How to replace temp-sensitive block-heater plug

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This article was published 06/02/2009 (6100 days ago), so information in it may no longer be current.

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ANSWER — Our seemingly extra-cold winter temperatures this year are causing more of a problem with these block-heater cords than they did in the past. The factory block-heater cord end turns off above -18 C. If a regular plug is installed — or because the factory plug, outside the engine bay, cannot track the engine’s temperature as the block heater warms it up — the engine temperature gets too warm and codes are set by the engine computer because of a wide temperature difference between the outside air temperature, sensed by the air intake temperature sensor, and the coolant temperature. The best fix is to install a factory block-heater cord and clamp the plug-in end onto the top rad hose. Use a short extension cord to run out to the front of the vehicle. This will keep the engine temperature within the proper range.

External timers and temperature-sensing cords will also work, but the trick here is to not get the engine too warm. By the way, the factory block-heater cord isn’t connected directly to the engine computer. If it isn’t working the way it was designed, the problem is detected by the computer comparing temperature-sensor inputs.

QUESTION — I am having problems with cold starting with my 2001 Nissan Sentra. I have installed new fuel injectors and tested fuel pressure, new engine-temperature sensor and a new MAF (mass airflow) sensor. Currently, the car gives a fault code for the transmission lockup torque converter and for the catalytic converter. The old cat was plugged and I have taken it out, but would like to get the vehicle running properly before installing a new cat. I wonder if these fault codes could somehow be interfering with the cold start of the engine?

When starting in cold weather, the vehicle takes off fine at a high idle, then after a couple of seconds the idle speed drops too low and the vehicle stalls. In warmer weather, the engine will not idle. You have to blip the throttle constantly till the water temp starts to read on the temp gauge. Could I have some kind of bad ground? Any insight into this problem would be appreciated.

ANSWER — There are very few sensors that are required for basic engine functions such as cold-start idle, warm idle and acceleration. The main ones are the temperature sensor, manifold pressure sensor (your car doesn’t have one), mass airflow sensor, throttle-position sensor and crank or RPM sensor. With these inputs, the computer can inject the correct amount of fuel and control the idle speed properly. The codes that you have for the transmission and catalytic converter shouldn’t affect starting and idle.

Because the engine starts, the crank or r.p.m. sensor must be sending information to the computer. The temperature and MAF sensor have been changed, so that leaves the throttle-position sensor. Instead of a variable sensor that is found on many vehicles, your Sentra uses a throttle switch (TPS). It looks like a throttle-position sensor, but sends only on/off signals at closed throttle and wide-open throttle instead. If the TPS is not sending a closed-throttle signal, the idle speed won’t be controlled properly by the engine computer. Surging, incorrect idle speeds or stalling can occur. The fault may be internal in the switch or even at the wiring connection to the TPS. It could also be caused by someone adjusting the throttle stop screw to move the throttle plates for a faster idle. I often see this happen. Because the throttle plates are set incorrectly, the TPS signal now doesn’t show closed throttle. If the switch circuit tests good, then look to see if the throttle stop screw has been tampered with. If it has, it will need to be reset and this can be done by monitoring the TPS signal voltage for closed throttle while turning the stop screw back. While you are at it, clean out any gum deposits from around the throttle plate housing bore, as that will cause incorrect idle speed and stalling too.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net