98 explorer, bad idle and rough start --MAP, MAF?

sss

Try cleaning the MAF/IACT with MAF sensor cleaner 1st,
If this doesn't work, just replace both.

1) New air filter
2) Clean IAC, MAF, and IAT 1st.

How have you narrowed to the MAP or MAF? I believe your Explorer has a MAF, not a MAP. But, your symptoms sound more indicative of a bad Idle Air Control (IAC) which is a pretty common issue with the Explorer. You could start by removing it and making sure the passages and valve are clean. If clean, there's not a lot of chance cleaning it will remedy your issue, you probably would just need to replace it.

Does it run smoothly if you just barely keep the throttle open, or is it pretty rough? Has your fuel economy dropped at all recently? Does it tend to idle fine once the engine is up to temp?

The Mass Air Flow (MAF) sensor measures the amount of air entering the throttle body so the computer can adjust the fuel injector pulse width to maintain as close as possible to stoichiometric air-fuel ratio.

-Rod

Every fuel injected engine, as far as I know, has a MAP sensor or a sensor that measures manifold pressure. I would agree, it sounds like it has something to do with the idle control system. Generally a problem with the MAP or MAF sensor(s) sets a code. I would clean the throttlebody and see if the IAC motor is all gummed up. You can try cleaning that too but it usually doesn't help or doesn't help for very long. MAF sensors tend to build up gunk over time which usually leads to lack of power and sometimes an unstable or rough idle. I wouldn't blow money on something until you narrow it down better. Try starting your truck cold with the IAC unplugged, it should default to a partially open position and allow the truck to idle a bit higher so it will run, just not idle where it should.

There's no mention of a MAP sensor in the factory service manual, but there is a MAF. What would be the purpose of a MAP sensor if the vehicle has a MAF? The vacuum in the manifold should be directly proportional to the air entering the throttle body. Back when I was still working on cars while in high school cars had one or the other, but I don't recall any instances where they had both. Alldata suggests a few Ford products (2.3L Ranger, 2.3L Focus PZEV, and 4.6L SC Mustang) use both Thermal MAP (TMAP) and MAF sensors, but that appears to be a relatively recent (relative to most of my experience) and not widespread incorporation.

-Rod

The engine runs relatively smoothly as long as I keep the throttle open... It idles pretty rough even when it's warmed up-- idles rough in the sense that the engine shakes pretty bad, but it usually stays on after it's warmed up.

Okay, this confuses me. Is the idle pretty rough just when warmed up, and smooth when cold, or pretty rough cold or warm (or relatively smooth?), it only stalls when cold? If pretty rough all the time, then the problem may be more related to a vacuum leak, coolant temp sensor, EGR valve, or plugs and/or plug wires more than an IAC. When holding the throttle open to keep the engine running when cold, what engine speed to you hold it at - something quite a bit higher than normal idle speed?

But if it really is relatively smooth even at normal idle speeds, I'd still suspect the IAC as the primary cause. Sounds like you still need to perform a tune up. The IAC will attempt to maintain idle speed as the load on the engine changes, so it makes complete sense that it would stall when put in drive/reverse. Same would go for turning on the A/C/defrost or turning the steering to one of the locks.

-Rod

2000 RPM is a very high idle speed. Did you let it run long enough to see if the idle would slow down, and if so, did it then idle roughly again? It sounds like if you were to hold the throttle open at 2000 rpm it is nice and smooth and remains running as well.

-Rod

Since when you unplug the MAF the engine idle speed goes up to the same range that you can hold the throttle at and still have a smooth idle, I don't really think the MAF is the culprit. However, it is very inexpensive to clean the MAF and that might be worth a shot. If holding the throttle to keep the engine at around 700-800 rpms will not keep the engine idling smoothly you probably have a vacuum leak or other misfire issue. If at 700-800 rpms the engine idles fine, then I think you're looking at the IAC.

-Rod

Well, it sounds like you have a vacuum leak or other misfire issue if it's also acting up when warm. If you haven't changed the spark plugs, wires, and thoroughly checked for vacuum leaks doing so would be money well spent. Without being able to witness the issues you're experiencing it is tough to give you clear direction forward.

-Rod

See smoke test for vaccum leaks www.youtube.com/watch?v=FfiQ7qWiG-M (http://www.youtube.com/watch?v=FfiQ7qWiG-M)

Since the smoke test requires a very expensive piece of equipment, I'll assume you don't have easy access to one. There are other ways to check for vacuum leaks. The common, albeit not real safe, way is to use a can of throttle body cleaner and to carefully spray it around areas of potential vacuum leaks with the engine running. If you spray over a leak, the engine idle will change. As you might imagine, you need to be very aware of where you're spraying since throttle body cleaner is highly flammable and the engine will get hot.

The poor idle didn't start shortly after replacing your plugs and wires by chance, did it?

-Rod

It's worth the money to find a Mechanic who has one todo the test only!

Since the smoke test requires a very expensive piece of equipment, I'll assume you don't have easy access to one. There are other ways to check for vacuum leaks. The common, albeit not real safe, way is to use a can of throttle body cleaner and to carefully spray it around areas of potential vacuum leaks with the engine running. If you spray over a leak, the engine idle will change. As you might imagine, you need to be very aware of where you're spraying since throttle body cleaner is highly flammable and the engine will get hot.

The poor idle didn't start shortly after replacing your plugs and wires by chance, did it?

-Rod


No. The idling actually improved after the plugs and wires.

BUT... I am a complete idiot. There is the biggest hissing noise coming from the engine, and for some reason I went with the worst case scenario and my ears ignored the hissing. It's coming from the rear end of the engine block. I've recently replaced the PCV which was bad, and the intake-PCV hose, which was bad. It's likely that the second hose going to the PCV is bad, as well.


--will post further results.

Good job!, you found the unmetered air leak.
Disconnect the PCV tubing, clean, and use intake sealent for fix.

So the PCV hoses weren't tight enough on the PCV. I tightened them, and the problem has been mostly alleviated. I can now start my car without it dying. Although the majority of the shaking has gone, some of it remains, but I'm not going to complain too much.

Thank you for the follow-up post, and congrats on finding the vacuum leak. Luckily your fix was VERY inexpensive.

-Rod