Hesitation in warm(er) weather - '98 Explorer 70K miles

Hi - I am 2nd owner of 98 Ford Explorer Eddie Bauer V6SOHC. Vehicle is almost showroom condition w/ 70,000 miles and properly maintained.
Problem: engine seems to stumble quite frequently and is most noticeable when ambient temp is above 60*. Highway speeds , 40MPH doesn’t matter. RPMS drop but it has completely shut down only twice in the last 9 months.
There are no codes. Plugs and wires changed several months ago, fuel pressure checked by Ford and it’s ok, injectors cleaned at dealer, MAF seems ok so I am at a loss. Since there are no codes diagnostics don’t help.
Except for the issue above, this is a very nice reliable vehicle.
Your advise is appreciated.

Sounds like you’ve already covered the basics. One thing, was the fuel pressure tested while driving at highway speeds? If not, that might be something to ask your shop to try. Sometimes the fuel pressure will fall (when it shouldn’t), but only w/ high fuel consumption.

Still, due to the temperature issue, I’m thinking this is an ignition problem. Some part is starting to fail under higher temperature operation. I’m not familiar w/your ign system design, but w/no codes you don’t have much to go on. In that case, if this were my car, probably what I’d do – if I didn’t have the kind of sophisticated ignition diagnostic equipment dealership shops have – is just replace the crankshaft sensor to see if that makes any difference. Failures of that part is a common report here, not necessarily on your car, but generally that part seems to fail a lot.

Edit: One other possibility is the cat is plugging up. The symptom is the car will start and run fine, but when you try a hard accel or try to drive fast, esp if the engine is fully warmed up and you’ve been driving for 30 minutes or more, then it will start to feel like the engine is stumbling or just doesn’t have the same power as it usually has. A shop can do a fairly simple backpressure test of the exhaust system to test for this.

The problem might be caused by a faulty crankshaft position sensor being effected by heat.


But here’s the problem. As the article states, a faulty crankshaft position sensor won’t always cause the Check Engine light to come on and set a code.

So, sometimes you can’t determine if the problem is with crank sensor unless it’s replaced.


Good advise … I will start with replacing the crankshaft sensor + cat backpressure test.
Thank You!

If I’m not mistaken, this engine uses a coil pack. A coil pack this age is very likely to be heat sensitive. If you have access to a heat gun this will be very easy to test for. Simply run the engine while heating the coil pack. If it starts to stumble, you have confirmation. Of course, if you have a scope it’ll be even easier and more definitive to use that.

Maybe this vehicle has more than one issue. The dying could be the crank sensor as mentioned. AutoZone shows one at 15 bucks. For that bargain basement price just throw one at it and see what happens rather than fret over diagnostics that may lead nowhere.

The stumble (assuming it’s not ignition related) could point to a lack of fuel problem. That doesn’t mean lack of fuel pressure so much as a lean fuel/air mix or fuel volume problem.

Just curious, but what about the fuel filter on this vehicle? Has that been changed in recent memory? If not, or never, then it should certainly be done as that can cause a stumble and even kill a fuel pump over time.

ok4450 - the fuel filter was changed the same time the injectors were cleaned ( about 5K miles ago). Thanks

Thank you for clarifying the part about the fuel filter. Have they considered a vacuum leak related to the intake manifold and hoses, etc or n air leak in the intake tract between the throttlebody and MAF sensor? Any leak at all in either of those areas has an effect on the MAF sensor.

Not too many years ago I had a stumble in one of my Fords that was random at best. It seemed to be worst when it did occur during acceleration out of a sweeping curve with no CEL or DTCs being set. I traced this down to a small razor type cut in the hose on the PCV valve. Visually the hose looked fine but during acceleration and based on manifold vacuum the softened hose would deform and the split would open up. Air pulled in that way then had an effect on the MAF sensor.

You could also try unplugging the MAF sensor and taking a test drive. If the problem goes away maybe a MAF replacement is in order as cleaning does not always work. Yes, the CEL will illuminate and a code will be set but it will go away later. This is only a backyard test so to speak.

Ok4450 - you bring up a good point that seemed so far fetched I didn’t mention it before …one condition that will almost guarantee the hesitation is on an incline and on a curve. Yesterday it hesitated 3 times while on an exit ramp after doing 70MPH for about 30 mins. So curves and inclines aggrevate the hesitation.
Will leave the MAF unplugged for a while to see what happens.
Thanks again

I have the 4.0L SOHC engine in my 2000 Explorer Limited. I also had a stumble and had a real bad idle at start-up on cold mornings. I seemed to take forever for a DTC to pop up. After a year or so of dealing with it, I finally found the problem was the intake manifold gaskets. The replacements were under $30 for the upper and lower gaskets, and I managed the job within a couple of hours. It’s been 3 years and no other problems since.

Thanks BustedKnuckles - mine starts right up and idles with no issues but I will add gaskets to my (ever growing) list of possibilities.

If any vacuum leak is suspected the best way of determining that is with a vacuum gauge. They’re about 15 bucks and can be hooked up in seconds. They will also let you know instantly if there’s a leak anywhere in the intake tract or its offshoots including the inner dashboard.

Intake leaks above the throttle plate and up to the MAF sensor can be done visually or with a smoke machine as vacuum gauges in that area are a bit more dicy on the readings.

I sometimes do electronics hobby stuff involving printed circuit boards, and if I am suspicious a part on the pcb that might be failing b/c it is heat sensitive, I spray it with a aerosol can of something that quickly cools it off. If the pcb starts working, I know I’ve found the problem.

I forget what the name of that cool-in-a-can stuff is, you can get it at any electronics lab supply store, but it is pretty effective at pinpointing heat-failed electronic parts. It’s sort of like spraying wd-40, comes with one of those pin point tubes to direct the flow. But it doesn’t lube, only cools. Do mechanics use that stuff too for diagnosing heat related electronics failures in cars?

I’ve known guys to dump cold water on the crank sensor, if the car won’t start, and they suspect the sensor

I haven’t personally seen anybody do it, but I’ve had a few guys tell me about it

The only problem is, the guys that told me about it, I loathe and despise them to no end, so I’m not sure I believe them

If these guys were burning, I wouldn’t . . . . on them to put out the fire