Rough Idle Only When In Gear

[Windstartled]

This is a little odd. Over the last two years or so the van had developed a rough idle issue. When left to idle in Park or Neutral the roughness would usually lead to the engine stalling after a few minutes.This happened both in and out of gear but only when the engine was still cold, idle returned to normal once normal op temp had been reached. Cleaning the IAC got rid of the problem... sort of. Now it idles fine in Park or Neutral but when in gear and engine/tranny still cold it is still very rough.

When waiting at a traffic light with my foot on the brakes the van shudders as it tries to lurch forward, the shaking is very noticeable even to those outside the van. This has become so bad I shift it into Neutral every time I need to stop at a traffic light, and it instantly settles into steady smooth idle at 800 rpm. However once the engine is warm everything returns to normal. Another thing I noticed is that when in gear and cold the roughness only occurs with brakes applied. Feels like whenever the roughness in the engine occurs the transmission attempts to override the brakes, which causes the lurching.

While the problem only lasts 5 to 10 minutes until op temp is reached it is nevertheless very annoying and driving me nuts. My main concern is that it may eventually cause damage to the transmission to be tugged at and jerked around in such a way. I am running out of reasonable explanations as tho what may be causing this and have begun elaborating more esoteric theories. My latest one is that since this only happens when the brakes are applied then the tiny voltage required to activate the brake lights is somehow causing some other electric part to fail, and that the rough idle is somewhat related to this. Shows you just how desperate I'm getting.

Anyone here ever encountered this?

[wiswind]

I'm also wondering if the tranny is fully easing up.
It should still provide some engagement...but it would have enough slip in the torque converter to avoid bogging down the engine.

I guess the FIRST thing I would address is any routine maintenance items....including transmission fluid (use a fluid that meets the Mercon V specification).

Vaccum leaks are another thing that I would look into.....as in condition of all the vaccum lines...including the PCV hose.

[catvents]

One point mentioned by Wiswind is a vacuum leak and according to your post, condition seem to be worst when you apply the brake which use (if I could say) a big amount of vac to operate the booster. If not already done I would take a serious look at the few vac hoses between the engine and the firewall, in addition to the brake booster, there is another big one connected to the base of the intake plenum on the passenger side and also a few little which feed the Vapor Canister Purge Valve and the EGR Vacuum Solenoid (on the firewall). Just a few idea if not already done.

[Windstartled]

A-ha! Now we're getting somewhere. Thank you gentlemen, off to check vacuum lines. This killer heat will make things even more interesting. Forecast heat index for today was supposed to be around 90F but at 10 am was almost 100F already

[12Ounce]

Ditto on vacuum system. And there may be a leak in the brake booster itself.

That hot in Montreal?

[Windstartled]

Originally Posted by 12Ounce

That hot in Montreal?

Sure is, Montreal is noted for its perfect incarnation of the Humid Continental climate zone the main feature of which being temperature extremes exacerbated by consistently high humidity levels. I have lived some years in Phoenix and I can tell you 100 degrees there feels quite cooler than 80 degrees here and it's way above 80 now. If one wants to experience what it feels like to live in Africa and then what it's like to live in Antarctica, Montreal offers both every single year. In summertime it is often hotter than Miami here, and no soothing sea breeze to cool you down in the evening, often no wind at all at night. In winter it is sometimes colder than Moscow and always, always that dang humidity. I have no idea how people managed to survive here before the advent of AC and central heating.

Needless to say it's kinda rough on vehicles

So when it became obvious that I couldn't perform a vacuum check without removing the cowl I opted to postpone the procedure until this evening and do my coolant flush this pm instead. Got two gallons of demineralized water and a quart of concentrated coolant at the ready, that's my van's summertime cocktail. Beamer only gets water with a dash of rust-proofing compound. I only drive it in summer and it has never tasted antifreeze according to previous owner, I purchased this car in Arizona (Tucson) where it had spent its whole and rather sedate life. I bought it on a whim because I had never seen such a small V8 before, it's only 4.0L. It is sluggish and Winnie beats it on acceleration lol

[wiswind]

Cool front is working its way toward you......it moved into Wisconsin a little later than expected....but it feels MUCH better now than it did earlier.

