I have a 1997 Mazda B4000, 5 speed, 4x4. I have no CEL on. Truck starts and runs fine and when driving easy on it youll notice no problems at all. When driving normal and accelerating at normal rates sometimes you'll feel the truck jerk forward and backward like a cylinder is misfiring randomly. It's not a consistent misfire like it's the same cylinder all the time, and it's only when there is a load on the engine. If I'm driving along just maintaining speed through town it runs smooth. but as soon as I press the gas pedal down a little more and there's more load on the motor it start bucking and jerking. if I let off of the gas pedal some, it goes away. Sometimes the problem is worse than others and sometimes is goes away completely and I can drive it any way I want and it never messes up. I used to have about 5-6 DTC's when I scanned the truck one day after buying it. I didn't know there were issues because the CEL was burned out. I went through and replaced a bunch of parts to take care of all of the codes and everything was great for a few months until this started happening.

Here's a list of the parts I've replaced:
Fuel Pump
Fuel Pressure Regulator
EGR Valve
Purge Solenoid and Thermistor
Cleaned the MAF with MAF cleaner
Engine Coolant Temp Sensor (the 2 wire sensor)
I also found a vacuum leak and fixed that
DPFE sensor
Fuel Filter
Spark Plugs
Coil Pack
and I have a new filler neck on the way to replace it since it leaks gas when I fill it up all the way.

Someone please make me look like a newb and tell me it's something simple I've missed! I also have a scan tool that will data log, if you need me to check anything else.

 

someone has got to have an idea....

 

Welcome to the forum

I would invest $40 in a Bluetooth OBD2 reader
Assuming you have a smartphone or bluetooth tablet

These are universal, work on any vehicle made from 1995 and up so not a bad investment.
make sure the APP is free or cheap, some adapters are $15 but APP is $40, so check it out

These allow you to view computer and sensor data in real time, in driveway or while driving.

There are a couple of things that come to mind, the TPS(throttle position sensor), this is the "accelerator pump" for a fuel injected engine.
MAF(mass air flow) sensor is slow to react when you push gas pedal down, the TPS gives the computer an instant "heads up" to add more fuel, like accelerator pump did for carb engines, gave it that extra squirt of fuel until air flow passed the Jets could catchup.

You can test TPS with volt meter, but OBD2 reader is better because you can watch data live while engine is stumbling.

After engine is warmed up and idling unplug the IAC(idle air control) Valve, RPMs should drop to 500 or engine may even stall, either is good.
If RPMs stay above 700 then you still have a vacuum leak
leave IAC Valve unplugged while you search for it

 

Thanks so much for the reply. I already have a scan tool that is capable of data logging so I believe it'll be able to tell me those things. I've tried testing the throttle position sensor with a volt meter and it tested perfectly. I will try the iac thing like you suggested and let you know what I find. Should I try this while running and should ac be off or on?

 

While truck is warm and rpms are stead below 1000 and ac off. I removed the plug for the iac and no difference. I turned the ac on and rpms came up. I removed the plug and they dropped back down. When I plugged it back in they came back up. So I believe the iac is working correctly, but correct me if I'm wrong. I also drove it around with a warm engine and the data logger connected... Remember how I said sometimes it doesn't mess up and runs fine, this is one of those times. Figures lol. The maf_v reading seemed to increase with throttle and rpms up to about 600. I didn't even check the throttle position readings since it was driving correctly now. I will wait for the engine to cool or maybe I'll hose it down after a while and then drive it around to see if it's messing up again. One time it started running fine while I was looking things over so I let the motor cool and hosed it down and it started acting up again. So maybe if I do that again I can get it to act up then get the scanner on it.

 

Not a question of IAC valve working, don't care about IAC valve in this instance.

1997 Ranger 4.0l manual trans should have warm idle of 650, forget AC, leave it off.

In order for the IAC Valve to set idle at 650, then idle must be lower than 650 with no IAC valve, otherwise whats the point of the IAC Valve if it can't set minimum idle.

So warm engine, idling, AC off
Unplug IAC valve
idle needs to drop down below 650

If it doesn't then IAC Valve was closed all the way before unplugging it.
And if idle was higher than 650, then it is because of a vacuum leak.

