Ford Ranger hesitates on acceleration

My wife talked me into upgrading my landlord projects truck from an '89 Ford Ranger to a '97, because the new one has this amazing new thing called AC, which comes in handy here in Texas. For a while, though, the “ice cold AC” was about the only thing that worked well.

A shop that checked the truck for me before I bought it missed something and decided later that the truck needed a valve job to correct a misfire. That’s done, and the check-engine light stays off, but I still notice some brief hesitation (“bucking?”) on acceleration, mainly on hills. I tried cleaning the mass air sensor. Nope. I’ve ordered spark plugs and wires. We’ll see.

But of course this is the “throwing parts at it” strategy. Other suggestions I’ve seen involve the fuel injectors, the cam position sensor, a seemingly non-existent flex fuel sensor, vacuum leaks and oxygen sensors. The cam position sensor and hard-to-reach fuel injectors are beyond my comfort level to work on. What do you suggest?

I suggest you run a few diagnostic tests on the truck and stop trying to diagnose this from reading internet posts.

Do these things and post the results back here along with the mileage on this truck, the engine it has and transmission and we’ll Try to help from there.

Warm the truck up before you do these.

Check the vacuum the truck pulls at idle and note what the needle does… stable or wiggly.
Plug in an OBD2 scanner and check for codes, also take a highway drive and note the long term fuel trims for both engine banks at a steady highway speed. Also note at idle.
Next, remove the sparkplugs and run a dry and then wet compression test. Note the condition of each sparkplug and tell us what they look like.

Do that, report back and we’ll Go from there.


So you have recently purchased a 97 Ford Ranger pickup, your shop gave it a valve job, which corrected a misfire, but now it is hesitating on acceleration. hmmm … Well, when you accelerate you press on the gas pedal, which is going to rapidly allow more air into the engine, & tend to make the mixture go lean. The engine computer should compensate by injecting more gasoline. The sensors it uses to make that adjustment are

  • Pre-cat O2 sensor
  • MAF sensor and/or MAP sensor
  • Throttle position sensor
  • The crank & cam angle sensors

You know what? While it could be one of those, I don’t think the problem is any of those sensors. I think you’ve got some other problem that is either making the mixture too lean, or is obstructing air or exhaust flow. So before engaging in a round of swap-tronics, check for

  • vacuum leaks
  • air leaks bypassing the airflow sensor
  • intake air obstructions (like a clogged air filter)
  • clogged exhaust system, like a clogged cat. Given the valve problem, there’s at least a reasonably good chance the cat is clogged. Sometimes it is possible without too much effort to temporarily bypass the cat as a test.

This is the 5L v8 engine, right? Manual or automatic trans?

Not available in a Ranger. 2.3 liter 4 cylinder or 3.0 V6 or 4.0 V6

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Ford didn’t put “Flex Fuel” engines in Rangers until 98, and it was only on the 3.0, and not all 3.0 Rangers were “Flex Fuel”

So with your 97, you wouldn’t have the sensor.

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I wonder how many miles on this truck. It generally takes a bunch on an unleaded fuel engine before the valves need work.
That leads to questions of compression numbers and rings.

You might consider checking the fuel pressure. Maybe there’s an issue with the fuel pump, filter, or fuel pressure regulator.

An EGR problem could cause this symptom too. The EGR function is usually activated on acceleration. OP might be able to temporarily disable the EGR to see if that has any effect.

Thanks everyone for the suggestions.

The truck has 159,000 miles, a 2.3 liter, 4-cylinder engine with 8 spark plugs and a 5-speed manual transmission.

Mustangman, I don’t know how to do or don’t have the equipment for all the checks you suggest. For most of those I will have to look up details or buy tools. That’s why I was asking for educated guesses on what to pursue first.

George_San_Jose1, in case this indicates that the catalytic converter is OK, fuel economy is good for this model – about 26 mpg highway – and the air filter looks good. Also, what I have read about disabling the EGR valve is that it’s difficult and will throw OBD II codes. Why would disabling it make things better rather than worse?

It_s-Me, thanks for clarification on the flex fuel sensor. It’s good to cross that off the list.

ok4450, the shop that did the valve job checked fuel pressure. A minor problem, apparently common to Rangers, was that the dashboard fuel pressure gauge went down to 0 when the truck was idling and then jumped to the middle of the normal range when the engine was running faster than idle. The shop manager thought I just needed a new fuel pressure sending unit, so I got one. With the new one in, the needle still goes down to 0 at a slow idle with the engine warm and then jumps to the middle of the range at anything more than idle. The gauge stays at 0 a little less than it did with the old sending unit. This is really a fake gauge that should be a light, because the gauge works like an on-off switch. I’m willing to believe the oil pressure is OK.

I also have replaced the fuel filter.

Sounds like you are describing the oil pressure gauge. I referenced fuel pressure and there is no dash gauge for that unless someone has added an aftermarket unit.

If there is a fuel pressure problem due to the pump or fuel pressure regulator then a new filter will not help. However, congrats on changing the fuel filter. Failure to do that can lead to short pump life.

If the EGR was opening too much or too quickly when you step on the gas pedal, it could cause the engine to briefly stumble. When the EGR valve opens exhaust gas is routed into the air intake path. Exhaust gas doesn’t combust, so this reduces engine power. On the upside, it cools the combustion chambers, which reduces nitrogen components of pollution. If the EGR function is disabled (as an experiment) so that the valve doesn’t open at all, and the EGR is in fact the problem, the stumble should diminish or be eliminated.

Ford owners here have sometimes posted about problems with a related part, the DPFE sensor. If your truck uses this part, also consider it as a possible EGR related culprit.