Honda introduced the 1988 Civic with a completely new fuel injected engine. All traces of the venerable CVCC carbureted engine were gone, and everything in the Civic line was fuel injected. The CRX and Civic Si had a multiport fuel injected engine since 1985, but a new twist came in 1988 with the introduction of the Dual Point fuel injection system. This Dual Point engine came in two horse-power configurations, with the base four-speed hatchback getting less punch than the five-speed and A/T models.

Like a lot of folks, I started wearing bifocals when I turned 40. Heck, now I have to take off my glasses to read wiring diagrams or to work under dashes. Even with my new specs, I find that it’s easier than ever to miss things. I’ll give you an example: I was searching for a topic for my next article, and it turned out there was a pretty good idea parked right under my nose.

It was a 1991 five-speed Civic sedan. Not surprisingly, the Civic was getting a little ornery after 191,000 miles of service. It had regularly delivered 400 highway miles from a tank of gas, which works out to about 40 mpg. It wasn’t getting that kind of mileage any more, but that wasn’t an all-the-time occurrence. The owner started noticing that “hey, didn’t I just fill up this tank four days ago?”

There were other indications that everything wasn’t working the way it had in the past. The Civic occasionally refused to restart after a hot soak. During warm summer temperatures, the idle speed also occasionally stayed very high after the initial startup or surged during warm idle. Based on the symptoms, I was pretty sure I was onto a failing TW (temperature of water) sensor. But the more I poked around under the hood, the more I found.

The TW sensor can be problematic. The tough part is knowing all the symptoms that point to a failed TW sensor. Then you’ve got to have your meters attached to the right wires at the right time to catch the right values for the symptoms when the darn thing acts up. Years of listening to customers, watching coolant sensor values, and being lucky enough to catch one ‘in the act’ have given me an edge.

The most common complaint I’ve heard is “I stopped for gas this morning on the way to work, and after I filled up, it wouldn’t start.” That gives me the time frame involved (about five minutes or so), with an engine that has heat-soaked from a fully warmed-up condition.

Often a meter attached to the TW sensor will read normal voltage (starting at about 2.5 volts on a 70 degree F engine). The voltage signal should decrease as the engine warms up, down to about 0.5-0.6 volts by the time the cooling fan comes on. Turn the engine off and leave the meter attached. If you can’t afford to stand around and watch the engine hot soak, do something else. After five minutes have passed, try to restart the engine.

If the car will not start, the TW reading has probably risen from 0.5 volts when the hot engine was shut off to about 3.8 to 4.2 volts before turning the key to start it back up. This makes the PCM think the coolant temperature of a fully warmed-up engine is far below zero. The PCM will respond by gushing fuel into the engine and flooding it out —right now. I have also seen the TW voltage go from 0.6-0.7 right up to 3.2 while the engine was running, then settle right back to normal again.

If you have one of these vehicles in the failure mode (the car won’t start when hot), unplugging the TW sensor will probably allow it to start right up. This prompts the PCM to default to a simulated coolant temperature reading of about 170 degrees F. Another way to determine whether you’re on the right track is to check the injector pulse-width with a ‘noid light or a scope. A bright ‘noid light flash compared to a dull pulse you had been getting earlier on the warm engine indicates the pulse width has dramatically increased.

This system has no fast idle valve, so the TW sensor also helps the PCM determine idle speed. On a cold engine, the PCM uses the TW input to decide how much to open or close the Electronic Air Control Valve (EACV) and maintain proper idle.

If the idle speed flares, see whether the PCM is providing a longer ground duty cycle to the EACV. Read duty cycle or dwell on the blue wire at the EACV. The black/yellow wire is a 12-volt power supply from the ignition switch. Most times, if the idle speed has just flared, or the car has started to run ‘lumpy,’ something has changed on the red/white wire at the coolant sensor. I’ve caught this a number of times. There’s nothing like being sure when you tell your customer, “Yes, the car is fixed.”

That’s enough for this month. Now you know what to do with a Honda that’s hard to start hot. Next month I’ll show you a few other key elements to expand your vision when one of these Civics shows up acting cranky.

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