Replacing the O2 Sensor was easy!!! Still a rough idle at cold starting, but damn this car rolls soooo smooth!
I did a cursory cleaning of the injectors, and ran at 5000 rpm for a fews miles to impose a strict cleaning, and this car really rolls!!! I only obsess about about a change in my idle smoothness...

Sorry not to respond sooner. I use my "lemme halps you halps yer RWD Volvo" on the different face-space-bube-toob-book. The charge gets used up. Like writing "how to" novels for brick walls sometimes.

CEL=CHECK ENGINE indicator light on the dash speedo instrument cluster.

So far as I know, not having laid hands on a running 740/940 in now a few years, 700/900 normally aspired (not aspried, we tried!), meaning without Turbo, can mean a 740/940 equipped with the Bendix Rex (ignition) / Regina (fuel) engine control system. Unless a non-Turbo, normally aspired, Bosch engine control system on your 1991 740. Unless it is a 1991 Volvo 740 Turbo.

Look at this iPd article image.





So, what does the ignition coil look like in your 1991 740?

If round Bosch ignition, do you have a 1991 740 Turbo? Or no Turbo?

So, as you have EGR, you or your mechanic clearing fault code 2-4-1 does not mean the EGR fault has cleared. The EGR fault may return. If you do not hear an exhaust leak someplace at the EGR piping unions from the backside of the exhaust manifold to the EGR valve inlet, or the second EGR pipe between the EGR valve out and the second EGR pipe connection at the air intake manifold, you may have a clogged EGR piping. More so the case if you use mineral (not synthetic) oil.

The two worst possible failures of EGR is:
- The EGR pipe between the manifold and the EGR valve inlet is rusty and will fail, meaning an a rather noisy exhaust leak.
- The EGR valve fails and remains stuck open, so constant exhaust is pumped into the air intake manifold.

All EGR systems should be inspected and correspondingly serviced is fault is found. You to not need to necessarily open of the EGR pipe unions to check for clogs. Easiest at the union at the air intake manifold as combustion gasses condensate from the very end backwards through the EGR valve (causing it to get stuck open, more so, than closed) or anywhere near the EGR valve inlet.

The other issue may be a faulty EGR temperature sensor at the EGR valve (newer setup) or somewhere along the pipe between the EGR valve output and the air intake manifold connection.

You can perform a check for OBD codes yourself and save around 90-150$ hourly.

See:

https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

Perform "1. Diagnostic Test Mode 1: Fault Code Retrieval"

Repeat the test if you encounter error codes until no new codes display.



Whether Bosch or Bendix Rex / Regina, it is socket 2 for fuel system fault codes and socket 6 for ignition.

Also, fault code 3-1-1 means the Volvo 740 traveling speed is not relayed to the engine control system. The engine control system uses this as measure for load to better refine fuel trim for a traveled speed. With Bosch engine control, like on my 1992 240 GL, engine RPMs may be somewhat higher than idle after coming off the highway until you shut the engine off.

Your 740 speedo instrument cluster can have issues with cracked solder welds and other things I'd read about. I'd not ever cracked, inspected, or repaired a 700-900 speedo instrument cluster.

Fault Code 2-3-2, usually socket 2, is a general imbalance between what the engine heats air, fuel, and maybe spark) and what the engine farts out, exhaust gasses at low engine speeds. So, maybe a slight air intake leak is the easy answer at any point downstream of the air mass meter if Bosch / and the air pressure sensor and the air temp sensor if Bendix Rex / Regina. The throttle body gasket also ages and become stiff and loses seal.

How many miles?

Or - could be a minor exhaust leak ahead or a larger exhaust leak after the oxygen sensor.

Yes, as you are in NorCally, near my former Novato, Marin County home, you will want to ensure your EGR system works, lest you use an emissions (smog) inspection service that does not look to closely at check engine lights or at the emissions controls.

Also, if you do not have Turbo, inside your air filter box is a preheater flap valve that, when working normally, allows a measured amount of air, preheated under the exhaust manifold heat shield. That silver accordion style hose between your exhaust manifold heat shield and the air filter box is the preheated air conduit. These get stuck open on non-turbo 700-900s, will roast your Bosch AMM / MAF or your Bendox Rex / Regina fuel pressure and air temp sensor in time, and may be causing some rough idle.

You can open yoru air filter box, remove the air filter, and look at the flap valve. Is the flap valve turned hard over to the cold air intake, so only hot air gets in? Yes, it is winter, and it is cooler where you are in NorCally, yet if you've not replaced this in like the last two years or so, than, from a vendor like iPd, FCP Euro, and maybe AutoZone:

Wahler 70411, the air filter preheat flap valve thermostat:



At the age of your 740, some causes for rough idle may be:

- Old ignition wires (run engine at night in dark, see sparks?), old or worn / cracked rotor cap, old ignition rotor under the distributor cap, or old spark plugs or spark plugs that need a gap reset to .28"-.32".

- brittle air intake port manifold gasket leaks. Or other vacuum leaks downstream of the air mass meter / air pressure / air temp gauges.

Maybe just once, though 27 years is a bit late, you can check torque on the air intake manifold nuts with a 10 mm hex socket on an extension. I would have done this about 15 years ago or so. The gasket shrinks and gets brittle and loses seal, so checking torque may not be the best thing to do. You can check for vacuum leaks at the fuel injector seals and the air intake port manifold gasket using throttle body cleaner with the red straw and gently spray at the fuel injector seals and the air intake gasket on a cold engine.

Using a digital multimeter is easy to do and with observance for safety and not causing electrical shorts you can help yourself to diagnose problems should they be electrical.

My problem with fault diagnosis is that I treat the entire system at once when I buy these cars, so I'm piss poor at step-by-step diagnosis sometimes.

Anyhoo, check them codes maybe once a eek or twice that to see if any fault code turn back up again in either socket 2 (fuel) or 6 (ignition) and these folks will guide you so you can resolve any issue.

The EGR problem fault code may return. My bet is the piping is clogged. You or your repair service can correct the issue before the next CA-state emission test.

Questions?

Hope that halps.

Face-toob-space-book-Volvo 240 fan club responder boyeeeeee "lemme halps you and your face space 240" too often.
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