A few days ago, the Check Engine light displayed on my 91 Volvo 240 B230F. I pulled the engine codes, and it blinks "1-2-3". The FAQ says this is due to a "signal missing to/from cooland temp. sensor". I drove around another day or two, and suddenly the car is extremely hard to start when the engine is cold. Again, the FAQ says this is a symptom of a wiring fault to or in the ECT sensor.

I searched online for several sources, which display the ECT or water temp sensor with a wide plastic connector (Bosch #0280130032). This sensor fits under intake manifold port #3.

I went to my local Volvo dealership, and they sold me the "Temp. Sender" (Volvo #460191-0), a smaller sensor with a slender blade-type connector. This sensor fits underneath intake manifold intake port #2.

Which one do I need? What does the Temp Sender do? Should I replace it too, or go back for the right part?

BACKGROUND: Six weeks go, my coolant thermostat cracked, the engine overheated and blew the cylinder head gasket. I got the cylinder head resurfaced at a machine shop, cleaned both these sensors with a wire brush, and reinstalled them in the cylinder head. It ran OK, a bit hard to start when cold, but recently began to lope/idle erratically. After the check engine light came on, it takes several tries to start in the morning. Could it be something other than the sensors causing this problem?

[[ southern fried volvo '91 240 135k mi ]]

Your engine has TWO temperature sensors.

The single-blade sensor, under intake header #2, is for the temp gauge (only).

The double-blade sensor, under intake header #3, is for the FI ecu. It tells the fuel injection system whether your engine is hot, cold, or frigid, and the FI systems adjusts the fuel mixture accordingly.

The resistance of the sensor varies inversly with the temperature. A hot sensor (high temp) means a lower resistance. A cold sensor means high resistance.

If a wire broke at the push-on connector, the the FI ecu would measure infinitely high resistance, which it would interpret as a very cold engine. The ecu would pour in the fuel. On the other hand, if the wires were shorted or the sensor had an internal short, the resistance would be low and the ecu would think the engine was hot -- and reduce the fuel accordingly.

"Could it be something other than the sensors causing this problem?"

It could be a bad (or failing) fuel pressure regulator or a bad (or failing) AMM. I wouldn't expect your error code for either of these problems, however.
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Don Foster (near Cape Cod, MA)

Thanks for the quick response! I apologize for not browsing the FAQ in detail, BEFORE I posted this comment. I found lots of other helpful information a few seconds after I read your response earlier today.

I purchased the correct ECT sensor from independent Volvo mechanic shop today. Can you believe the owner actually ENCOURAGED me to do the job myself?!? The mechanics were tied up on some more detailed jobs, and couldn't replace the sensor for me. So the owner let me use an empty work area, and I replaced BOTH the sensors at their shop.

I reset the error codes, and the Check Engine light has not returned. However, the car still idles/lopes erratically. Next, I will recheck/reclean the IAC and Throttle Body, look for another vacuum leak... and take a look at the Fuel Pressure Regulator.

[[ southern fried volvo ]]

"Can you believe the owner actually ENCOURAGED me to do the job myself?"

Sure! Replacing that sensor is a royal PITA. And if something happens, it's your fault and your (new) part, not his or his.

"I will recheck/reclean the IAC and Throttle Body, look for another vacuum leak... and take a look at the Fuel Pressure Regulator..."

Yep. All the right things. Check along the intake manifold gasket for a leak.
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Don Foster (near Cape Cod, MA)