Trying to diagnose the rough idle on my ‘98 4.0 OHV. It’s got about 140,000mi on the clock. The engine runs great with plenty of power whenever I touch the gas pedal but sitting at idle it shakes a good bit which has been annoying me since ive owned it. So far I have replaced the IAC valve(which was a cheap $50 aftermarket) and cleaned the MAF sensor. I had known for several months the engine had a major vacuum leak at the intake (25%+ fuel trims and lean codes) and recently had the shop replace my upper intake & fuel rail gaskets to fix the vacuum leak and lean codes. The Lean codes are gone and combined fuel
trims have dropped to between 5% to 15%. While doing the intake gaskets the shop smoke tested the engine and said my EGR valve was also leaking which I declined to have them repair due to cost. I have tried spraying carb cleaner all over the EGR components but cannot seem to find this leak but I don’t doubt that it exists. I also did the test unplugging the IAC valve which does NOT drop the idle to 500rpms or stall the engine and I’m currently running 5% to 15% fuel trims so presumably vacuum leak somewhere still persists? I’m trying to narrow down which components of the EGR system may or may not be faulty as the shop only mentioned “EGR valve.” I came across a post by RonD about testing the EGR valve, EGR solenoid, and DPFE sensor. I pulled a vacuum on the EGR valve which made the engine stumble and almost die so that seems normal. I removed the EGR solenoid, tested wires for 12v which was present with key on. I connected both a 9v and 12v power source to the EGR solenoid and tried the blow test. I could hear a very faint click when energizing the solenoid but it made no difference with the blow test whether it was energized or not, I’m not sure how to interpret this result. When blowing thru the bottom tube no air passes through whether energized or not, when blowing thru the top tube a small amount of air comes through, energized or not. With the engine running at idle if I disconnect the skinny green EGR vacuum hose and put my thumb over it I can feel the solenoid pulsing which I would think is normal?



Any advice on tracking down the supposed EGR vacuum leak?

 

Did they say EGR "vacuum leak"?

Or is the EGR valve leaking, not closing all the way and leaking exhaust into intake which would cause rough idle

I would expect P04xx codes for either, but not always

The 4.0l OHV "eats" spark plugs lol
Motorcraft or Autolite last longest
Make sure gap is 0.054 at least for smooth idle

Have you tried Seafoam or similar injector cleaner in gas tank?
I do it once a year on my 4.0l and it smooths out the idle for next 10 months or so, original 1994 injectors still in there
If did replace spark plugs wires finally they "lasted" 16 years, well maybe 14 years, lol, because it really smoothed out idle with new motorcraft set
Ford dealer near me runs "tune up" specials twice a year, wires and plugs for same or less than good 3rd party brand

 

RonD, thanks for the reply. I believe they said it was a vacuum leak at the EGR valve, they said they could see smoke coming out at the valve. ( I have no idea where, and can’t get any response out of the engine by spraying carb cleaner onto the EGR valve) I just put in new plugs which didn’t seem to help much with the rough idle, I haven’t done new wires yet. I used Autolite iridium plugs which are .054 gap. I examined the old plugs which all looked good. They looked clean with no oil or carbon fouling, a uniform brown color and no signs of steam bleaching or anything like that. I did run a couple bottles of fuel injector cleaner in the tank which didn’t seem to help the shakiness at idle. I’m wondering if I should do spark plug wires next or if I need to look more into the EGR components? I should also try to specify what I mean when referring to the “rough idle.” I mean that the RPM’s are reasonably stable around 850 or so in park, RPM’s don’t surge up and down on the Tac or OBD2 scanner live stream data. The engine doesn’t chug or stumble, but there is a subtle engine shake that you can feel when sitting in the truck that comes and goes. The vacuum leak surely isn’t helping anything but I don’t know how to go about fixing that; whether to replace the EGR valve or not? Spark plug wires sound like they may be needed also.

 

Smoke out the EGR valve means EGR Valve leak, which could "technically" be called a vacuum leak, but not by any mechanic I know, lol
It would be like saying smoke coming out past the throttle plate was a vacuum leak

So not sure what to make of that

You can put a longer vacuum hose on the EGR valve and then suck on that hose, should hold vacuum, if not replace the valve, engine OFF for this test of course

What happens over the years is that carbon residue builds up in the EGR valve and then at some point valve won't close all the way
At idle intake vacuum is high, 17+ inches of vacuum
So that sucks in exhaust thru the partially open EGR valve which causes misfires

EGR normally opens only when engine is under a load, accelerating or cruising above 40mph, at idle the exhaust just fouls things up

 

How much would replacement of the EGR have cost? ...at the shop you paid to examine {smoke test} the vehicle?
So much points to the EGR valve. Changing it wouldn't be a shot in the dark.

At 23 years of age, are the vacuum lines themselves suspect? Do these smaller lines just pull off and push on?

On top of the source problem, can aged motor mounts contribute to shaking?

Sorry, no answers. Just additional questions.

 

I am also not sure what to make of the supposed “vacuum leak” at the EGR valve, I don’t understand where the smoke could have been coming out unless it were the flange/gasket area (which I can’t get a response out of the engine by spraying carb spray on.) I put a piece of hose onto EGR valve and sucked on it, it definitely holds vacuum. I’m not sure where to go from here. In my day job I work full time as a small engine mechanic and one thing I know well is the futility of throwing parts at a problem without a proper diagnosis. Part swapping 1) costs money and 2) doesn’t fix the problem so I hate to just go and replace working parts if I don’t suspect them to be faulty. Any ideas?

 

Yes, vacuum hoses "just pull off", but can be hard to remove after so many years, so twist them first to break them loose

Ford started using Hard Plastic vacuum lines because they are cheaper and can be color coded easier for easier assembly at the factory, these are shown in Ezyrider1581's picture
The "rubber" ends are what you grab to pull the hose off, the hard plastic part should NOT come off the "rubber" end
Any of these can be replaced with regular black "rubber" vacuum lines, if you find cracks
And you can splice the hard lines with correct size "rubber" lines if any are broken, just needs to be tight fit

These hoses have Negative pressure inside when engine is running, so no clamps are needed, they pull together, they don't try to push apart, like say coolant hoses that have positive pressure inside

 

I picked up some spark plug wires and going to put those on as soon as I get a chance, Is there any other way to assess whether or not I need to replace the EGR valve?

 

I have 380000Km on my '96 4 liter Ranger and it's on it's 5th set of autolite plugs so I'd hardly say it "eats" spark plugs.
I have, however, been fighting the lean lope for quite some time and finally got peaved enough to do something about it today. I sprayed some ether at where the EGR pipe enters the upper manifold and it became VERY obvious where the leak was. I just saturated some heavy cord eith silicone sealer and stuffed the gap all around the tube, the schmeered a coat of silicone sealer over the whole joint. I threw the heat gun at it for about half an hour as the stuff doesn't cure very well at +2C. That was about 3 hours ago - I just went out and checked - the sealer has firmed up pretty well so I started the truck and it quite quickly settled to a smooth 850RPM idle - which it has not done for several months. I will leave the EGR dissabled for at least a few more days - if it remains smooth with no lean codes I will likely reconnect the EGR and see if it stays smooth at idle or if the EGR valve also has a minor leak - which will NOT make it throw a lean code. Funny, running with the EGR disconnected did not throw any EGR codes - -