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Mechanics can't figure it out!

3 mechanics are stumped, can you help?

-1999 mercury sable wagon/ 80k miles

-engine stalls only on hot days (85 degrees)

-only stalls after highway driving of at least 30 minutes

-never stalls at high speed, only after exiting freeway and coming to idle

-after stall, will not start for 30 minutes; engine cranks fine; gets closer to “catching” the longer it sits

-engine runs smoothly otherwise

-spark and injector voltage normal even when stalled

-fuel pump looks fine on scope

-fuel pressure normal when engine running

Really appreciate any help you can provide!


You need to be able to diagnose the car when it is not running, It needs to be in failure mode to find the problem. You have a good idea of when it fails, try and stop it at the dealer while failure mode is probable, then if it fails and then they can isolate the problem. my guess fuel pump related.

What kind of deal will you make with a mechanic that takes the car on? What I am getting at is these three “stumped” mechanics may be looking at the money side of the deal and decided to “pass” on this one. you yourself can see there will be alot of test driving to duplicate and confirm a repair is a good one, will you pay the going rate ($80.00-110.00 for this service?).

It’s is nice to be able to say “no thanks” to a car that you probably will not break even on.

you’re right. problem has been that I have not been able to get it to fail at the shop. I need to wait for a very hot day and time everything just right. I was hoping to narrow down the list of causes so we can diagnose it in the 20 to 30 minutes we have before it starts running again. Frankly, it does nave symptoms of fuel starvation, but the pump looks good on the scope. could it still be the pump? thanks for your input.

what I’ve done with the repair shop is agree that I will do all the driving and get it to fail at their place. that really keeps their cost down to a regular diagnostic process. they are being very helpful but need to get their hands on it when its not running. I’ve not been able to get it to fail at the shop yet, and was hoping to narrow down the list of causes so I can compare that to how the shop is doing diagnostics and have some confidence they have the skills to fix this one. thanks for your input.

After 3 tows to the dealership and then started right up with my wifes ford I had her stop by the dealer every time she passed by as we had the warranty. It failed at the dealers, they were pre warned if it stops get out there fast,they diagnosed fuel pump replaced it and have been fine since, except for the passenger window and the backup sensors,

Yes I can see you want to prep them the best you can, it is good they are willing to take this on, that is their job, but sometimes you want to pass on a problem car and just work,others may relate to the same feeling in their chosen fields,getting held back in your day by a problem car does not feel good.

Your post is confusing. If no one has checked it while it is stalled, how do you know the “spark and injector voltage normal even when stalled”? “Frankly, it does have symptoms of fuel starvation, but, the pump looks good on the 'scope.” What are the symptoms of fuel starvation?
Can you be a participant in the troubleshooting? When the car stalls, can you spray Starting Fluid into the engine intake tube? I’d ask you to use a fuel pressure test gauge and actually measure the fuel pressure when it doesn’t restart; but,
A little spark induced test lamp check would be nice, also.
A check with a fuel injection 'noid test lamp would yield more useful information.

It seems that not one mechanic in the whole country has one of these diagnostic aids, which is the ideal tool for your kind of problem:

Just offhand, it sounds like the Idle Air Valve needs to be cleaned or replaced but you might clarify the part about not starting.

When it stalls are you attempting to restart it without depressing the accelerator pedal? If so, this could point to the IAC which can be problematic without leaving any codes.

Kit you are right on, but to tell you the truth when ours was in failure mode I was like man I have a warranty, let them fix it. A few times we did not have the tow she left it and it started right up for me, or I jumped it and it started fine, It was a very frustrating ordeal and I could not help the oh right glance when I got there and all was fine, but there was a problem that was solved as stated previously.

I understand. thanks for the responses.

The trouble may be with the fuel pump relay. Carry a can of starting fluid with you so when the trouble happens again you can spray some fluid into the intake and see if the engine runs. If it does, you know it is a fuel delivery problem.


per instructions from my shop, I checked for spark and used a noid lamp to test it when it was stalled and wouldn’t restart. the reason I say starvation is that when it stalls, the rpm wavers for about 5 to 10 seconds then it dies. when you first try to restart it just cranks, nothing more. after waiting 5 to 10 minutes, it will sputter and fire a few times, then just crank. after 20 minutes, it will sputter longer. so the more time it has to cool the more it sputters and gets closer to starting. finally after 30+ minutes it will restart like normal. I know that the pump is a likely suspect but my shop is trying to save me from spending $700 only to find out its something else - they feel its not the pump because the pump current waveform looks so good on the scope.

I did try the gas cap already. no change. I will try the starting fluid. great idea. thanks.

thanks for the suggestion. when I attempt to restart I do depress the accelerator as I would for a normal cold start.

I bought a replacement relay and will install it next time the car dies.

Sad to see you throwing parts at an unresolved problem, the possibilities are possibly potentially expensive and unnecessary, find the problem and fix it you can otherwise waste tons of money and time repairing possible problems, and if that is what you are on board with that is cool, but I prefer a proper diagnosis and repair to save wasted dollars.

Good idea. Cheaper and more convenient than buying a fuel pressure gauge to check the fuel pressure when the car isn’t starting. If the starter fluid somehow gives ambiguous results, a fuel pressure gauge is a possibility if the Sable has a Schraeder valve to attach the gauge to. A gauge that has to be plumbed in might take so long to install that the problem may fix itself before the fuel pressure can be measured.

Why would you depress the accelerator at any time when starting the car? Did I miss something in an earlier post? This vehicle has fuel injection, not a carburetor. Depressing the accelerator while starting hasn’t been necessary since the late 1980s.

Anyone think the problem may be the infamous Ford ignition module problem?