Subject is 87 Ranger w/2.9 v6 5 speed and factory FI. This is truck that has been off the road for a while and we're bringing back. Today's issue is serious surging at part-throttle cruising speeds. The truck will start surging and loose power until I floor it. When floored, it picks up speed again. Got it home yesterday using the method of full throttle and coasting in between. It idles Ok, too. Fair number of new ignition and fuel parts on this thing including new fuel system: pumps, filter, tank, lines; ignition system: cap, rotor, wires, ignition module, coil and plugs; emissions: O2 sensor. I thought I'd check for vacuum leaks today visually and with a can of carb cleaner to spray around the intake manifold. Any other ideas? Thoughts appreciated!

 

Ok lets think about this some more. Given the clue about it wanting to lay down at throttle up, but taking off at WOT, sorta sounds like a TPS problem. Given its vintage & coming out of non use, maybe it has a dead spot at the normal cruise position, such that the resistance isn't Smoothly changing as you move the throttle open, so the computer doesn't know the TPS sensor/Throttle has moved/opened, so the air fuel ratio goes lean as the throttle opened. SO the engine stumbles, until you move it a lot, go WOT & it gets past the dead spot & wakes up & outputs a voltage change the computer can detect.

So use your Analog multimeter to Slowly perform a TPS sensor resistance check, from Closed to WOT, while Closely watching the meter needle for a drop out, or dead spot where the resistance doesn't change as the throttle is Slowly opened. Watch the meter needle closely around the normal cruise position. This normal position is where most of the wear will take place on the TPS internally. A uneven resistance change, or drop out is easier to see with a Analog meter. EDIT: If you find the TPS sensor is faulty, consider opting for another Motorcraft, or BWD brand TPS, as they're known good quality parts

Since the vehicle had been setting a while, also check the TPS fastener to make sure its snug in place & its electrical connector pins / sockets for corrosion, loose fit, bent pins, spread sockets & the TPS wiring for varmint damage since its set for a while & mice like to nibble on the plastic insulation & build nests in there, like mud dobbers too.

In case you have more than one problem, also Scan the ECM for trouble code clues & post All pending, or set code Numbers, as they can help focus a trouble shoot.
Some beginning thoughts for consideration, let us know what you find.

 

Pawpaw, you rock it again. Thank you. Just tested the TPS and it indeed has a dead spot. Closest parts store didn't have it in stock so it will be a couple more days before I can install and, hopefully, share positive outcome.

 

Ok good trouble shooting, finding & feedback, we'll keep our fingers & toes crossed that this will put things right, so will look forward to hearing how it goes with a new TPS. Be sure to let us know the TPS Brand name too.

 

Forgot to suggest taking your Analog multimeter to the store to check the New TPS, so Murphy's Law has less chance to mess with you with a faulty new part. Here is a link you may find useful in maintaining your vintage Ranger, as it has a good bit of info on the early model sensors, scroll down to & click on the TPS sensor, for readings range, verses rotation angle. Enjoy the info, I sure have over the years keeping my 94 3.8L Taurus up.
Fuel Injection Technical Library » Home

 

Pawpaw -- Well, seems the TPS wasn't entirely the problem. After I installed it, I had my son rev the engine a bit. Wish I could post a pic of the black spot it left on the garage wall! We're obviously running waaay rich. Was going to do a fuel pressure test to check for leaky injectors but couldn't find the Schrader valve adapter for my fuel pressure gauge. It was gone from the loaner gauge set from my local auto parts store, too. So, suspecting leaky injectors, I dove into it this morning. Upper plenum removed and I can see five of the six ports are wet, sooty. One was dry, light tan. I think we're on to something. Took a break to do some other honey-dos and to watch NASCAR. Have reman injectors on the shelf but not O-ring kits so ordered those, replacement EGR and throttle body gaskets.

Glad to hear you're keeping a 94 Taurus with 3.8 going. I had a 93 with the 3.8. Bet you've gotten good with head gaskets, right? Mine went at 105K. Had it repaired and then traded it for an Escape.

 

Ok good feedback. Remember the faulty TPS probably corrupted fuel trim, so when your finished with the injector rework, be sure to disconnect the battery B- cable to wipe the KAM, so to wipe the old corrupt fuel trim tables so the computer can begin building new ones using the new TPS & reworked fuel injectors. To help things along, on the cold restart, perform the Cold & Warm idle strategy relearn, as it too also gets wiped on a KAM erase. Here is a OBD-1 idle relearn routine. https://www.ford-trucks.com/forums/s...9&postcount=12
Which brand IAC did you opt for & has it fixed the idle & go at throttle up problems??? Let us know how it goes

 

