'92 740T OBD 4-1-3 EGR Temperature Sensor - Ohm and Voltage & Falling MPG

Hello Fellow Brickers,

Soon after installing a new turbo CHRA (TD04-13c) I got a new, 4-1-3 OBD fault code. The code is for a faulty EGR temperature sensor/connection.

The first time it appeared I removed the sensor, and measured resistance at the pins while I heated the sensor gently with a lighter and all seemed to check out O.K - Ohms increased as temp went up. I then cleared the code and checked the voltage at the connector, the result was 5 volts.

With code cleared I drove about 40 miles before the engine light popped on, and again it was 4-1-3. An hour or so later after stopping, I again checked the feed voltage, wiggled some wires to check for loose connection, but found a solid 5volts. I then checked the resistance across the sensor pins, and found it to be 579 Ohm, and 15 minutes later 577 Ohm - dropping as it cooled.

Would anyone be able to give me an indication of what else might be the culprit for this fault code, other means of me testing? Perhaps 5v is not the correct voltage for the EGR temp sensor? I have been unable to find specs to that effect...

I should add that i have checked that the diaphragm in the EGR is working, and holds pressure, and that the car seems to have dropped from a nice 23/24mpg to 14/15mpg since the code appeared.

Any insights, suggestions or moral support is greatly appreciated .

Thanks for reading,

James

I have fixed this issue several times. I have about 4 sensors here and only one of them would keep the light off. It was a pipe-mounted one with approx 550 ohms cold. The EGR valve-mounted ones were approx 175 ohms cold and would instantly trigger a CEL.

5V reference should be normal. I don't think it's a 12V.


You have a bunk sensor, wiring, or EZK. You can check resistance all the way back at the EZK box to the sensor. Or try another sensor. These sensors are a known weak point to me. I've had them go full open circuit before.

Thank you for the fast and valuable insights, ZVOLV.

I can confirm it’s a pipe mounted sensor, so will check resistance back at the EZK box...which I’m researching is above the accelerator pedal...would you be able to advise off-hand where the cable for the EGR temp sensor inputs/ports?

Best,

J

EZK ignition box is on the pedal bracket. Consult a wiring diagram. If less than 2 ohms on each wire, end to end, fire a sensor at it. If it comes back, try a EZK box.

I bet it's a dying sensor based on experience.



You could swap to a non EGR ignition box, or get chips, to make the light go away, but that renders EGR inop and your NOX levels will go up, which you don't want at smog test time.

Thanks for keeping me afloat here, ZVOLV.

All options seems good, but thinking to keep the car as close to stock as possible - fixing the EGR rather than delete / chip...for now at least

I've now disconnected the harness from the EZK, and located pin #22 on the harness side as EGR temp sensor input...but stumbling over how to check Ohm reading at this point...

Would you mind expanding a little on how to test the end-to-end resistance - if facing just a single pin? Do i need to pull the Temp sensor wire out from the harness for this?

Understood that its likely the sensor, will begin looking for replacement in the meantime.

Thanks again for your insights,

J

**

As a foot note, incase others stumble across this thread - here are the 3 most useful wiring diagram sites i found for both ECU and EZK.

http://www.lintsi.info/wx/volvo/lh_2.4/B230FT_LH2.4.htm
http://www.autoelectric.ru/auto/volvo/940/1993/940-93.htm
https://www.davebarton.com/pdf/Harness_LH-2.4_240_EZKPinFunctions2020.pdf

You disconnect the EZK connector and the temp sensor. Then you check the resistance if each wire tip-to-tip (sounds gay) and make sure there isn't an open or high resistance. I use a 10 foot retractable test lead set and the proper test probes which allow me to front probe without damaging the terminal by spreading it out.

You should BACKPROBE unless you have the proper FRONTROBE tool. You can make some from old male terminals. This will also allow you to check for proper terminal tension. You slide the probe in and out and FEEL that the male terminal is sitting tight in the female terminal.

Thanks ZVOLV, for taking the time to explain a little further.

I followed your instructions and completed the tip-to-tip test. The reading sat at 0.8 Ohm, and connections also felt good so guess that means its not the harness.

The only other symptom beside engine light / 4-1-3 is a deterioration in my MPG, now around 14/15 (when it was around 24/25). Also smelling gas vapours shortly after hard acceleration...Do you think that either of these symptoms point to the Temp Sensor or EZK being the issue more than the other? If so, will start there.

It's funny that the EGR checks out O.K. with with temp/resistance readings, but would trigger the fault at random. I'm assuming the EZK is contantly reading the temp sensor, and making adjustments, so wonder why does the engine light not come on as soon as ive rolled down the driveway? ay ay ay!

Thanks again,

J

Maybe try a fresh O2 sensor for the MPG concern.

I recently failed smog for NOX at 900ppm. Tech recommended a new O2, confirm EGR working well, and fix lean running issues. Fresh Bosch O2, installed EGR temp sensor correctly, fixed a potential air leak:I got NOX to 450 and barely passed. I think 507ppm was the limit at 25 mph at 15 mph I was in the 200's A fresh cat would get it to zeroes.

Also, disconnect the pipe at the manifold and clean the hole. It's probably clogged. I don't think that would throw a code. I had my EGR temp sensor plugged in, but just hanging in the breeze and it wouldn't throw a light. FYI.

You could also monitor the voltage of the temp sensor, apply vac to the EGR valve, the engine should stumble, and voltage should change on the sensor as EGR flow occurs.

Thanks for the tips, ZVOLV. And sorry for the delay.

Cleaned the piping and all seemed clear (had also done it when replaced the oil separator about a year ago). Also tracked down a new EGR temp sensor, and will update after it arrives and have installed it.

Nice work on the NOX drop. And good reminder for me to check for vac leaks.

A new bosch O2 was installed about 6 months ago, so unless running the car with a faulty EGR sensor could turn it bad (?). Will check resistance next time i'm there.
Thanks again

It may be worth checking your coolant temp sensor resistance at each computer.

A skewed MAF could cause poor MPG.

You could also pull the fuel ressure regulator vac hose, engine running, and check for raw fuel coming out.

Thanks for the electric test pointers. I'll find a known good MAF to switch in or run a check on the existing when i find one online. (engine idles poorly if unplugged)

New sensor arrived, unfortunately its a Negative Temp Coefficient, which isn't compatible. Assuming this is the one for valve mounting, rather than pipe mounted. The hunt continues...

Meanwhile, would you happen to know the Ohm reading of the working pipe-mounted EGR temp sensor you installed, when engine is hot?

My hot engine sensor reading was 680, and room temp is 550. A difference of 130 doesnt seem alot?

TY

Having been there, got a few suggestions.

1. Check the resistance between both pins for the temp sensor, make sure that resistance matches what you have at the sensor itself. This also checks out both wires in the same shot.
2. While you're still connected, wiggle the wires to the temp sensor. 2 of the 3 I had actually had broken wires so they would test fine, then fail while driving.
3. With the engine cold, start the engine, let it idle, check if the pipe the sensor is in is getting warm. If so, the valve is leaking, as it shouldn't be open at idle. It's very common for them to get carbon buildup over the years.

As for the mileage concern, when the EZK throws that CEL, it will throw the LH box into limp mode so it will run rich, down on power slightly, and lower mileage. I saw roughly 2-3mpg drop when mine was acting up.