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'94 4.0 Ranger - Misses Under Load and at WOT

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Old 12-27-2009, 05:32 PM   #1
Kennett Square, PA
'94 Ranger XLT
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'94 4.0 Ranger - Misses Under Load and at WOT

1994 Ranger 4.0 idles fine but has been running real rough since this past summer when it started suddenly missing while driving. For a while the rough running would come and go. Now it runs rough all the time, though sometimes lots worse than other times. It starts and idles fine.

Yesterday when I first started it, it ran so badly I couldn't get it over 15mph. When I gave it gas, it backfired (which it often does when it's acting up like this and I give it more gas). After it "warmed up" it drove, but real rough. It runs worst the more I press on the pedal. It runs best when I really baby it and keep the RPMs as low as possible.

Stuff I've done so far:
1. Replaced the MAF sensor (with a used one, after cleaning it with electronic cleaner spray (not brake cleaner)) - no change
2. Replace the fuel filter - no change
3. Replaced the FPR - no change (fuel rail pressure is 29psi or better, and goes up to maybe 35 psi when driving at higher RPMs)
4. Replaced the coil pack with a used one - no change
5. Tested TPS - .95v - 4.6v nice and smooth from close to full open

Any thoughts on this? I'm thinking it's just running way too rough for it to be an ignition wire or plug problem (i.e. too many cylinders missing - can't be a single plug or wire, and there's no "central" distributor wire given that it has a coil pack). The fact that it runs fine at idle was leading me to think it was a fuel pressure problem, but after testing, the pressure seems fine. Could I have a sensor problem? What sensors would come heavily into play with higher RPMs?

Aside from sensors, I'm starting to run out of things to check/swap out, and hoping the evidence isn't pointing toward a bad ECU.

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Old 12-27-2009, 08:02 PM   #2
topeka, kansas
91 navajo
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since he says it so well:

Originally Posted by manaen View Post
OK I couldn't find the thread, so here are the basic steps.

1) fuel pressure test - yours should be somewhere around 35psi if it is higher then that your FPR is bad and is causing too much fuel to be dumped into the engine. I had that exact problem on a gen1 sport many years ago, I was getting 10mpg, there was a definite sign of black soot on the rear of the truck and the plugs were burning very black.

2) vacuum test - disconnect you brake booster and hookup your vacuum gauge to the line. Start the truck and let it warm up. Your vacuum should settle, take a reading. Then rev your engine to 2500 to 3000 rpms and hold it there. Watch your vacuum it should settle to about the same (maybe a little lower) as the idle reading. If you vacuum drops drastically and stays low you have something blocking your exhaust. IF you vacuum goes higher then idle your intake is too restrictive etc... get the picture?

it is a set of basic test that can tell you so much about and engine...

The basic three is, Fuel Pressure/Read Plugs/Vacuum test
it sure would help if you had any codes it's outputting.
That leaves a bad injector or two.
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Old 01-01-2010, 11:34 AM   #3
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Great State of Texas
1999 Sport
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Rough running and backfire when opening throttle...

Sound like a lean fuel condition to me...That or the firing order is wrong... Do you have any obvious vacuum leaks? Any unobvious leaks like intake manifold gaskets?

As you are monitoring the fuel pressure as the rpm increase, does the fuel pressure ever go over 40 psi? As the throttle opens, the vacuum signal drops and the FPR should increase the the fuel pressure to 35-40-45 psi in some cases.. If the vacuum line is removed and plugged on the FPR, what does the pressure go up to and how does the engine run?

Lastely, if you put a vacuum gauge on the engine, what level does it go to and how steady is the needle?

The idle pressure is in the ballpark but it sounds like the FPR is not increasing the fuel pressure as the vacuum signal drops...

When you changed the fuel filter, was the fuel that drained out of the inlet clean or was it brown and dirty looking?

BTW the OHV firing order is

3 4
2 6
1 5

This is on the coil pack with #1 on the passengers front corner...

Let us know what you find out...

When in doubt, use a bigger hammer!

2014 Jeep Compass Limited [Company Vehicle]
2003 Ford F-150 XLT Supercab
1999 Ford Explorer Sport [new playtoy]
1983 Ford Ranger V8 [soon to be gone]
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Old 01-01-2010, 12:00 PM   #4
Novi, MI
'99 EX 4.0 OHV
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I had a similar problem with my truck, my friends dad suggested getting the injectors cleaned, so I went to a "20 minute oil change" place and asked for it, and they did, I went on the freeway and woo buddy, there was a difference, lol. It was about $40 IIRC.

Not sure if it's the same for you, I just kind of skimmed the thread, but hope that helps.

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Old 01-01-2010, 07:24 PM   #5
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The "fuel injector cleaning" at those oil change places, and even the dealership, usually consists of a concentrated cleaning solution poured into your fuel tank, and the engine being run at high RPM/redline for an extended period. NOT how I want my fuel injectors cleaned. I'd think you could get the same effect with throwing in some Seafoam yourself. Real injector cleaning is done with the injectors removed from the vehicle. You can also use "top tier" gas from certain stations that will clean the injectors over a period of time if you keep using it.

It's kind of odd it would run that poorly and not throw any codes, but have you at least checked the spark plugs to see what shape they're in? They could be very worn down or even have damaged electrodes. Even if they're just old and have a gap well over spec that could cause running issues. How old are the spark plug wires? If they are original, you should consider replacing them. Usually they just cause a slight miss or hesitation, but it might get severe after enough mileage. You might also consider dropping the fuel tank for inspection, if the factory fuel pump assembly rusted off enough, it could have rust chunks in the tank that are clogging up the in-tank fuel filter. The fuel pump itself could be going out or having delivery issues, replacement might be in order.

You've probably already looked over it some, but if you're lucky the problem might just be a bad sensor or unplugged connector somewhere. If it's not ignition/fuel/air related, maybe it's timing or the Crankshaft Position Sensor (which sometimes just needs to be cleaned), or the ECU. An ECU wouldn't be so bad, someone parting out on here will probably sell theirs for a deal.

Also, the Check Engine light might be out, so you might hook up a code reader or limp it to Autozone for a free code reading, just in case.

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Old 05-15-2010, 12:06 AM   #6
91 XLT
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I know the the guy that asked the ? about the fuel psi, and this is because I asked him to. I am not a ford guy and know that in these early years ther'er terminalilogy was different then GM. My gut feeling on this truck, is that it is not getting enought fuel.{voloume} the fuel trims are at 33% at idle, and increase when brake torqueing. P.S. if you had low fuel psi, would youn get a dtc22?

91 xlt 4.0, auto, shortened hi-pin d44, disc8.8 5.13's detroit lockers, 35'' mud terrains=money pit
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