2000 Jeep Grand Cherokee / 4.7 liter engine/ stalling near idle speeds

This car has 245K miles on it. At very low speeds of maybe 10-15 mph, or
when stopped at a stop sign, the engine will stall. No problems on initial
crank, and NO problems at any speed other than at or near low idle. Engine
will always restart, but sometimes hard to get into gear again and get
going, as it stalls again at such low speed. So, to get it going, I have to
keep the idle up to about 1000 rpm and then shift it real quick to drive,
and take off.

I have noticed however, that in order to get it cranked again, I have to
hold my accelerator pedal down to the floor. This has **never** been
needed before, either when the engine is cold is hot.

Put in new CPS about a year ago. I am NOT a mechanic, but this seems to
me like a fuel related problem, such as a fuel filter or fuel pump. Is it
hard to "test" a fuel pump ?

Please give me some tips on this so that I can better discuss it with my
mechanic.

Remember, no problems EVER above 20 mph, and no problems ever on initial
crank.

Thank you !!

James

Vacuum leak?

James wrote:
> This car has 245K miles on it. At very low speeds of maybe 10-15 mph, or
> when stopped at a stop sign, the engine will stall. No problems on initial
> crank, and NO problems at any speed other than at or near low idle. Engine
> will always restart, but sometimes hard to get into gear again and get
> going, as it stalls again at such low speed. So, to get it going, I have to
> keep the idle up to about 1000 rpm and then shift it real quick to drive,
> and take off.
>
> I have noticed however, that in order to get it cranked again, I have to
> hold my accelerator pedal down to the floor. This has **never** been
> needed before, either when the engine is cold is hot.
>
> Put in new CPS about a year ago. I am NOT a mechanic, but this seems to
> me like a fuel related problem, such as a fuel filter or fuel pump. Is it
> hard to "test" a fuel pump ?
>
> Please give me some tips on this so that I can better discuss it with my
> mechanic.
>
> Remember, no problems EVER above 20 mph, and no problems ever on initial
> crank.
>
> Thank you !!
>
> James
>
>

James wrote:
> This car has 245K miles on it. At very low speeds of maybe 10-15
> mph, or when stopped at a stop sign, the engine will stall. No
> problems on initial crank, and NO problems at any speed other than at
> or near low idle. Engine will always restart, but sometimes hard to
> get into gear again and get going, as it stalls again at such low
> speed. So, to get it going, I have to keep the idle up to about 1000 rpm and then shift it real quick to drive, and take off.
>
> I have noticed however, that in order to get it cranked again, I have
> to hold my accelerator pedal down to the floor. This has **never** been needed before, either when the engine is cold is hot.

First off you need to pull the engine codes.
http://www.wjjeeps.com/faultcodes.htm
By the way, that's also a good site for information.


WOT (Wide Open Throttle) signals the ECU to go into "bypass mode"
meaning a sensor is failing. But not always.

How dirty is the throttle body and the idle-air-passage?
From a 4.0. http://revbeergoggles.com/tb/tb.html
If that gets crudded up it can cause stalls.

Will the Jeep start with just a light amount of depression on the
gas pedal? If so then I'd suspect the IAP. This also makes the
engine run rough.

> Put in new CPS about a year ago. I am NOT a mechanic, but this
> seems to me like a fuel related problem, such as a fuel filter or
> fuel pump. Is it hard to "test" a fuel pump ?

It could still be the CPS.

Fuel filter and pump commonly cause pinging at high RPM and power sag
on the highway.

Another possibility is a failing or weak coil or in the older system,
bad spark plug cables. Even a bad plug. But that usually causes a
stammer/shake and a WOT start won't help.

> Please give me some tips on this so that I can better discuss it with
> my mechanic.
>
> Remember, no problems EVER above 20 mph, and no problems ever on
> initial crank.

Could be the CPS plug is dirty. Follow the CPS lead up to the connector
and disconnect/clean/reconnect.


--
-- DougW -- 93 ZJ 4.0 http://revbeergoggles.com
HESCO Supercharger - 300W IASCA Stereo - Edelbrock IAS Shocks
Gibson Exhaust - rear DCpower - custom gauge install - Stillen Rotors
Banks Header - and BEER, in the fridge!

Thanks Doug and Scott, good ideas !!


Any other comments/ advice from the group ?

