This is a 1991 740 turbo californian model.

I bought this car from a friend a year ago.
This friend who owned this car since 1991 drove it with regular gas for 20 years (this car has a intercooled turbocharger), and meantime it developed dirty ICV, dirty EGR valve, and leaking vacuum lines. When I first got the car, it was knocking and was hesistant at the dabbing the throttle.

So as I clean and replace these systems and abovementioned sysmptons are gone.
However the mileage stayed same or improved a little. The highway mileage stayed same at 26mpg and the city mileage improved a little from 14 to 16.

Two weeks ago, I found leaking vacuum lines around intake manifold and replaced them and now they are tight. However the mileage actually went down. Highway mileage registered only 20mpg and the city when down to 14mpg.

What could be the cause of this? I feel like the motor is not adjusting to the increased amount of air which the new vacuum lines bring. Is there any way I could make the ECU to adapt to the new vacuum lines? I'm really getting confused.

 

the ECU should learn over a day or two worth of driving. FWIW, I get 16-18MPG around town/county in my 1992 740T wagon (California) and no better than about 23mpg on highway runs, and mine is running about as well as these ever can. My town mileage varies with how much leadfooting I do, and ditto highway mileage, cruising at a smooth steady speed is mandantory for getting that 23-ish number.

 

18mpg in city is my dream number right now. On fuelly I come across 740t drivers with 20 mpg in the city!

 

no OBD codes, right? if there are any, read them, report them here, and reset them. see Engine and OBD Diagnostic Codes for how to read and what they mean, and how to reset them.

also, the LH2.4 'learns' the optimal lambda settings, but it should pretty quickly adapt to changes when you drive.... if you want to force it to start over, disconnecting the battery for a minute 'forgets' the learning, so it learns all over, doesn't take it very long.

 

Here's the epa figures when new...

Real numbers are lower, particularly as turbos run rich. Try running premium to avoid detonation. Quite important in California where the gasoline is particularly weak. Eliminating detonation will keep the icu from retarding the timing and provide more power. As some have alluded, repairing the vac lines means the ecu needs to relearn. Odds are you need a new O2 sensor as well for optimum mileage...but that's another thread...

 

ah, thats the 'revised' MPG, which is supposed to be more typical of real world sue. the original numbers were 19 city, 22 highway, 20 combined. you have to try to get those numbers, on long highway trips going a steady 65, you might do a little better, 23-24 sustained. doesn't take much stopping and speeding up to drop back to the 20 number, though.

 

It's my understanding the knock sensor system adds fuel (to cool combustion) vs taking timing like most set ups. Maybe you're (falsely) setting yours off too much. Or your fuel is just junk.

24 MPG on my 1991 940 turbo wagon.

 

the knock sensor is wired to the ICU not the ECU on LH 2.4 systems, and all the greenbooks show it retarding timing.

 

pierce// just pulled obd codes and got 241 from the number 6. Didn't know it the whole time because the warning light didn't come on. Now the 241 code confuses me because I don't know if my Californian 740 Turbo is a Bosch, EGK or Regina. It seems like they all have to do with the EGR though. Strangely, one of the vacuum hoses I replaced and stopped the leak was the one feeding vacuum from the intake manifold into the EGR cylinder on the driver's side strut tower. Is this cylinder the EGR controller or EGR vacuum booster?

swiftjustice44// Since I got this car, I switched to premium and the detonation was gone. There are people on fuelly who has logged 20 mpg in the city. One of them is doing it with regular gas with his 740 turbo!

eljefino// I didn't touch the timing at all. May be should I advance it a little?

 

a turbo is always bosch. if its got OBD, its got to be LH 2.4 (LH2.2 didn't have OBD).

timing on LH2.4 is electronic, there's no adjusting it, it comes from the crank position sensor.

the EGR vacuum controller is on the left strut tower, it looks like a little round flying saucer or hat shaped thing, has a 2 wire electrical connector on the bottom, and 2 vacuum lines going into the lower side towards the engine. top vacuum line is vacuum from the intake manifold, this 'top hat' is a electrically accutated vacuum switch, and when its turned on, it enables vacuum to the lower hose, which goes down to the EGR valve, which is under the intake manifold...

do you have (or can you borrow) a 'mity-vac' ? its like a hand trigger vacuum pump, with a bunch of hose adapters....

disconnect the lower hose from the EGR vacuum controller, and hook the hose up to the mityvac. start the car, and pump a bit of vacuum with the mity-vac and the engine should choke almost immediately, maybe even stall, and the vacuum gauge on the mity-vac should hold constant. release the vacuum and the engine goes back to normal idle.

