Hello! A while back, an elderly gentleman donated his little pickup to the agency where I work. It wasn't running particularly well and had some shifting trouble, but I claimed it to use to haul materials and supplies for a carpentry class. The pickup is a 1991 Ranger, 2.3L, 2wd, with about 114,000 miles. It is nearly the basest of base models, with absolutely no options save for automatic transmission. It even still has its factory radio and antenna delete plates on it. It appears to have been well maintained for most of its life; it isn't leaky and someone even treated it to a new seat at some point.

I didn't have funds to do much to it all at once, but I started bit by bit. So far I've changed the thermostat, fuel filter, and transmission vacuum modulator. The first two improved its running considerably and the third solved the shifting problems. I pulled a few plugs and they looked reasonably clean and good. Also the fuel pressure regulator appeared to be new, though I can't prove that it is.

After those things, it was a pretty useful little pickup. All the same, it never really seemed to run quite as well as I thought it should. Nothing major, just a little pinging here and there and the fact that the temperature gauge still barely comes up into the normal range (it didn't move at all before the thermostat). I've been figuring that it needed a tune-up and probably will do that soon. Recently though, it started up with a new problem. It starts great in the morning, I let it run a moment, and then I start off. That's when it begins to run a little roughly. If I let it idle along, the trouble clears out after a few moments and it is good to go the rest of the day. If I try to accelerate, the engine bogs down badly (though it never totally dies) and will not accelerate until I let it drop back to idle. Again, the trouble clears out after a few moments. I have no check engine light or codes in the OBD I computer. I have cleaned the MAF sensor and throttle body, checked the vacuum lines, and also cleaned and checked the electrical grounds. We use Chevron gas, and I also added a dose of Seafoam to the last tank. None of these last things have seemed to affect it.

I've owned over 30 vehicles myself and have worked on many more, but relatively few of them have been Fords and even fewer were from this era. Rather than throwing parts at it I figured I should get some insight from folks who know them better. Any help would be appreciated!

 

Welcome to FTE.

41 reads but no replies yet, so I'll see if I can get things started. Good idea to begin getting scheduled maintenance replacement items brought up to date, checking vacuum lines & inspecting battery to engine & body grounds.

With the mileage on this puppy a number of things are due & you've begun by replacing some of them. Spark Plugs, Wires, PCV valve, O2 sensor, Air Filter, Fuel Filter, come to mind.

Was it doing this bog down thing when you go at throttle up, Before you cleaned the MAF sensor???? If not, did you remove it so you could get to all sides & clean it with a plastic safe, non residual cleaner, like CRC MAF spray, let it dry & made sure to have kept it squeaky clean when reinstalling????. Bogging down when the throttle is opened suggests the air / fuel mixture is going lean for some reason & several things can cause that. like a dirty MAF sensor, faulty throttle position sensor, low fuel pressure, non adequate fuel delivery rate from a wimpy fuel pump, pinched / kinked or otherwise restricted fuel, or return line, faulty fuel pressure regulator, old, slow to respond (sorta like me) O2 sensor, faulty DPFE sensor, cooked or dry rotted rubber plumbing hoses from the manifold up to the DPFE sensor, are some things that come to mind that could corrupt fuel trim, when we go at throttle-up.

Does the CEL light up & turn off for its self check, when you turn the ignition switch from Off to On, before cranking the engine??
If you used a scan tool to look for trouble code clues on this OBD-1 system, will it display live sensor PID's & if so have it take a look at the ECM coolant temp sensor, TPS, MAF & O2 sensors readings & response when you go at throttle up. With the coolant temp showing its still low, it could be several things, like a faulty new thermostat (you did check it to make sure it opens fully at its rated temp....Right???), faulty dash gauge temp sensor, or a faulty ECM temp sensor. The ECM looks to the O2 & ECM Coolant Sensors to know when to begin using the O2 sensor to begin controlling fuel injector squirt time to try & keep the fuel trim in balance with engine temp. SO, if any of those sensor inputs to the ECM are corrupt, they can cause the ECM to corrupt fuel trim by commanding wrong fuel injector squirt time. SO, if your using a scan tool like say a Actron CP9145, or the like scan tool, that can hook up to the under hood OBD-1 DLC, & have it take a look at the TPS, O2, ECM coolant temp & MAF sensors & maybe they'll offer up some clues on why this puppy wants to lay down when you want it to go at throttle up. Some initial trouble shooting thoughts for consideration, let us know what you find.

