93 4.0 black smoke and rough idle

[hunter29078]

93 4.0 MAF Voltage???

Been looking at the issue for a few days with no luck. Checked the codes by jumping the eec wires and I am getting a mass airflow sensor code and a fuel pump relay code? I replaced the mass airflow sensor but the code is still being thrown. I did check fuel pressure and it is reading a 40psi with engine running and 38psi when off. Any sugestions? Is it likely the new MAF is defective?

 

[RonD]

MAF(mass air flow) sensor tells computer how much air is coming into the intake.
Computer then uses that data to calculate how much fuel to add base on gasoline's 14:1 Air:Fuel ratio.
So if 14 pounds of air comes in computer adds 1pound of fuel, it's done by weight not volume, which is why "pre-vaporizers" don't work, lol, another story.

Computer is also programmed for a 4.0l engine, 4 Liter engine, that means engine sucks in 4 liters of air every 2 RPM(4 stroke engine)

So if MAF sensor is unplugged computer will switch to this factory table of RPM based air weight and add fuel weight accordingly, this is call "limp mode", because you can limp home, lol.

Unplug the MAF sensor and see if it runs any better, if so then maybe new MAF isn't working or.............wiring to MAF is an issue or computer's MAF circuit isn't working.

Look here under Testing: http://www.therangerstation.com/tech_library/CleanMAF.shtml

You can test MAF sensor voltages

Does fuel pressure hold at 38psi for awhile, it should stay above 30psi for a few months.

On the fuel rail is the FPR(fuel pressure regulator) it will have the fuel return line and a vacuum hose attached, front passenger side of lower intake.
Remove the vacuum hose and check it for fuel or fuel smell.
If FPR diaphragm leaks, raw fuel is sucked into the intake via this vacuum line causing very rich running, i.e. black smoke and rough idle

 

[hunter29078]

Thanks Ron.

I haven't done a voltage test yet. However there are multiple issues with it. I was messing with the fpr and got shocked by the wires. The wires and plugs need to be changed. I am thinking this is a result of the truck being run rich for a period of time. This truck is new to me and had these issues before I got it. The plugs and wires are on order so when I get them in I'll take another look.

The plugs have been had excess fuel for awhile. I am leaning towards a leaking injector as the culprit but maybe fpr. Pressure did hold 38 with key on and 40 running but it did leak down fairly quick. Not instant but over 45 minutes it was at 0. I pressurized it again and watched it and it was dropping, just slowly.

I have read some of your write ups regarding injector testing. If I turn the key on press the pedal to the floor and turn it over no gas should leave the injector. Since I am taking the plugs out can I just pull the plugs and do that test and see if gas comes out of the spark plug hole?

Thanks.

 

[RonD]

Yes, with gas pedal to floor injectors will be off and when cranking engine it should not fire at all, if it does fire even on one cylinder then you have fuel coming in from somewhere, FPR or injector, and the 0psi fuel pressure would indicate that.

Yes, you can disable spark and then do the same test, then pull out spark plugs, the wet one is the leaking injector cylinder.

0psi can also mean the check valve in the fuel pump is not holding pressure, it is part of the pump so whole pump must be replace.
Although check valve leaking wouldn't cause rich fuel mix.

If spark plugs show signs of running rich then O2 sensors may need to be replaced because of carbon fouling.
O2 sensors use a chemical reaction to detect Oxygen levels in the exhaust, usually they are good for about 100k miles before chemicals are used up, running rich over long periods shorts that life considerably, along with the carbon coating they get.
They can be cleaned but.............no real way to test them as to accuracy so best to just replace them.

 

[hunter29078]

Yeah I have had the same thought on the o2 sensors as well as the converter. once I get the plugs in i'll update. I think the truck has been run rich for a while. thanks will let you know in a few days.

 

[hunter29078]

Ok it runs much better on 6 cylinders. It was having a stall on idle after the plug and wire replacement, I removed the iac and cleaned it real well and it seems to be idling as it should with a slight hesitation once it starts moving but goes away as the rpms increase. I am still getting the MAF code 159 but it is working correctly. So other things that can trip this code from what I have researched is a faulty IAC which I am starting to suspect, also an air leak after the MAF which is possible as well.

If it is leaking, it is at those rubber adapters that connect the MAF to the intake hose, or the other one on the throttle body end. Is it possible to buy those rubber adapters without buying the entire hose? If so can anyone provide a link or name of what they are called?

Also RonD I did the injector test after the plug and wire install and all is good there. No leaking injectors. I did order the FPR so it is on the way, if that is not the cause of the leak down then I will check the fuel pump check valve when I replace the sending unit.

 

[hunter29078]

Ok pulled all codes today. Here is what I have

Key On Engine Off

111
10
121
122
157
158
172
177
543
556
121


Key on Engine Running
157
998

Any help or thought is greatly appreciated

 

[hunter29078]

Also tested the new mass airflow sensor and it is out of range? Is there anything that would cause the voltage to be at 2.3V across the signal wires at idle other the MAF? The voltage doesn't fluctuate either it stay constant. Thanks.

 

[hunter29078]

If I unplug the MAF should the signal wire have voltage?? If so where does it come from?

 

[hunter29078]

Anyone??

I am starting to thing the MAF signal wire is supplied its voltage by the PCM.