Now there is one other thing, some "smart guy" who doesn't know fuel injected engines don't have Idle Screws, may have goofed up and adjusted the anti-diesel screw on the throttle linkage, thinking it was an Idle Screw.
Look on the throttle linkage.
Find the screw, same setup, warm engine, idling, IAC unplugged, turn the screw.
If idle drops then turn it so engine is barely running, stumbling idle, if it stalls PERFECT, leave it off.
Then plug the IAC valve back in and engine should be at a lower idle than before, 650.
My 4.0l has a warm idle of 625.
After resetting this screw idle may wander a bit until computer relearns IAC valve setting for correct warm idle

If you have to adjust this screw and do get a lower idle then no vacuum leak, if idle stays above 700 then you do.

 

That's a great point and I will definitely try that. But wouldn't it be easier to look at the short and long term fuel trims with the scanner?

 

Truck warmed up and a scanner showed the engine coolant temp was 180 something. It shows that the desired rpm was and what the actual rpm was. Desired was 752 and it was staying right around that. I unplugged the iac and the rpms did drop some but not close to dying. I think they dropped about 50 while unplugged then after plugging in went up to a little over 1000 then came back to 750. The long term fuel trims were 1.00 on bank 1 and 1.03 on bank 2. Short term fuel trims were staying almost exactly around 1.00 at different rpms.

 

Ford must have increased target RPM for warm idle to 750 on manuals, for emissions reasons.

As long as it dropped when IAC was unplugged then no vacuum leak.

180degF is low operating temp, should be around 195degF, stock thermostat should be 192/195degF, higher temp cleans oil better and gets better MPG

Watch MAF sensor data during acceleration, and switch it to grams/second, g/s
At warm idle it should be about 4, engine displacement.

Also check IAT(intake air temp) sensor read out, it is in the MAF sensor in '97, make sure it is reasonably close to outside air temp

14.7:1 air:fuel mix is a weight ratio, 14.7 pounds of air will need 1 pound of fuel added in a gasoline engine.

The MAF sensor data sets the 0 for short term fuel(STFT) trim in the computer
-1 and +1 is the pulse width(open time) for the fuel injectors, "-" is shorter open time, "+" longer open time
Computer calculates 0 pulse width based on MAF data then adjusts the pulse width based on O2 sensor feedback data, O2 only works when it is above 650degF so not used until engine/exhaust gets warm.
-5 to +5 is normal for warm engine, normal because as a system gets older the MAF changes, fuel pressure changes, spark efficiency changes, O2 sensor changes.

When average STFT or LTFT gets to -20 or +20 then you will get Rich/Lean codes.

Engine is never running Rich or Lean, the codes are to let driver know that fuel calculations are too far off based on MAF sensor data and O2 feedback.

When you accelerate, push down on gas pedal, STFT should go high into the "+" numbers, computer is reacting to the TPS and performance is put ahead of fuel economy during this time.
Once TPS matches MAF flow and RPM, cruising, computer will reduce pulse width for best economy.

Long term fuel trim is used on cold start or if MAF or O2 sensor should be out of range, code is set for that issue, and computer runs LTFT as default.
On cold start there is no O2 feedback so LTFT is used with MAF data

 

Thanks a lot for all your input and help. My wife and I went to the movies and the truck was warm and I had the scanner on during the drive there. I had her watch the short term fuel trims. While on highway in 5th gear maintaining speed, the short terms fuel trims were both staying right at 1.00. I slowly pressed down the gas pedal more and more and the truck would start to accelerate and trims stayed around 1.00 but once I pressed the pedal down enough for it to start misfiring, at the same exact time the short term fuel trims dropped down to 0.87 and stayed exactly there without changing at all, until I let off the pedal. Once I let off the pedal enough to maintain speed the misfiring stopped and fuel trims went back to 1.00. So it seems the misfiring and fuel trims are exactly related. What should I check next to see what's going on?

 

I don't know if I can change the readout to grams per second.
While driving the engine temp was barely over 190.
I believe when accelerating the short temp fuel trims were still around 1.00

 

What does the TPS Voltage or % show when you push down hard on the gas pedal.
It should go up fast.
TPS is the same as a light dimmer or volume control, instant feedback when you change it.
Also watch O2 sensors when accelerating, .9 is rich, .1 is lean, should jump up to close to or over .9volt
If it drops then computer is not adding more fuel like STFT show, and thats the reason for the hesitation.

Yes slow acceleration wouldn't change fuel trims much, fuel economy is the default for the computer, performance switch over would require faster change in TPS.

