'94 4.0 Knocking, bucking, hesitating, getting worse

[swtjames23]

UPDATE, SOLVED 1-18-16: The problem ended up being caused by two things: Leaking plenum-to-fuel rail gasket and bad wires/plugs (not sure which, I changed both)

Hey guys, haven't posted in a while because I usually find my answers by searching the forum but not this time. My latest ranger is a '94 4.0 and it ran pretty good when I bought it with 250k on the odometer. PO says it was rebuilt but who knows. It had a high idle so I chased that for a while, fixed it. It has since developed a much bigger problem. It has a miss at idle but runs ok under light throttle. As soon as you give more than 1/4 throttle it loses power, cuts out, misses, knocks. It started out at 3/4 throttle or more but has gotten progressively worse.

So far what I have done (that hasn't helped):

New fuel pump
New fuel filter
New EGR valve and sensor
Checked fuel pressure at rail
Checked air filter
Checked plugs (visually, look good)
Checked wires (ohms checked out)
Checked coil (ohms checked out and each plug has spark)
Cleaned and checked MAF (voltage checked out at idle)
Cleaned IAC
Replaced fuel filler neck (unrelated, just saying )
Checked vacuum (~22)

Hoping you guys can give me some ideas as to what to check next. Does it sound like an internal mechanical problem? Like a burnt valve? Or would a leaking intake gasket cause this? Keep in mind, it didn't do this when I bought it a couple months ago, so maybe I did something to cause it?

BTW no CEL and no codes

 

[RonD]

Burnt valve isn't common on the 4.0l OHV so way down on the list.

What was fuel pressure?
and did it hold pressure after engine was shut off, it should stay above 20psi for a few MONTHS , not minutes, days or weeks....months

What was the fix for the high idle?


4.0l is known for the lower intake bolts to loosen up, that can effect only 1 or 2 cylinders at low RPM then get better at higher RPM, and then could start knocking/pinging because of lean mix under load.

Just to test, try Clear Flooded Engine Routine to see if an injector is leaking and TPS is working.
Key on
Push gas pedal to the floor and hold it down
Try to start while still holding gas pedal to the floor

Engine shouldn't start or even fire, fuel injectors are shut off, but spark is still on.

If engine fires, even a little gas is leaking in, leaking injector or fuel pressure regulator's vacuum hose.
If it starts up then TPS isn't working right.

TPS(throttle position sensor) gets 5 volts from computer, when throttle is closed it sends computer under 1volt on it's center wire, when throttle is wide open it sends computer 4.5volts or higher, voltage on center wire increases as throttle opens, this tells computer to increase fuel faster than MAF sensor can, TPS is EFI's version of carbs accelerator pump.

Clear Flooded Engine Routine works when computer is on(key on), TPS voltage is above 4.5volts(gas pedal to the floor), and engine is at 0 RPMs, Computer will then shut off injectors.
When you try to start there is no fuel so no start, as soon as TPS voltage is below 4.5v injectors will start working.

It is easy to test TPS with voltmeter
Set for DC volts, use engine as meter ground, pierce center wire with a sewing pin, meters red probe goes here.
key on
you should see .6-.99 volts
slowly open throttle while watching voltage, should be a slow steady climb to wide open, and 4.6-4.9volts
If there are any signs of sudden voltage drop or increase then replace TPS

 

[BLOODBANE]

I had problems with the ECU on my 94 Explorer once. I chased all kinds of codes, and replaced a ton of different things. Finally took it to FORD and they diagnosed the ECU. Replaced it and it ran fine. I picked up a used comp. on Ebay just recently (another set of problems and to have a spare) and only gave around 50-60 dollars for it. From FORD it was 600. Just get the numbers off of yours ,and type it in the search criteria.

 

[swtjames23]

Fuel pressure was within spec, I don't remember exactly what it was. But I didn't check it after leaving the truck overnight, maybe I should.

The fix for the high idle was cleaning the IAC and a vacuum hose was disconnected.

Intake bolts are tight.

I'll try the clear flooded engine test and I'll check the TPS.

Is there way to test the ECU? I've thrown enough money at this thing without a result so I'd like to test things wherever possible instead of just throwing parts at it and crossing my fingers.


Sent from my iPhone using Tapatalk

 

[RonD]

No, no way to test ECU(PCM).
You can remove it and open the top to see if circuit board looks good, capacitors on the circuit board can leak after 20 years causing damage to the board and of course that circuit to go out of it's voltage range.
Look here for what it should look like: http://www.auto-diagnostics.info/ford_eec_iv

There are a few things the computer uses that you can use to ID problems.
On PCM startup the computer checks the ECT(engine coolant temp) sensor, if ECT shows under 170degF the computer runs in Open Loop.
Open Loop can also be Choke Mode when ECT shows lower temps.
Open Loop is needed because O2 sensors won't work right when they are under 600degF, so they need to be warmed up by internal heaters and the exhaust.

