Late breaking news - and finally some solid facts (I think).
I decided to bring the Volvo for diagnosis ($120) to an independent provider where , I am told, one of the mechanics has been working on 240's since that were introduced. Results - both the original and factory replacement EGR temperature sensors are defective. This was determined by back probing the sensor wires with the car running to determine the amount of output voltage. Also that the there was a fault code (#2 diagnostic slot on the OBD) with regard to the mass air flow sensor. Specifically that the contacts are not good, resulting in inaccurate readings. I will get another sensor from FCP. Probably won't get it in time to meet the deadline for renewing the resistration.
Anyone know of a good explanation of how to do back probing? Especially with regard to these specific wires.

K (sometimes 1K on the Ohm test dial) means 1000

As in 1000 ohms.

"2.75 K" =

Or 2.75 x 1000 =

2750 ohms.

The value the above nominal for the EGR temp sensor for your and my 1991 Volvo 240 with EGR.

I may have gotten figures wrong in my earlier email as I'm horribly dyslexic. I originally saw "2.75 K" as 275,000 ohms, yet I know to check values several times.

Your other option, if you can, is to remove the speedo instrument cluster and surrounding dash bits, and pop out the light bulb for the EGR cluster, and reinstall it. Though you must be mindful to reconnect connectors and wires correctly or risk damaging it if you connect the wrong wire spade connector to the wrong terminal.

Perhaps the most direct way to test the sensor is to use the two probes at the end of the EGR temperature sensor. Some probe options can end in small alligator-style clips.

Here is how I test the oxygen sensor heater element using my multimeter. Though it is a cheapie Harbor Freight multimeter. Pressing the probe ends againts the pin. Art did say when I posted this the value may not be quite in range.



You are in CA-state, THE state that hates cars (or, rather, makes older vehicle ownership difficult), was built on the car, yet loves all the revenues the 40 million cars in the state generate for CA-state. EGR is a state mandate for a 1991 240 if sold there. My 1991 Volvo was sold in Germany and Germany required it. The same system version as on your 1991 240.

Though you can research this stuff on the i-net, EGR is a systes to direct some 10-15% or so of exhaust gases back through the engine to reduce nitrogen gas oxides, or oxides of nitrogen, or NOx. On Volvo 240, it reduces the mileage by 1 mile per gallon. Atmosphere is some, what, 768% nitrogen. So, fuel, air, compression, spark and heat oxidizes the nitrogen gas, and NOx can be a health hazard.

Political environmental policy made the need based on too much smog. Smog may be caused / comprised by NOx produced by transportation vehicle or any sort of carbon based power generation.

CA-state inspects emissions controls. Yet how closely depends on the SMOG test facility you visit. They will want to see that you have the silver preheat hose connected between the exhaust manifold. Cannot drive to a test with the EGR dash light on, as you know.

Questions?

Hope that helps you.

PS ......




Off topic, sort of:
Do you know whether the preheat flap valve in your air filter box works, or is it stuck to hot air inlet only. If stuck to hot air inlet only, that can raise NOx. Some will block the bottom air filter box air inlet somehow.



--
Give your brickboard.com a big thumbs up! Way up! - Roger Ebert.

Let me start out with an apology to Kitty'sgreyvolvo. Somehow, I did not notice that you had submitted a response and instructions on Oct. 26. I just noticed it today. So what is written below was written without having read that submission. Rather than change what I wrote below, I will add comments at the end after studying what Kitty'sgrey had to say.
Perhaps I should not have named the two black boxes BB1 and BB2 given that the Brickboard is known as BB. While on the subject does anyone know what that little black box that I named BB2 is called and whether or not it is discussed in the Bentley Manual? Phil wrote:
" it powers the headlight relay for your high beams.
This relay provides a flash feature that can be used anytime whether your ignition switch is on or not.
It can be used to attract attention to your car if you are alone in a parking lot or want to tell someone where you are parked. This without blowing your horn, that I think, does not work if your ignition switch is off!"
Here is my experience with that "little red wire" that Phil (Machine Man) referred to in a previous comment. It originates in the little black box (BB1) under the hood, at the top of the left fender. That black box has a manifold of lugs in it (not sure if these are the correct terms). That manifold is directly connected by wire to the positive battery post so any wire connected to the lugs is always hot if the battery cable is attached to the battery post. This red wire we are talking about is attached by a spade clip to a lug on the always-hot electrical manifold. Using a six volt tester, I found that the red wire dives into a wire harness and emerges below where it enters a wiring harness connector (The connector consists of four wires, two red and two white. One of those red wires is the red wire we are discussing here.) Since that particular wire is hot, I assume it is providing power to the black box (BB2) that lies just below the vacuum booster (the term used by the Bentley manual). Here is how that red wire fits into my experience with the car.
Four days ago, it had been a week since I had deleted the 4-1-3 trouble code with it not returning, so I figured I could run it over for the needed smog check. Just after I exited the freeway a couple of blocks from the test site, the "Check Engine" light(CEL)came on! How aggravating. I am wondering if my driving five fast miles on an unusually hot day might have been a factor. I abandoned that mission, stopped, and deleted the code, but it came right back after two blocks. So much for outsmarting the car. I deleted the trouble code again. Then I explored that red wire discussed above that comes out of the "little Black Box" (BB1) and goes to the other box (BB2) just below the Vac motor. I opened up BB1 and slipped off the red wire going to BB2. I then confirmed that the CEL still lights up (along with all the other red lights on the dash) when the key is turned to position #2 (like you see in a normal start up). I then started the engine and drove home without the red wire connected. I was not aware of anything not operating, even though the red wire was not connected. When I got home, the Check Engine Light (CEL) had not been triggered, and I then put the red wire back in place on the hot lug. Yesterday, three days later, the CEL came on again - no hot ride involved. I again erased the code and the next day the CEL came on again. Each time the CEL comes on, I erase it. Seems that interval between the appearances of the CEL is getting shorter. It seems that the "little red wire" has not yielded any useful information, so let's say we are done with that.

