4.0 will not idle

[Exenos]

I have a 97 4.0 that will not idle for more than a second or two on its own. I can keep it running if I give it throttle but even then it surges badly. I am getting a "low mas air flow" code. I've already replaced the maf with a known good used one and it did not solve the issue. Replacing it with a 3.0 maf would let the truck idle and drive but the rpms would hang. Replaced the bank 1 o2 sensor because of a bank 1 lean code, didn't help it either.

The exhaust smells very rich so I'm not going to rule out an ignition problem but I'm more leaning to a bad intake gasket. Any thoughts? I cant afford to just start throwing parts at it.

 

[ford4thot]

Clogged cats?

 

[Exenos]

Ok so I put full vacuum to the EGR valve and the surging went away and it will idle now. The idle is still rough but it will idle. However I have noticed that the exhaust coming out the tailpipe is extremely hot , could this be from unburnt fuel being burned by the cats?

 

[chewy012]

Sounds like something is "telling" your engine to run rich. If MAF is ok, I'd check IAT and IAC.


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[RonD]

Yes, hot exhaust and popping noises from CAT means rich exhaust

Does it run OK at speed?
If not then yes could be clogged Cats or exhaust blockage

Pull a couple of spark plugs and see if they have blackish tips, sign of Rich fuel mix.

You should get a Volt meter and test the MAF sensor voltage while engine is running, sensors rarely fail, not never just rarely, most problems are with the wiring, dirty or corroded connectors.

So YES, don't throw money at it.

The computer knows it is running a 4.0 Liter engine, so it knows how much air the engine should be sucking in at 700RPM, or at 2,000RPM, the MAF sensor is there to fine tune the exact amount of air for best MPG and performance.
So if the computer sets a MAF code it means the RPM air that should be coming in doesn't match what the MAF sensor is showing.
Could be MAF sensor is dirty or the voltage coming from MAF sensor is low(or high), MAF sensor voltage is 0 to 5 volts, so even a little corrosion on a wire connection makes a big difference.

When you opened the EGR valve at idle engine will stall, if it didn't then you have a problem.
EGR is only used when engine is fully warmed up and only when accelerating or cruising at highway speeds, purpose of EGR system is to Cool down the cylinders to lower NOx emissions.

So if opening the EGR at idle made idle smoother then you have restricted air flow into the engine, opening EGR valve lets in more "air".
Pull a vacuum hose off the vacuum manifold, upper intake drivers side toward the back, see if engine idles better with a vacuum leak, confirming restricted air flow.
Remove air filter cover and air filter, see if engine runs better, could be the Cold Air Intake tube is plugged.

On the upper intake near the front is the IAC(idle air control) valve, looks like a can mounted sideways.
IAC Valve lets in air so computer can set idle RPMs, it is, in essence, a controlled vacuum leak.
It has a two wire connector, start cold engine idle should be above 1,100rpm if not then IAC Valve is not opening, so not enough air is getting into the engine.
Remove IAC Valve, 2 bolts, plug in its connector, turn key on, IAC valve should move fully opened, unplug connector and it should move closed, repeat a few times.
If IAC Valve is not moving at all you should replace it, these don't fail often so wrecking yard replacements are not a big risk.
The 2 wires should show 12volts, key on, but it is Pulsed Voltage, so can look odd depending on the meter being used.

 

[Exenos]

So this morning is much colder than yesterday ( 15C vs 29C) and it will idle today. Its still rough and you can hear fuel burning in the exhaust though. I checked the IAC and while it sounds like it is actuating (you can hear the solenoid) there is no visual movement of the plunger/rod. Tried the engine with the IAC connected and not and it idles better with it connected, but not by much. I did disconnect a vacuum line on the manifold and it almost killed the engine.

I'm beginning to think this is an ignition issue but I really dont want to tear into that since it would appear that the plugs are rusted solid in place.

 

[monstermazda]

Discount your cats see if it runs better if it idle fine you have clogged CATS. There is a way to test them on the car has to be at operating temperature measure the temperature on the outlet ring and the inlet ring of the catalyst converter

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[chewy012]

So this morning is much colder than yesterday ( 15C vs 29C) and it will idle today. Its still rough and you can hear fuel burning in the exhaust though. I checked the IAC and while it sounds like it is actuating (you can hear the solenoid) there is no visual movement of the plunger/rod. Tried the engine with the IAC connected and not and it idles better with it connected, but not by much. I did disconnect a vacuum line on the manifold and it almost killed the engine.

I'm beginning to think this is an ignition issue but I really dont want to tear into that since it would appear that the plugs are rusted solid in place.

By "15C vs 29C", did you actually mean "59F vs 84F"? ..... what's Celsius?

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[firehawk618]

It could be several things.

Instead of throwing any money at parts what I would do is pick up some sort of scan tool that has the ability to read sensors.

With this tool you can plug it in and see what the PCM is seeing.

Example. You could pull up the ECT sensor and see if it reads within reason given whatever temperature the engine is at.

You could pull up the O2 sensors and see if they're fluctuating normally.

You could pull up the MAF and see if it's readings are linear or not.

You could pull up the TPS and see if it's reading linear and within specs.


etc etc etc.

Seriously. Save yourself a bunch of time and $$$ because your time has to be worth something. Pick up a GOOD inexpensive scan tool that can read PCM parameters and not just codes.

Go from there.