Bosche ignition not "regina"
old style distributor hall effect sensor, not the crank sensor on bellhousing.

So I started a thread entitled "My 740 lost it's mojo"

it got a bit crowded with info so I decided to try to recap my findings.


- It was shaking abnormally at idle

- during takeoff when cold it seemed to have low power accompanied by a whitish cloud of exhaust followed by better performance, but not good performance.

- it seemed to sort of "come alive" after about 2-3K RPM but had weak power at low revs


checked plugs, noting abnormal but #3 seemed to smell of gas.

changed all 4 injectors and fuel rail from another volvo of similar age, no big difference but it "may have solved the cloud of smoke issue. I think I had a leaky injector # 3

checked timing , it was off but no great improvement. Its at 12 degrees BTDC now.

checked compression all around 150 PSI, so no seriously burned up valves.

engine sounds normal as far as it's internal mechanics are concerned.

changed for another idle air valve, little difference noticed. I cleaned the one I put in before installing with carb cleaner , I didn' t dry it so much.. a day later with a different AMM I could smell the carb cleaner, maybe it started opening? when I installed it I unplugged it during idle and saw no change. I was expecting unplugging it should change the idle, this lead me to thinking the ECU isn't "thinking right" adding an indicator lamp shuld allow me to watch to see if or when it has power.

changed AMM for another used one. dramatic difference.

now instead of it being weak on power at low RPM's it is weak if I give it throttle at about 2K or 3K , it seems like maybe it's too rich and sort of flooding, it stumbles , if I back off on throttle it feels more normal. now it's almost the opposite situation weak at high revs, before it was weak at low revs.. I dont know the symptoms of how the AMM "normally fail"

the idle now seems rich, I can smell it in the exhaust and it idles as if it's running a bit rich whereas before it was lean. it kind of goes BLUB BLUB BLUB BLUB at idle now, hard to describe in words.. ;-)

now it takes a few attempts to start. Its cold out , we had snow I think its just to rich upon startup and wetting plugs but cranking seems to get it going.


things I plan to do about it:

1- try other Air Mass Meters, or replace for a new one, a new bosche AMM is over a thousand so maybe an aftermarket one? it nmde a difference and that causes a situation where I dont know if i have two bad ones. I have a couple more in cars that I can swap and try.

2 - I want to check the temp sensors, maybe the one on the block or the one in the air filter box? if the computer thinks the engine is warm whan it's not or the reverse it may cause strange behavior.

3 - block off the air filter flapper so it can't get warm air and blow the AMM from excess heat.

4 - hook a test lamp up to the idle air valve so I can see when it has power or when it's on , difficult to see changes and I'm not sure if it's operating. I did that with the fuel pump too.

5 - check idle switch , swap the TPS ( throttle position sensor) it could be bad or maybe the switch is not working electrically , it does click. verify with a meter. maybe I can check the continuity and see if the TPS is somewhat linear or if it's jumpy. I can check out out of the car and then swap it in there.

6 - replace hoses and clean flame trap. maybe I can get some appropriate line locally for the hose to the canister and from fuel pressure regulator to the manifold. they seem sealed but hose is too old to trust. I did not find leaks, want to try propane or quick start maybe a bad manifold gasket , I have a spare gasket. I might pull the manifold ot get to the flame trap hoses, they are tight in there..

7 Watch and observe to see if I get any more issues white clouds of smoke for the first block after startup in the morning. I think I had a leaky injector and that may be resolved. I can try to crack open the fuel rail after an hour and see if it looses it's pressure.

8 it still shakes a lot at idle. I'll plan to drain tranny and refill , maybe I need to drain the torque converter too to get a full change. I can try to remove the pan or check any screens.. not in the snow so I need to wait. check tranny fluid level during idling.

9 fix oil leak , I think it's the pan gasket but I can add oil rather than crawling in the snow to do this,

I think that for some reason, maybe a bad contact or a bed temp sensor or for some unknown reason the computer isn't controlling what it should at idle and it's just causing this shaking at idle and overall really poor performance. maybe runing in limp home mode..

10 - I will verify that the timing belt hasn't skipped a tooth. I dont think that is the issue but it is something I can verify. I know the engine can make full power as it does at times, it has compression , it could be out a tooth, maybe from a backfire. I changed it at 80 K so I guess it's due.

