1994. pushrod 4.0. Truck sits there and idles well. when i hit the throttle she runs like crap, acts like it wants to die. Anybody have any clue why my truck might be doing this? this is my daily driver right now so I really need to get this figured out asap. Any and all help is greatly appreciated.
Thanks,
Art

 

Could be a few things.
Has it been losing power for awhile, like at higher speeds it wouldn't go any faster.
That would be a fuel filter clogging up.
Idle would be fine.

TPS(throttle position sensor) this tells the computer you want to go.........you push down on the gas pedal and the TPS rotates to send computer a higher voltage which means send more fuel to engine.
Computer sends TPS 5 volts, TPS sends back under 1volt for idle and over 4.7volts for full throttle, so it works like a light dimmer, as it turns more voltage is sent.
Easy to test this sensor with a volt or OHM meter, also just check to make sure is isn't unplugged.

Fuel pump or Fuel pressure regulator(FPR) is going bad.
If fuel pressure is low engine will idle but not "go"....
FPR is on the fuel rail and it sends fuel back to the gas tank when it is not needed, it has a spring inside to hold it closed when engine is off or at high rpms, and it has a vacuum line that pulls it open at low rpms when more fuel is not needed.
Check this vacuum line for fuel smell, replace FPR is there is gas in that vacuum line.

To test fuel pump you need to get/borrow/rent a fuel pressure gauge, there is a schrader valve(looks like a tire air valve) on the fuel rail.
Engine off pressure 30psi
Engine running pressure 30-35psi
If pressure is lower and/or starts to drop after key is off then there is a leak in the system.
Fuel pumps on fuel injected engine do not run all the time, they are off more than on, computer turns on fuel pump for 2 seconds when key is turn on, then it won't come on again until rpms are above 500 and then it will go on and off based on rpms, TPS and MAF data.
Turn key on count to 3 then turn key off, repeat 3 times, then start engine and see if you get good throttle response.
What you are doing is building up pressure in the system in case it was leaking pressure.


MAF sensor, on air cleaner, measures air flow into the engine, these need to be cleaned every now and then, symptoms of a dirty MAF sensors is bucking and stumbling along with other fuel/air mix issues, the MAF is the main sensor used to set fuel/air mix, O2 sensors are just for fine tuning mix.
If MAF gets it wrong then O2s can't fix it

 

I do all the regular maintenance on this vehicle, Fuel filter is changed at regular intervals, clogged or not. I will check out the TPS sensor and the MAF sensor. I have a fuel pressure gauge. Fuel pressure is fine. Best way to check MAF is just to unplug it?

 

The above is a great I would also check and clean the idle air control valve. Get some contact cleaner and clean the mass air, you can find the contact cleaner at a electrical supply house.

 

Yes, you can but I would still clean it.

When you unplug the MAF it will put computer into "limp mode", and CEL will come on, similar to open loop when engine is cold.
Limp mode is so named so you can limp home or to a shop for repairs.
In limp mode computer will run preset rich fuel air mixes based on RPM and TPS.
When you plug MAF back in everything will go back to normal so it is fine to unplug it.


Another thing that popped into my head was water in the fuel, could you have gotten some bad gas last time?
It would idle fine but wouldn't support any load at all

 

Only thing like that that I've noticed before this started happening was if i put the pedal straight to the floor it would bog down and not give me much acceleration

 

tried unplugging the tps sensor, no difference. tried unplugging the MAF sensor, no difference.

 

Already cleaned the IAC and double checked to make sure it's working

 

I mean, I try to get gas from the same place every time but it's not impossible that I may have gotten bad gas...it idles like a champ though doesn't miss or sputter or do anything weird

 

Clean the mass air and report back.

 

If you unplugged the TPS and pushed open the throttle engine should die or almost die, so should have been a big difference.

So your computer may be the issue.

Just to be sure I am understanding the symptoms.
Idle is OK
When you open the throttle(not in gear) the engine starts to die and sputter, is that right?

If engine only sputters under load, in gear, then that's different, EGR valve, if so equipped, could be opening to soon.
My '94 4.0l doesn't have one.


Water in the fuel can act a bit funny, engine can idle OK and RPMs can even be raised, not in gear, but as soon as you put a load on the engine the limited power of the water/fuel shows up.

