Stalling Jeep 5.2, any ideas?

[HENCHR]

My Girlfriend has a 94 Grand Cherokee with the 5. 2 in it. Just recently it has started to have trouble with idling. When you first start it, the engine will go up to about 1200 rpm and then drop almost all the way down to zero, vibrate and then level off and idle, you can give it gas and it picks up fine but when you let off it drops all the way back down again before returning to its normal idle. When your driving it, it seems to run fine until you stop, then the rpm drops all the way down and will even stall sometimes and if it doesn't stall the idle is pretty rough but maintains about 500 rpm. I've also noticed that the fumes coming off of it are unbearable, you can't stand next to the thing without it burning your eyes. The computer isn't throwing any codes either which is kinda strange. I was thinking the stalling and rough idle might be the idle air control valve and a dirty throttle body, but I'm not sure about the fumes. I replaced the plugs, wires and cap, but that didn't improve anything with the idle. Anyone have any ideas?

 

[jgillott]

I have a '96 GC 5. 2L that is filled with wierd problems, so I've had a fair amout of experience dealing with these type of issues.



What is the wierd smell that it has? Does it have a sulfur smell to it? My first guess to your problem is a plugged CAT. When they plug on these vehicles, they tend to throw the ECM into a rich condition that for some reason messes with the idle speed. Along that same line, check the condition of the O2 sensor, which could be effecting the condition.



If the first item checks out ok, then you are probably seeing a BCM (body control module) failure. Now I know that it doesn't make sense that the Idle speed would be effected by something other than the ECM, but on these crazy things, the BCM also supports engine Idle and engine RPM. The following is taken from pg4 of the 1994-1997 Jeep GC electronic service manual... ...





"The body control module provides or supports the following features:





Intermittent Wipe control

Door, Hood, Liftgate, Rear Glass Ajar Status

Reading and Transmitting of External Temperature

A/C Select Switch Status

Chime

Courtesy Lamps (including halo) with time-out feature

Automatic Headlamp Control

Headlamp Time Delay

Automatic Funeral Mode (with automatic headlamp only)

Gulf Coast Country Overspeed Warning

Illuminated Entry

Instrument Panel Dimming

Vehicle Theft Security System with Panic Mode

Door Lock Inhibit (key in ignition or headlamps/parklamps on)

Electronic Odometer Support

Brake Pressure/Park Brake Switch diagnostics

High Beam Indicator

Seatbelt Reminder

Speed Sensitive Intermittent Wiper Control

Fog Lamp Control

Remote Radio Controls and Radio Support

Mechanical Instrument Cluster Support

BCM Diagnostic Support

VIC (vehicle information center) support

Liftgate Courtesy Lamp Disable

Rear Window Defogger Control

The BCM receives information over the CCD Bus from the PCM in order to support certain features. The required information is as follows:





Engine RPM

Injector on Time and Distance Pulses

Vehicle Speed

Engine and Battery Temperature

Charging System Failure, Engine Temperature Limp-In, VTSS Arming Status

Engine Model

"Check Engine" lamp status

The BCM provides the PCM with information on the A/C switch status and the `OK to Start' or VTSS status. "



Hope this helps... .



Let me know if you have any other specific questions.

 

[B.G. Smith]

Also might want to check the EGR valve. bg

 

[HENCHR]

The smell is not a sulfur smell, it just running really rich I guess, smells like my old dodge farm truck that has no exhaust on it. I was also thinking the O2 sensors as well, but since the check engine light never came on I started to think of other things, but now that you mention the BCM controls the light who knows, it could be the BCM, I suppose I could switch the one from my 97 into and see if that makes a difference, should be similar, they didnt change a whole lot. Thanks for the info.

 

[rcaldwell]

Sounds like the idle Air control valve is worn out. this can be checked with a scan tool. The tool should tell you how far it has to open to keep the car running. I belive anything over 40% is bad i am not sure of this number. But the sympotms you give sounds like the IAC motor. hope this helps



Bob

 

[MKellam]

Hench, how is the condition of the battery? Does it appear to crank a little slowly? I would take a standard voltmeter and connect it to the battery. Monitor the voltage while cranking and see what to total voltage drops to. If it falls near or below 9 volts when cranking, then it is possible that the PCM is losing memory when cranking. Tow of the MAJOR sensor/actuator memories are the IAC (Idle Air Control) motor STEPS and the ADAPTIVE MEMORY.



First the IAC- these 'steps' tell the PCM where to put the pintle to allow a calibrated amount of air at start up. It work exactly like a choke on a cold start, but also as a 'throttle stop' to rpevent the vehicle from stalling when put in gear or when the A/C comes on. If the battery is starting to go, then the PCM will 'reset' and lose the IAC memory. So it has to learn the position all over again. It will cycle the engine speed high to prevent a stall and GRADUALLY, and I mean GRADUALLY lower the RPMs to as low as it can go before stalling to establish a 'range' of steps and recalibrate. Now if the engine stalls when it drops low, then you recrank, and since the battery is already low, you start the viscious cycle all over, only worse, because the engine didn't run long enough to even begine recharging the battery after the first start, so now your battery state of charge is even lower. See the problem here?



The same thing will happen with the O2 sensor adaptive memories. By default, when you disconnect the battery power to the PCM, the memory is reset ro a default enriched value to prevent the engine from running lean and causing damage. At which point, the PCM will monitor the exhaust stream and 'trim' the fuel to achieve the most efficient engine operation based on Oxygen content in the exhaust stream. Also, you can easily check the O2 operation by using your trusty DVOM. Backprobe the sensor signal circuit and watch the voltage switch. You are looking for a 0. 2-0. 9V signal that 'switches' back and forth above and below 0. 5V. SOrry to be so long winded, but I hope this helps solve the dilemma.



MKellam