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Well, let's see. Your 740 GL is LH2.4 Runs in closed loop but not in open loop. Individual tests have not pinpointed the issue. Excellent suggestions have been made and, btw, yes n/a 240's and 740's use the same ecu's and icu's as well as the 016 amm.
I have an 89 245 that gave me similar fits when I bought it. Using a digital volt meter, all my sensors were within the specified "range". I detest throwing parts at a car until it works again. But, this is when I had my Volvo epiphone...While many running issues are easily traced to a single item gone bad, there are those times when the issue is caused by a series of sensors all functioning on the low side of the acceptable "range". Individually, they check ok, but the combined almost synergistic effect is poor running. In a nearly 20 year old car, even the connectors are suspect...the pins in the amm plug can get loose and pushed back rather than connect. I know you've swapped amms...check the connectors for loosness and corrosion. Using your Bentley's, ascertain the correct voltage is going to the amm. This will tell you the ecu is doing its job and the connectors between are ok. If it's not acceptable voltage, pull the kick panel and test the correct pins on your ecu. If the voltage there is ok you have a break in the harness.
I doubt it's your coil or icu...these two items are the most durable in the system. Assuming you have checked your plugs, wires, rotor and cap and have good spark when at operating temp you can rule them out. I will say IMHO Bosch standard plugs and NGK standard plugs have given me the best service. Platinums haven't done as well for me.
The engine coolant sensor is a PITA to swap w/o removing the intake but w/ an inspection mirror you can make sure it's not pulled the wires loose from the connector...that wire is too short in every LH Volvo I've seen!
A bad ecu is always a possibility...smell it! Burnt electronics, no matter how small, have a distinct aroma that stays for months, if not years!.
My plan?
1. Check all voltages as mentioned earlier; write them down. When you're done, compare the voltages w/ the book. If you have 2 or more sensors reading on the low side of acceptable, welcome to Volvo Land!
2. Scrupiously examine your ignition wiring and the insulators on your plugs. I've had cracked plugs that opened up when warm.
3. Check vacuum for leaks. The two usual methods are a. spraying carb cleaner on the hoses (don't forget the big ones below intake at oil trap), bases of injectors, intake gasket, throttle body...a leak will be indicated by an increase in rpm or b. using propane (unlit) in a similar way, you have to have a long piece of vac hose on the end of the propane neck and have it VERY close to the area you are checking.
4. Have someone rev the engine in park/e brake on while you hold your hand near the exhaust pipe. If you don't have an appreciable increase in flow and/or small particles hit your hand...your cat is bad.
5. If you do not know how old your O2 sensor is...replace it. All 3 wire sensors are the same; you can spend as much or as little as you like. I go the cheap route and solder the wires. Some people do not feel they can do less than buy o.e. Bosch, connector and all. I tend to change mine rather often as they never cost me more than $30 and a lazy O2 can send you all over the engine bay looking for answers.
6. If the new O2 sensor didn't fix it and your cat is ok; swap out the amm w/ a known good one.
The amm tells the ecu how much air is flowing through it, the O2 sensor tells the ecu how rich/lean the exhaust is. Based on this information (w/ help from the cam sensor, crank sensor, ecs et al...the ecu determines how much fuel to send to the injectors. That's why leaks in the air duct between the amm and intake can mess things up. The O2 sensor says lean so the car runs rich. Without accurate input from the O2, the ecu has to "wing it"; generally not too well. But I must say IMHO, rwd Bricks will run poorly longer than any other vehicle I've seen. Might not be right...but they get you there!!! I know I've over simplified, but sometimes it's good to revisit the basics.
Your car runs best when first started cold (closed loop) and runs like crud when sensors come on line (open loop). The joker in the deck is the cat; rule that out asap. Then, install a new O2 sensor unless you know the history of it. If you know that it is relatively new (less than 15k) measure the voltage output at the firewall connector. Then measure at the ecu. Then, do likewise for the amm. Although there is a litany of issues that can cause running problems on rwd Bricks I generally find it to be a combination of culprits w/ an overlooked O2 in the middle of it all. Good luck!!
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When Volvos are outlawed, only outlaws will have Volvos!!
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