So, to all of you that don't remember:



"I searched the forums, found some helpful info, but still no answer, so I post this in hopes for some more info.

So, here's the deal.
89 Volvo 740 turbo wagon. 146k Summer mileage (in a hilly, poor road condition state) in town 20-21. Highway 24-26.5 (and has been getting better steadily) Winter mileage (same conditions, no snow until today) in town 12-16. Highway no more than 20.
Work done/mods:
Plugs, wires, cap, rotor button, vaccumed cleaned all four injectors, cleaned throttle body, adjusted TPS correctly, cleaned IAC, engine temp sensor and guage sensor, fuel system check valve, fuel filter, checked both fuel pumps for operation, fuel pressure regulator brand new, k and n drop in filter, synthetic engine oil, brakes-none dragging, four studded snows on steel rims (this may be part of the problem as the stock alloy rims/tires were about 7 pounds lighter each), adjusted ignition timing to 13 BTDC (book says 12, did this mod to try and resolve poor fuel economy) as it was at 10 BTDC, camshaft seal, timing belt/tensioner, thermostat, new flame trap (called something else on a 740) complete assembly and cleaned all hoses, fittings, etc, checked turbo for wear (no axial play at all), battery, camshaft cover gasket, new headliner, cleaned MAF and connector, probably more I can't remember right now.
Reduced the curb weight by about 100# by removing- frame "vibration dampeners", A/C all components, interior floor mats (under carpet mats), stock radio amp, all extra brackets under the hood, and a few other things I can't remember. Air box is intact, thermostat is fully functional.

No codes, or at least no codes that produce a light. Cannot find a module inside the engine bay to check for codes. All books/stuff on the net says it's on the drivers fender well and show photos of theirs; mine is absent.

Outside temp above 50, great fuel economy for a heavy car. In fact, with a 2500# trailer behind it on the highway I still saw 18mpg. Our weather has been intermitent as of late. I check fuel economy everytime I fill up. One week it will be warm and I get great economy. The next week it's cold and I get terrible economy.

I figured winter gas formula, sure. 1-2mpg. But it doesn't fit because I'm using the same gas station, and one week to the next then back again I'm getting good, bad, good, etc. No snow to speak of so no tire spin, or extra drag, etc. No extra ballast in the boot until today.

I was thinking tires/wheels but that would be bad all the time after putting them on. No beans there either, although they probably aren't helping.

Running out of ideas. Never changed the 02 sensor. Who knows how old it is. Car does use some oil, say a quart in 5k miles at most. Still, I would assume this to be bad fuel economy no matter what the temp is outside.

Thinking of picking up an air/fuel guage. Not sure how to install it, but probably worth a shot."





NEW INFO:
Installed new 02 sensor. Also noticed a low voltage condition at full throttle with the headlights on. Read the article on alternators, replaced my regulator/brushes. The existing brushes were less than 1/8". So, that was time.
The good: Car charges better, feels better and a little more powerful. Appears to be smoother too.
The Bad: Fuel economy has not improved much. I have not tested in town fuel economy, but I did highway. I stopped at a gas station with a fully warmed up car. Filled to the top. Set the odo. Set the GPS (to verify the odo) Stopped about 40 miles later. Drove very economically, cruise set to 70mph. Outdoor temp: 5degrees F. MPG: 20.5

The weather here has been cold. It gets to around 15 during the day, and at night it is around 4-5.
So, starting at 24-26, assuming I'm getting winter mix fuel, I should be seeing a loss of at most 2mpg. I'm seeing 4-6.

Now completely out of ideas. Anyone wanna muster a guess? I'd try about anything at this point. Maybe the mobil ONE 10w 30 I'm running? I ran 15 weight in the summer.