 

[hunter29078]

Ok. I need the know what resistance should be between the MAF Signal and MAF RTN (Ground). I checked but got no continuity reading. I unplugged the pcm and checked for continuity between the MAF wires going to the PCM and they were good so my wires to that point should be ok.

My question is should the two pins on the pcm where the wires go give me any resistance? Right now it is showing as an open circuit. Not sure if that tells me anything, let me know. Thanks.

 

[RonD]

I wouldn't think Signal and Ground would read not connected, so it is probably OK

I would test voltage at MAF with engine running as per test here: http://www.therangerstation.com/tech_library/CleanMAF.shtml

About a 3rd of the way down

Testing:
With the engine running at idle, use a DVOM to verify there is at least 10.5 volts between terminals A and B of the MAF sensor connector. This indicates the power input to the sensor is correct. Then, measure the voltage between MAF sensor connector terminals C and D. If the reading is approximately 0.34-1.96 volts, the sensor is functioning properly.

Use sewing pins to pierce the wires for live testing

Computer sends sensor 5volts so reading should be approx 1v at idle(1,000rpm, cold idle) up to 4.5volts at high RPM, similar to TPS return(signal) voltage.
Computer "Learns" MAF voltage, but it does have limits, like TPS, which is also a "learned" voltage for the computer.
So there is no absolute voltage, just a range

 

[hunter29078]

Thanks Ron, I did the engine running test and on three different MAF the reading where consistently too high. Which makes me think it not the MAF but something else. I know the wires coming to the MAF are good and not shorted out.

I took the pcm out and the circuit board looks ok with nothing obviously burnt.

My biggest concern is with the MAF signal wire. Does the pcm send voltage through that wire to the maf?

If I unhook my MAF and turn the key on and the engine off there is voltage on the MAF signal wire. Is it suppose to be there?

 

[RonD]

No, technically C and D are both grounds, one is reference ground the other is "floating" ground being changed by resistance from temp changes in MAF sensor wire.

In the diagram in the link above the 4 wire MAF terminal A should have battery voltage, B is the battery Ground.
Set voltage meter for DC Volts, 20vDC if available, then test battery voltage across battery terminals, this number will be used to see if a Ground is good, if battery voltage is 12.5v at battery, then when testing a ground it should be 12.5v

First test would be to make certain B is a battery Ground, key on, MAF connector pulled out, put red meter probe on Battery + , and black probe on B wire, should read full battery voltage, if not full voltage or it's no voltage then you have a loose ground wire.
Put red probe on A wire and black probe on Battery -, turn key on, should read full battery voltage, A's 12 volts is from EEC relay.
A and B is the Heater circuit for the MAF wire, a Ford MAF sensor works by heating a thin wire, when engine is running the air flowing past this wire cools it off, the rate of cooling is used to calculate the "weight" of the air that the engine is using, IAT(intake air temp) sensor is also part of this calculation, for obvious reasons.

C wire is the Computer ground, a computer has several Grounds and they are not shared internally so a loose ground wire for the computer may only effect one thing.
Same test as above, red battery +, black to C, key on, should read Full battery voltage, if computers ground for this circuit is good.

C and D work together, like most newer engine control systems the Ground is used for control wiring instead of 12 volt wiring, it is less troublesome to have "ground" wires running all over engine bay than 12v wires, so whenever possible ground wires are used for control or sensor feedback.
i.e. fuel injectors, coil, and the Temp sender, which works similar to MAF signal.

Key on will supply dashboard temp gauge with voltage, the sender on the engine is the Ground for that voltage, as sender heats up it becomes a better ground(lower resistance) and gauge needle moves up, if you were to use tape on the threads of new sender that could effect the Ground so gauge would not have the correct reference Ground and read incorrectly.

MAF is similar, as Heated wire temp changes with air flow resistance in this ground circuit changes so it becomes a better or worse ground, which varies the voltage the computer sees internally.
It uses 2 wires unlike most senders because sender uses engine/intake it is screwed into as the Ground reference, MAF is plastic so needs a reference ground wire, provided by the computer.

So if Reference ground is not a full ground the computer isn't reading MAF correctly

 

[hunter29078]

Figured some of the issue out and maybe all. This truck is new to me so I don't know much about the state the po drove it. Only that the truck started having issues and he drove it with problems until a brake caliper locked up on it, then it was parked.

So I replaced the fpr because vacuum hose was wet. Changed the plugs, wires, and coil because they needed it from being run rich. No MAF helped, could find no vacuum leak. I put the pcm back in because I really felt that was not the issue (rarely is). Took the intake snorkel off to check it for leaks once again (still would like to know if those rubber pieces on the end can be purchased separately). And there it was red fluid in the throttle body. Someone has been adding automatic transmission fluid to the engine haha.

Actually the transmission modulator diaphragm is ruptured, and it is pouring ATF into the intake. The FPR vacuum line wasn't picking up fuel but the ATF was going to the fpr via tha vacuum line. So i reckon I had a vacuum leak but not in a tradition sense. Probably been doing it for years, cant believe the truck was still running. I will replace it and report back. Just want to post findings as many threads never have a solution posted.

Also it is weird because the black smoke stopped when I changed plugs and wires, so apparently ATF doesn't blow smoke out like oil or a rich condition.