I got some info from the Porsche forum, I have a /85 944 that I bought dead from a friend for a good guy price. its been dead for about a dozen years..but I don' t think it has a lot wrong with it. the Porsche has a bosche ignition system too but it has a "barn door" and basically a rheostat or potentiometer , It seems a lot more simple but like the volvo it was made before much diagnostics were there to help.

I want to get that thing running ,, Im excited about that.. Ill wait till summer.. anyway on the porsche forum they found a temp sensor responsible for a similar issue and it was a bad temperature sensor in the head. the engine wasn't reacting to the temp correctly, the sensor can be checked for continuity at various temps.

11.5 - I might be able to compare to some other used ones or just replace that. Ill first just try putting my meter across the terminals of any other temp sensors and see if I see consistency between any I have around.

- I think if the ECU get bad readings from the temp sensor it may adjust my timing and this may have caused me to THINK i got the timing right but It may have been compensating by adjusting the timing to compensate for a temp that was false.. In my 88 van you unplug the "spud connector" that disables the timing adjustment by the ECU while you set the timing, then it is reconnected. The volvo doesn't seem to mention unplugging anything during timing adjustment. If the ECU is trying to adjust the timing and I set the timing without knowing that, then maybe it caused me to set it wrong.

I think there is a temp sensor in the water-jacket of the head , is this the one the ECU looks at or is there a second one I missed? I need to verify that.

- there could be another temp sensor that I'm unaware of and need to discover and check. I'm looking for a fault that is confusing the computer.

11 - I can try "normal" plugs, maybe it doesn't like these new "iradium" ones with the tiny electrodes, they are supposed to last longer but maybe they aren't right.

12 - I can try swapping plug wires they aren't old but they are not OEM "boogilchord" ones either.

13 - I can yank the fuel rail and try "starting" to see what the spray looks like I haven't tried this before. I'll be careful of the gas causing a possible fire. extinguisher at the ready.

14 - I have some filters that go inside the injectors. , maybe I can hook the fuel rail to compressed air at about 30 PSI and check by applying power and see if I detect a leak. I have some filters and the pintle and Orings so maybe I can disassemble the injectors to see if I can fix or clean or see anything there..

15 - the car does not have the little diagnostics check box on the drivers side fender, its earlier. I may be able to make up a test LED and see if I can get any codes that way. Im trying to decipher that procedure. I need to look for the test wire near the coil and see if I can get any codes..

- I printed out the great info that was provided earlier in the thread so I can take it by the car and go through some of the checks mentioned.

16 - I want to open the ECU and ICU and replace any electrolytic capacitors. I have spares but I might have a bunch of them all with bad caps, that could improve them but it's somewhat of an experiment.

- I can swap the ECU from my 240 and it will start. but the ICU is different. the plug is different. If I can verify I have a working spare then I can try to "recapacitorize" one and see if it changes things. I dont think soldering in new electrolytic caps of the same value will harm them and they may improve.I found signs of leaky electrolytic caps in my van's ECU it's an 88 ford. also bosche ignition.. the board was discolored a bit near the electolytic caps. these volvo's I own and all my spares are about the same age give or take a year or two. I think all the electrolytic caps might be simply expiring due to age.

I'll try to report my findings , It may take time as working in the snow and dark after work might get trumped by more fun and warm things.

I managed to back into something in the snow and damaged the lower portion of my wagons driver side tail light so maybe I can find a new/ used one or glue it up with a patch, red tape for now.. it's just broken near the very bottom of the lens.

I know the back lenses for the 88 740 sedan are really hard to get. I had advertised as I was wrecking a 740 and had a real lot of requests for those, wagons have different tail lights though. It seems some cars have aftermarket solutions and some do not.
I was able to find new repro front indicator lenses..


I like the car, I want to fix it, Its at 160 K , not bad for it's age.. it is a frustrating thing to solve because there is a lot to understand in order to be very proficient at troubleshooting. The great shops that I have dealt with in the past seem to be closing down.

I really appreciate the replies to the original thread. I'm learning as I struggle.. I'm determined to solve this, somehow !

I put it back on the road for this week, because my van is completely scary in the snow.