3.2 Carrera CHT sensor question:

speeder · speeder's Garage

I've searched the archives but didn't exactly see my question answered. Most of the CHT sensor failure modes mentioned were associated w/ poor idling, cutting out, etc...

Can a CHT sensor just make the car run rich? I was talking to Steve Wong tonight about the '89 Carrera that we are about to chip, and I noticed that the O2 sensor was black/rich looking when I installed the test pipe today. The general engine health of this car is excellent w/ smooth power delivery and healthy sound, plus assumed tight compression/LD since it has a recent top-end from a respected shop and only ~130k total miles.

The car does not smoke, but it is generally a little sluggish the way that many stock Carreras that I've driven are. The discovery that it's running rich makes perfect sense, that will make a car slower.

Obviously we are going to make sure that all sensors and systems are in good "baseline" condition before chipping, can a CHT sensor just be "off" and cause rich running? What else to look for on this car? TIA.

speeder · speeder's Garage

Quote:

Originally posted by SoCal911SC
I would think that if the CHT is not functioning properly, and signals the DME that the engine is dead cold when the engine is in fact at full operating temp, it would run too rich.

Kind of like the coolant temp sensor on a H20 cooled DME car.

Without a doubt. What I'm wondering is if the sensor could be just *a little bit off*, ie. the car runs OK but a little rich.

Early_S_Man · 04-09-2005, 12:18 PM

Denis,

There is a spec in Bentley for testing the CHT sensor, but I can't recall if they give just a test point or two, or the full temp vs resistance range. Is your sensor the updated, two-wire version? If not, I suggest replacing it just on principle ... and you could test the new one for your records and posterity!

speeder · speeder's Garage

Warren, that's not a bad idea, I'm going to look through the reciepts for this car and see whether it has ever been replaced. Does anyone know exactly where it is located under the hood? (I'm new to Carrera maintenence).

When I did my search, all I saw was total values from cold

Fl Ted · 04-09-2005, 01:13 PM

Speeder;
I'm in that area now, checking for cause of cutting-out. Last drive /test pointed to CHT (unplugged and jumped DME input part of plug with 270 ohm resistor, problem went away.
Basically Bently book lists 2000-3000 ohms@68deg F, 250-400 ohms@ 176 deg F. so if the CHT is faulty it CAN mess-up your mixture.
As to location, yeah they hid the actual probe real good ! You get at it starting in the left rear wheel well. I'm not sure about an '89 Carrera, but my '86 CHT connection is right forward ( looking from behind the engine) of the heater boost fan. I removed the duct going to the fan and that gave me a clear shot at a vertical bracket that has 3 connectors on it. The CHT is the top one. Take off the outboard plug (that part goes to the DME) Get a mirror and light and look at the connector still on the bracket. If it has only one contact, it's the old style and should be replaced. You can use your ohmmeter across the contacts to measure its resistence to see if its outside the normal range, but to really test it, you have to remove it. Do a search, theres a thread on how to change it.
Good luck, I'm off to try to fix my MAF !!!

greglepore · 04-09-2005, 02:03 PM

Yes, it'll run rich-when I accidentially dislodged the connector on mine recently, the car went to so rich it wouldn't start, unless you held the pedal to the floor and cranked for a while, then it was still so rich it wouldn't really run.

When the previous one failed, it went to the cutout mode.

However, before I'd suspect a bad cht, in a car that is otherwise running fine, I'd check the baseline mixture.