D1, 202K, head gasket done already. MPG dropping below 12. Carbon fouling all plugs on cylinders on left side of engine. Plugs on other side look great. Have observed black smoke output at high idle. Black, not blue, white, gray, or brown. Suspected stuck open thermostat because low coolant temp sensor readings. Coolant temp readings did not agree with dash gauge (8:30) or infrared thermometer reading of radiator inlet. Changed thermostat from 180 (have run for a few years) to 195 (what store had in stock).

With new thermostat the coolant temp sensor is reading 15 or more degrees lower (185) than IR sensor used to spot check radiator inlet temp (202F) Coolant sensor (two wire, not the one wire gauge sender) removed, clean and shiny, no coral build up. Coolant flows out the open port vigorously, do not think it is internally blocked.

Scanner says we are in closed loop (both sides) well before thermostat opens. On the road going to wide open throttle makes system go to open loop as expected, drop back to cruise and it is closed loop on both sides.

Scanner shows downstream O2 sensor on side with fouling has very low output voltage. Like .005, does change, but no where near the voltages of the other three sensors.

So is the poor mpg and fouling more from sensor telling CPU the wrong coolant temp, or the down stream O2 sensor? Or both? Anyone have a chart of expected resistance vs temp for the coolant sensor?

Many codes, as expected, related to misfire, emissions, etc.

Thoughts?

 

I would speculate the coolant temperature sensor is not the culprit because that would affect the fuel trim for both banks. Temp sense is not per bank. The O2's are per bank. However, I thought the downstream O2 were primarily relevant to catalyst efficiency and not fuel trim. I don't really know. Are you sure it's not actually the upstream O2 that is bad? Do you also see abnormal fuel trim on the bank with fouling?

 

BUZZ ITS GREAT TO SEE YOU!


what do the readings look like on live data stream for the upstream o2, the upstream should both be rapid fluxuating on closed loop


I wonder if that fuel rail coolant sensor would screw it up?
guess there is also the possibility of a vacume leak on one side leaning it out and causing other side to appear rich


downstream are just for cat efficiency, will trip bad cat code that's it afaik

 

Hey Buzz....welcome back!

 

Buzz, great to see your still kicking. Miss your words of wisdom from time to time.

Coolant temp sensor could be failing, hence the difference between actual temp(ir gun) and measured temp(sensor)

Or the connector itself could have corrosion, throwing off the reading.

O2 sensor sound weak based on your description.

And as Tom said, the possibility of a vacuum leak is very high.

 

You may be able get rough idea from this if not seen yet. A 15 F temp sending discrepancy shouldn't cause a rich condition on both banks much less one.

Yours is a good method to find difference between sensor output and actual coolant temps. Thanks for that and will be doing the same soon to ensure temps I'm seeing are accurate. It would probably a good cooling system PM item.
......

 

Readings from scanner at idle:

ST FTRM1(%).......24.2
LT FTRM1(%)......-24.2
ST FTRM2(%)......-25.0
LT FTRM2(%)......-24.2

FUEL SYS 1.........CLOSED
FUEL SYS 2.........CLOSED

COOLANT............189F

O2S11 (v)...........0.990 (not charging much)
O2S12 (v)...........0.000 (goes up to .005 at 2K rpm)

O2S21 (v)...........0.015
O2S22 (v)...........0.025

ECT OHMS = 258 at 185F, so somewhat out of spec, but most likely no problem. Did not find any vac lines loose, etc.

Work and health issues have kept me off the site, plus college offspring was driving Rover maybe 75 miles a week. I was just doing underhood check. Bought her another ride, retired my Mercedes wagon, and starting driving the Rover again. So it may have been having problems and just haven't been looking. CEL is burned out from previous owner, I go by the MIL indicator flag on the scanner when hooked up.

So which O2 sensor should I swap out?

 

That one. Bank 1 rear.
That will affect trims, which affect fuel economy.
Once switched out, if it doesnt affect fuel trims, them you either have a leak , and may require a smoke test, or the MAF is on its way out.

I hear you, life catches up with us all. Good to see you're still kicking old friend.

 

When I said vacuum leak I meant plenum or intake gasket

Down streams do nothing on these old obd2 mine are bypassed with no cats

the one that's not changing much, its showing how rich you are running

Ntk are cheap on amazon if you wanted to do both upstream for good measure, pull coil packs to access the plugs

I'm not sure it the issue

 

I'd replace both upper O2's. Read the GEMS pdf on sensor values, they don't swing far +1/-1 lean/rich but like Tom said, the values move often.