Temperature Sensor and Idle

[Dave Bell]

Well I have moved along in my search for improving the idle on my 75 1.8 with L-jetronic. The valves are adjusted, I have checked the intake air sensor, O.K., Throttle Switch, O.K., Auxiliary air regulator, O.K., took off the throttle body and cleaned out oily junk + other ugly looking crap from the intake side...looked like it got sucked in from the oil breather because it was in the air intake channel just after the air cleaner and the air cleaner was of course.... quite clean!... Found the same stuff in the oil breather, (aka oil vent), when I took it apart. One of the vacuum ports on the throttle body was completely clogged with brownish crap... got that all cleaned out. After re-assembly, everything seems to idle better but I am still having a warm start problem... after the engine is warm and off for awhile, the idle wants to drop out after restart until the car has been running for a while again.

So I made it tonight to checking what Haynes calls, "Temperature sensor II." Per Haynes, I disconnected the plug to the sensor, hooked up an Ohm meter and with the engine very cold I read about 1 - 2 ohms only... but Haynes says that I should only see 0.5 ohms to 200 ohms if I am reading the instructions correctly? It was real cold tonite here in LA area and I did not see much if any increase in resistance "due to ohm meter current" as Haynes suggests.

Can anybody confirm that I am doing the test , "right" or if there is a better way to test the Temperature sensor.

Also, I would like to take the oil breather out again and let it soak in some engine or carb cleaner to get the rest of the crap out of there that I found in the intake system... Is there any reason I should not do that... i.e. any hidden parts in side I don't want to dissolve?


Thanks,

- Dave

[Pritchard]

To test the Temp Sensor II or Cyl.head temp. sensor: connect the ground/negative lead of your ohm meter to the negative side of battery and positive lead of ohm meter to temp sensor. You should get a reading of 1500 - 2500 ohms when cold and <100 ohms when hot.

Good luck!

Pritchard

[Dave Bell]

Pritchard -

Thanks for the input... if that is correct, then my temp sensor is shot... I did the test you described and got a reading of about 2 ohms only! I suppose that with a failed Cylinder Temp Sensor the FI brain can't tell when the engine is warmed up and thus the source of my warm start problems...

Looks like it is time to place a Pelican Order.

- Dave

[Dave at Pelican Parts]

A couple things:

First, make SURE you measured correctly! If you've only got 2 ohms on the HTS, the engine will run HORRIBLY lean and probably will not even start when cold! Make sure you didn't have the meter set on the "Kohms" scale--2 Kohms, 2000 ohms, would be pretty much right on.

Next, if the sensor really IS bad, be VERY VERY VERY CAREFUL about removing it! It may be best to let the engine get all the way warmed up, since aluminum expands more than steel. Penetrating oil is your friend. So is patience, and some gentle back-and-forth movement of the wrench/ratchet/whatever.

It is incredibly easy to strip the hole in the head where the HTS goes. None of us who did it were applying enough force to strip it, but it stripped anyway.

In fact, if the sensor hasn't been replaced in years and years, it might be best to let a shop do it. They may have tricks and techniques that we don't know about.

--DD

[Dave Bell]

Thanks for the warnings Dave...

I only checked the temp sensor while the engine was bone cold. Again, per Haynes, I removed the plug, then hooked up the ohm meter between the sensor and the battery ground. I flipped the leads on the ohm meter just to make sure becuase on many ohm meters, the red lead, positive lead for voltage measurement, will actually act as the negative lead for a resistance measurement.. not much difference though.... the meter was set on the 0 - 200 ohm scale. When set on the 2k ohm scale the reading was .00X ohms.

I was a little confused by the Haynes short description of this sensor, but from the reply I got from Dave Pritchard, it seems that I should expect a higher resistance at low temperatures and resistance should descrease as the temperature goes up. Do I have that correct now?

