Rough Idle no DTC's

[viking74]

Hello, I have a '98 Explorer XLT with 4.0 SOHC. I am having an issue with a rough/maybe even low idle when warm. Usually occurs when sitting at traffic light. Just a rough idle for a couple of months now. Tach reads around 600 at warm idle. Engine performance is great/smooth above 1k rpm. Also, no check engine light or DTCs for months now. I have replaced several items since this problem with no solution yet. Here's what has been replaced: Crank & cam sensors, IAT sensor, cleaned MAF, new fuel injectors, new fuel pump and filter, new PCV valve and replaced degraded PCV hoses, new intake gaskets, cleaned IAC, new coil, wires and plugs. No smell of gas when idling, no black smoke from exhaust. New air filter. Plugs looked decent when recently pulled...not to full of carbon and gap was in spec. Tested fuel pressure...is at 30psi at idle. Pulled EGR valve and verified no leakage when valve closed. I'm out of ideas! Help me out guys!

I also tried driving car with MAF disconnected for testing purposes. The car still had idle issue. When MAF was disconnected, trans shifted erratic and lots of hesitation when starting from a standstill. Also, noticed that the CEL did not come on after disconnecting MAF and driving for approx 10 miles. CEL does come on when key on engine off. So, CEL bulb works. So, I just connected MAF back again and runs fine with the exception of this idle issue.

Also, I have the obd2.com scanning software I've used for years. I noticed that this Explorer shows long and short fuel trims constant at -100% all the time, both banks. Seems strange to me. Here's some other sensor info at idle:
Running in closed loop, Load: 29.8%, ECT 187, all fuel trims -100%, rpm 645, sparkadv 18.5,iat 86,maf 0.57 #/min,TP 18.8%, misfire all cylinders at 0%.

 

[2000StreetRod]

scanning software

Obviously your long and short fuel trims are not -100%. Another member reported their scanning software could not read the trims on their Explorer. I don't know if it was the same generic software package but I suspect it was. When I purchased the Auterra software package it showed several different long and short fuel trim parameters. Some of them were always at the extremes. Your package is not reading the correct parameter. Too bad since it would be very helpful in diagnosing the problem.

Can you read the commanded idle speed to see if the IAC valve is responding properly? You may want to try my Idle Adjustment Procedure. See My Helpful Threads for a link. Your IAC valve may not have adequate control range. It is an electro-mechanical device prone to failure. Cleaning sometimes is not adequate to restore the control range. My guess is you need a new one. Performing the idle adjustment procedure may allow you to get by with a reduced IAC valve control range.

 

[viking74]

Good thought! I will order a new IAC today from Ford and then try the idle adjustment/setting procedure! I'll let you know how it goes. Thanks!

 

[2000StreetRod]

EGR tube seal

It is easy to damage the large O ring seal around the EGR tube when the upper intake manifold is removed. Try spraying aerosol where the EGR tube enters the intake manifold to check for leak.

 

[viking74]

Ok. Will do. Thanks!

 

[viking74]

Ok. I replaced the IAC, tried the idle adjustment procedure as well as changed EGR gasket. Unplugged battery for an hour. Still having same symptoms. Checked again around intake for vacuum leaks. Found no leaks.

Thought possible o2 issue.....So, I also tried unplugging the o2 sensors and drove for a day in open loop mode. CEL came on. Idle still feels the same. So, I don't think it is an o2 issue.

I was giving this problem some more thought. When I replaced the injectors, I did notice the grommets the injectors snap into in the head did have a hairline crack on a couple of them. I'm thinking about taking upper/lower intake off this weekend and replacing those grommets and check for damanged intake gaskets. Now, where to get those grommets from. Probably dealer, any ideas?

Thanks again for everyone's advice. Once I get this issue fixed. I'll have a ton of new parts in this engine and should get another 100k out of this.

 

[2000StreetRod]

injector insulator PN

Send a PM to shelbygt and ask him for the part number. He just determined the PN for a cracked insulator for one of his fuel injectors. $5.60 each at the Ford dealer.

http://www.explorerforum.com/forums/showpost.php?p=2408784&postcount=12

 

[viking74]

Good information. Thank you. I just posted on that thread.

 

[viking74]

Ok.....I think I'm set! Smooth idle now!

Thanks for the ideas. Here's what I did last weekend which I think worked... Idle is smooth now. I removed upper and lower intake. Then, replaced all the fuel injector adaptors (Thanks to shelbygt for those part numbers). Then, I spent a bit of time cleaning the carbon from the upper intake, lower intake and throttle body with two cans of air intake cleaner. What a mess they were! Then, put new intake and throttle body gaskets on. I greased the fuel injector adaptors when installing. Also, inspected the EGR o-ring and greased when installing intake again. Fired it up and it's smooth! I did notice when cleaning the lower intake that there was some carbon buildup just on the inside of the seal which looks like it should have been cleaned off the last time I had the lower intake off. I think between the injector adaptors and removing the carbon from intake (which has been now under my fingernails for 4 days), did the trick. I also noticed a small hole in the middle of the throttle plate which was also plugged with carbon. That looks to be some sort of idle air passage. So, that being cleaned probably helped as well. Thanks again to this forum for all of the good ideas!