Hi everybody,

I recently bought my first truck, and it is a pretty old ranger, 1997 2wd manual 5spd 4.0L engine with 155k miles on it. The truck runs strong, transmission works nice, there seems to be no leaks, and the price was good. But it runs rich and gets about ~12mpg on the highway, ~10 in the city.

Now, to the issue. There are bunch of codes regarding the O2 sensors: P1152, P0155, P1132, P0135, P0141. I know that they are known to fail (many I've looked had similar problems), so I bought 2 upstream NGK O2 sensors. They were stuck, so I got it to the shop, the installed them, and also replaced a fuel filter.

However, after resetting all codes they came back ~10 miles later (all of them).

Another issue is that engine temperature gauge does not work, but I checked hoses and it felt fine (so no overheating). The shop was not able to figure it out, they said the sensor was alright, and the harness was also working properly. I got an OBDII scanner, and got the following information:

RPM during idle was ~1400 when cold, got down to ~900 after warming up (seems normal)
Coolant engine temperature went up only up to ~153. I read that the operating temperature should be around ~190, so does it mean that thermostat is stuck open, or for lower temperature?
Fuel system status said: open loop due to detected system fault. I've researched and it seems that on some vehicles, especially old, it can be stuck open by PCM if you have any error codes.

Things I will be doing this weekend:

- clean MAF
- replace ECT sensor (I have the part already, and will test the temperature after)
- replace spark plugs/wires (just as a part of the tuneup, I don't expect it to fix anything)

Should I replace the thermostat as well, if another sensor shows the same?
What else can be a problem?

Also, there are no misfiring, or rough idling, any hesitation, or lack of power. Only running rich.

Thanks!

 

I think your 4.0L has two downstream O2 sensors, too...one after each catalytic converter. See attached TSB.

Is the coolant temp gauge needle completely off the scale? Or, is it just low on the scale?

Does you OBDII scanner have accurate conversion firmware for you particular vehicle so that the voltage readings get translated into the correct temperature?

Yes, if the motor is fully warmed up and you're only getting 152 degrees at the thermostat, then the thermostat is the incorrect temperature range or is malfunctioning. Do you have an IR thermometer?

Your plan sounds good. All O2 sensors, the thermostat, and the ECT sensor all play a part in the PCM determining fuel trim and when it'll drop into closed loop mode.

 

Do you mean it has two catalytic converters? When I looked underneath, I think I saw only one. Also, error code only for bank 1 sensor 2, so I assume there is only one. Initially I was under impression that it does not affect anything (except emissions warning), so I was planning to replace it later, if needed. Now, though, after looking into this scanned info, I am not so sure.

Coolant temp gauge does not even flick, even if you floor it. I mean, if the data is correct, that it is only 150, it still should go up, but it does not. I checked the upper radiator hose after driving, it is hot, but not unbearable. Unfortunately, I can't distinct by hand what is 190 and what is 150, so hard to say how correct this 150 degrees measurement is. I don't have an IR thermometer. Will I use it after driving to ensure the coolant temp?

My OBDII scanner is a Bluedriver, and honestly I have no idea whether it support correct conversion between voltage on ECT sensor and actual temperature. How can I check it?

My only concern is that it tells me in my scanner that it is stuck in open loop due to system fault, which I assume DCTs are. And I changed upstream sensors, but the problem still persists.
Now I feel I should change the downstream sensor as well, just for the peace of mind.

 

Step one in fixing problems of this type is to always do a visual and fix these and any other obvious problems. In this case there is an obvious problem, the engine temp is low. The cause of this should be determined via diagnostics, and be repaired. Knowing that an engine running below a pre-determined threshold can cause the PCM to remain in open loop, you should make repairing the obvious problem your #1 weekend repair priority. Repair, verify it's a valid repair, clear DTC's, test drive and see what happens. The engines and cooling systems used in Rangers don't tend to take well to aftermarket thermostats brands, as such this is one of those times to insist on a Motorcraft.

***Some engines in Rangers have two temp senders, one for the gauge and the other to feed data to the PCM; something to be aware of if you haven't yet determined this. As for testing your thermostat, sometimes people put them in a pan of water on the stove, insert a thermometer into the water, and see if the thermostat begins to open at the temperature stamped on it. Crude but simple and effective test.

EDIT: Not sure if you know, but a DTC means there is a fault in a system, and unless the DTC specifically points at part (very infrequent), then there could be several reasons for the DTC OTHER than a part. Sounds like those O2 sensors for example, were a guess rather than confirmed via diagnostics to be needed parts, especially two of them at once. Me uses the diagnostic flow charts in Ford FSM's to resolve problems but since you're seeing DTC's for no heat in the O2's, then it only stands to reason that wiring should be suspect, and on that note, the cause could be as simple as a toasted fifty cent fuse for that circuit. But...now we know.

 

Sorry. I think you're right. My '97 4.0L has three O2 sensors: Bank 1 is passenger’s side. Bank 2 is driver’s side. Bank 1, Sensor 1 is before the catalytic converter. Bank 1, Sensor 2 is after the catalytic converter. Bank 2, Sensor 1 is on the driver’s side before the catalytic converter on the exhaust manifold downpipe.

If you had an IR thermometer you could hit the thermostat housing to get a pretty close read of the coolant temp independent of your OBDII scanner or dash gauge. Since you can't trust your dash gauge and you're not sure if the Bluedriver has the right conversion factors in it, it's the next best way to determine if your thermostat is misbehaving.

Go to the manufacturer's website and see if it is compatible right out of the box or whether you need to update it with additional firmware and/or conversion tables for your specific vehicle.

