Unstable idle with Drive on

Topan · **Posted:** Sun Sep 28, 2014 8:22 pm

I noticed this week end that my idle is fluctuating from 750 to approx. 900 rpm non stop.
It happens only when I am in Drive , not in Neutral or Park.
Otherwise, idle is smooth and power is normal when driving.
I have a fuel pump and a new generation fuel filter head.
Any idea ?

Auberon · **Posted:** Mon Sep 29, 2014 4:36 am

Probably a bit obvious to ask this one but how long ago (distance) was it that the fuel and air filter elements were changed.
Given it only happens in drive - am I correct in assuming it does NOT happen when reverse is selected?
On that - how recently was a service done on the auto trans and were ATF +4 and Mopar filters used?

Topan · **Posted:** Mon Sep 29, 2014 12:11 pm

My KJ has 143k miles.
The fuel filter was upgraded to the new version about 50k miles ago.
Last trans fluid, filter and grid replacements about 1000 miles ago. I used ATF4+.

One thing to mention is that my transmission has been overheating since 50k miles ago, approx.
I have mounted a by-pass trans cooler since then.

No ideas if this unstable idle is linked to the trans,,,,,is it ?

I will test if the problem is only in D , or as well in 2, 1 and reverse.

Auberon · **Posted:** Tue Sep 30, 2014 2:33 am

Hi. It could, of course, be the transmission. Could be anything.
Please forgive the obvious nature of this post but it is where to start, I guess.

Note you wrote ATF 4+. It needs to be ATF+4.....I'm not being picky but the tranny has a tendency to be that way. I trust this is a typo. If not, that could well be the cause.
It could well be as simple as that.
Failing that, it could also be one of a couple of dozen things ranging from t/f case viscosity getting up there if this hasn't been changed to diff pumpkin fluids becoming more viscous with age/distance or ..........sheesh it goes on into a bit of a headspin really as to potential causes.
It will be interesting to read your evaluation of other gear lever selections.

Perhaps you could look at the most obvious first (the tranny fluid you used ) and then if I'm off the beam there, do a quick bleed of the fuel system.
I'd then be suspicious of the fuel filter head not being faulty but just having a little air accumulated. It could well draw through as you get it going (wound up) unless the hunting is present even after a run at speed or is it just after a period at rest?
Again an obvious one....Can you pull codes?
Did the elevated temperature issue creep in over time or did it happen quickly?
These are all part of the info needed to offer informed help to you.

Hope that gives you some directions to explore in a good way.

flash7210 · **Posted:** Tue Sep 30, 2014 8:33 am

There are a couple of things that can make your idle speed go up and down.
1. Air conditioning: as the compressor cycles on and off the idle speed may go down or up
2. Viscous heater: same as the AC, but should stop once things warm up
3. Power steering pump: usually you would only notice this while turning the steerin wheel

In all three the cases the ECU is supposed to compensate for any of the above actions and maintain a steady idle.
I kinda suspect its the pressure sensor on the power steering pump. If that sensor were sending a false pressure indication, the ECU might be trying to compensate for additional load that isnt there.

But i could be totally wrong about all of this. I just dont see how it could be the transmission. Once put in D, the RPM should go down sligtly and stay down. If RPM comes back up then that would mean the transmission lost engagement. If your trans fluid is full then the only other possibility is a sticky valve or solenoid in the valve body.

Topan · **Posted:** Wed Oct 08, 2014 7:09 am

Auberon wrote:

Hi. It could, of course, be the transmission. Could be anything.
Please forgive the obvious nature of this post but it is where to start, I guess.

Note you wrote ATF 4+. It needs to be ATF+4.....I'm not being picky but the tranny has a tendency to be that way. I trust this is a typo. If not, that could well be the cause.
It could well be as simple as that.

.

I have used the Mopar ATF+4 transmission fuild. i am still very surprised that i have reached the max level and i have already poured 8 liters after my fuild replacement.
I have now an extra liter at home, i will add it up.

Topan · **Posted:** Wed Oct 08, 2014 7:11 am

Auberon wrote:

Hi. It could, of course, be the transmission. Could be anything.
It will be interesting to read your evaluation of other gear lever selections.

The idle is unstable in all gears except neutral and Park.
It does it only when the engine is at running temp.
It does it very ramdomly, sometimes it is stable, sometime not at all....

Topan · **Posted:** Wed Oct 08, 2014 7:14 am

[quote="Auberon"]Hi. It could, of course, be the transmission. Could be anything.
Did the elevated temperature issue creep in over time or did it happen quickly?
quote]

yes it did dome out suddenly. During one off-road trip, i took a rocky uphill path and the gauge went suddenly in the red.
Before that the neddle never passed the 11:58 mark...