Clarification of fault codes. I found no fault code in position 2 of the on board diagnostics (OBD). Position 6 (EZ-116 ignition systems) of the OBD showed the fault code of 4-1-3. This code is not mentioned in the Bentley Manual. I believe it is listed erroneously (page 280-4) as 4-3-1 in Bentley ("EGR temperature sender signal missing or faulty 1991 and later California cars"). That is the only fault code that has come up. Bentley (page 253-6 "Emission Control" chapter) tells how to test the temperature sensor. As stated previously, my test showed it within normal limits. Machine Man suggested looking to the Vacuum Booster as the source of the problem. I tested the vac booster using the procedure in Bentley (pages 253-5 and 253-6). It passed that test. That same section mentions that "The self diagnostic program of the ignition system unit can be used to activate the vacuum booster and perform a functional test". I have not found that section to try it.
Question: Should I do further testing of the vacuum booster, given that the test specified in the manual seemed to indicate that it was working properly? That is, grounding the wire caused the engine to stumble which happened because the vacuum booster let vacuum into the EGR valve and opened it. If further testing is recommended, are there specific instructions on how to go about that (what do I attach and where)? Can it be done leaving the booster on the car?

Following the recommendation of Machine Man, I removed the plastic clip on top of the vac booster. I did not find noticeable deterioration of the mesh filter in there, but I removed it to see if that changes anything. I then started the engine and immediately the CEL lit up.

I have not yet followed Kitty'sGreyVolvo advice to check the temp sensor by seeing if it continues to have the correct resistance when the wires are wiggling. That will involve taking off the bellows between the mass air sensor and the throttle body to get access.

The car's latest behavior now is to consistently throw the 4-1-3 code after about one mile of driving.

This would all be more interesting and enjoyable if I were not so inundated at this time with work and other demands on my time. Don't these cars know that there are other objects that need attention? Not to mention wives, children, etc.

Okay, I just reviewed kittysgrey's comments. Thanks for the clarification of the junction block (power distribution block), and thanks for the info on your car. Would your car have the same EGR setup that mine does?

I hope I have now been clear that the only fault code is 4-1-3.

You write:
"You have some manner of out of resistance range as a wiring or continuity fault between the sensor all the way to the ICU connector. "
AND someone wrote:
" you may wanna back probe the connector at the ECU in the event it is a wiring problem beyond the EGR temp sensor lead and connector.
What specifically should I do to find out what I need to know?



I did not understand when you wrote:
"- - - you may leave the old EGR valve in the pipe, get a replacement, if you can, that has no EGR temp sensor hole. Leave the old sensor in the hole, though it may clog, eventually, or seal it up with appropriate hardware (Short machine screw / washer, some sealant). "
However, I believe we are not at the point where replacing the EGR is necessary. If I decide to replace the EGR valve, do you know if new part(s) is available? Looks like a big job.

One last question: I delete the code each time by following the instructions on the OBD.Is there another way to reset the computer after it throws a code?

Today I hope to have time to check the temp sensor wires by wiggling them.

Jerry

Phil (Machine Man):
Thanks again, this time for clarifying the issues I brought up in my first message to you.
I located the "little black box" (we'll call that black box 1, or BB1). That terminal has lugs on it that are all "hot" because the thick wire feeding the box (as you said) goes directly to the positive battery cable.
QUESTION: Are you are referring to the red wire going from one of the lugs on BB1 directly to a small black box (we'll call this BB2) directly underneath the "vacuum booster"(VB) ? That box has three other wires attached to it - two whites and another red. I do not know the purpose of that box. Do you know the purpose of that black box? What happens if that (hot) red wire going to BB2 is disconnected? Would that cause a fault code?
Before attempting to do what you suggested, I followed instructions in the manual to test both the EGR valve and the VB. When I applied vacuum (using a handheld vacuum pump you recommended) to the EGR valve with the engine off, I could hear it moving. When I repeated that at idle, the engine stalled. That means the EGR valve is working. To test the VB, I followed the manual instructions to isolate one of the two wires and grounded that wire which also caused the engine to stumble, but not as definitively as directly applying vacuum to the EGR valve. This confirmed that the vacuum device works OK, or so says the manual.
I have erased the code and it has not returned - yet. Given these findings, I decided to do no further troubleshooting at this time. My next step is to get the smog check while it is being nice. After that I will return to my troubleshooting. I continue to be concerned that I might damage the VB by trying to pry the plastic cap off the top. I have learned over the years that old plastic, such as the interior trim, can become brittle and break easily.
Meanwhile I see that there is a new response to this thread which I will study and see what I can learn. Again, thanks for your help. I hope that future users of this forum might benefit from this exchange.