So I got a junk yard ignition power stage and swapped it in. It still misfires. It's still getting worse. I'm going to drop in the new coil I got next. I decided to paint it white since the new one was left bare metal and the original is black.

 

Yup, dead on. Well, now I dropped in a new OEM coil. No difference. I have a new cap and rotor that are going on next. I think I'll check all the engine grounds too.

 

So I need suggestions on how I should go about testing and isolating this misfire. I'm going to replace the car and rotor, but pretty sure that won't do anything, as I tried an old one that never gave me any problems. So where should I head next?

 

What about the Hall sensor and/or wiring in that vicinity? Also, did/do you have an extra ecu to try?

 

I'm out of spare ECU's to try. I did have a slight suspicion about the hall sensor. I put this one in about 15k miles ago. This one would rub the shutter wheel if you put an radial load on the shaft. I'll pull that out and check for wearing action.

 

I got it! I think I got it! Non Han Solo style. I had an old junk yard distributor core of unknown age kicking around with hall effect sensor of unknown condition on it. Since it was suspeced by more than one person, I figured that it was worth a try even though I replaced it just over 2 years ago. So I cleaned it up, secured the connector and the misfire seems to have evaporated into the cold thin air! I may invest in a new hall sensor since I have no idea how many miles are on this guy.

 

I take that all back, it's started doing it again. Bah!

 

I hate intermittent problems like this. everything you do seems to fix it until it comes back a day or two later.

gremlins, must be gremlins!

or bad wiring somewhere.

 

Gremlins indeed. Wiring is suspect I suppose. If only it weren't below freezing now and or I had a garage.

I got a few more clues to throw into the mix:

Maybe it's just one. The misfire seems to happen when the car is running at a "balanced throttle." What I mean is it's not really accelerating or decelerating. When I punch it, the car takes off fine. When I engine brake, once again no problems. It really only happening when I'm holding a cruise speed. I've never had a misfire in the boost zone. So on an interstate cruising at a constant speed, the car will start intermittently missing.

Also, the car bucks at slow speeds when holding a speed. I thought this had something to do with the hunting that the car does at an idle. I was thinking that there's still a little vacuum leak. I can't find it. I've found several and plugged them pressurizing the intake system and spraying soapy water on it, but I still hear a really small one near the throttle body. I can't find it, though. Honestly I don't think that this misfire issue is caused by a vacuum leak.

Cold temperatures make it worse and it wasn't doing this until last winter. I'm going to pull my distributor again and check for oiliness. I have no idea what condition the shaft seal is in on the unit I just installed.

Anyway, that's some more stuff to chew on.

 

I don't know much about 740's or turbos. But a misfire is almost always a ignition/timing problem. It has the distributor mounted in the back of the head, correct? (vs. block mounted). Does it have a hall sensor in the distributor or a crank position sensor?

Will it misfire if you give it throttle in neutral? If so watch under the hood when it's dark, to see if you have stray sparks off the coil or any of the ignition wires.

 

Agreed,

I have new plugs, wires, cleaned the distributor (I have a new cap a rotor, but the one one isn't that old, I just keep forgetting to swap it on), and new coil and a junk yard powerstage andl recently done.

 

Okay, new cap and rotor are installed and the problem persists. I'm driving it home for Christmas tomorrow, I hope it makes it. It's going to be about 40 hours on the road. May the norse gods protect!

 

40 hours?!? thats darn near coast to coast !!!

 

Yeah, I made a stop in Wichita, but I crossed the Rockies and and the Appalachians on a severely loaded Volvo. At least it won't be that loaded on the way back after the first of the year. It's good to have it back at home in its garage. We only have one spot in the garage and of course I took it.

The car seemed prone to missfire in certain boost and vacuum ranges. The beginning boost range and the lower vacuum range. I'm still confused. MOst of the time in these ranges it would not really missfre, but it felt a little rough, like one cylinder fires good and hard and the next not so much and in no rhythm. I think I'm going to pick up a new fuel injection computer. Maybe it's lost some data or something. I found this guy on ebay:1988-1989 VOLVO 740 2.4L ECU ECM # 0 280 000 591 | eBay

The PN is 280 000 591 not the 280 000 941 as posted earlier, so I'm not sure it this is the 'smarter' computer or not. Mine should be 280 000 541. Anyway, I'm going to see what the junk yard holds for me.

 

Having been another driver of this car for the last couple years, I thought I would add some comments that Titan Joe might have missed. It was mentioned, but I'm not sure emphasized enough--the problem really only seems to happen when the outside temperature is cold. When Joe reported that he thought he had it fixed, it had gotten warmer for a day or two, and when I had been driving it, there hadn't been an issue. But when I started it up on a cold morning, it started missing again.

The other thing is it happens in a sort of two stage style: when the motor is cold and the outside temperature is cold, you can almost count on it misfiring (sporadically) until the motor warms up. But then typically nothing happens for a while, and it is only after driving for a while (over 20 minutes) with a warm motor, it begins to start missing again. It is sporadic again--you drive for a while, and nothing may happen for a bit.

Anyway... at this point I rather am inclined to think it is the Ignition computer. We'll see if Joe can find one at a junk yard tomorrow.

 

Well, at least I used a question mark! LOL...The 591 is the dealer replacement ecu for the 541. A little smarter than the 541. Those are the only two turbo ecu's for LH2.2 cars.

 

I was reading last night and thought that I read 941 was correct after I posted, so I checked around a bit more now because I was surprised at swift's last comment (thinking I goofed). But what we have proven here is that numbers I confusing anyway.

I called around some local yards, besides them telling me that a 1991 N/A computer will work in an 98 turbo car (not true at all) they were 150 to 300 bucks, and then they told me that they are sold as is and non refundable. Obviously the guy is not a stellar salesman. I'm getting a feeling that this is not going to solve my problem. I'm not sure how these guys regularly fail. Do they just give you a no run condition? Or driving problems? If this is a good thing to do I'll spent a c note on getting a new computer, but throwing money at it is a good way to end up poor and still have the problem continue. I think that I still have a minor vacuum leak, but I've never heard of one causing this. I left my green books at home at school, anyone have the CO content setting procedure by using an LED? I can never remember which way to turn the screw or if light on or off means rich of lean. I'm thinking about setting it a bit rich and see what happens. I've adjusted it, but I may still have a vacuum leak and maybe I threw everything off.

 

I popped a plug. I think it looks a little lean. For under 5K mi I think that they have a lot of white ashes on them. I have no idea where a serious vacuum leak could be.

 

I reset to CO levels, and it enriched it a bit. The car still had trouble, but it did a 4350 mile trip in the mean time. It's time for a good break and really tackle these problems.

 

I will be the first to admit, although I have read this epic thread from stem to stern, it has been quite some time and I've forgotten what all you have checked/replaced. So...some of this may be redundant. At some point have you replaced all the plugs and it not change anything? Turbos will break the ceramic w/ detonation. Not running E85 are you? O2 sensor?