if it won't hold a vacuum, or the engine doesnt choke/bog, I'd be looking at that hose, and the EGR valve itself under the manifold.

also, you can test the EGR vacuum controller, its DTM 3 on the OBD block, in the #6 ignition position... this test mode 3 will cause the EGR vacuum controller to cycle on/off/on/off, so if you disconnect both hoses from it, and hook a clean hose up to the top, and gently suck on that hose with your mouth, you'll feel the vacuum going on/off... if thats not working, the problem is the vacuum control thing, or electrical

if the EGR valve holds a vacuum, and the vacuum controller is working, then its either a clogged EGR pipe (goes from the exhaust manifold around the back of the engine to the EGR valve), or a clogged EGR valve, or both. since you got a 'low EGR flow' fault, this sounds most likely to me.

 

I put new o2 sensor three months ago. Another one for $80 again? =-(

Pierce, thank you very much for teaching me how to test those egr parts. Will let you know when I test them.

 

o2 sensors should last for 100,000 miles or so. unless the engine is running way too rich, they can get fouled fairly quickly..

 

Typically, not at all. Remember...I said "Odds are...". Most folks neglect to service the O2. It should be fine unless, as Pierce mentioned, it is running so rich it leaves a black cloud everywhere. Running rich will ruin the cat as well.
I was re-reading your original post to see if I missed where you mentioned replacing the O2 and saw where you were having a little hesitation on "tip-in" with the throttle. These cars are notorious for having fouled throttle bodies. The flame trap (PCV) feeds crank case vapors straight into the TB and the blade can get so fouled the TPS doesn't know when you're off throttle.

 

Pierce, I tested the both EGR valve and its vacuum controller.
When I connected the mityvac onto the EGR valve and pulled the vacuum out of it with the car idling, the idling went down slowly and the mityvac lost the vacuum quickly that I had keep pumping to retain the vacuum.
For the vacuum controller, while the socket 6 was cycling the controller on and off, I tried to feel some vacuum out of the top port and didn't feel anything. In the worst case I assume both are kaput however I doubt that. A month ago when I took the EGR valve out to clean it, I test it by sucking air out of it and the valve moved up to open the cylinder in a free manner. Thank you very much for your tips. Now will have to get a replacement for this vacuum controller. These simple plastic pieces go for $200!

How do you test the EGR vacuum controller electronically?

 

Cleaned up the TB as well a few weeks ago. It had soots around the butterfly valve but wasn't too thick. Cleaning the TB didn't do anything.

 

if the bottom hose won't hold a vacuum, I'd try replacing that hose before doing anything else. it doesn't take much of a crack to leak vacuum. ok, I suppose its possible your mity-vac leaks, I've seen that too, put your finger tightly over the end, and pump the mity-vac and see if it holds a vacuum against your finger, if not, then the mity-vac's internal seals are dirty, and it needs to be cleaned and lubed.


when its cycling in test mode, there's no vacuum as the engine is off. i suggested disconnecting both hoses, attaching a clean hose to the top port, and sucking on that top hose with your mouth, and you should feel it going on/off/on/off...


the test is just what you did above that makes it cycle. its a simple solenoid on/off air valve. the vacuum itself comes from the running engine, intake manifold vacuum.

 

I will double check my mityvac.
Forgot to tell you this. Suspecting the mityvac was leaking, I sucked the hose myself and the motor almost died severtal times. Of course I had to maintain the vacuum myself as well.

When I sucked it, I was able to suck it all the way through without any blocking by the controller. I will do it again tomorrow to make sure.

 

well, if you think you need a vacuum controller, most any 89+ 240/740/940 (90+ if its a turbo) in a California junkyard should have that controller. they are all the same.

the on-off-on-off should be really obvious. when its not being tested and the engine is off, it should be closed.

 

Not sure if the replacement out of another 150k miles 740 would hold up well. Do the 740 none turbo and 740 turbo share the same EGR vacuum controller?

 

afaik they are the same. guess I can check the part numbers in VADIS...

k, lets see... 740 non-turbo says...

3501695 up to 1989, REPL 3531138-0, 9130436-0, 3523442-6
3531138 1990 and later.


a 1992 turbo 740 uses 3531138
same on a 1995 turbo 940

740 turbos before 1988 don't list any EGR. a 1988 740 turbo lists 3531138

hah, Tasca says that PN is used on V90 and S90 up to 1998, and on 850's too. so you can get that from a wide range of Volvos from 1988 up to 1998, as long as they have EGR.


my 740T has 220K on it, the EGR system works crisply.