 

I agree with Kevin here, after your comment ... My truck behaved like your quoted comment, I cleaned the mass air sensor, and Poof, ran like a champ.

 

Well pushing open the throttle and having the motor almost die was the problem in the first place.

And yea I just did a bunch of work on this motor recently, and it idles great, but sitting there with the stick in neutral if i hit the throttle at all it acts like ****.

 

Then either computer doesn't know you want more RPMs, TPS signal(voltage increase) is not getting to the computer.
Or computer is getting the voltage but is not responding, this is assuming you have fuel pressure.

Get a sewing pin and volt meter, the TPS should have 3 wires, 1 with 5v(key on), 1 with ground and 1 with 1v(key on, throttle closed), this is the one you want to test for changes as throttle is opened.

Use the sewing pin(s) to pierce the wire(s) as this test has to be done with TPS hooked up and key on, engine can be off.

Did the Check Engine light(CEL) come on when TPS was unhooked?
Same thing for MAF did CEL come on?
It should for either, if it doesn't then computer may be the issue

Do you get black smoke out the tail pipe after pushing gas pedal down?

 

CEL always on, has been since i straight piped years ago and disconnected 02 sensor. Negative on black smoke out of the tail pipe. I found an injector o-ring that was leaking fuel, put a new one on, and now im having different symptoms. At idle it will miss intermittently, if i throttle up and then let go it backfires. when i hit the throttle it still runs like crap but not as bad as before. If i gradually throttle up it will throttle up but something is wrong here she still aint happy.

 

Recheck you firing order, then check it again
3 4
2 6
1 5
Front

I know you know it is correct, humor me :)
Also check the wires

The Waste Spark system fires spark plugs on power AND exhaust strokes

 

Waste Spark System? What is that?

 

I used a spark checker, I am getting spark at all the plugs

 

Traced each wire from the coil to the plugs again, they are all correct.

 

Yes, you are getting spark but is it at the right spark plug at the right time??

Check firing order on Coil Pack
Coil pack should be
3 4
2 6
1 5
Front

Cylinders are numbered
3 6
2 5
1 4
Front

So coil wires are different on the drivers side of the coil, if these are mixed up you will get what you have described.


Waste spark is an ignition system that sparks a cylinder twice in its 4 strokes(1 power cycle), regular systems spark once in 4 strokes.

There are only 3 coils in the coil pack, not 6.
They are
[3 4]
[2 6]
[1 5]

So when computer(ignition module) wants to spark cylinder 3 it must also spark cylinder 4, so cylinder 4 is a "wasted" spark, it is not igniting a fuel/air mix, it is firing at the end of the exhaust stroke, "burning any left over fuel/air", yea right, lol.



Any way that is the Waste Spark system.

 

Yes i have a chilton book with a diagram that verified those orders, I traced every wire from the coil back they are all hooked up correctly

 

Well with no O2 sensor the only thing the computer has to adjust fuel/air mixture is the MAF sensor.
Make sure there is no extra air getting in between MAF and intake, make sure vent hose on that air tube is good an tight and oil cap is tight along with PCV valve in valve cover.
Any extra air coming in on this side of the throttle plate(ported vacuum) will cause a lean mix because MAF isn't "seeing" that air so isn't adding extra gas for it.
Normally O2 sensor data could compensate for a small leak.

If you get a spray can of starting fluid(ether), and on idling Cold engine spray some around lower intake, throttle body, and vacuum lines, the engine RPM will change if ether is sucked in, this can help find an air leak.

 

I went and bought a MAF sensor, turned the truck on, and.....big difference. I can rev it up and everything I just drove it around. if i really whomp on the throttle and let go it will still backfire but it runs and drives thanks a million to RonD and everyone else who posted to help me out here.

 

Hey, good work.


Backfire is probably the result of the changed exhaust pressure, i.e. no cat or muffler.

Just FYI, driving on public roads without a Cat Converter(if vehicle came with one originally) is a $2,500 Federal Fine, that's Federal nothing to do with State emissions.

 

Good work yourself, sir, you were the one who told me it might be the maf.
I didn't know that, i thought that it was only based off state emissions. $2500 is more than I paid for my truck!