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Downhill Engine Misfire

2005 Ford Escape, 4-cylinder with ~75,000 miles. I experience engine misfire when driving on the highway after 1+ hours that only occurs when I’m going DOWNHILL. The misfire will last between 36-41 seconds and then goes back to operating fine. Doesn’t occur on every downhill and doesn’t occur on steep downhills during in-town driving. The diagnostics always show the error for cylinders 1 and 4. The following repairs have been made:

Replaced all 4 spark plugs and ignition coils, replaced fuel filter, fuel injectors cleaned and tested showing no concerns.

Misfires reoccur during next long drive 6 months later, compression test performed and failed. All four fuel injectors replaced and one connector replaced.

Misfires reoccurred again during next long drive 4 months later.

I am stumped as to why the problem keeps happening, why it only occurs when going downhill, and why it seems to last for a fairly consistent length of time. Any thoughts or suggestions would be appreciated.

I suggest taking the car to someone who is very familiar with the engine management system on that Ford. If the spark plugs, coils, etc that were installed were OE or equivalent I suspect a problem with MAF or the computer.

Thanks for the suggestion. All of the diagnosis and repairs have been done by Ford dealerships so I’ve been trusting of the parts that have been used. Replacing the computer is currently on my short list for the next repair. How would the orientation of the car result in the problem stemming from the computer?

I forgot to mention. All the diagnostics and repairs have been done by three different Ford service departments so it hasn’t been the same place/person doing all of the work but they have seen all of the previous service reports.

[quote=“p1bar, post:1, topic:96261, full:true”] compression test performed and failed.
[/quote]Please provide more information on this. What were the numbers and what was done to fix the problem?

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Wild guess: Your intake manifold produces a small vacuum leak (or leaks) when fully warmed (after an hour). Downhill the throttle is nearly closed and vacuum is at max (lean mix) -> misfire. The computer compensates (richer mix) then it runs OK. Between these episodes, the computer re-compensates to normal around-town driving.

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Yeah… What he said.

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Long down-hills are similar to extended decelerating, and it is a challenge to get the fuel/air ratio just right. It used to be cars had a gadget called a “decleration valve” to handle that situation to prevent misfiring and backfiring on down-hills. Some cars may still have one. But precise throttle management is part of the problem, so you may have some issue with your idle air control system. A good clean of that system and the throttle body is probably worth a try. During this, look for any split rubber boots which might be allowing unmetered air into the engine. Might also want to take a look at this recall. It doesn’t seem to apply for your model year, but it might provide your shop a clue how to proceed. Best of luck.

Ford has also issued a recall (04S13) for 2001-2003 Ford Escape sport utility vehicles with 3.0L V6 engines for an intermittent stalling problem. The stalling typically occurs while decelerating at speeds below 40 miles per hour. The problem is caused by the calibration of the idle air control valve and evaporative emissions system. The fix here is to reflash the PCM with updated information.

Sadly I only have the limited service report so I have no numbers and I’m largely going off of memory. They said they did a pressure test and found that one of the cylinders tested low but when replacing the fuel injector and connector that the pressure was then fine. All four injectors were replaced because the labor would have been the same for them to do all four compared to just the one and I was worried they would later have to go in and replace another injector at a later date. It is very possible that replacing the fuel injector is a bit of a red herring but rules out the injectors as being the root of the problem.

Cylinders 1 and 4 are companion cylinders

I’m just speculating perhaps it’s no coincidence that those 2 particular cylinders are/were missing . . . /

Sounds like they did a fuel pressure test, not a compression test. You may want that done.

Since you had the fuel injectors simply cleaned at first, and that seemed to fix the problem for 6 months, it could be you’ve got some bad gasoline left in your tank that eventually fouls the injectors again. Grit or something which sinks to the bottom of the tank so it is hard to get it out. It might be possible for a shop to take a fuel sample from the bottom of the tank, then you’d know for sure.

The counterpoint to this idea as the cause is that electronic injectors – b/c they are either fully open, or fully closed – they tend to be self cleaning, at least from small grit in the gas supply. But they can get gummed up. I had a problem on my Corolla where it misbehaved during slow speed driving, like in first gear neighborhood driving where I was trying to vary the speed smoothly between 5 and 10 mph. Instead it would vary the speed in jerks, jerking forward when speeding up, then jerking again when slowing down… An injector cleaning treatment and cleaning the throttle position sensor fixed that problem.