Temp gauge issue

[dunny]

Lately I've been fighting with my 318i....trying to figure out why the temp gauge needle keeps bouncing all over the place.  

So basically the car seems to run fine, but then the little needle starts moving very strangely once the car heats up to operating temps.  I have an infrared heat gun that I can measure the temp of the hoses and the top hose connected to the radiator is never above 170 yet the gauge will have the needle pegged to nearly the red.  I can then turn on the fans/wipers/lights (anything that pulls some current) and the needle moves around a lot.  Shut all of the accessories off and then it goes back to normal.  I'm not certain if the gauge is correct or not.

I had the alternator tested, it checked out good.  The belt was a little loose so I tightened it.  On the hot side of the radiator I've never measured above 180....cold side though is a little strange.....sometimes it will read 90-100 (or hotter) but often it will be 40-50 (temps are in the 30s in ohio right now).  So I sort of suspect maybe a stuck thermostat or a clogged radiator?  But both are new (<6k miles).  

FWIW, the MPG has dropped significantly as well.  Used to get 31-34MPG, now it's 25-27MPG.  Plugs look fine.  There is a lot of valve noise on cold startups but otherwise nothing out of the ordinary.  I do have a slight oil leak towards the front driver's side, not sure what that is yet or if it has something to do with my problem.

Any ideas on what I should investigate?  Thanks!

[DesktopDave]

I'd check the coolant gauge temp sensor on the cyl head.  Might just be a bad connection or a failing sender.  On the e30 there are two...the blue two-pin DME sender and the brown one-pin gauge sender.  We're interested in the brown one...should be the one furthest back.  It only has a single wire, grounds through the head, so grounding issues in the engine bay will make it flake out.  IIRC the sender should show about 1400 ohms cold and 300 ohms at operating temp.  Check it in the ETM or the Bentley if you have them handy (I don't).

Here's a thread over at BF that shows a few low-quality pics.

Other potential issues: the grounding nut on the back of the cluster has a habit of backing off, and capacitor/SI board batteries are a persistent problem.  The gauge isn't all that accurate anyway...good thing you're using an IR thermometer instead of guessing what the gauge is trying to tell.

Don't worry too much about the fuel mileage.  Winter mix fuel has more aromatics for easier cold starting...that can drop mileage significantly.  A slow O2 sensor or the two DME temp senders can also hurt your mileage.  The DME temp senders (one on the block & one in the AFM) strongly control Motronic's fuel & timing calculations.  If they're wrong the DME will be running the injectors rich.

[dunny]

I'd check the coolant gauge temp sensor on the cyl head.  Might just be a bad connection or a failing sender.  On the e30 there are two...the blue two-pin DME sender and the brown one-pin gauge sender.  We're interested in the brown one...should be the one furthest back.  It only has a single wire, grounds through the head, so grounding issues in the engine bay will make it flake out.  IIRC the sender should show about 1400 ohms cold and 300 ohms at operating temp.  Check it in the ETM or the Bentley if you have them handy (I don't).

Here's a thread over at BF that shows a few low-quality pics.

Other potential issues: the grounding nut on the back of the cluster has a habit of backing off, and capacitor/SI board batteries are a persistent problem.  The gauge isn't all that accurate anyway...good thing you're using an IR thermometer instead of guessing what the gauge is trying to tell.

Don't worry too much about the fuel mileage.  Winter mix fuel has more aromatics for easier cold starting...that can drop mileage significantly.  A slow O2 sensor or the two DME temp senders can also hurt your mileage.  The DME temp senders (one on the block & one in the AFM) strongly control Motronic's fuel & timing calculations.  If they're wrong the DME will be running the injectors rich.

Thanks Dave. So if I'm not seeing very high temps at the hoses coming off the radiator then it really isn't overheating, correct?  I just don't want to warp the head.

Dumb question - but what about the temp sensor that is located on the side of the radiator?  For some reason I thought that was what the temp gauge used.

