Evening everyone, I've got a 1996 XJ6 Vanden Plas 4.0 that is having a weird issue with dying after idling or driving for a bit. It has new injectors, spark plugs, fuel pump and filter, and I've been swapping out between different ecus and ckps's, no change. O2 sensors all unplugged to isolate, no change. Unplugging maf sensor and it dies immediately or won't start at all. I believe it may have a clogged upstream cat as it smells like firecrackers and has uneven temps fore and aft of said cat. Would that cause stalling though? And why does it idle so beautifully for 15-30 mins before dying? Any ideas on what to test for at the sensors?

 

EGR Valve perhaps?

Unplugging MAF sensor should kill the engine.

Without 02 sensors, fuelling stays in open loop. Its fueling off of a program instead of sensors. I wouldn't unplug those to find the problem.

Do you have a check engine light on? Can you read the codes.

 

I removed the EGR valve recently and there slwasnt much carbon buildup, way less than the last XJ6 I owned, and I also swapped it with a valve from said previous XJ6. I have also run it disconnected without change, same issue.

I'll hook the O2 sensors back up and check for changes.

No codes currently besides a stored P0727.

 

With a good working catalytic converter, the temperature at the outlet should be significantly hotter than the temp at the inlet, typically at least 100° Fahrenheit higher. If the outlet temp is the same as the inlet temp, the cat is not lighting up. If the cat is obstructed, it can be tested for back pressure with a gauge that threads into the upstream O2S port. You may be able to rent a gauge set from a local auto parts store.


Here is the definition of P0727 from the ZF 4HP24 Transmission Diagnostic Trouble Codes Summaries manual. I don't believe a loss of engine speed signal at the Transmission Control Module will cause the engine to stall. More likely, the engine stalling is triggering the P0727 code:




I recall that with the 4HP24 transmissions, engine stalling when slowing to stop or make a turn could be caused by low transmission fluid. My assumption is that the torque converter was not releasing properly. Have you checked your fluid level?

Another possibility could be a failed fuel pressure regulator (FPR). One quick check is to pull the vacuum hose off of the regulator, crank the engine and look for wet fuel at the vacuum hose fitting on the FPR, which indicates a ruptured diaphragm.

Cheers,

Don

 

May be due for a main fuel filter change to ensure the fuel pressure at the idle is enough

And Dons mentioning of the Trans / Engine talking to each other as they dance together , clean the on the transmission body electrical connector , do not just twist off , trick involved

To know the O2 sensors and cats are regulating the total package correctly would be a short term fuel trim target 0 % ( this is a sample of the exhaust products ) , there is 1 fuse for all 4 ( sometimes 2 in Europe ) in the sensors heating elements for them to read correctly ( sensor physics )

The hook up of the O2 sensor connectors can be crossed that has caused others problems that do resolve on correct hook up

The O2 sensors return wire does have a needed ground on the rear engine firewall to protect the signal

The engine ECU does have a dedicated external ground strap for further protection

And the engine to starter to frame large ground strap , the frame surface condition effects many different sensors , the strap itself should be fine

By swapping ECU it is luck of the draw that the throttle position sensor at idle is a " match " with the different ECU , this match is referred to as " orientation " and is the special electrical test equipment adjusting the ECU to the TPS " null " or idle reading , on paper this is 0.60 volts DC on the middle wire with the TPS still connected

Someone by cleaning the EGR did recover the Idle and may have to consider the Rochester valve in the EVAP system

 

I had the same problem one year ago. I try to change everything. Finally i solved the issue when i replaced the O2 sensors.