If this were my truck, I'd put a new thermostat and new downstream O2 sensor in it. Then I'd troubleshoot the ECTS wiring harness. Then I'd replace the ECTS. If I had the patience that day, I'd replace one component at a time, reset the codes, and see if the problem goes away so that I could narrow down the actual problem it turns out to be.

I don't know if our '97s have two separate ECT sensors like CBB9M suggests. Search elsewhere on the forum for that answer.

 

On the website, they just say the vehicle is supported. Also, it is a bluetooth scanner, so I have an application on my phone. So I can speculate that OBDII scanner just sends the voltage values, and the actual app decodes that. Although not so sure, this is all quite new to me.

It does have 2, that's right; that's why I was not so concerned about the coolant temperature in general – the gauge and the computer readings are unrelated.

Thanks a lot for the suggestion! I will go with your suggested route, sounds reasonable to me.

Meanwhile, I replaced spark plugs and wires today, plus cleaned the MAF sensor. The MAF sensor looked pretty good, but I discovered that the hose clamps were pretty loose, so I tightened them. The ride became more smooth, but the error codes are still there.

 

Check the app website. I know some OBDII apps need conversion parameters to translate values properly. For example, a bluetooth OBDII transmitter and app for my 2013 Toyota RAV4 requires the user to enter conversion factors into the app in order to translate the signals correctly. I'm not saying that's the case with your app...only that it's a possibility...and it's good to confirm that the equipment is working properly so that its reading can be trusted. I suppose you'll know after you put a new thermostat in if it shows the proper temperature.

Our motors are also known for having failed fuel rail gaskets and lower intake gaskets, which can lead to P0141 & P0171 codes. If you change out components and are still having the same codes, that would be a good place to look. Search for "fuel rail gasket" and "lower intake" or "P0141 P0171".
https://www.ford-trucks.com/forums/2...ke-gasket.html

 

first, on the o2 sensor codes and you are getting a heater error, I would check for a blown fuse in your fuse box.the fuse supplies power to all 3 sensors but is controlled by the PCM on individual lines. This would be the first place I would check. it will either be a 10A or 15A fuse.

since you are always running in open loop, it probably is not even using the downstream sensor yet.

on the temps, I suspect you have an aftermarket thermostat in it. I tried 2 of them and both always opened up too early and engine would never get up to full temp. As soon as I went back to a ford motorcraft original, the low temps have gone away.

 

OP, is there something from my earlier post and now the newest one immediately before this one that doesn't make sense or is unclear and being ignored? And all the talk about voltages and conversions to temp are actually completely irrelevant to solving the problem, and a waste of time to think about (and it's why I ignored it until now). This because, the ability to access "live", aka, real time engine temp is part of the OBDII diagnostics, a Federal requirement of all vehicles sold in the US beginning in with model year 1996. Vehicle make/model are not relevant, you're good to go. Focus on diagnosing and solving the vehicle problems, which at this time there appear to be two of them, neither of which should be very complicated to correct.

 

Your comment is demonstrably false, @CBB9M . Some bluetooth apps like Torque Pro require the use to input parameters (PIDs) in order to read the bluetooth transmitter data properly. For example, here are the transmission temp parameters I have to enter for my 2013 Rav4.



If the OP wants to ensure his smart phone app is giving accurate results for coolant temp he needs to confirm whether or not he needs PIDs (or whatever his specific app might require for customization).

 

Same thought RE the above false statement . PID needed to display tranny fluid temp is not an OBDII requirement; PID that provides access to live engine temp for all vehicles built in MY 1996 and beyond is.

 

You've confirmed that all bluetooth OBDII transmitters and the various smartphone apps are universally compatible and require no additional user customization for proper function?

 

Our tangential disagreement aside, @seva.zaikov , how's your troubleshooting going?

 

Thanks for all your tips. I can't contribute to the discussion about the conversion, but it seems that my tool shows the correct value.

So, I switched the downstream sensor, the thermostat and ECT sensor (the one which sends the data to computer), and idled it for about ~15 mins. Not only in the tool it got up to 190 degrees F, the gauge actually moved to the normal temperature range! So, it was all false speculations, gauge worked from the beginning, the problem was the cold engine.
Interesting, that inside there was a Motorcraft thermostat (the same I installed after). I haven't managed to test it in boiling water yet to see when it opens, but it seems that it is working properly now.

However, it seems that the trouble codes are back. I have not driven it yet, just idled, but in the scan tool they all reappeared under "pending codes".

So, tomorrow I am planning to look at the realtime voltage of upstream O2 sensors. I have read that closer to 1V indicates rich, and closer to 0 lean. Ideally, it should constantly fluctuate, which will prove that it adjusts, in my case I expect to see close to 1 (since it is quite rich). I hope my tool supports this type of live data.
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Now, if that proves wrong, I am thinking about fuel pressure. When I changed spark plugs, they all were equally coated in carbon (plenty of black residue), so I don't think it is individual injectors failing. So, if I am still clueless, I will probably do a fuel pressure test.

I actually checked them all (visually) for an unrelated reason (combination switch was not working properly) before. I think that there are additional fuses, so I can just try to swap a fuse for O2 sensors with a spare one, just in case. But I don't think it looks blown.

 

I looked at the realtime voltage of O2 sensors (via OBD scanner) and it shows steady 0 on all of them. Assuming that the data is correct, this is probably the culprit, not the wrong O2 sensors or something else. I looked at the fuse, it seems fine, and checked for continuity (was fine). I replaced it with a spare fuse anyway, but no change. Does it mean that there is no power coming to all O2 sensors?

I looked at the wiring in Haynes manual, and they say that it goes from pins 35 and 60 from PCM. So, these sensors are going through different wires – how they all not work then? And how to check these wires in general? I have only an idea to go from the O2 sensors and check wires manually going up, but again, if all 3 of them are not powered, I'd expect something wrong much earlier.