James

DougW wrote:

> James wrote:
>> This car has 245K miles on it. At very low speeds of maybe 10-15
>> mph, or when stopped at a stop sign, the engine will stall. No
>> problems on initial crank, and NO problems at any speed other than at
>> or near low idle. Engine will always restart, but sometimes hard to
>> get into gear again and get going, as it stalls again at such low
>> speed. So, to get it going, I have to keep the idle up to about 1000 rpm
>> and then shift it real quick to drive, and take off.
>>
>> I have noticed however, that in order to get it cranked again, I have
>> to hold my accelerator pedal down to the floor. This has **never** been
>> needed before, either when the engine is cold is hot.
>
> First off you need to pull the engine codes.
> http://www.wjjeeps.com/faultcodes.htm
> By the way, that's also a good site for information.
>
>
> WOT (Wide Open Throttle) signals the ECU to go into "bypass mode"
> meaning a sensor is failing. But not always.
>
> How dirty is the throttle body and the idle-air-passage?
> From a 4.0. http://revbeergoggles.com/tb/tb.html
> If that gets crudded up it can cause stalls.
>
> Will the Jeep start with just a light amount of depression on the
> gas pedal? If so then I'd suspect the IAP. This also makes the
> engine run rough.
>
>> Put in new CPS about a year ago. I am NOT a mechanic, but this
>> seems to me like a fuel related problem, such as a fuel filter or
>> fuel pump. Is it hard to "test" a fuel pump ?
>
> It could still be the CPS.
>
> Fuel filter and pump commonly cause pinging at high RPM and power sag
> on the highway.
>
> Another possibility is a failing or weak coil or in the older system,
> bad spark plug cables. Even a bad plug. But that usually causes a
> stammer/shake and a WOT start won't help.
>
>> Please give me some tips on this so that I can better discuss it with
>> my mechanic.
>>
>> Remember, no problems EVER above 20 mph, and no problems ever on
>> initial crank.
>
> Could be the CPS plug is dirty. Follow the CPS lead up to the connector
> and disconnect/clean/reconnect.

My 4.0 (and several GM products) had more trouble with the TPS (sensor or
connector) than anything else at idle but I know several mechanics who make
a practice of cleaning the IAC port and pintle as the very first step.

--
Will Honea

James wrote:

> Any more ideas here ? Why is it more severe when engine is cold, first
> thing in the morning. I am not a mechanic, but it seems that should be a
> big hint....

Maybe a clue here - it took me months to get an 88 MJ to idle right with
similar cold clues. When I had tried everything else, I changed out the O2
sensor. I didn't think about it causing the idle issue but the tailpipe
told me that it was running rich at idle and changing the sensor did the
trick. I've since learned a bit more about the O2 sensor and the one I have
has a heater in it that can fail. The sensor will work fine once it heats
up but without the heater element working it takes a while for the exhaust
heat to do the trick.

I still have a nuisance with cool (not cold) days. Seems like as the
weather warms in the Spring and cools in the Fall there is a range where the
!@#$% thing dies at the first stop sign. Starts fine, idles fine but that
first stop kills it. All is well if it's warm or cold but that in-between
temp is a PITA. I still can't figure that one out but after 10 years I
guess I can live with it.

This is an outside WAG but you've tried everything else so look for
smoke/soot on that cold start that would indicate a rich mixture and a lazy
O2 sensor. Easiest way to see it is to start it when it's still dark out.
Put a strong flashlight or spot light shining through the exhaust stream and
it will make it easy to see the fine emissions.

--
Will Honea

James wrote:
> Update:
>
>
> Auto shop did a scan with OBD tool, but it pulled no codes.

Not everything will store a code. So when you have problems
and no apparent code it's time to check them.

- weak O2 sensor (usually also gives you crappy gas mileage)
- spark/coil/plugs (no sensors on them)

> weak coil/ and/or bad spark plug or cables/ ?? has original coil

I'd zero in on this next.

There is a test device for coil strength.
http://www.toolssuperstore.com/produ...roducts_id=773
Several variants on that.

The problem is you have COP (Coil On Plug) so the ol standby
won't help. You can install one of these between the COP
and the spark plug but there is an easier way.

So let's get low tech on it's ass.
http://www.youtube.com/watch?v=pxRx31f6BXQ

The high tech method. Most folks don't have this at home.
http://www.youtube.com/watch?v=n5W2DXboy7M

--
DougW

Thanks, Will and Doug !!

James