 

Hello, and thanks for the reply! Yes, it did start doing this prior to cleaning the MAF sensor and you nailed it exactly with the CRC MAF cleaner. I also used CRC Throttle Body Cleaner when cleaning out the throttle plates. I was considering the throttle position sensor, but I suspect that it would show up more regularly if that were the culprit. I did not check the new thermostat because brand new thermostats always work perfectly right out of the box (I read that in a supermarket tabloid). The Check Engine light does cycle as it always had on startup, but I haven't watched the live data since the CEL didn't remain on; I really should do that. I might even try to replicate the fault (it is pretty consistent) while the live data is going and hope I don't run into the side of the shop while I'm watching the scan tool.

Hopefully I can give that a whirl tomorrow and get some more information. I'll let you know what I find when I do!

 

OK, so the CEL isn't burned out, as it lights up for its self check & winks out. Still you might have a pending code that's been stored but hasn't happened often, or long enough to set a hard code that turns the CEL on, so look for any pending codes & post the Numbers, as they can help focus a trouble shoot.

Seeing as how this is a vintage Ranger, you might find some useful info poking around this site Ford Fuel Injection » Throttle Position Sensor (TPS) for monitoring the ECM & Dash engine temp gauge senders, tps, etc. sensors test info.

In the page side bar menu, click on Sensors to open the sensor list page, then click on the title at the top of each sensors general info stack, to open more Detailed information about the sensor of interest that can be used to electrically check the input to & output that each sensor is sending the engine computer (ECM).

On the TPS sensor, use a Analog meter to watch for drop outs or unsmooth voltage or resistance changes as the throttle is moved from Closed to fully open. Pay close attention to what happens as the throttle begins to open, as that's where your lay down problem is happening & where the most TPS sensor wear is happening.

If for example the O2 sensor switching range or speed is lazy / slow to respond, or the TPS sensor has a dead spot at, or close to the Closed throttle position, it'll corrupt info sent to the ECM.

When we go at throttle up, the throttle plate can move open but the Dead spot in the TPS sensor won't register the movement, so the air / fuel ratio goes lean because the computer never got input from the TPS that the throttle plate had opened, so it could increase fuel injector squirt time to keep the air/ fuel ratio in balance with the extra incoming air as the throttle plate opens, because of a TPS dead spot. Same scenario for a old lazy O2 sensor, or dirty or faulty MAF sensor, or low reference voltage to the sensors. All a vicious circle & on this vintage vehicle, with the posted symptoms, the TPS, O2 & MAF sensors belong on the suspect list for right now. More thoughts for consideration.

 

Took me a while to find my OBD 1 stuff, but I did finally locate it this morning! KOEO tests came back clear, but my tool was not able to pull codes or live info in KOER. While running the KOER, the Check Engine light blinked out several codes (I presume), but the numbers I wrote down didn't seem to line up in any meaningful way with the code lists I found online. An analog meter is more my cup of tea anyway, so I'll move into the tests you described above next.

While running the truck today, I was watching the coolant temp gauge pretty closely; I think the thermostat is opening and closing because the needle kept going back and forth between a touch into the normal range and a touch below the normal range. My thinking is telling me that either the thermostat IS faulty and opening at a temperature that is too low, or that the temp sending unit is out of whack. Am I reading correct in your previous post that there are separate sending units for the gauge and the ECM? It really wouldn't be that hard to pull the thermostat and test it, so I may do that too.

 

Yes the Ranger has a separate temp sender for the ECM, its a Two wire sender, the one for the dash temp gauge is a Single wire sender.

 

Finally got a chance to tinker a little bit; I went ahead and changed the thermostat again and got the exact same results on the gauge. The acceleration problem went away for a while and so I procrastinated on further tests but it happened again yesterday. I had a lot of errands to run around town so I was able to get the truck up to temperature as much as it can (barely above normal). As soon as I got back to the shop I checked the upper radiator hose with an infrared thermometer and got an average of 155 degrees. Even if this isn't the cause of the problem, I can't imagine it is helping things (this seems to be a well-documented problem and I'm finding good info on it- looks like a fan clutch might be on the list). I want to do the TPS test cold (since that's when the problem happens) so I'll try that Monday.