RPM Fluctuations and Engine Runaway at Stops after Start Up

M06Man · **Joined:** Fri May 16, 2014 3:47 pm **Posts:** 5

Hello all,

For approximately a year now my KJ Crd (2005) Sport with 280,000 km (174,000 miles) has been acting up on occasion at the first stop after start up. The jeep will start up, and sound great, no rocking, loud grinding or any of the sort. However, immediately after backing out of the driveway and switching into Drive the engine rpm dives 200-500 rpm then immediately climes back to normal. What happens next is the frustrating part. At the first stop, usually not that far from the house the engine starts to gurgle and rpm dives then rises again. This occasionally leads to a stall unless power is applied and the KJ gets moving again. Today however things were more severe. At a stop sign the KJ rpm dived down to 500 rpm or so then shot up to 3200 rpm for a few seconds. With the torque this engine has the brakes were straining a little to contain it. So I had to switch to Neutral to prevent lunging into the intersection. After 5 seconds it levelled out again and acted like nothing happened so I was able to get into Drive and get home. Today was a severe escalation of this little "hiccups" that causes the occasional stall. Rather then a mild issue the KJ tried to runaway if you will and barely contained itself. Now I am not sure if it was compensating for the dip in RPM and went out of control or what it was. All I am sure of was it was the KJ own doings because when the RPM spiked it was in Neutral with no driver inputs. I would really appreciate some advice on this as finding a KJ mechanic in Newfoundland, Canada is darn near impossible and in a month I have to drive the Jeep 3,000 km back to Ontario.

flash7210 · **Posted:** Thu Mar 17, 2016 3:08 pm

Quote:

However, immediately after backing out of the driveway and switching into Drive the engine rpm dives 200-500 rpm then immediately climes back to normal.

This part sounds like the typical transmission drain back problem.
The solution is to let your engine idle in neutral for a minute after startup, then put it in gear.
Also check your transmission fluid level.

Quote:

At a stop sign the KJ rpm dived down to 500 rpm or so then shot up to 3200 rpm for a few seconds.

This part is weird.
Actual runaway engines a rare.
Best I can tell you is to check your boost hoses for oil and either clean them or replace them.
Wouldn't hurt to clean or replace your MAP sensor too.

M06Man · **Joined:** Fri May 16, 2014 3:47 pm **Posts:** 5

Thanks,

I was not aware of a transmission drain back issue with the engine. However now that you mention it the longer it idles the less likely major engine hiccups are. Also as soon as it hits 1500-2000 rpm on the road its almost like the issue is purged and no rumbling occurs even at the first stop. That or a stall occurs and the engine starts back up and runs great. I think I will have to look at the boost hoses as I am not sure when it was last serviced or changed. The map hasn't been cleaned in a year either so I will look at that as well. Hopefully its an easy fix and this minor stalling inconvenience can be fixed for good.

ebbnflow · **Posted:** Thu Mar 17, 2016 4:24 pm

Next time engine is cold, put it in neutral and let it idle for 30 seconds before you drive off. I bet you won't stall at your 1st stop. This is a common problem. The fix is a new front pump for the transmission.

olypopper · **Posted:** Thu Mar 17, 2016 4:57 pm

" The fix is a new front pump for the transmission."

Not necessarily. Often times the spin on filter within the transmission pan is the culprit for these symptoms as it contains the anti-drainback mechanism. Aftermarket trans filters should be avoided on these transmissions (and most modern Chrysler transmissions in general).

ebbnflow · **Posted:** Thu Mar 17, 2016 7:05 pm

olypopper wrote:

" The fix is a new front pump for the transmission."

Not necessarily. Often times the spin on filter within the transmission pan is the culprit for these symptoms as it contains the anti-drainback mechanism. Aftermarket trans filters should be avoided on these transmissions (and most modern Chrysler transmissions in general).

I'm just going off my personal experience. New filters never fixed that problem on my Jeep.

olypopper · **Posted:** Thu Mar 17, 2016 8:49 pm

ebbnflow wrote:

olypopper wrote:

" The fix is a new front pump for the transmission."

Not necessarily. Often times the spin on filter within the transmission pan is the culprit for these symptoms as it contains the anti-drainback mechanism. Aftermarket trans filters should be avoided on these transmissions (and most modern Chrysler transmissions in general).

I'm just going off my personal experience. New filters never fixed that problem on my Jeep.

I totally understand. The problem most folks run into is that a transmission shop will rarely ever open up a low mileage transmission and just fix what is wrong. They will often require a complete rebuild not fully understanding what the root cause of the failure is. These particular transmissions usually fail from a valve body failure that causes pressure loss leading to torque converter failure. Once the converter fails......it's all over with but the crying.

I'd wager to say that if 90% of the transmissions that have failed were opened up prior to major failure and had a valve body upgrade and improved torque converter installed, the front pumps would last for a very long time. The front pump usually only goes bad from eating debris caused by other components grenading.

papaindigo · **Posted:** Fri Mar 18, 2016 9:10 am

The drain back issue is/has been around for DCJ, perhaps other, vehicles for decades. Essentially the "check valve" in the system that is supposed to keep fluid in the torque converter leaks and over a period of ca. 6-8+ hrs the fluid drains out of the TC which has to be "refilled" by the tranny pump at startup. When that happens the at start up the transmission may or may not engage and if it engages it may or may not do very weird things in the first few hundred yards although in my experience I don't recall the high RPM spike.

Causes on the CRD
1. tranny filters not seated properly - replace/reseat depending on age
2. bad check valve in one of the 2 tranny pan filters - replace may or may not help
3. leaky valve in tranny front pump - replace or see below
4. loose tranny pan valve body side plate - ideally install Transgo kit
5. low fluid - tranny likes to be "really" full of ATF+4 - add fluid

If holding a high idle (say 1200+- rpm) in neutral (not park - fluid circulates faster in neutral than park) cures your problem I'd have the tranny serviced with quality filters; check the valve body side plate, and ATF+4. If that doesn't fix it but the high idle in neutral continues to work you can drive that way for thousands of miles with no harm. Translation I would not leap to replace the front pump as that requires dropping the tranny. However, if you do decide to replace the front pump then I would consider since you are in that far going ahead and replacing the torque converter with the "Euro" one from idParts.

RPM Fluctuations and Engine Runaway at Stops after Start Up

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