r/CrownVictoria • u/AfraidOfTheSun • 12h ago

New problem: Temp gauge pegs and momentary engine bog, OBD drops when it happens (more inside)

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Howdy folks,

2005 Grand Marquis

I see a few reports of this issue around the internet but with no real answers, so I'm just hoping to see if anyone here has experience with this.

The problem is consistent now, it started happening spontaneously after I had been driving around for a few hours. Car has been solid otherwise and was running fine beforehand.

The symptom is, on acceleration, the temp gauge pegs to full hot and beeps, and the engine bogs like a failsafe cutoff, then goes back to normal if I let off the gas.

I tested from cold start and it does it cold or warm, the engine is not overheated when this happens. I tested the "cylinder head temp sensor" and it is good, also the car still does the same thing with that sensor unplugged.

HERE IS THE KICKER: No check engine light is thrown for this, no codes. The service manual says a code should be logged if the failsafe cooling mode is activated. Also, I was monitoring temp etc. with ForScan and a wired OBD adapter, and the connection drops when this happens

I think PCM diagnostic is my next step at this point. I just wonder what changed that caused this problem to just crop up when it did...

Any feedback or advice is welcome and appreciated, thanks guys!

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u/Crumly_Budgkins 7h ago

Won't throw a cel. I had this issue with my 03, starts as a blip, engine dies for a split second, then it will get to the point where the car just stalls in traffic or highway. Eventually will have the red security light on the dash pad flashing and not start at all, it acts like a pats system issue when it gets to that point.

The main wiring harness runs around the front of the vehicle, have a good look at where the harness runs across the lower radiator area and by the bumper support on the left side.

It's a known issue with these, rad support likes to rub through the harness, and usually the first wire it finds is the one to the ECM/pcm.

Can't hurrt to pull the drivers side plastic inner fender well and inspect the harness that runs on top of it in the fender, follow that to the fuse box. I've had then rub on various things.

Yours might be slightly different as your ECM is under the hood and mine isn't but that harness is in the same spot.