So, I installed the new thermostat's. Both the hotter cooling system sensor and the air box sensor.
The airbox temp has been much cooler, 48-54df. It stays much more consistent. It does not go up above 100df anymore! Who knows what the effect will be on fuel economy. I'll know that more in the next few days.
Also:
Tore apart what I think is part of the wastegate actuator. It was located on my Mitsu turbo on the front. It has a vacuum line coming from the manifold. It is just above the wastegate actuator. It looks just like an EGR valve from a GMC I own. It was sticking, not horrible, but obviously not functioning as intended. After a good cleaning with a rag and some q-tips, it moves freely. The diaphram that moves it does not leak. Adding vacuum to the hose will keep it open.
I'm hoping this will help the problem.
I finally got rid of all of the clips holding the intake hoses to the clamps. Found one hose that was a little mangled at the clamp end. I cut it shorter, and life is good for now.

I'll give an update when I get the new MPG's.
B

I'm looking forward to update!

I've got the MPG Blues too on my 1986 740 Turbo.

I've just replaced the AMM* and will be replacing the thermostat within the next week.

* Autozone offers a remanufactured Bosch AMM by Cardone brand (hopefully I'm not done/dumb..) $116 before $40 core charge. Too early to know effect on mpg.

LATEST MPG UPDATE:
In town 60%, highway 40%, one tank, 17.8 mpg. Better. Still not where it should be. But, 2mpg better average than before repairs began. In town is probably about the same, but highway seems to be a lot better. I'll have an accurrate highway mpg tomorrow when I fill up.

All in all, I lost about 4.2mpg total average from summer to now. I figure 2 of those are in the fuel. Probably 1 of those must be the studded snows. Hard to say, but I guess I have to deal now. The new thermo in the air box will increase the lifespan of my AMM. So, no harm done. The heat is much warmer now. That's nice because it was struggling to keep me warm in the in town short runs before. No effect on the dash needle to speak of.

So, I guess that's all that can be done until summer, with new tires and hotter temps.
B

That thing on the turbo is the compressor bypass valve. CPV for short. It's supposed to dump any excess intake system turbo pressure when you let up on the gas and close the throttle plate. If it's not working the rubber intake hoses tend to get oily due to the excess pressure forcing oil vapor through the rubber. It doesn't get a whole lot of use with automatic cars but the stick drivers open and close the throttle plate alot more.

Ops..I mean CBV.

So, it's update time.
Re-installed all of the stock pre-heat hosing/air box. Everything is bone stock on the intake side now. I put a digital thermometer sensor in the box to monitor intake air temp. My results: 95-100df if you let it idle during warm up until it's warm. Temp goes down and fluctuates between 70 and 90df. Outside temp doesn't seem to matter. My concern is, I've heard the temp is supposed to be between 50 and 70? Not sure on this.
Fuel economy is 20.9 on the highway. Slight improvement. Probably due to the new 02 sensor.

Went through and replaced all un-replaced vacuum lines today, before checking fuel economy. No improvement there in mpg.

Tires are at 40psi. They are rated for up to 44.

Have not done the pressure test on the exhaust system yet. It is the next step, I guess.

Anyone know of a source for that air box thermostat besides the dealer? It could be heating that air way too much? I don't know.

Ideas?

I have experimented with tire pressures and seen no detectable difference on fuel economy - even though in theory there should be some. Conclusion? it's too small to matter, (within the normal range of pressures). Running your tires at 40psi will just wear out the centre part of the tread, and may actually reduce their traction ability because the entire width of the tread is not equally flat on the ground/snow.

Your intake air temperature is OK at those numbers. Consider that Bosch designed their AMM's to handle 110F+ Arizona summer air temps, and then be left to cook underhood while you shopped for groceries. The little thermostat device in the airbox is a rather crude piece and won't respond quickly to varying conditions. All it is meant to do is to heat up cold air to some degree (Ho!).

Just keep an eye on those temperatures when the weather turns warm, to make sure it is not significantly overheating the intake.