Topan · **Posted:** Wed Oct 08, 2014 7:22 am

flash7210 wrote:

There are a couple of things that can make your idle speed go up and down.
1. Air conditioning: as the compressor cycles on and off the idle speed may go down or up
2. Viscous heater: same as the AC, but should stop once things warm up
3. Power steering pump: usually you would only notice this while turning the steerin wheel

In all three the cases the ECU is supposed to compensate for any of the above actions and maintain a steady idle.
I kinda suspect its the pressure sensor on the power steering pump. If that sensor were sending a false pressure indication, the ECU might be trying to compensate for additional load that isnt there.

But i could be totally wrong about all of this. I just dont see how it could be the transmission. Once put in D, the RPM should go down sligtly and stay down. If RPM comes back up then that would mean the transmission lost engagement. If your trans fluid is full then the only other possibility is a sticky valve or solenoid in the valve body.

1. It happens even when the aircon is off
2. do you mean the clutch for the main fan ? I have just repalced it by a heavy duty Hayden
3. It happens even when i don't touch the steering wheel

True that i have just replaced the transmission fuid and have still not reached the max level. I have now added 8 liters and will add another one.
This is how I measure the trans fuild level, is it correct ?
Car in Park + engine heat-up at running temperature + engine on, iddle during min. 60 seconds + flat horizontal surface ==> the level on the dipstick is still below min after i have added 8 liters of ATF+4.

Auberon · **Posted:** Sat Oct 11, 2014 6:44 am

Per the book:
For fill tube equipped with indicator (read Diptsick with indicator marks):
1 Start the engine and apply the parking brake.
2 Shift the transmission into DRIVE for approximately 2 seconds
3 Shift the transmission into REVERSE for approximately 2 seconds
4 Shift the transmission into PARK
5 (It says to hook up a / the scan tool and select transmission)
6 Select sensors
7 Read the transmission temperature value
8 Compare to the reference chart.
9 Adjust transmission fluid level shown on the dipstick according to the Transmission Temperature Reference Chart.

For those in the real world:
Given not many can pull the temperatures of the transmission....normal running temperature is good.

You would read it on a level surface, normal running temp, select the gears DRIVE, REVERSE, then PARK and Check against the Dipstick.
It can also be a benefit to select each gear in turn but D & R the P should do it.
It must be shuffled back and forth through the forward and reverse selectors to fill the TC and valve body.
I like to leave the motor idling.
If any fluid is added, then allow at least 2 minutes for the fluid to drain into the pan before re-checking the level against the dipstick.

papaindigo · **Posted:** Sat Oct 11, 2014 11:38 am

Topan - just a reminder even with AC off the AC compressor cycles in several vent modes to keep cabin air dry, check the Owner's Manual for which modes or just put the vent position switch to Off and see if the problem goes away.

Auberon · **Posted:** Thu Oct 16, 2014 5:14 am

It sounds like what I would call "hunting" - revs going up and down.
Does it happen as a cyclical surging...is it rhythmic or random in its occurrence?
What kind of order is the fuel filter in?
It could more likely be a clogged fuel filter or air in the fuel system.
Am trying to suggest the more obvious things that spring to my mind (that don't cost the earth too).
The fat that you noted that it cam eon suddenly leads me to write the above - you could've got a bad batch of fuel or two or it could just be time to change it.

The hunting could well be due to the engine being only slightly starved for fuel but not enough to lean out and blow white smoke.

Topan · **Posted:** Fri Oct 17, 2014 7:21 am

papaindigo wrote:

Topan - just a reminder even with AC off the AC compressor cycles in several vent modes to keep cabin air dry, check the Owner's Manual for which modes or just put the vent position switch to Off and see if the problem goes away.

very interesting point , I never thought that feature exists.
however, the ups and downs in the rev, are very regular cycles, very maybe 2 seconds per cycle.
I wonder if the vents modes to keep the air cabin dry hit so frequently. no?

Topan · **Posted:** Fri Oct 17, 2014 7:29 am

i have added another liter of ATF+4 , a total of 9 liters after my oil flush.
The ups and downs are still here.

Strangly , yesterday night i tried to purge the air out of the filter head.
The pump was so hard that i could not give it a full compression.
Then I opened the valves to let the air out, nothing hapenned: no air , no diesel....
Tried to repeat the operation several times : same thing...nothing out :shock:

next thing i will replace my gas filter.
True that when I installed my intank pump, a bit of dirt fell into the gas tank...

Auberon · **Posted:** Fri Oct 17, 2014 10:18 am

Hmmm. That last point is very curious. Maybe the search for the culprit is going in the right direction now. It is a guessing game for us on the other side of the country or world.
Hope you pin it down now.
Please keep us informed.
I'm sure we can come up with other suggestions if this doesn't pan out.
Good luck.

greiswig · **Posted:** Fri Oct 17, 2014 11:20 am

Topan wrote:

Strangly , yesterday night i tried to purge the air out of the filter head.
The pump was so hard that i could not give it a full compression.
Then I opened the valves to let the air out, nothing hapenned: no air , no diesel....

