the MAF I got out of the junk yard is also toast, as these symptoms are similar though much worse than one time after a long day of dusty logging roads my MAF needed a good cleaning. The probability of that is slim, though the donor vehicle did not have any obvious signs of damage. I have a friend that has a ranger 4.0 MAF that he is giving me, so I will eliminate that possibility for good next.

 

[Mazanger]

As I thought, it turned out to be the ECU. I took the ECU apart and could not see any obvious issues, but a new ECU fixed it. I'm crossing my fingers that there isn't a short wire somewhere that will make quick work of the new one, but the harness appears to be in good order. Thanks to everyone that replied and helped out!

 

[RonD]

Thanks for the update.
Good work