 

Thanks a lot for you help man. Maybe my scanner is limited on what it'll do. When I have the engine off and am reading throttle position is says 200 when not pressing it. It'll slowly rise as I press it all the way down and the reading then is about 915 I think.

 

It doesn't have hesitation just misfiring when under load

 

Under more load when throttle is opened more there would be less vacuum. Think this could be related?

 

There is no vacuum sensor on 1997, yes vacuum decreases when throttle is opened more then comes back up when RPMs match throttle/air flow.

Fuel injection doesn't rely on vacuum like carbs did, but a vacuum leak throws off fuel mix because of MAF sensor being bypassed.

What I would look at is why STFT is not jumping up when you "give it gas" by pushing on the gas pedal.
You could be getting lean misfire at that time, i.e. air flow increases but fuel doesn't, so lean mix.
Point of the TPS is to prevent that from happening.
MAF sensor takes over mix once TPS is fixed, cruising

Also at warm idle STFT should be +5 to +7, idle needs to be rich, computer ignores O2 sensor feedback at idle, also at wide open throttle(WOT), STFT can go up to +10, performance not MPG at work there, lol.


Not sure what scale that is for the TPS, 200-900??
Doesn't make sense to me

 

i would start looking at the fuel injectors

they are NOT ethanol resistant , if you are running 87 octane fuel

try running through 1 tank of 91 octane fuel ( to dilute the 87 octane fuel )

91 octane fuel does not contain any ethanol

then take the test again halfway through the 2nd tankful of 91 octane fuel

the ECM of that year was not programmed to detect burned ethanol in the exhaust emissions

ethanol eats older rubber fuel hoses for breakfast ( meaning tiny bits of rubber have broken off inside
the fuel hoses are probably clogging up your fuel injectors

 

The way I was thinking the vacuum would affect the misfiring was if the fuel pressure regulator was acting up since its operated by vacuum. One thing I did yesterday was take a short drive and the truck was acting up the whole drive. When I got home I disconnected the negative terminal on the battery for almost 4 hours. Then after driving the truck it ran fine. But since the problem comes and goes sometimes I don't know if that actuall changed anything or not. I knew if the better was disconnected long enough the computer would have to relearn the tune. I also removed the bed removed the old leaking fuel filler neck pumped all the gas out that I could while it was up on ramps to make the fuel run to the back of the tank, then I installed a new filler neck. I had a 2 gallon gas can with 87 octane I put into the trucks tank. Then I drove to a near by gas station and put 89 octane gas into the tank. On the way there and back it drove fine. Another thing I didn't mention at the beginning of the thread is when the gas level is below 1/4 the truck will stall at stops sometimes or almost stall then go back to idling fine after sitting there a couple seconds. I didn't think they were related at the time.

 

if the bed is still off of the vehicle

i would pull the fuel pump out and check the pickup screen at the very bottom of the pickup tube

they will disintegrate over time clogging the pickup tube to some extent

disconnect the fuel pump to fuel filter hose and blow compressed air into the connector

( reverse flushing the fuel pump to blow out any debris that is in the fuel pump pickup tube

 

With lower fuel tank levels hard turns or quick stops can uncover the pick up on the fuel pump, air gets sucked in, fuel pressure suddenly drops and engine stalls or nearly stalls.

Mine does that on down hill 90deg right hand turns when under 1/4 tank, I have that turn on the way home every day so know the symptom, lol, engine has never stalled completely through

 

I noticed it's worse on turns and sudden stops. I bet your right! Thanks!

As far as the problem it's still doing it. Trying to reset the tune didn't help. I'll try to watch the maf sensor readings. It sounds likely that it a lean misfire like said above. But I don't know what would cause it. When the computer shows the STFT reading at 0.87 that means it's taking away fuel, right? Not adding fuel

 

When I replaced the fuel pump I put a new screen and rubber hose on it then too.

 

yes. .87 is less fuel being added than 1.00

But really haven't seen fuel trims like that
+1 to +25 and -1 to -25 is what is normally used, 0 of course is the computers calculated fuel trim based on MAF, RPM, 38psi fuel pressure, outside air temp and TPS

What brand is your scanner/reader?

 

Sct sf3

I think this scanner is more meant for flashing computers on mustangs but works to read codes and data log info on the truck. I tried cleaning the MAF again and so far so good but not getting my hopes up again. What would cause the computer to take away fuel when the pedal is pressed down more?

 

i would guess that the fuel injectors are partially clogged