If you have time, perhaps you could explain a little more about the function of this sensor with respect to what the FI does with the info... it would be informative to me at the very least. I would have thought that the cold start valve, the aux air regulator and a high idle setting, (which I had to run to keep from stalling) would get the engine going)... and then when it warmed up... the "failed" sensor would already be reading some low resistance, like it was warmed up, so I would just be left with my high idle and the warm start problem... that is my simple logic with my limited and perhaps flawed understanding of how the sensor and FI are interacting)

I had planned to go through a few rounds of penetrating oil as you suggested... but now you really have me scared. I will take any suggestions from others that have been successful or not successful at pulling this sensor without stripping the threads.

- Dave

[This message has been edited by Dave Bell (edited 02-22-2000).]

[Joe Bob]

Hi Dave, I see by your profile that you are a La Cresenta/La Canada/Flintridge person...cool, I survived Eagle Rock and the Angeles Crest Hwy...but I digress....the sensor is a weak link, it has probably been there awhile and was installed w/o any
anti-seize lube.

The advise from Dr D3 is very good, wait until the engine is at operating temp before trying to get that sensor out. If you have trouble getting it to op/temp let someone who has done the removal before, do it for you. Not knowing your expertise, it is very easy to faux paus on this...ask me how I know.....the T4 motor needs to run "slightly" rich to start, but not too rich....I would HIGHLY suggest that you have an exhaust gas tester to verify the emissions. It is a window into the soul of the T4 motor. You need to know if your motor is running lean or fat....

Good Luck....

[Alpine]

Dave,

You can find some trouble shooting and testing information for the L-jet system at my page http://persweb.direct.ca/aschwenk/earlyefi.htm . This information is from a VW manual but a lot of it should still apply to your 914.

Alpine

[Pritchard]

Dave-

Here's my best guess on the Temp. Sensor II.

The CHT sensor is a resistor that changes resistance with temp. If the engine is cold, it needs a little more gas to operate. The CHT sensor tells the computer to prolong the injector duration by sending a higher resistance value (1500- 2500 ohms). As the engine warms up to operating temp., the CHT sensor reduces its resistance value (<100 ohms when at operating temp.) and the computer shortens the duration of injection.

I played around with a potentiometer connected in-line with my CHT sensor to "dial in" my engine. The changes in resistance had a very noticeable effect on idling. Radio Shack has a 0-5000 ohm pot. for a few bucks if you want to try this before you try to remove the CHT sensor. I have replaced the CHT once and had no trouble removing it. Maybe I was just lucky.

Good luck.

Pritchard

[Alexander]

I would agree with Pritchards suggestion of trying the potentiometer in line with the sensor before removing it. I replaced my sensor only to find I was reading my meter incorrectly and the temp sensor was just fine. The potentiometer only cost $1.99 at Radio shack and I got a smooth idle with about 550-780 ohms of resistance added to the line. A 10 watt resistor of 680 ohms was then installed in line (cost $0.50). My engine now starts easier, idles better than before, and solved my idling problem with an investment of $2.49 and that was after I spent the time and money to replace both temp sensors and a few other things.
Good luck.

Alex

[Dave Bell]

Thanks guys... these are excellent suggestions... which I will try... especially since I found out recently that I have had my head up my a@# on this one. I can blame it on the 914 still being a new car to me and having just now done work on the engine... I found that I had been looking at a similar shaped sensor screwed into ??? up near the distributor instead of the Cylinder Head Temperature sensor. When I found my error and located the real Temp Sensor II.. it seemed to check out O.K.

So I will return the one I ordered and try some dialing in as you both suggested.

What sensor was I looking at on my 1.8L up near the distributor?... same shape as the Temp Sensor II only a little bigger.

- Dave

[mejulihn]

Dave,
Sounds like you found the oil pressure sender. No wonder your readings were off! Most of us visit this BB because we learn a lot -- even when it comes at the expense of someone else! Thanks for sharing your honest mistake. This thread generated some great advice.

[JP Noonan]

...my favorite was my fist valve adjustment. "Man were those valves off from the PO", I thought as I adjusted away. Well, they were off if you adjusted them like a 90 degree Chevy V-8. Suffice to say it is NOT the same procedure as a 180 degree VW H-4. All I got out of the engine was a few back fires (luckily I didn't smack any valves), of coarse I then went through the entire rest of the motor before I realized that maybe, just maybe I didn't time the valves right. Helps to read the book.