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Turbo 740 14 MPG gentle driving

comanche250

Active member
Joined
Jul 12, 2024
Location
Okanagan, Canada
I've now gone 2 whole months of gentle city driving in my 1991 740 wagon with a turbo redblock and AW71L. I have been running stock boost, stock exhaust (minus the cat eww). Just to work and back with at least 20% highways driving. Every acceleration from a red light I get it just past the middle mark on the boost gauge to keep up with traffic, one would say just regular driving, how every normal person accelerates.

Here, just like this, this is how I drive
Note my fuel level, after 214 km of driving, or 132 miles. That's about a week of driving. I drive gently, do the speed limit, ect. Yet without fail, at 300 km or 186 miles, my fuel light comes on, and I gotta throw $70 CAD worth of fuel in it again. I average 14 MPG. Now if I was driving quick and being stupid yeah I'd expect that, but I was getting the same mileage in the fall when I had my MBC in and set to 12 psi, running it hard and driving dumb. I had my fun for those few weeks just to see what it was like, not my thing anymore I'd rather not risk killing my nice daily. But it gets the damn same mileage as my 3/4 ton carbureted squarebody chevy!

So lets talk about what I've done.
The motor has around 130k km on it. I swapped it and the transmission (and entire engine and dash wiring harness out of a 1992 940 sedan) it's in incredible condition, no ticking or tapping or leaks or anything, I could eat my lunch off the painted block. I've replaced the spark plugs with NGK BPR6ES, and replaced the leads, they're bright blue kinda annoying and flashy but whatever. New volvo OEM thermostat. I have tested the ECT sensor right at the EZK side of the connector and it is within spec. no vacuum leaks, I even resealed the entire HVAC unit, it'll hold vacuum overnight I can go out to the car the next day and twist the selector knob and I'll hear the flaps move still. I am pretty confident there is no fuel leaking itself, everything is dry and doesn't smell. I also tested the fuel rail pressure since I noticed if you don't let the pump prime for a few seconds it'll stutter when you crank it, but the pressure holds 41 psi at idle and goes to 47 when you rev it. I popped off the vacuum hose the the FPR it doesn't leak fuel but it does smell of fuel.

I replaced the OEM bosch oxygen sensor just the other day with an NGK unit, I noticed the engine has slightly differnt power but I can't really explain it, it just feels like the turbo has a fraction longer lag, it could just be placebo. I will have to drive a full tank and see if my mileage is different. I can usually tell right away because the fuel gauge unpegs off of full after like 13 km or 8 miles and hits the halfway mark around 150-160 km 93-100 miles. One thing I should mention is the bottom stud on the turbo to the downpipe flange has snapped, so its just the two studs holding that coupler on, right before the o2 sensor. I don't hear a leak and I don't feel one so I'm assuming its not leaking air in and skewing the sensor.
I just used my shitty scope on it this weekend since I was curious of the waveform, it seemed normal to me maybe one of you would know better?

I dunno I'm out of ideas, I keep getting the same "hurr durr you must be driving it wrong" responses and honestly the next person to say that I'm gonna grab their balls and fuken twist them so hard. I'm close to just selling it and buying a shitty little diesel golf or something.
 
I should also mention I tested the AMM briefly by probing pin 3 when idling. I got 2.6 volts. Online says it should be approximately 2.3 though?
 
Paging @240 240
Might be worth a read: https://www.turbobricks.com/index.p...ht-foot-omg-dont-stop-im-almost-there.374713/

I'm usually getting 25-ish MPG out of my 940T, sometimes a bit more or less, but usually never below 23.
Most of the driving I do is on rural roads between 60-100kph without much stop and go though, which I think makes a big difference.
These engines just don't like driving in the city or in traffic a lot when it comes to fuel usage, I believe.
 
Yeah the spikes and there are multiple center crossings that shouldn't be there. It should only cross the center point (0.4-0.5 volt) about 0.5 to 2 times per second at idle. Keep in mind LH doesn't read the voltage per se, it counts the voltage rising/falling across the center point in normal operation.
 
Yeah the spikes and there are multiple center crossings that shouldn't be there. It should only cross the center point (0.4-0.5 volt) about 0.5 to 2 times per second at idle. Keep in mind LH doesn't read the voltage per se, it counts the voltage rising/falling across the center point in normal operation.
Oh okay that's good information maybe the new O2 sensor is faulty? Go figure in today's day and age of "just send it out" mentality. I'm not sure if the spikes were just from my scope, it's not the best and I had it alligator clipped onto the connector under the hood and grounded on the coolant reservoir bracket.
 
Paging @240 240
Might be worth a read: https://www.turbobricks.com/index.p...ht-foot-omg-dont-stop-im-almost-there.374713/

I'm usually getting 25-ish MPG out of my 940T, sometimes a bit more or less, but usually never below 23.
Most of the driving I do is on rural roads between 60-100kph without much stop and go though, which I think makes a big difference.
These engines just don't like driving in the city or in traffic a lot when it comes to fuel usage, I believe.


I've replaced everything. My ecu shows no flaws all voltage readings good. Plugs slightly lean.

I haven't had a free weekend to swap out the turbo since it's leaking from the waste gate.

Op fix your exhaust leaks
 
Is there a gasket that's supposed to go on the turbo flange to the downpipe too?

I think I'll just try and pull the turbo and drill the snapped stud out.
 
Great O2 video - as esmth says, try fixing your exhaust leak.

Normally, LH 2.4 cycles the O2 signal up and down roughly once per second. Ignoring the noise spikes, you have sections where the O2 is cycling roughly 10 times a second. 1200 rpm would be 40 exhaust pulses per second, so it doesn't seem to be cycling every pulse. If fixing the exhaust leak doesn't help, try a compression test or hook up a vacuum gauge and see if one cylinder is low.
 
I'm following this thread. I basically get 15mpg outta my 1991 745, but I drive it pretty hard

Maybe try a different set of injectors and/or a different MAF.

Smoke test the induction system for leaks. A pressure test and soapy water would also be a good idea.

Other than that, IDk. That O2 sensor trace has lots of noise. You could probably get a nice clean trace if you play with the settings and probes. I used a 2008'ish SnapOn scope and it always put out clean traces compared to other scopes due to the filtering.


I honestly don't know what a perfectly good trace of the 02 sensor at idle would look like, but I don't think it's gonna look like a perfect sine wave. Try it again at 2000 rpms and a better ground point.
 
driving around yeah I'm noticing its so dead out of boost it only really pulls in boost just driving the highway today. I'm gonna see if I can find another AMM to test.
 
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