Tickover problem with low speed lurching

[gazhawkins]

1999 Toyota Corolla 1.6SR petrol, 'T' plate, 4A-FE Engine (4 cylinder Fuel Injection), manual, 3 door/hatch, 164000 miles

Dear All,

I have an engine tickover problem problem on the above vehicle - hope some folks can diagnose what the problem might be?

When the vehicle is started from cold (even in Summer), the auto choke does come in at 1200-1400 rpm. However, once this comes down as the engine warms up the tachometer/rev counter needle swings up and down from about 800 to 400 rpm. Generally, the rpm recovers to 800 of its own accord - but sometimes the engine stalls completely if the car stops quickly - rev counter dives from 800 to zero.

At lowish revs, with the car running in any of gears 1 to 5, this can result in the car 'lurching' - as opposed to proceeding smoothly which it normally would.

Does this suggest a choke problem, or perhaps HT Leads?

Car was last fully serviced at 160,000 miles, doesn't use much oil - but it does do a lot of mileage mainly on motorways.

 

Any suggestions?

 

Many thanks in advance to any contributors.


Regards,


Gazhawkins.

 

 

 

 

 

[garethpaul]

I would be suspicious of the MAP sensor, its wiring or its associated vacuum tubing. A fault here would exhibit the symptoms you are experiencing.

The MAP sensor is mounted on the rear wall of the engine bay, usually at the top in the middle. Small black square unit with a red/blue label. Has a 3 wire connector and one silicone vacuum tube The picture below shows two items mounted to the rear wall in the middle. The MAP sensor is the left one as you look at it.

Inspect the vacuum hose for any splits, and that it is not loose on the fittings both ends. Ideally, replace the hose if it is original.

If the hose is ok, there are procedures to test the operation of the MAP. Requires a multimeter and a vacuum pump with gauge. If you don't have a vacuum pump you can test that it is generally working by sucking on the tube  and watching the reading on the multimeter, but this will only tell you if the sensor is dead or not.  You wouldn't be able to test its accuracy.

[gazhawkins]

Thanks Garethpaul - I will take a look; or at least, I will get my friendly mechanic to get his multimeter out - if I can't spot anything obvious.

 

Gazhawkins.

[garethpaul]

Found the test procedure below. Good luck. Let us know the outcome.

1.Disconnect the electrical connector from the MAP sensor. Turn the ignition On(do not start the engine) and check the reference voltage on the MAP sensor electrical connector between the yellow (+) and brown (-) terminals. It should be approximately 5.0 volts.

2. Disconnect the vacuum hose from the MAP sensor. Reconnect the electrical connector to the MAP sensor.

3 Using a voltmeter and the appropriate probes, backprobe the MAP sensor harness connector and connect the voltmeter to the light green/red wire (+) and the brown wire(-). turn the ignition key On. Without vacuum, the sensor voltage should be approximately 3.0 to 4.0 volts.

4 Using the hand-held vacuum pump, apply 15-25in-hg of vacuum to the MAP sensor and observe the voltage readings. The voltage should decrease to approximately 0.5-1.5 volts.

If the test results are incorrect, replace the MAP sensor.