Hey guys, I'm new to the site here, so I'm sorry if this post is in a wrong field.

I just picked up a 98 explorer with the 4.0 ohv motor. The car has just under 210,xxx miles...and keeps on going! The car has been in my girlfriends family since 98, her grandfather owned it, and maintained it religiously. I just bought it off her brother, he's had it for 3 years. He hasn't neglected it, but hasn't maintained it like it used to be...

I've drove the car many times before it came into the option of be being able to buy it, and it's never had a single problem, EVER.

The car was sitting for about a month now, and I was approached if I wanted to buy it. I needed a new car, yadda yadda (sorry if it seems I'm giving my life story here...)

About a week before I bought it, my girls brother started it up to move it, and it had a really rough idle. Can't figure out whats up. It did not have this problem prior to it sitting for a little under a month.....now it does?

I've read a couple things about the MAF's being prone to cause the 4.0 to idle rough, and also IAC. I cleaned the MAF on the truck with MAF cleaner, didn't help. But when I unplug the MAF, the idle straightens up like nothing is wrong. It doesn't have any hesitation when driving at all. Only at an idle either in park, or when stopped. Cleaned the IAC with MAF cleaner, didn't help. Did new plugs and wires, didn't help. Went to a local pick and pull and got 2 MAF's, one off a 97 explorer 4.0, and one off a aerostar (all part numbers matched up so I assume it was the same). Brought them home, plugged them in, neither of them helped. I tried resetting the computer, didn't help. Cleaned out the 2 MAF's I bought, didn't help.

I'm not sure if I either got really "lucky" and bought 2 bad MAF's? Or if there is something else wrong I should possibly be looking at?

One again, I'm sorry if I gave my life story, but I want my problem to be as best explained as possible so hopefully some of your guys can give me some help.

Thanks in advance!

-Brad

 

Had over 40 views, and no one has any help? Looking for help here please.

 

Welcome to the forum!

Sounds like you have a vacuum leak, and when you unplug the MAF the ECU no longer knows how much air is entering the engine, therefore it uses the O2 sensors to determine how much air it is getting. If you spent some time while waiting for replies searching this forum, you might have learned that it's pretty common for these engines to have vacuum leaks at the PCV elbow and with the intake manifold gaskets.

If you have access to a vacuum gauge, with the engine idling watch the vacuum gauge. The engine should have around 20 in Hg of vacuum at normal idle speed. If it's less than that you probably have a vacuum leak.

-Rod

 

___________

 

Ok, ran a vacuum test, it has 17 in Hg at idle. I sprayed carb cleaner all around the intake and all vacuum fittings, nothing happened. I used a fog machine to check for something, and still nothing. Checked PCV valve and elbow, those are both good, and no cracks. Ran a EEC test, and it gave me a P0455, EVAP gross leak. Is this something that could be giving a rough idle?

 

Yes that would cause a leak in the vacuum side.....

FYI the 98 is OBDII not EEC.

 

From the factory service manual for code P0455:

Continuous Memory Diagnostic Trouble Code (DTC) P0455 indicates that the Evaporative Emission (EVAP) control system detected a large leak or no purge flow.

Possible causes:

-- After market parts and accessories nonconforming to the EVAP system specifications.

-- Damaged, missing or improperly installed fuel filler cap.

-- Blockage in fuel vapor hoses/tubes (EVAP canister tube, EVAP canister purge outlet tube or EVAP return tube) in EVAP system.

-- Disconnected or cracked fuel vapor hoses/tubes in EVAP system.

-- EVAP canister purge valve stuck closed.

-- Canister vent (CV) solenoid stuck open.

-- Fuel tank pressure (FTP) sensor malfunction.

-- EVAP canister leaking.

-- Fuel vapor control valve tube assembly blocked.

-- Fuel vapor vent valve assembly blocked.

-Rod

 

I dont understand how a gas cap can go bad, or be put on wrong? A dude told me that the only thing that could cause the rough idle is some type of valve thats located on the firewall, that allows precise amounts of unburned exhaust to recycle through the intake. He said it has a line coming from the gas tank, and a line going to the intake, and it's controlled by a vacuum line. I'm not sure where exactly on the firewall this is, or what line it is going to the intake? Does that sound possible?

 

The valve that your friend is referring to is the "EVAP canister purge valve" listed above.

That valve being stuck closed should not cause a vacuum leak and subsequent misfire, but a hole in one of the hoses for that system might.

-Rod

 

The EVAP purge valve.....is it the little thing under the battery? One line coming in, one line going out to the bottom of the TB, and one vacuum line?

 

Yes - Your "dude" is in error about the exhaust gases - they are handled by the EGR system. The EVAP system takes vapor from the fuel tank and feeds it into the throttle body. If the gas cap is not sealing properly, it produces an air source not measured by the MAF - hence acting like a vacuum leak - I had to throw away my aftermarket gas cap and get a replacement from my Ford dealer. Also, "toppping off" the gas tank can cause liquid fuel to be drawn into the EVAP canister Purge valve ruining it.

 

So should I bother with the gas cap thing, or should I take it to a shop to be checked out? Before I got the car, when my girlfriends brother had it for about 2 1/2 years, the explorer never saw a full tank. Maybe 3/4 at the absolute most. I filled it up when I got it, and as the pump kicked off because it was "full", there was a little bit of gas that came out of the filler neck, and splashed onto me, the truck and ground.

When I unplugged the line going TO the TB, and blocked it off, and unplugged the little plastic nipple by the EGR pressure sensor and blocked that off, it still didn't fix the idle problem.

When the car is cold and first starts up, it runs fine for about 1-2 minutes. As soon as I drive it, and get to the first stop sign at the end of my street, is when the idle acts up.

 

Do you have access to a scanner capable of live data??

 

I don't think so. Only thing I've got is a OBD I,II tester.

 

What else can I do from here?

The gas mileage on this truck is absolutely horrible. I know 200,000+ miles will have some factor, but I'm at 180 miles trip, and at a 1/4 tank. ALL city driving.