CHT Sensor Testing

[R. Aster]

Hi guys,

I have been running down a surging problem (mostly on decel) and had a fun day checking and cleaning the ISV, checking the idle switch, etc. I think I have a couple clues and need some input

• The surging on decel really only starts when the car is pretty hot -- almost the midpoint of the oil temp gauge -- perhaps 210-220 if I understand the gauge right
• My resistance readings from my 1-wire CHT sensor seem a bit off:
• Cold - 1.39 k ohms
• 150F - 276 ohms
• 180F - 85 ohms
• 210F - 70 ohms

From these readings, obviously the sensor is sort-of doing the job, but all the data points are quite a bit lower than spec, if we assume that CHT approximates oil temp.

So -- are these readings enough off-spec to make any difference, and if they are, what does the DME do with a 70 ohm input that implies CHT=265+? Would it go leaner than it should (for an engine actually at 210), or richer to cool the engine down? Or is the CHT actually 265 when the oil temp is 180-210 so it's not that far off?

Thanks,
RA

[stlrj]

Your cylinder head temp sensor readings look fine. Surging on decel or light throttle is usually due to a rich condition on DME.

Could be the SW chip or a rich signal from your air flow meter. What was it like before the chip?

Joe

[R. Aster]

Hey Joe,

Thanks for the response.

The car did this even before the chip upgrade, perhaps a little less. It took me quite a while to notice that it only does it when the engine gets reasonably hot, as I mentioned. I'm only in that temp range occasionally (mostly short trips). Now that I have the pattern figured, it's obvious (as most things are in retrospect -- ask me about some of my old girlfriends...).


I'll check out the AFM next weekend if it's not cured. As a problem determination step, I wired a 100 ohm resistor in series with the sensor today, but the jury is still out -- the surge appeared to be gone or very much improved, but it wasn't really hot enough today to get the temp up enough to be quite sure.

If we assume it's not the CHT for a minute, what would make the mixture go way off only when hot? I've checked my O2 sensor output and it looks like the engine runs somewhat rich without the O2 feedback into the DME -- voltage is around .75v at 2000 RPM when warm but not hot. (I'll try it hot when the weather turns.)

Thanks again for the thoughts.

RA

[WydRyd]

BUMP... How did you go with this R.Aster?

I seem to have a similar problem with my car, but mine seems to be poor idle and running when cold. When it warms up, it's fine.

I suspect it's the CHT sensor, but haven't had a chance to perform some resistance measurements. I'm told it SHOULD read closer to 2.2-2.8K Ohms, so your readings above seem a little low?!?

[R. Aster]

Hi Merv,

I'm still goofing around with this. Emphasis on 'goofing.' As you noted, my CHT outputs seem consistently low, although the smarter guys here indicate they are OK. This weekend it got warm enough to check the car when hot (~225F oil temp). CHT reading were as before. I measured the O2 sensor and compared it to some prior readings and here's the net:

engine warm -- 200F O2 = .79V -> Indicates rich. Car runs well.
engine hot -- 225F O2 = .49V -> Indicates perfect mix. Car runs worse. (Surges at low / closed throttle.)

Mixed in with this, I also get the hunting idle problem sometimes. This seems to be related to ambient and engine temp both.

I think all this is telling either my O2 sensor and/or my CHT sensor is lying to the DME (and me) since it should run better at ideal mixture than the (indicated) rich.

So. I am still not getting very far with this. So far I have

• Checked the distributor cap / rotor. 10K old and look great
• Done the dark garage check of the plug wires. Started to feel woozy after a while...
• Checked the ICV and cleaned it
• Adjusted the idle speed screw on the TB
• Run several tanks of Chevron and Shell 93 Octane through it
• Checked the air temp sensor resistance
• Put 200 ohms resistance in series with the CHT since it seems to read low
• Done the oil cap off test for air leaks
• Checked the idle on/off switch

But the problem remains annoyingly erratic.

I think my next steps are

• Check the AFM flapper output voltage
• Adjust the AFM mixture screw
• Replace the O2 and CHT

Anyway, the CHT is very easy to check. Let me know what yours reads. And hopefully some of the actual experts here will jump in.

RA

[ChrisBennet]

I'm not expert but to me, those readings look low. Cold car on a summer day is around 3Kohm. (Or 3.6K ohms on a Canadian summer day ). Hot engine is around 270ohm.
-Chris