Bad MAF's

[Joes4x4Ranger]

My 91 XL has had a ongoing rough idle problem since the day I got it 4 years ago. I put a reman long block in it last summer, still idles like S$#T. With the MAF plugged in it idles around 500 and rough like a vacuum leak. I un-plug the MAF it idles high around 900, but much smoother. The MAF is clean I sprayed it and 3 other used MAF's I have with CRC. No CEL and the truck runs decent but idles like sh*t.

I have sprayed a freaking case of carb cleaner around the entire intake system and cant find a vacuum leak. I plugged all the vacuum taps at the back of the plenum, still idles like S$^T. Pulled all the plugs, all look the same, kind of lean/white lookin.

A new MAF is over $100 how can I test my 3 used ones to be sure they are bad. None of the 3 used makes any difference, all idle like sh*T, the truck runs best with the *******s un-plugged.

What else might cause a rough idle other than the MAF? I have a new O2 sensor, new injectors, new IAC, new external gaskets, TPS is .95, no codes other than 13 and 411. Could it be possible I have 3 bad MAF's???

 

[2000StreetRod]

MAF sensor test

I suggest that you perform my MAF Sensor Test Procedure

It's written for my 2000 SOHC V6 but the voltages should be similar for your OHV.

 

[Joes4x4Ranger]

Thanks streetrod. My engine runs real good at every RPM except idle. Acceleration is strong up to 2500-3000. I do not notice any hesitation or surge at crusing speeds or when I jump on the motor.

My problem is a rough, shaky idle. I've been in 2 other Gen 1's and I could not even feel them running. Both idled smooth as silk at about 650rpm. Mine idles like **** and fluctuates from 500-600. I did a vacuum test and the idle vacuum is HIGH about 24-26 hg, but steady. Normal is around 20 hg an intake vaccum leak should give a 14-17 hg reading.

One real weird thing is the idle does not change when I pull the PCV out of the valve cover grommet, or put my finger over its hole, or disconnect it all together and plug the hose. It rattles and DEFENATLY has vacuum. This problem has kicked my ass for 4 years now.

 

[2000StreetRod]

Not the MAF

I doubt that the MAF sensor is your problem since you're on your 3rd one but measuring the voltage output or reading the airflow with a scanner would confirm it is good.

I would have suspected your IAC valve but you have replaced it.

Sometimes a weak spark can cause an unsteady idle but your engine idles steady with the MAF sensor disconnected. I know that the PCM utilizes default parameters when there is no MAF sensor but don't know if the PCM reverts to open loop. If the idle is steady in open loop but not in closed loop that would indicate deteriorated O2 sensors but you have replaced one of them. You really need a scanner to see what's going on with your engine.

Intake leaks are a likely cause but you've sprayed for leaks, checked hoses and the engine idles good with the MAF sensor disconnected. However, it is hard to find gasket leaks with aerosols. You may want to try my Idle adjustment procedure . It might help until you are able to use a scanner for diagnosis.

 

[Joes4x4Ranger]

I do have a scan tool. What tests should I run with it?? I did the complete run through the tool calls for, I got DTC codes 411 & 13. I think it threw these codes because my A/C is empty right now and the clutch will not come on.

 

[2000StreetRod]

Scan tool vs code reader

Your 1991 model utilizes OBD-I. Usually only very expensive scanners are compatible with it. I think you're confusing a code reader with a scanner. A scanner can read parameters such as MAF sensor airflow in pounds per minute, engine coolant temperature, etc. A code reader can only read DTCs reported by the PCM. You can test your MAF sensor output voltage with a voltmeter.

DTC 411 (R) Idle speed system not controlling idle properly (generally idle too low) - ISC

DTC 13 (O) Idle speed control (ISC) did not respond properly (extends to touch throttle then retracts for KOEO) - ISC

What happens if you start your engine with the MAF sensor and IAC valve connected and then disconnect the IAC valve electrical connector?

 

[Joes4x4Ranger]

At cold start-up my idle is around 1000 if I pull the IAC plug the idle drops to 500 and rough. Once the engine is warm and idleing rough at 500-600 nothing happens when I pull the IAC plug, however if I pull the MAF plug the Idle raises to 1000 and seems less shaky.


