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MPG will suffer if the t-stat is stuck open...

I had the stuck-open t-stat issue on my M52. On a trip to VT, where the normal mpg was about 33, it dropped drastically to 25 to 27. During this time the temp gage read at a constant 10:00. Prior to this, I first noticed the gage would only drop from the center to the 10:00 position if I were going down a long hill, and not applying the gas pedal. When I gave it gas again, the temp would go back to normal 12:00 position.
Once the t-stat was replaced, all was back to normal: 25 around town, 33 highway.
Your poor mileage may in fact be due to a cold engine.
Ed CT
1998 528i (aka, best car I ever owned!)
5-Speed
Aspen Silver
Aubergine Leather
Zeckhausen CDV
Nokia CK-7
D-Con Cache on Fuel Pump Cover

My E36 M3 S50 engine has metal thermostat housing

I've heard with the aftermarket aluminium housings that they sometimes have problems sealing against the engine. And these engines are a metal cocktail of alloys.

Yes BMW do stuff up and change their designs but at least from a meterials point of view I assume they test them (even though in 1992 they obviously didn't with the head gasket material they first used in the 318is engine - it was replaced under warranty, but I was already wallet raped with the initial purchase price)

New oxygen sensors fixed my poor fuel economy

Yeah, the car does cost money to keep going at it's age, but it is still cheaper than buying a new one with even more reliability issues plus still to be known issues (based on my father's experience with the E46 and E90).

If I didn't like the feel and looks of the E39 the car would be gone from my garage, but I'm hooked. At least with a message board like this one it helps remain hooked.

hehehe I agree with the savings of weight !

Lotus said - "...if you add HP you will be faster in a straight but if you loose weight you will be faster everywhere..."


I agree with all that but at what cost ? Aluminium might weighs a few ounces more but last 4 times longer and safer. No need to recycle 3 plastic crap because the aluminium keeps working !

Its like creating a crap item so we can recycle instead of using the same one forever. No need to recycle if we don't have to throw things out !



the car heavy as it is, so I will put a new plastic on it... :-)


1997 BMW 840CiA
1990 BMW Z1
1999 BMW 528iT (Hauler mobile)
2002 Astro Van AWD
2003 Nissan 350Z Enthusiast Package
1987 BMW 535i with 5spd manual (for sale)
1992 BMW 735iL (needs head and interior)
1977 BMW 320i (project racecar)
2007 Mazda CX9 AWD (Wife mobile)
1998 Yamaha Blaster+Breeze

1995 850CSI CD00177 (SOLD)

overheating was most likely a not complete

bleeding of the system. it shut up to the red and then once I turned on the a/c it went to normal. it would stay in there and if a/c was shut it might went up but sometimes not. I checked the coolant level and it needed some. It has been working fine with or w/o the a/c since the air is hopefully out of the system, but the temp gauge reading is cold. It takes a good 25 minutes of driving to get the operating temp according to the gauge.

I order the coolant sensor then... I thought the sensor in the thermostat housing was for the gauge. Brilliant design, you need to buy a whole unit instead of a 5 usd thermostat and still fails at the same time as the old units. Where is the advantage in this then ?

thanks for the help will definitely check all.

as far as gas mileage, this car sux. My 840 gets about the same as this car. 20-21 mpg average and 25-26 on the highway. The 840 with a 5spd auto and 3.15 LSD does 10% worse then this and has a lot more power !

So I do need to fix the Vanos and this to see if the economy improves or not. I did not expect a fuel sipper but a 20 average is hardly good for a smallish 2.8 engine.



1997 BMW 840CiA
1990 BMW Z1
1999 BMW 528iT (Hauler mobile)
2002 Astro Van AWD
2003 Nissan 350Z Enthusiast Package
1987 BMW 535i with 5spd manual (for sale)
1992 BMW 735iL (needs head and interior)
1977 BMW 320i (project racecar)
2007 Mazda CX9 AWD (Wife mobile)
1998 Yamaha Blaster+Breeze

1995 850CSI CD00177 (SOLD)

Even a few ounces saved with plastic adds up>>

Also, apparently, plastic is easier to recycle than metal, and the Europeans are rabid about car recycling.

As Carroll Shelby said, "Saving weight (on a car) is free horsepower that you never have to fix."
Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

Thermostat is electronically controlled

The M52TU engine has a re-designed cooling system , head and block. The thermostat is an electronically controlled thermostat so that is the sensor you're refering to. The Coolant temperature sensor for the engine and cluster gauge is the "double temperature switch" 4 -wire sensor that is located at the back of the left side of the head on the M52TU engine. This is the sensor you probably need to replace.


Do you know the cause of the over-heating of the engine? It looks like the M52TU engine still used the mechanical fan clutch which I believe is the first design cause of my M52 head demise.

I note on the current model BMWs all the fans are electric and not connected to the engine moving parts at all and they initially removed the temperature gauge from the cluster !!!!!!! $hit

looks like the thermostat housing comes with

the coolant sensor as well. Its a whole unit - housing, sensor, thermostat all mounted and probably glued together.

