Stalls when decelerating

JORUMA216

MEMBER
2003 MERCURY MARQUIS
4.6L
V8
2WD
AUTOMATIC
98,000 MILES

Car has been maintained properly. The following items have been replaced by mechanics in regard to that problem: Oxygen sensor, camshaft position sensor, crankshaft position sensor, throttle position sensor, BDI sensor, MAP sensor, EGR valve and last the PCM unit. You guys are my last hope before I give up with this vehicle. 3 times with different mechanics a scan has been used, with negative results. The motor stalls when the speed is reduced likes traffic lights or intersections and sometimes when I stop completely.

3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

We will give it our best shot. What does the vehicle do when it stalls? Does it hesitate then shut off like it is running out of fuel or does it just stop running like you flipped a switch?

If there are no codes then we are going to need to monitor the live data from the o2 sensors to see if this is an air/fuel issue. If this is normal then we need to suspect the power and ground source to the PCM. If the PCM loses the power or ground basically the injectors and spark plugs just stop firing and the engine shuts off.

Let me know this info and we can see where we go from there. Thanks

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Monday, June 8th, 2020 AT 3:24 PM

JORUMA216

MEMBER
2 POSTS

Thanks Kenny1 for your help. Appreciated it. The vehicle stall like flipping a switch. No hesitation. Just stall all the time at less than 5 mph. Never at a speed more than that. Your idea of the PCM losing power or ground makes sense, but, the warning lights on the dashboard are on. After every stalling, I just, place the car on park, turn the ignition switch off and start the vehicle as nothing happened. The frustrating thing is in the scanning there are no codes and everything seems to work fine. Any other suggestion, please.

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Tuesday, June 9th, 2020 AT 9:25 AM

KASEKENNY

MECHANIC
18,907 POSTS

I can assume you are pressing brakes wen this stalls each time. If this is the case, try finding a parking lot where you can try this safely. Drive the vehicle under 5 MPH where it would stall and press the brake pedal real hard to jerk it to a stop. See if will stall each time doing this. I am wondering if we have a brake booster vacuum issue.

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Tuesday, June 9th, 2020 AT 7:15 PM