Last spring my 91 740 started having intermittent fuel problems that got worse. The car would not rev well over initially 4000 rpms and then later 3000 rpms. In the process of chasing this I changed, the plugs, cap, rotor, wires, radio suppression relays, ignition module, fuel pressure regulator, all turbo hoses, AMM, air filter, fuel filter, vacuum lines and I think a few other things that I have forgotten about.

I brought the car to two mechanics; both said they saw nothing wrong with it.

Eventually I noticed that the car would act up, then if I shut it off, and turned it on it would be fine. while a mechanic told me it was likely not the cause, i changed the original ECU (i want to say it was a 951 or an 851, which ever one is the original for a 91 turbo 5 speed) and replaced it with a 937. I then drove the car for 10,000 miles and 8 months with no problems.

With the cold weather in New England here the car has again started acting up. Most of the time I drive it is fine, but now lately sometimes when I turn it on, it bogs down at around 3500 RPMS and will then buck and hesitate, it does not die it just feels like its getting no gas. I brought it to a new mechanic that seems decent and works mainly on older European cars and when I dropped it off it was acting up and they witnessed the problem and drove the car while it was doing this. Then with out doing anything, when they started it up again the car has been fine and revs right up to the redline.

Both fuel pumps seem to work fine and my understanding is that they do not fail intermittently, also the level of the gas tank does not seem to effect the problem.

Do the 937 ecu's crap out like the earlier ecu's as well? The FAQ seems to mention some issues with failing causing the pre-pump to not work, and a deterioration of the hybrid chip. Is there anything else this could be besides a failing ECU and if it is likely the ECU does it pay to go with a rebuilt unit and if so from where?

Is there a way to test the ECU? In the spring when this last happened I talked to a Volvo dealer about it and they assured me that ECU could not fail intermittently.

I am pretty sure its not the AMM as the car throws no codes when it acts up, it seems to think that it is working correctly.

In two weeks I have to start driving 100 miles a day with this car. Any thoughts would be appreciated.

well i had another used ecu put in last week and so far 100 miles with no problems.

so today i decided to open up the ecu and see if i could see a bad solder joint or something.

i immediately noticed upon getting it open that on the right side (plug facing forward) there is a 1" x 2" board that appears to be coated in silicon with these two raised pyramid things on it that have a black cross across the top of them. on one of them the surface is broken a bit and there appears to be a hole in it (the metal cross is exposed). could this be the cause of my ecu woes? i am guess it should not be like this.

if i can find my torx bits i will remove the board to look for cracked solder joints on the other side.

i will try to take a picture later tonight.

I'm dying to see those pictures.

Bill

bill,

i am sorry to say i donot know how to resize the photos to fir the brickboard server, email me at msunkenberg at hotmail.com and i will send them along, i would love to get an opinion of them.

I am going through the EXACT same problem. Just when I think I found it and fixed it, it pops back up. Just tonight I was coming home from getting dinner and it happened. With the gas fully depressed, the car surges and falls repeatedly and back fires a little too. I haven't tried it yet, but in a post that I made on the subject, someone suggested unplugging the O2 sensor. The idea is that either the ecu, or the sensor is going bad, and causing a lean condition. By unplugging the sensor, the ECU should be tricked into causing a rich condition, suggesting that the O2 sensor is going. If the condition continues, then suspect the ECU. Please post your findings, and I will make sure to do the same. John

i got my car back today, i had my mechanic go through it and he checked the ignition system, my turbo, fuel pumps etc... everything was fine. i had him swap the ecu with another used one as i think that it likely what it is.

my car did this last spring, and i went through all sorts of hoops and finally i noticed that when it acted up, if i shut the car off and turned it back on it would be fine. i changed the ecu and it has been tip top for 10k miles.

when your car acts up, does the check engine light come on? mine does not, but did when my AMM went bad. also how are your turbo hoses? i changed all mine as they were old and it caused a similar condition.

good luck, and here's hoping it was my ecu.

Can you get that old ECU and get it apart?
Perhaps there's some parts such as electrolytic caps that are famous
for going away with age. The silicon stuff is usually bulletproof.

It would be interesting to see if ECU's can be rehabbed... certainly
old caps, even corroded connectors could cause those funky problems.

Bill
in balmy Swampscott

it is in my trunk, i suppose that i could open it up.

when i put this ecu in my car didn't idle so well, sometimes it would go low, and it did not do that with the old ecu and the new ecu idles better.

I've replaced all of the hoses, and the AMM. When I shut my car off, the problem vanishes as well.

I guess you had a 563 originally and they are certainly known to fail fairly often. I think the 937, 967 and 954 all work well in those cars and are more reliable. I am not saying this is your problem, just that 563 have a good faolure rate.