If you have a 1999 or newer windstar, do not forget the isolator bolt TSB........folks have posted that this vaccum leak can be worse when cold.

[Windstartled]

Originally Posted by wiswind

Cool front is working its way toward you......it moved into Wisconsin a little later than expected....but it feels MUCH better now than it did earlier.

If you have a 1999 or newer windstar, do not forget the isolator bolt TSB........folks have posted that this vaccum leak can be worse when cold.

Cool front will be greeted with open arms and open windows yay!!!

Isolator bolt check added to to-do list, thank you

[Windstartled]

I believe I have finally narrowed down the list of vacuum leak suspects to one: the brake booster itself. I have noticed that if I turn the engine off while stepping on the brake the pedal immediately becomes stiff as rock. This is not supposed to happen, normally the brake booster should hold enough air for a few assisted applications even after the engine is shut off. My theory is as follows: if the booster's diaphragm is ruptured but the booster casing is intact, power brakes will still operate but only as long as there is "live" vacuum (i.e. engine is running). Loss of diaphragm should normally prevent the booster from keeping any "reserve vacuum" once the engine is turned off.

As I was expecting a new brake booster costs a small fortune, so I guess a trip to the junk yard is in order.

[akakwo]

Instead of purchasing a brake booster at the moment, I think you should be able to remove the vacuum hose from the booster, plug it, and let it run. The hose does not need to be connected to the brake booster to run the engine. If the engine runs well, problem found. If not, back to the grind stone.

[Windstartled]

Originally Posted by akakwo

Instead of purchasing a brake booster at the moment, I think you should be able to remove the vacuum hose from the booster, plug it, and let it run. The hose does not need to be connected to the brake booster to run the engine. If the engine runs well, problem found. If not, back to the grind stone.

I purchased two boosters

But those are junk yard units at $8 apiece, and I will do as you say prior to installing anything

[scubacat]

If none of this proves to be the issue, I'd check the motor mounts next. Broken or worn motor mounts often exhibit symptoms that can be difficult to pinpoint.

[Windstartled]

Originally Posted by scubacat

If none of this proves to be the issue, I'd check the motor mounts next. Broken or worn motor mounts often exhibit symptoms that can be difficult to pinpoint.

Interesting. It just so happens that the motor and tranny mounts are due for replacement, one is obviously on life support. I keep postponing this job because it involves using an engine cradle and special jacks. Also the mount and subframe bolts were unremovable even with a 42-inch breaker bar but I purchased an IR air impact wrench and it appears to have no trouble at all spinning off those rust-welded nuts, judging from a few trial runs. Still, it's a day-long job I would guess.

With regards to the brake booster I did as suggested and it didn't change anything, even with the booster off the vacuum stepping on the brakes still causes rough idle that returns to normal as soon as I let go of the pedal. This only happens when tranny is in gear, stepping on the brakes when idling in Park or Neutral has no effect, idle remains normal. It would seem that a vacuum leak can be ruled out as the cause.

[12Ounce]

When the tranny is engaged, the engine twists a bit under load ... not so in N or P. What air leak would be more apparent if the engine twists?

Under load, the idle is slower ... could it be the IAC is at fault?

[Windstartled]

Originally Posted by 12Ounce

When the tranny is engaged, the engine twists a bit under load ... not so in N or P. What air leak would be more apparent if the engine twists?

Under load, the idle is slower ... could it be the IAC is at fault?

Been wondering about the IAC myself. I used to have a rough idle issue only when on N or P so I cleaned the IAC and that fixed the issue. But now that I think about it, this was around the same time the rough idle in D issue appeared. I suppose it's possible I inadvertently performed a "politician fix", solving one problem by creating a new one that's worse