I think you're supposed to do it the other way 'round: open the bleeder first, engine off, then start pumping. If you do it in the order you describe, and you have no air in the system (i.e. no compressible gas), then I might expect what you describe to happen.

FWIW, the "hunting" that Auberon described above... if it is as regular as you describe, it does sound electrical, or like something the ECU is telling the system to do. I've seen a throttle position switch (TPS) go bad and cause this, but I don't know if the CRD has a potentiometer like that associated with the pedal, let alone how it works at idle.

papaindigo · **Posted:** Fri Oct 17, 2014 12:20 pm

Sorry but no you do not open the bleeder first; see https://www.youtube.com/watch?v=qPwGcWi3gE0 for video on how to bleed although rather than use paper towels I prefer a bit of vinyl tubing over the bleeder running to a catch can.

The primer pump essentially pulls fuel from the tank and pressurizes the fuel filter, before you ask yes there is a check valve that keeps fuel from immediately running back into the tank. Cracking the bleeder screw releases that pressure by discharging whatever (air or fuel or a mixture) is at the top of the fuel filter head. So bleed as follows a) pump primer until firm; b) crack bleeder; c) close bleeder; d) repeat those 3 steps until all the air is out of the system.

If no fuel comes out when you crack the bleeder either a) the primer pump is not building pressure or b) the bleeder port is clogged. There has been at least one case of the primer pump check valve failing, in which case the pump never gets hard, and on case of the "O" ring or whatever that functions as a shaft seal on the primer pump failing, in which case fuel comes out around the primer pump shaft. I am not at all sure how or if the pump could fail in a manner where it would appear firm but not actually to have built pressure in the fuel filter head. The bleeder is identical to a brake bleeder; hollow inside/"V" shaped end matching and inverted "V" seat in the fuel filter head/small hole in the "V" thru which fuel runs when the bleeder is unscrewed. If that hole or the passage to the inverted "V" seat get gummed up the bleeder won't work until the passage(s) are cleared. Remove the bleeder screw and clear passage(s) with something like a pipe cleaner.

AC compressor cycles, even if AC switch is off, when the vent control is in the Floor only setting or in either of the 2 settings with the Defrost symbol although I don't think that would explain your unstable idle. If I have not said it before set the vent control to "O" for Off which should prevent the compressor from running at all and see if anything changes.

PS. I wonder if the primer pump check valve could fail in such a manner as to make the primer fail "hard" such that it won't pump fuel and if so doing could cause a weird idle. Way way out on a speculation limb here but could you rig a temporary filtered (just connect the inflow and outflow lines together with a basic cheap inline filter) bypass of the fuel filter head to run things long enough to see if the idle problem goes away? If it does replace the filter head.

greiswig · **Posted:** Fri Oct 17, 2014 12:54 pm

Thanks for clarifying, papaindigo. Been too long since I had a stock filter head, obviously. On my Racor as well as on my Unimog primer, you want to open the bleeder a bit first, otherwise you risk pressing too hard and blowing out the primer diaphragm.

Auberon · **Posted:** Sat Oct 18, 2014 9:35 am

Agree with Papaindigo.
Topan, you may also consider draining the contents of the fuel filter into a clean container and examining.
and releasing the nipple, placing some clear plastic hose on it and pressing the primer button in one direction only then closing the nipple off before releasing (much as you would bleed brakes on a car) and see if that frees it all up. Can't go past Papaindigos' suggestion of the pipecleaner though.
Also would like to reiterate the points of a bit of plastic and into a bottle to catch the bled-off fuel rather than cloth or paper towel method.
I was kind of encouraged by the revelation that it was quick at onset and that it's persisted for while plus the history although it would be much better for the problem not to be bugging you.
So it's a matter of working with the fuel head/filter etc and then moving on to other possibilities.

Can't see the possibility of the ancillary items developing enough synchronicity to cause the hunting long-term.

Topan · **Posted:** Fri Nov 07, 2014 3:13 am

Sorry guys, I was out a few days. I made a full service on the kj.
Replaced engine oil, (10w40), oil filter, air filter and....fuel filter.
After replacing the fuel filter , I bled the air out.
This time , I could pump the primer till hard. I did the bleeding (pumping then air out through the bleeder) 4 times before a regular flow of fuel went through.

The engine run much more smoothly now, but ....the irregular idle is still here.
It is much less obvious, but it's still here.

Papaidingo, first , thanks for the detailed explanation on the bleeder and fuel filter head function.
I will try to bypass the entire fuel filter head and post the results.
It is a new generation that I installed already sometime ago, approx. 2 years ago.
Some of my friends here directed me more towards a leaking pipe on the injection, or on the vacuum lines.

Unstable idle with Drive on

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