During Open Loop PCM uses MAF sensor air flow to calculate the 14:1 air:fuel ratio based on tables in its memory, also RPM and TPS data, long tern fuel trims are also used to off-set memory parameters.

In Choke Mode it will set a higher idle, run richer fuel mix and advanced spark timing, just like a choke plate did on a Carb engine.
As ECT sensor warms up idle should go down until "target" idle is achieved, for manual trans that is 600-650, automatic 700-750.

When temp is at approx. 170degF the PCM switches to Closed Loop, the O2 sensors are now hot enough to give valid oxygen levels in exhaust, so PCM has instant feedback on its fuel calculations, the Loop in air:fuel mix is Closed.

If there is a difference in how the engine runs cold and warm then that will help you narrow down what to look at, and it will, most of the time, mean there is an electronics issue.
If same problem occurs cold or warm then it is usually a mechanical issue, something is physically wrong, low compression, bad injector, bad spark plug, ect...............


If a main sensor is unplugged or fails the PCM will switch to Open Loop regardless of ECT temp, this is often called "Limp Mode" because performance will often go down but this allows you to "limp" home, lol.

MAF sensor is a main sensor, it is used cold or warm, but PCM "knows" the engine is a 4.0l(or 2.3l or 3.0l) so it knows how much air should be coming in at 700rpm for example, it can calculate 14:1 ratio just based on RPM, although not as well
Unplugging the MAF sensor will put the PCM in limp mode, open loop, if this allows the engine to run better it doesn't mean the MAF sensor is bad, could be the PCM

Also after engine is warmed up, unplug the IAC valve it will close all the way, engine RPMs should drop down to 500 or engine may even stall, either is good it means no more vacuum leaks, if idle stays higher then there is still a leak.

 

[swtjames23]

Thanks for all the info Ron, sounds like I should also test the ECT. Although, I don't think just that sensor alone could cause this much of a problem, do you? What does "limp mode" feel like?

 

[RonD]

If ECT sensor was a problem engine would run poorly cold, and then better warm, or you would get a Check Engine Light(CEL) if ECT wasn't warming up with the engine.

Computer doesn't have a clock, so it doesn't know how long an engine has been shut off.
If, for example, ECT sensor was bad and showing 150degF when temp was really 60degF.
When you turn on the key after truck sat over night, the computer would see 150degF(so "it thinks" you ran into the store and came back out after 10minutes, lol), and real temp is 60degF, when you start it it would stumble and run rough because cold engine needs extra fuel.
As it warmed up it would start to run better and if ECT went up as well computer thinks everything is fine, so no code.

If ECT showed 60degF(correct temp) engine world run fine cold, but after about 8 minutes the ECT should be at or above 170degF, if it wasn't then computer would set a code, turn on CEL, telling you about engine temp problem.
Computer does have an ECT timer.

So one way ECT could fail you would have rough running cold, good running warm, and no CEL.
Other way on ECT failure, good running cold and CEL after a few minutes, probably short time, under a minute of rough running, but maybe not, O2 sensors would be used after 5 minutes so engine should run fine thru out.

ECT gets 5 volts from computer on 1 wire.
It is a resistance sensor, warmer it is the higher the resistance.
The other wire will have about 3volts if engine is at 60degF
When engine is warmed up(185degF) this other wire will be under 1 volt, .5-.7volts is average

Limp Mode would have sluggish acceleration, lesser throttle response, but not rough running, computer is running preset air/fuel mix, not relying on most sensors.

 

[msubully82]

If you unplug the MAF sensor and it runs better, I would suspect the sensor even if you are not getting the Check Engine Light on. I just went thru this on my truck with somewhat similar symptom and no CEL. Before the stalling/bucking symptoms appeared, I had pinging in hot weather that required premium gas to make it stop. I changed the MAF and all is OK. And yes, I tried cleaning it with no improvement.

I got a Motorcraft reman. sensor from Rock Auto. It required a core charge, but when I sent the old one back I got the credit for it.

I have had the truck since new in 1995 and this is the 2nd time to replace the MAF sensor. So I consider them a common failure item.

Don

 

[swtjames23]

When I unplug the MAF it runs worse, almost wants to die. And I tested it with a VOM and it was within spec with the engine running

 

[swtjames23]

Update on this; I pulled the intake plenum to check the injectors and found the plenum-to-fuel rail gasket had to cracks in it and thought "great! that must be it." So I went ahead and pulled the injectors, they looked normal, put new orings on them and reinstalled everything with the new gasket.

So now, it's definitely better but I'm getting the same symptoms only not as severe. Miss/hiccup at idle (inconsistent) and significant backfiring when under load.

I also did the leaky injector test and the truck just cranked but no sign of starting so I don't think any of them are leaking. Any other ideas? I'm still considering the ECU but I just don't think that's it.

Also want to add a symptom I didn't mention before that it's had this entire time. When decelerating in 3rd gear (ONLY 3rd) the truck bucks and jerks until I put the clutch in. It's done this since I bought the truck.