At this point about the only things I have not replaced on this motor are spark plug wires and coil pack. The plug wires are aftermarket so someone has changed them but the coil pack looks factory and has over 200K. I am so frustrated with this rig I'm trying to sell it. I want it running good I'm asking $4,500. Even if I get that, I still wont get back half the money I've put into this POS.

 

[Turdle]

At cold start-up my idle is around 1000 if I pull the IAC plug the idle drops to 500 and rough. Once the engine is warm and idleing rough at 500-600 nothing happens when I pull the IAC plug, however if I pull the MAF plug the Idle raises to 1000 and seems less shaky.


At this point about the only things I have not replaced on this motor are spark plug wires and coil pack. The plug wires are aftermarket so someone has changed them but the coil pack looks factory and has over 200K. I am so frustrated with this rig I'm trying to sell it. I want it running good I'm asking $4,500. Even if I get that, I still wont get back half the money I've put into this POS.

Better fix it. You'll be doing good to get 2k for a 91

 

[2000StreetRod]

IAC valve appears to be working

At cold start-up my idle is around 1000 if I pull the IAC plug the idle drops to 500 and rough. Once the engine is warm and idleing rough at 500-600 nothing happens when I pull the IAC plug, however if I pull the MAF plug the Idle raises to 1000 and seems less shaky. . . .

It appears that your IAC valve is working normally. The factory idle speed is usually about 500 rpm with no IAC valve assistance.

When you disconnect the MAF sensor the PCM detects that and reverts to preset rich conditions (limp home mode).

Has it been more than 30,000 miles since you replaced your fuel filter? Have you tested the fuel pressure?

 

[Joes4x4Ranger]

Fuel filter is going to be done Monday. The fuel pressure looks good 38lbs with vac line on 45lbs when I pull the vac line. With all the receipts I have for new parts along with the aftermarket suspension $4500 is fair.

What I have replaced All with warranty time remaining: Engine Long block, Manual Transmission, Starter, Alternator, P/S Pump, P/S Gearbox, IAC, MAF(used), O2 Sensor, Temp Switch, Thermostat, Radiator, Clutch fan, Fan, Water pump, Fuel Injectors, A/C Compressor & Hoses, Orifice Tube, Clutch, Pressure Plate,Throwout Bearing, Slave cylinder, Front Rotors, Brake Pads, Front Calipers. All this and it still idles like ****.

As you can see this truck has been a nightmare to own, and when I sell it, I'll never touch another Ford.

 

[2000StreetRod]

Ignition or vacuum

Since you installed a remanufactured long block your compression should be good. Your fuel injectors are new. About the only thing left is faulty ignition or a vacuum leak. Is there EGR on your vehicle? If so, you could have a sticking open EGR valve.

 

[Joes4x4Ranger]

No EGR on this year motor. Would the cylinder balance test on my scan tool detect a weak spark at idle? What about coil and plug wire resistance, I cant find them in my Haynes manual.

What other way than carb cleaner can I use to find a vacuum leak. I'm not perfect and there is always the possibility I did not get the lower intake on correctly. The intake could also be warped or cracked. The previous engine idled rough as well. I have not been able to find a leak with carb cleaner or MAF cleaner. What about valve covers is there anyway possible they could be causing a vacuum leak through the PCV???

Are there any engine wire harness issues that could cause this problem and not set codes??

 

[2000StreetRod]

ECT sensor

You might check your engine coolant temperature (ECT) sensor. I'm not familiar with OBD-I but if the ECT is out of range the PCM should detect it and set code 21. However, if the values are in range but incorrect the PCM will not detect it. Your engine idles as if the mixture is lean. If the ECT sensor tells the PCM that the engine is warm when it's actually not then the mixture will be lean. BrooklynBay has a table of the ECT sensor resistance vs temperature in his list of useful threads. The ECT sensor may be more accessible on your OHV than it is on the SOHC. I have to pull the intake manifold to get to mine.