[e9nine]

When your car is at idle and tap on the gauge cluster, does the needle move? It's a common e30 issue. You can fix by removing the cluster, inspecting to see if the little 8mm or so (someone correct me on the size if I am wrong) nut is threaded on securely. If the issue is elsewhere, swap in a good used temp sensor.

The temp sensor on the pass side of the radiator is for the aux fan.
The temp gauge is fed off a single pole sensor in the head
The ECU temp reading is from another sensor in the head

[DesktopDave]

+1 - the radiator sensor (fanstat) is a two-way switch that only controls the aux fan in front of the radiator.  It's not tied into anything else, just switches the aux fan off/low/high depending on the temperature in the rad end tank.

I haven't done any cooling system work on my e30, so I'm no expert yet.  But here's a few general tips I've learned along the way:

Low temperatures on the driver's side of the rad make it sound like the motor is healthy.  I'm pretty sure that's the "cold" end tank in our cars.  But if you have a pressure leak and the system isn't getting up to 30psi or so, the radiator end tank temperatures won't reflect what's going on inside the head.  Low temperatures on the cold tank could also mean a broken water pump or plugged radiator.

BTW, if you're having trouble cooling the car off, pressing the A/C switch will turn on the aux fan at low speed.

[dunny]

+1 - the radiator sensor (fanstat) is a two-way switch that only controls the aux fan in front of the radiator.  It's not tied into anything else, just switches the aux fan off/low/high depending on the temperature in the rad end tank.

I haven't done any cooling system work on my e30, so I'm no expert yet.  But here's a few general tips I've learned along the way:

Low temperatures on the driver's side of the rad make it sound like the motor is healthy.  I'm pretty sure that's the "cold" end tank in our cars.  But if you have a pressure leak and the system isn't getting up to 30psi or so, the radiator end tank temperatures won't reflect what's going on inside the head.  Low temperatures on the cold tank could also mean a broken water pump or plugged radiator.

BTW, if you're having trouble cooling the car off, pressing the A/C switch will turn on the aux fan at low speed.

Thanks!

I think my aux fan is not functioning....I remember us IM'ing a while back about this.  I never messed with it because after getting a new radiator and radiator cap my car ran great for 10k miles / 6 months.

When you say "low" temps on the driver's side....what temp range are you talking about?  Anything under 180 (which is when the Tstat should open)?  The weird thing I'm seeing is the drivers side will be 160-170 and the passenger side will be anywhere from 40 all the way up to 90.  

As for PSI - when I take off the cap after running the car and letting it sit I do hear air escaping so it must be holding some pressure.  

I suspect that the water pump isn't working properly, but I can't be sure.  I plan on replacing that plus timing chain/guides/etc.  But currently when I run the car without the radiator cap on I do get coolant flowing through the little opening at the top of the radiator (where the cap goes)....hold the revs at 3k or so and it seems to flow great.  Would that be the case if the water pump was bad?

Thanks for the info in this thread.

[DesktopDave]

You might also want to verify that both your fans are working.  The engine fan should have little resistance to spinning if it's cold.  When you try to spin it it should take noticeably more effort if the car is warmed up.  Some test it with the car running...a hot fan should be able to shred newspaper, but ours have plastic fins and I'm reluctant to try that.

You can test the aux fan easily by pulling the plug off the fanstat.  Be careful because it tends to dissolve over the years.  There are three pins inside that plug.  Green is ground, black/red is high speed and black is low speed  (if I recall correctly).

Get a short bit of jumper wire, turn the ignition on, and plug one side of the jumper wire into the green wire (ground) socket.  Then touch the other side of that jumper wire to one of the black wire sockets.  Fan should immediately turn on.  Pull the wire out of the black socket and put it in the other black wire socket.  Fan should immediately turn on at a different speed.

If either setting fails, check fuses and relays as described in the ETM.  If the low speed fails, check (& likely replace) the resistor pack on the fan's frame.  If the A/C button won't run the fan at low speed but the resistor is OK, disconnect the battery & check the inline diode inside the fuse box.