I have had 44 max psi tires for years, and always run them at 38-40 psi. Years ago with bias ply tires, or maybe older radials the center would wear fast if at too high a pressure, but I have not had that problem with the many different brands of tires I have used.
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84 242Ti IPD bars&springs, 89 745 16v M46 IPD bars, 89 744 16v M46 IPD bars, 93 945 Turbo AW71, 91 245SE AW70 IPD bars, 93 245 CLassic M47

Hey-
I appreciate the info. The airbox t-stat made it to 111df today. That tells me it's time to replace it. It reacts very slowly. I don't want to see it cook the AMM. I'm also going to pick up the hotter radiator t-stat. I have the 87c right now. Probably will help to have things a bit hotter.
B

Not sure if it will make any difference. I can't believe your door sticker has your tires at 40. I run mine at 32. The rating on the tire is only there for maximum.
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Post Back. That's whats makes this forum work.

Door sticker says 35 or 32, if memory serves. I don't mind the rougher ride, it's not enough to kill my handling, and a little higher psi should give me slightly less rolling resistance, thus, slightly better fuel economy. More importantly, when the temp drops to 0 the pressure is a closer to what it should be.
B

I got mine at iPd and I have seen them at FCP groton as well. It is not too hard to replace. They are the same for the 200 series as well. Good Luck- Scot

No ideas but have to relate that my uncle had a 140 series volvo back in the 70's and was always bragging about what wonderful mileage he got. His lovely neighbors played with him by adding gas to the car at irregular intervals over a few months till his mileage seemed infinate, then they started siphoning the tank.

Some owner's manuals specify the 92C coolant thermostat for the turbos, and supposedly some call for the 87C. I would try the 92 - certainly in winter conditions. I use the 92 year 'round in all cars.

Mobil-1 5W30 has lower viscosity at cold (startup) conditions and is actually slightly "thicker" at high temperatures than their 10W30, according to Mobil's published specs. Thus, nothing to lose by going with the 5W30.

I keep the preheat tubing and valve assembly functioning on my cars, and use those cheap little indoor/outdoor digital thermometers to keep tabs on the actual temp. in the airfilter housing. In cold weather, the preheated intake air (to about 50-70F) should give better fuel vapourization, hence improved MPG and lower emissions.

Cold weather is pretty certain to reduce fuel economy for many reasons, but your experience seems excessive. In my attempts to keep winter MPG's up, I use Redline synthetic 75W90 axle lube, keep the snow tires at 32psi, use the aforementioned Mobil-1 5W30 (year 'round), try to combine my errands to avoid short trips, switch on the block heater for about 2-3 hrs before the morning start, and use a "mask" in front of the grille when it's really cold out.

Still, I see fuel economy go down about 2MPG overall in cold weather.

Thanks for your prompt to get the airbox preheater hose going again, because my winter mileage is not good. I had *disabled* the [expletive] thermostatic operator, because it fried at least one AMM.

After hunting through my extra stores I was delighted to find 2 of the little $10 operators. Never been used. (To be fair, I think one had ridden around atop a car's radiator, waiting to be installed.)

Data point: Neither operator worked. Not a bit.

Overall length did not change, whether placed in our freezer (about equals night-time outdoor temperature her in MN), or atop a radiator in our house (about 100 deg, F).

IF they poof out on the shelf, how little can we expect their life to be in service? :-(

OTOH, I probably bought them from a popular vendor which offers good prices, on parts both cheap (ScanTech, MTC, no-name, bad-name) and superior. These may not be equal to what has been available through Mother Volvo. They are marked only 7041.10 0703. I suppose the last 4 digits are date of manufacture.

I'd sure like a simple, reliable solution to this MPG-preheater-AMM thing! :-)

Hey-
I'm going to price them out from the dealer today. Wish me luck. I'm pretty certain this is most of my problem, because the air box temp was ambient temp without the thermo, and the car got crap economy. Now the temp varies from 80df to 111df. I suspect that over 100df the car would run less efficiently. Also, I'm gonna order the 197df thermostat for the coolant. I've been running the 87c and I want to experiment with the hotter one.
B

$50 from the dealer. And they can have it here tomorrow!
I might try my luck with an aftermarket, and keep my household thermometer wired and installed on the dash. I only need this thermostat for the winter anyhow. It'll be back to the cold air intake with no pre-heat for the summer.
B

The airbox tstat isn't used on turbos.

