35d worsening gas mileage - solved- thermostat/fan/EGR cooler - DIY with pics

[Rigmaster]

Originally Posted by AlpineX

Do you think you got a defective part?

I assume so. I will ask the vendor but not expecting them to handle it very easily so I'll probably just eat it and buy another one elsewhere.



Honestly not a huge deal, I can probably swap out the t-stat in about 1-1.5 hrs from start to finish now that I've done it. Just a PITA that I have to re-do it again.

[apw2607]

Many thanks for this OP. Just did this on my 35d with 135K on the clock. Average mpg went from around ~21mpg to 25mpg combined !!!

No error codes.

Also, all the vacuum lines were replaced .. and the "lifetime oil" in the transfer box.

Hey, 135k .. things wear out.

[josiahg52]

I'm getting ready to have my thermostats and glow plugs/module replaced. In the winter, it runs at 79C and greater. In the warmer months, 82C and higher. I average over 26 mpg in mixed driving and +/- 30 if I stay on the highway. The 'temperature implausibility' DTC has been set twice over the last year/10k miles.

There is currently a DTC set for glow plug 6 and control and there is some smoke on startup. Oil analysis every 6000 to 7500 miles using Mobil 1 ESP Formula 5W30 and OE filters. The last oil change I went 9000 miles to evaluate the longer intervals. Today I changed the oil with Liqui Moly Top Tec 4605 5W30.

[crystalworks]

^Best we've ever managed out of ours is about ~27mpg highway (@75mph) and 23mpg mixed. That's after thermostat, glow plugs, and module replacement. Worst fuel consumption was 21mpg and some change. Truck just went over 153,000 miles. Wish diesel fuel prices weren't on the rise as we love putting miles on the 35d.

Still much better than the mileage out of my 4.8 e53 which averages about 15mpg mixed and 18 highway.

[PGas32]

Just replaced my thermostat again yesterday, and everything is back to normal (85-90C on the highway). I initially replaced it at ~79k in December of 2015, and just recently noticed poor mileage and low temps again at 115k miles. Pretty disappointed to have the tstat only last 36k miles, but at least it's an easy fix...especially with no EGR to mess with!

[josiahg52]

I did replace the coolant and transmission thermostats and the glow plugs. I had to replace #1 glow plug again due it failing. BMW replaced the part free of charge. Looks like the EGR cooler may be next on the list.

Coolant temperature stays above 85C even in -10C air temperature. It's easily increased the minimums by 5C and it does reach 89C+ now. Fuel efficiency hasn't changed much, maybe a few tenths but it's within my normal variance, 26+ mixed, so I'm not going to say that it's really made a difference.

[noodle654]

Just replaced the thermostat, mechanical water pump, and flushed the coolant system on my 2012 X5 35D. Had the dealer do it along with oil change, tire rotation, and idle pulley bolt recall. I have 150k miles and its been running great, but mpg has improved significantly after the thermostat change. Was averaging 21mpg on highway here in VT, now it’s more like 26mpg. Great improvement overall. Before the change motor never got over 155F, even during the summer.

[robnitro]

I have a question as to how to test a thermostat that is weak but not failing. I can hit 80-85C and stay there but my stop and go commute makes it hard to judge as opposed to a steady highway run.

I was idling the car and checked around 65-70 C the other day which lines are hot/cold. It was about 55F outside.
(refer to image of cooling system below)
I noticed that the left side coolant hose to radiator (that runs into the thermostat) was cold. (near the number 1/2 in diagram- blue colored in diagram)
The right side one to radiator was very warm (near "16" big red colored hose) and the radiator itself was hard to check because the AC one is in front of it.

Being that the coolant flow is from block to right side of radiator, through radiator and exits left side to be taken into the thermostat I'd think if my thermostat was shut (below 80C or so opening point), the red hose on the right would be much cooler- unless it heats up by sitting on the "hot side"

If the thermostat was prematurely open, the left side that goes into the thermostat would be running warm- right?

If anyone has recently done their thermostat on their 35d or 335d could check these 2 hoses at a coolant temp before 75C or so- but above 60, it would be helpful to diagnose a thermostat that opens early (or "leaks through")

[josiahg52]

Ambient temperature and the status of the HVAC system will affect the coolant temperature. I find that in 15C ambient with the HVAC system on and set at 21C, >/=89C coolant temperature is attainable in non-highway use after about 30 minutes.

>85C coolant temperature is attainable in sub-0C temperatures with the HVAC system on after ~15 minutes at highway speed. >87C coolant temperature is pretty easy to attain in most conditions. Highway operation is rhe hardest.

I had a recent experience driving through cold and wet conditions. The conditions were unique in that after several hours of driving, a 1/8" sheet of ice formed over the entire vehicle. In some areas, there was almost 1/4" of ice. There was also ice accumulated on the radiator. Even in -10C conditions and driving about 30 mph, coolant temperature was in the low 90C range.

As the outside temperature increased, average speed increased and the ice melted, coolant temperature decreased to the 87C. It makes me wonder if an air restriction device would be in order.

[jav1er]

Thank you for the great info Gents, I tackled my Thermostat and this was very helpful. I was trying to take out the EGR unit to avoid the fan removal, as I thought it was much harder then it actually was. Take the fan out, one guy said he did it without it, no point to struggle take the fan out, Also I didn't drain any of the antifreeze out, I just pulled on waterpump slowly and let it drained out, caught about 4 liters of coolant. I should have done this earlier, much easier then I thought. If any folks in the Toronto GTA area need a hand, happy to help! Cheers!