Low mpg 1988 735i 8mpg city - solved

[Cactuar]

That's good to hear, I hope it keeps starting reliably for you.

Yes I believe the filter is installed correctly. The arrow is pointing towards the center of the car. I do wonder if my filter is correct however. It's a somewhat generic Bosch unit I picked up from Rockauto, part number F5030. It was listed as compatible with my car but I've always wondered if it might not be too restrictive or something. I should change it for the stock Mahle unit just in case.

[slowcar88]

That's good to hear, I hope it keeps starting reliably for you.

Yes I believe the filter is installed correctly. The arrow is pointing towards the center of the car. I do wonder if my filter is correct however. It's a somewhat generic Bosch unit I picked up from Rockauto, part number F5030. It was listed as compatible with my car but I've always wondered if it might not be too restrictive or something. I should change it for the stock Mahle unit just in case.

That sounds right (that’s how I installed the filter anyways).

Not sure if it makes a difference but I am using the OE unit: Mahle KL9

[zapass]

IMO intermittent start issue is solved because you removed leak. That prevents fuel pressure drop. When engine is hot that does matter because when pressure is low fuel starts to boil in fuel rail. So fuel pump needs to provide cooler fuel and you need to crank longer.
I do not think oem/not oem filter changes anything. This system is robust enough. Pump provides amount of fuel way biggest than engine needs. If not you should experience power loss or power delivery issue during hard acceleration when fuel consumption is highest.
Of course you need to change fuel filter from time to time.

[slowcar88]

Another update.

Replaced oxygen sensor (looked old)
Replaced engine and coolant temp sensors
Ran a couple cans of fuel cleaners

It has only been around 100 miles, but the mpg has slightly improved to around 10mpg in the city

I might have spoke too soon about solving the intermittent start - it’s having some trouble during immediate hot starts

[shogun]

I might have spoke too soon about solving the intermittent start - it’s having some trouble during immediate hot starts

That might be a problem of a defective internal check valve in the fuel pump, when engine is switched off, fuel runs back into the tank till pressure is released, then in hot engine bay fuel vapor starts to build up in the fuel line.
Years ago BMW sold a separate fuel check valve to repair this, NLA, but there are plenty available in the aftermarket, install one on top of the tank, here pics from an E30 http://www.bimmerboard.com/forums/posts/496880

[youngbimmer]

Wrote a small guide for this a while back:

https://www.bimmerforums.com/forum/s...k-valve-repair

[slowcar88]

Wrote a small guide for this a while back:

https://www.bimmerforums.com/forum/s...k-valve-repair

This is great / will try it out. Thanks

[slowcar88]

Update:

Mpg has gotten worse (back to around 6-8mpg).

Installed the check valve in the engine bay (pics below).

Good news is that I think I may have found a fuel leak. Where the feed hose/hardline connects in the engine bay is dirty/wet. It sits close to the lower control arm/frame so maybe that’s why I wasn’t smelling any fuel.

[slowcar88]

Quick update. Not sure if any of this will actually help my poor mpg problem, but at least can cross a few items off the list.

I relocated the fuel check valve closer to the tank (as others have done) - this is because I wanted it to be before the fuel filter (who knows what can break inside the valve, and hopefully the filter will catch any debris). Here's how it was installed.







Next, I added an inline fuel pressure gauge.
*Marshall Instruments MSB00100 Liquid Filled Fuel/Oil Pressure Gauge Black - https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
*Earl's 5/16 Vapor Guard Barb To 1/8 Npt Tee - https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
*ABA 21014 Mini Fuel Injection Style Clamp 10-PK - https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
*+ fuel hose, hose cutter, thread sealing plumber tape



Replaced:
*Fuel Pump Relay: 61311373585
*Main Relay: 61361729004

[slowcar88]

Wanted to provide a comprehensive update.

City MPG has not improved. I am getting low/mid single digits in the city, which translates to about 20miles per 1/4 tank. Full tank used in ~100 miles. Have noticed slightly better MPG with 87 octane, worse MPG with 93.

Highway MPG is great (22mpg+).

So far I have replaced with new parts:

Fuel

• Fuel filter - MAH-KL9
• Fuel pressure regulator - STANDARD MOTOR PRODUCTS PR60
• Engine fuel lines
• One way fuel check valve
• ~5 cans of fuel injector cleaner over 4k miles
• Fuel injectors (what a bitch DIY job) - BOSCH 62417

Intake

• Air filter
• Intake vacuum hoses
• Main intake boot and breather hoses
• Throttle position sensor (TPS)
• Throttle body
• Mass air flow meter (AFM)

Ignition

• Spark plugs - NGK-5077
• Spark plug wires/ignition harness, distributor and rotor

Rolling resistance?

• New tires, alignment, air pressure check
• Brake rotors, pads, fluid
• Front control arms (upper and lower)

Other

• Various sensors: O2 sensor, engine coolant temp, engine temp, aux fan coolant temp
• Various relays: main relay, fuel pump relay
• 2 oil changes - LM 10w-40
• Radiator
• Valve cover gasket
• AC aux fan
• No engine codes

The hunt continues...

[RobertRO]

Frustrating, your mpg issue.
Most odd is that you get decent mpg on highway, but horrible mpg in the city.
To get only 100 miles out of a full tank it's outrageous. However, your old spark plugs (ref: your post on Feb. 05, 2022) do not seem to indicate a very rich mixture condition.

I believe that your engine is all right. The problem may be somewhere else. Like one or several sticking brake calipers, that let go on the highway. Or a fuel leak still to be detected - but a leak should also impact your highway mpg. I assume your air filter is in good condition.

[slowcar88]

Thank you RobertRO.

Good call on calipers sticking. I am noticing drivers front rotor/caliper runs slightly hotter (250-300F vs. 200F for others, when it gets hot), so maybe something is sticking.

Air filter has also been replaced (updated above).

Open to all suggestions!

[zapass]

Long shot: AT converter issue- excessive slip. On highway speed you get correct mpg because of lockup.
No idea how to diagnose it though

[shogun]

torque converter testing info https://www.bimmerforums.com/forum/s...ter-stall-test

[slowcar88]

now I think I’ve solved it : sticking caliper on the driver side

Strange because I’ve driven 5k miles over this year without any serious broken caliper symptoms (not pulling to one side when braking)

Recently I had intermittent wheel wobble at 60-65mph

Then it got so bad I couldn’t really drive over 40mph

Then I noticed driver front wheel / lug nuts much hotter than other wheels

Remanufactured centric caliper from rock auto installed. The old one rubber seal had dislodged

It solved my wobble and noise issue. Over 100miles and I’m getting mid teens mpg

CHECK YOUR CALIPER / WHEEL TEMPS!!

[Sansouci]

Plug #5 shouldn't have any crap on the shell. Do you have a leak of some sort?
BTW is the 6 an auto or 5 speed?

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