First off, to answer your questions, anything in terms of electronics/computers can act up intermittently.
It sounds to me like a fuel mixture issue, given that when I had a nasty intake leak (read fuel mixture issue) it would do the same thing: rev up fine to almost 3K if given steady pressure, but bog down if punched or pushed past a certain point.
Given that it comes and goes, I would agree with the ECU issues.
You said you got all the turbo hoses, and I wanted to point out the obvious and ask about the short 4-incher from the intercooler to the metal intake tube. It's easy to overlook, especially if someone else replaces "them all" for you.
This is a stretch too, but I'm trying to be creative - what about the O2 sensor? Ever replaced? I don't see how it would work intermittently though.

Good Luck,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

I did all of the turbo hoses and I believe there were 4 of them, I also sprung for the volvo hose with the sensor.

when you had the intake leak can I assume that that was not an intermittent problem?

i am leaning towards getting another used ECU.

Also fuel pumps generally fail, they dont' go intermitently right?

When I say it acts up intermittenly i would say right now its like once a week and I am using the car to commute 35 miles each way daily. when it does not act up the car runs like new, and pulls like a bastard right to 5500 rpms.

Four sounds right. Airbox to turbo, turbo to ic., ic. to intake tube, intake tube to throttle body.
Though... there is the one for the idle valve too.

The intake leak wasn't really intermittent though, it was more like I drove like a grandma all the time, so it wasn't apparent until I tried to really push it. There was a time when I drove in high winds and noticed it wasn't able the keep up speed without constantly spooling the turbo. It did pull okay for a while, but not really well at all. I think it just degraded slowly into undrivable. It was not awesome and then crap and then awesome.

I've replaced my fuel pump because it went bad. The in tank pump was bad for a long time, and eventually the main pump died too. It sounded like a hornets' nest for months and always worked, until one day.... nothing. Dead as a doornail.

I'm guessing that the power stage/amplifier/ignition unit on the firewall is not overheating at this time of year. How about water getting into somewhere like the distributor?

I'm thinking right now you could just pick and pull another ECU more easily than track down all these other possibilities. I've learned to listen to my hunches. I would check the idle air valve hose just for kicks though.

good luck,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

Hello
i have not known the white label computers to fail, well maybe two in the history of reading the forums.
i dont know how many miles are on your car but i would check the fuel pressure.
i would also swap in a known good AMM to rule out the two items.
i also suggest to check/replace crank position sensor.
cant walk you through any process like some here.
good luck
Mike

the AMM is a rebuilt unit from FCP groton with about 10 months and 10,000 miles on it, also when that acts up won't it usually throw a code?

can the crank sensor fail intermittently, i have asked the mechanic twice to check the fuel pressure. the FPR is new and the systems primes and turns off like it should when i start it. also can a fuel pressure (fuel pump) problem happen intermittently.

when it acts up if you accelerate from a low rpm to a high rpm at a constant pressure it hesitates at high rpm, but if when it starts to hesitate you push in more on the gas it will lose rpms.

Hello
from reading the volvo forums for like 6-7 years the rebuilt amm have a bad reputation. i just assumed everyone keeps a extra JY amm around for problems like this.
the crank sensor can fail intermittently as the motor moves around and the wires may flex.
the advise to clean the grounds is a good one.
although im no mechanic i can and have fixed all the problems on my volvos i ever ran across. (well exept one cluster problem)
but like i said others here no better than me.
how many miles on that car of yours?
at least this will bump up your post.
good luck
Mike

the mechanic who has it thought it might be the AMM but when that goes, doesn't that usually throw a code and trip teh check engine light? i do have a spare junk yard one that i think works fine in my trunk.

i have also thought about the reference sensor but willt hat cause this or more of a no start?

i had the shop check the fuel pressure and the ground, i will pick it up tomorrow and deal furtehr with it myself.

i still think its the ecu as this is exactly what happened last time but who knows.

thank you for the ideas.

I have a 1990 740 turbo with over 200,000 miles on it and recently went through the same experience. Relucently I took it to the dealership and after a month of trial and error they thoujght it was the ecu, which turned out to be false. At the end they changed a fuel rail and it has been running fine since. Cost with tune up and labor and parts $900.

when you sau fuel rail, do you mean just the rail, or all of the injectors along with it?

Not being a mechanic I would say it was just the rail. However the manager told me that the mechanic working on the car had an old one and he put it on my car visually it looks like a used part, the whole unit.

"Intermittant fuel problems" ? Could you elaborate?

Hunch:
Service the engine grounds especially the ones on the intake manifold which are for the fuel injection. wire brush, and reattach with conductive grease-
never seez is OK.

Where in New England?

Bill
Swampscott,MA

Bill, allow me to jump in, if his problem is like mine, which it sounds like it is, I believe he is saying most of the time, when he starts the car, there is no sign of any problems. However, sometimes, completely at random, when the car is started, and put into gear, it will accelerate fine up to boost conditions but then runs horribly until he lifts off the pedal and reapplies the gas. Once again, when it hits boost or slightly after, the engine starts backfiring and looses power to the point of decelerating. If the car is shut off, and restarted, the problem vanishes until the next time it decides to pop up.

i will ask them to check the grounds, i checked them last time this happened and they were okay.

i'm in lovely south worcester.... my little slice of heaven on earth.