Thanks for letting me know about the $50 cost of the preheat thermostat at the dealer.

Looks like gold, made of gold? Nah, gold's >$1,000/oz.

Still... WORTH it if demonstrably superior.

AND you can pawn it if necessary. ;-)

I wonder if your units actually failed. On part of that picture not included in your tests is spring tension. It is my understanding that these units are being constantly applied pressure via the spring inside the airbox. I may be incorrect in this, as it has been a few weeks since I last had it apart. I thought the flap was either held open or held closed by spring tension, and the thermo pushed against that to overcome the spring.
It would be easy enough to test; assemble the off the shelf thermo in the box outside this time of year. Note the position of the flapper. Bring it inside to the 68df of your house and note the change in the flapper, if at all.

Let me know what you find out.
What temp coolant thermostat are you running?
B

Just a data point: I got a couple of the airbox preheat operators (cartridges) yesterday. About $10 each. Dunno who made them.

When applying 20 pounds, using a bathroom scale:
cold: 1-5/8 inch long overall
warm: 2 inches long overall

Unlike, for instance, tailgate gas-filled lift operators, these are quite inelastic. I mean, I think you would get the same length whether pushing with 10 pounds, or 30 pounds.

I DID cut open one of the dead cartridges. Inside, best as I could see, was brilliant copper-colored particles in an emulsion. If I worked on a TV crime show I could have sent this to the lab, and let you know what it is, but I don't, and I can't. :-)

Doh! You are right. (And I am also right, by chance.)

Taking the flapper apparatus out of the box (in this case a 240), and using a bathroom scale, I see it takes 10-20 pounds force (!) to swing the flapper.

Applying force to the naked operator, when warm and when cold, I now see that its length does change, but TOO little.

It DID indeed poof out on the shelf.

I installed it anyway in the box, hoping against hope. Anything is better than nothing? Until I get a replacement.

I am still hunting for the other aged-on-shelf operator, to test it.
It's either adrift here somewhere, or already in the garbage, or near the Dremel, awaiting destruction... will test first.

Thanks for the wakeup call!

Thanks for mentioning the spring. I didn't think of that. I thought that the operator would move of its own accord. But, like tailgate lifts, maybe their natural state is fixed... despite the temperature.

I DID think of taking a Dremel + cutoff wheel to the bugger, to see what was inside. Now I am glad I waited.

I'll try again.

Let me know what you come up with. I'd be interested to hear how long my new thermo will last. I'll have that this monday and I'll update everyone about fuel economy.
Thanks for your info.
B

I pulled the t-stat today. I do have the 87. I'm gonna order up a 92 from IPD tomorrow. I need the airbox t-stat too, as the air box temp made it to 111df today. That might help, who knows.
Thanks for the info.

Thanks for the input, some great ideas.
I've put some thought into 5w30, but I'm concerned about turbo oil consumption. It isn't excessive, but I do find it in the hoses between the turbo and the intake. I'm attempting to keep it from being excessive, for obvious reasons. Any thoughts on that point? Maybe run 5w30 in the winter and 10w30 in the summer?

I'll take a look at the auto parts store for that axle lube; mine's got a small leak anyway. I've been topping it off from time to time. Hoping to get enough extra bread at some point to rebuild the rear with a limited slip too.

I've been running the snow tires a little higher in terms of PSI. I figure a little over is better for fuel economy and it keeps them up in the coldest of weather. (0 to -10 around here)

I believe the thermostat is 192f? I threw out the receipt a while ago. The temp needle stays just a hair above center and didn't change when I replaced the Tstat.

I'm gonna start carrying some cardboard and zip ties for when it gets really cold.

I will pick up one of those thermometers tomorrow and get an idea of what the temp is inside that box. Soon to be bought and installed are: fuel/air mixture gauge and an accurate boost gauge. Then I'll really know what I'm looking at.