I found an aluminium housing version but the seller is a bit questionable - fcgroton - on ebay. I bought a few items from them and they seem to be bending the truth their way. Also if things proven to be wrong they resist the change/refund until you force them with paypal/ebay. I ordered the oem unit at the end, 79usd versus about 54-56 for the aftermarket.




1997 BMW 840CiA
1990 BMW Z1
1999 BMW 528iT (Hauler mobile)
2002 Astro Van AWD
2003 Nissan 350Z Enthusiast Package
1987 BMW 535i with 5spd manual (for sale)
1992 BMW 735iL (needs head and interior)
1977 BMW 320i (project racecar)
2007 Mazda CX9 AWD (Wife mobile)
1998 Yamaha Blaster+Breeze

1995 850CSI CD00177 (SOLD)

25 minutes is a long time to warm up

but the cluster gauge is only the electronically converted measure of the engine temperature.

On my '96 528i after the temperature overheats my gauge started going dead cold intermittently. After replacing the resistor temp sensor in the front of the head on my M52 engine the gauge problem was solved. I'd say the over heating probably damaged the operation of the resistor.

I think the M52TU engine still has a temperature sensor, perhaps you need to replace this if the only symptom you have is the gauge staying cold for 25 minutes.

thanks guys. looks like the whole unit is whats

available for about 54usd. (Motorad) not too bad but why does it have to be plastic housing ? whats wrong with aluminium ? Why does BMW sticks with plastic everywhere ?


anyway I ordered it and it will be replaced on the weekend. I ordered a gallon of coolant too from BMW.
Sounds like an another job on the 528iT ... it keeps draining my valet and I only drove it a 1000 miles...



1997 BMW 840CiA
1990 BMW Z1
1999 BMW 528iT (Hauler mobile)
2002 Astro Van AWD
2003 Nissan 350Z Enthusiast Package
1987 BMW 535i with 5spd manual (for sale)
1992 BMW 735iL (needs head and interior)
1977 BMW 320i (project racecar)
2007 Mazda CX9 AWD (Wife mobile)
1998 Yamaha Blaster+Breeze

1995 850CSI CD00177 (SOLD)

Re: That sounds more like thermostat stuck open,

+1 on the T-stat. Same thing with my 528 when I got it. I agree about replacing the housing and the water pump as well. Just make sure you get a decent pump...one with a metal impeller. I forget what I got but I think it was German. The cost of the housing is minimal and worth replacing while you're doing the job IMO.

IMO, you really shouldn't drive around too much like that because your computer will run the motor rich which will not only give you poor fuel economy but also eventually lead to other problems like your cats burning out, and possible cylinder wall damage from running rich all the time.

Stuck thermostat...

... so the engine either runs too cold or too hot. I had the same problem and on top of that my aux fan was faulty as well. Just replace the thermostat together with the housing and the waterpump. These parts are weak links. Make sure you bleed the system properly afterwards. Jim Cash described how to bleed properly: do a search.

Gadjo
2002 525iT

That sounds more like thermostat stuck open,

rather than air in the system. Since you have DME controled thermostat, good code reader would show if there is a fault there.



1998 BMW 540i 6 speed
Arctic silver, M sport suspension (euro delivery), prod. date 05/98, non VANOS
Staggered 18" style 32 OEM wheels
3.15 open differential
Remus exhaust
AFE CAI with improved heat shield
Z3 shifter
VDO oil pressure and temperature gauges in place of headlight/fog switches
Powder coated valve covers-satin black
CDV delete
Debadged
AEA DRL module (aftermarket daytime running lights)

1997 BMW 528iA
Alpine white, premium package, prod. date 04/97, single VANOS
Staggered 17" style 32 OEM wheels
Debadged
AEA DRL module
Custom made sun blinds for rear seating area

2000 Nissan Frontier Desert Runner, AKA "The Work Horse"

Garage aids:
Direct Lift Pro Park 8
Campbell Hausfeld 28 gallon oil lubricated air compressor
Newair ACP-1400H portable air conditioner and heater
Auto Enginuity scan tool with BMW enhanced option
Actron 9135 scan tool-for quick readouts

528iT temp gauge is slow set to middle

on a mornings it takes about 25 minutes of driving for the temp gauge to reach the middle. At the same time it has very little heat although there is heat, just not that hot.

I'm thinking air in the system (it was open and many hoses were changed a few months ago)


The car "overheated" or actually went into the red previously with a/c turned off, but never with a/c on. 3 times it happened and I turned a/c on and turned engine off after all went back to normal.
I don't have overheating, I have under-heating issue.

what could it be ? My aux fan is not on, most days the a/c is completely off now, and still....

1997 BMW 840CiA
1990 BMW Z1
1999 BMW 528iT (Hauler mobile)
2002 Astro Van AWD
2003 Nissan 350Z Enthusiast Package
1987 BMW 535i with 5spd manual (for sale)
1992 BMW 735iL (needs head and interior)
1977 BMW 320i (project racecar)
2007 Mazda CX9 AWD (Wife mobile)
1998 Yamaha Blaster+Breeze

1995 850CSI CD00177 (SOLD)