Missed your comment about the 3.8L Taurus. Back in the 90's I got Two recall notices from Ford to take it in to have the Dealer replace the head gaskets, its still leak tight but it never did have a coolant consumption problem.
BUT I've had Two OEM radiators put on it & both were bad plastic coolant tank molding cold knit line weep problems. Ford's vendor seems to not have good temperature control over his molding process. Both looked like a cold mold, or material, or a combo of both, so since it was gonna be my dime this time, I took control & opted for an aftermarket American Radiator & its been fine for about 10 years now. Looked to be Way better workmanship & material than the 2 OEM radiators!!!!
Of note, had to replace the wife's 2000 Neon radiator a couple of years ago, for its OEM radiator top plastic tank material deterioration from a shiny black, to a cooked looking rust brown color & it too began weeping below the upper radiator hose fitting nipple, like both Taurus OEM radiators, but the Taurus radiators didn't change color & looked cooked. They both leaked at a cold flow knit line in the plastic drivers side tank.

Yah I really like how the 94 Taurus sits & drives, it fits me like a glove & the engine is quiet but plenty of torque on demand, so is a nice around town & highway cruiser with the 3.8L torque curve. With all that torque down low, it doesn't need 6000 rpm to move out when you want to go at throttle up, as you likely remember. Gets very good mileage on the road, still around 35 at the posted speed limit. It'll still do 25-27mpg around town in Spring, Summer & Fall, but drops below that in winter. They got the 94-95 models mostly right imo, then got carried away with those awful overly Oval theme 96 up models. That's when they lost me & I knew the 94 was gonna be around my garage for a looooong time!!!! lol

Anyway I use mostly OEM, if they've proven to have good service, or BWD replacement parts, so was interested in what you opted for on the TPS replacement.

 

TPS is Standard brand (but not the T-series). I liked our 93 Taurus, too. Head gaskets went at a little over 100K in 2001. Local shop replaced them and I sold the car on, replacing it with an 2001 Escape which I kept until last year before selling it to a neighbor's daughter.

But back to the Ranger for a sec. New injectors are installed. Today, I decided to hook up the battery and check for fuel leaks while upper plenum was still off. No leaks! Also decided to do a fuel pressure test now that I have an adapter for my gauge. At each turn of the key, pump ran for a couple of seconds and the gauge jumped. But soon, pressure dropped and I saw fuel coming from the disconnected vacuum line to the regulator. Does that seem right to you? Bad regulator? Pinched fuel return line?

As always, thanks for your guidance and that of others on this forum.

 

OK, another good find & feedback & yes the failure mode your reporting is a common one for the FPR internal diaphragm leaking & sucking fuel into the vacuum system. Seeing as how this leak is downstream of & not monitored by the MAF sensor, it'll corrupt fuel trim & drive the computer crazy trying to figure out what to tell the fuel injectors to do for squirt time, so it can get fuel trim back in spec. If its just sucking air the fuel trim will go lean & if its sucking fuel it'll go rich, so if the leak is bad enough the computer will loose any control & we'll get a O2 sensor code for non switching, or stuck lean, or stuck rich. Opt for a Motorcraft, or BWD brand FPR & double check the vacuum line to it for dry rot or soft loose fit. I think your about to get this puppy running right, so keep us posted on how it goes.

 

Thanks, Pawpaw! Just ordered the FPR and got the BWD, too. I'll keep you posted.

 

Pawpaw, thanks again for your encouragement. Replaced all six injectors with reman units from ebay (replacements are four hole design versus the single hole of the originals) as well as installed a new BWD fuel pressure regulator. Truck is showing 35 pounds of fuel pressure (my chilton's says 30-40) and ran well on a 10 mile test drive last night and another one this morning. Much more power than before as well as lower idle speed and easier starting. My son and I both very happy. Please know I appreciate the encouragement and brainstorming. Next is a new BWD idle control valve, mostly because of problems I've had with them in other Fords, so pre-emptive maintenance. Also ordered a reman water pump after noticing ours is weeping slightly (again, no surprise since the truck sat so long - already replaced the heater core).

Bottom line, we've kept another old Ford on the road!

 

Ok good feedback & to hear the efforts have so far resulted in positive results, so maybe your finally getting this puppy dialed in. Are the Lean codes for both cyl banks now gone???

 

No lean codes that I was aware of -- it's an 87 and I honestly don't know how to read them on something this old. If anything, judging by the amount of black soot when we first got it going, I think we were running way rich. But seems to be behind us now. My son just took his mom on an errand to donate some old furniture to Goodwill so an installment on its usefulness and her patience with us. If Momma ain't happy, no body is, right?

 

Look in post #5 link right side table of contents, to find out how to use a jumper to have the in dash CEL flash stored codes. You can also visit this site, https://www.troublecodes.net/ford/
to wire the under hood DLC to wave a analog meter needle, flash a lamp, or you could sub a 12 volt beeper / buzzer to sound the codes. So on OBD-1 vehicles we can get trouble codes without a scan tool, or code reader, but they are easier & more accurate. A scantool can also monitor some sensors, which is nice when the failure mode isn't obvious.