Thanks again for all the info. I'll keep you posted when I find out more.
B

I'm in CA now but am from Massachusetts, where I was when I went through 6 or 7 140s and 240s. This was on the coast so not as cold as inland, but still below freezing a lot. IIRC, I was using 192 tstats. I got good at finding and cutting to size a heavy duty piece of cardboard that would come up above and be bent over the radiator support so that it wouldn't slip down, and sized to cover about half the radiator, laterally. This made the cars run better and made the heat quite pleasant also. I would do this well above 0, and just leave it in for the season unless there was a warm spell, which had an obvious effect on the temp gauge.

The 544s had an aluminum 'blind' in front of the radiator that pulled up and down with a small chain, from the driver's seat. I always thought that was a cool, interactive idea.
Thank heaven we now have computers.... hehe

Maybe the cold weather is somehow causing your brakes to not release and drag some??

I'll jack up the wheels today and verify that there isn't any dragging. Thanks for the info.
B

Sorry just noticed the new O2 sensor. Also try losing the K&N filter. Somewhere on Brickboard I read the K&N flowed less air.

I'd still do the vacuum cleaner blow test on the exhaust system if it's rich.
http://www.brickboard.com/RWD/index.htm?id=1383995

good luck

The exhaust has been black. It was obvious that it was running rich, you could smell it if you stuck your head near the pipe.

I think I may have found the culprit, but I'm still going to try your vacuum trick tomorrow.
The culprit (based on miles on the odo and position of the fuel gauge, I'll know for sure after a few more miles and a fill up) seems to be the preheat hose. Or, to be correct, the lack of heat. I know the preheat hose was done away with in the early 90's, but I have a theory about it. All of my sensors are basically new. No vacuum leaks. Good turbo pressure, etc. I think the ECU from 89' requires the incoming air to be at 50degrees F or higher. If it is not, the ECU richens the fuel mixture because it senses the car is "cold." After all, if it were warm, the preheat hose would be making the airbox toasty. That thermostatic valve is open even at 45d. (less than have at this temp) I have two of these boxes. I modified one to have a 4" intake pipe that has Ram air forced in from the front air dam. I put this back in the other day at 15d f. and drove on the highway, mpg went down to 16! Put the old air box back in, still only 20.5 on the highway.

One thing I forgot to do was plug the ram air hose with a rag or something. My theory is that the ram duct was still blowing cold air (which terminates one inch in front of the box, pointed directly at it. At highway speed, this would cool the box down dramatically. Further, I had removed the baffle inside the cold air side of the stock box that feeds from the fender. Thus, distrupting the balance from the cold intake and the hot intake. This would have caused the thermostat to be un-calibrated. As a result, more cold air than is supposed to be coming into the box was being funneled in.
This is all theory as of now, but I'll post an update with photos of my mod and it will be more obvious.

Thanks for the ideas and the help.

Will do the vacuum test, sounds like a good idea. I appreciate the info, I'll keep everyone up to date.
B

My kid ran out of gas recently (broken gas gauge and heeps track of miles). He's been letting it defrost/warm up before going to school. Wow you've tried everything. I feel for you. How about the cat? Maybe it's making the engine work harder?

What's the tailpipe look like? When I had a rich issue (exhaust leak & O2 sensor)the pipe was black. After fixing greyish & clean. Both my B230FT wagons 1991 are like this.

I don't think my O2 sensor problem was the body leaked air @ one of the joints. It was a generic fit for a V8 Mustang. For the extra $40.00 I'm buying Bosch (no seams).

Check the O2 sensor see if it responds properly to rich/lean conditions & if it sweeps without you skewing the fuel mixture.

The cat was brand new 2 months ago. It was an aftermarket model, same inlet and outlet size as stock. Made by magnaflow, I think. Supposed to be better flow than stock. Muffler is also brand new, as is complete exhaust system from the after the header pipe back.
I'm gonna try running a little higher tire pressure tomorrow. I figured the snow tires are probably loosing about a mile per gallon too.
Still scratching my head!
B

Could a weak AMM be causing this? Any thoughts?
B