 

[RonD]

One of the MPG advantages of fuel injection is that fuel flow to the engine can be cut off.

In a carb when you were going down hill or decelerating, in gear, foot off the gas, the engine vacuum was high and so extra fuel was sucked in from the idle jets.
People would often shift to neutral and coast with engine at idle to save fuel.

With fuel injection the computer will turn off the injectors while decelerating.
Parameters vary but example would be:
Speed above 10mph
RPMs above 1,200
TPS(throttle position sensor) under 1volt(foot off the gas pedal)
Injectors would be off

So with fuel injection it is better to stay in gear using engine compression as a brake, as needed, because that saves fuel vs neutral coasting and idling.

At approx, 1,200 rpm computer will turn injectors back on at idle levels.

Your bucking while decelerating means fuel is being added, no other way you would feel bucking with foot off the gas pedal.
It would feel like you were tapping the gas pedal while decelerating.
Why only in 3rd I don't know, could be because engine is above 1,200rpm in that gear and not in 4th or 5th?

I would investigate TPS and its wiring, it could be "tapping the gas pedal" as far as computer is concerned, or could be TPS circuit in the computer itself.
TPS gets 5volts from computer on upper wire
Sends back under 1volt(.69-.99v) with throttle closed
Lower wire is Ground.
I would test center wire while it is hooked up Key On engine off.
If it is within spec, start moving wiring harness around see if it effects that voltage.

If should be within spec or computer would set a TPS "out of range" code, but if voltage varies it won't set a code, computer just "thinks" driver is tapping the gas pedal.

You could also unplug the TPS and go for a test drive, you will get a code and CEL but see if does the bucking while decelerating.
With TPS unplugged computer will be in open loop, it will have sluggish throttle response since computer only has MAF air flow to know throttle position

 

[swtjames23]

Checked the TPS; KOEO and throttle closed it read 1.2 volts. I was able to adjust the throttle screw and loosen the TPS and adjust it a hair to get it down to about 1.09, still >1 obviously so should I replace it?

It did bring my idle down to about 600 rpm, which is perfect but it still has a miss. Here's a video I just took:

https://www.youtube.com/watch?v=6vKoWtJLock

 

[RonD]

Could be the voltage meter, what was the reference voltage at?
5.0volts even or ?

Any jumping around of voltage when you moved wires around?

And if you opened throttle did voltage steadily climb, no dropping out or jumping up?

And just out curiosity what spark plugs are you using?
Motorcraft or Autolite work best in the 4.0l OHV, mine would "eat" other brands, would get missing at idle and even had one just die driving down the highway, NGK that time.
Never had a spark plug just quite working like that, lol, bang steady miss, kept driving the rest of the day with dead cylinder, never came back on.
Pulled it out the next morning and it looked just fine, tested on an old coil and no spark, just dead.
Put an old one in and was up and running, got a set of autolites the next week.

I would run a can of Seafoam or similar in the gas tank, I do that once a year on each fuel injected vehicle, notice a smoother idle after about 1/2 tank, so should probably do it more, lol.


Engine has good strong vacuum and drops to 0 when you blip the throttle so clear exhaust, you get a bit of valve floating(pointer jumping around) at high rpms but miles and valve spring wear will do that, wouldn't be cause for alarm IMO.

 

[swtjames23]

Well, problem solved! It's funny that you should mention spark plugs because it was either them or the wires that were causing the problem. I changed the plugs from Autolite double platinum to Bosch OE type finewire platinums (6907) and duralast gold wires (the best I could get on a Sunday). Between the plugs, wires, adjusting the throttle stop, TPS and the intake gasket this thing is running great.

The voltage did increase steadily as I opened the throttle but it still rests at about 1.09 and reads 4.6 at WOT. Should it be 5 volts? Would there be anything to gain by getting a new one?

Thanks for all of the input Ron, you're a wealth of knowledge.

 

[RonD]

Good work


TPS is a "learned" sensor, because over the years wiring and power supplies change, the computer will "learn" new values for TPS, if 1.09v is the lowest it "sees" then that is throttle closed, if 4.6v is the highest then that is WOT
If that changes it will "remember" lowest and highest and if it is repeated enough that voltage would be the new "lowest" or "highest" it uses.
I learned .69-.99v for low parameter and above 4.5v as WOT, 4.6v is fine 4.8v is highest I have seen

IAC Valve is similar, in that the computer "learns" how to set IAC Valve to get 600rpm, 625rpm, 650rpm, 1,100rpm, ect....

When you change a sensor or IAC, or sometimes even if you have computer unplugged from power for 10+ minutes, it can loose some of the "learned" data, you might notice wander idle or slower throttle response as computer relearns.
Learning can take 3 to 10 drive cycles, heat up/cool down = 1 cycle

Not sure on the TPS, normally I would always want under 1volt, I would wait and see if a friend has a different volt meter borrow it and see what it says, we are only talking .1 volt from 1.09 to .99


And thanks
But you did all the work all I did was type