Toyota IAC strange "Hiccup" upon cold start

Hi,

I have a 1998 Toyota Corolla 1.6 (4A-FE) automatic with 44,000 miles on the clock. The Corolla is a UK OBD1 vehicle.

Symptoms

The following complaint only ever occurs on a cold start, never when the vehicle is at normal operating temperature.

Cold start issue: Cranking the engine and just at the moment the engine catches (to start), there is a "hiccup" and then the engine catches and runs at about 1300 rpm. Sometimes the engine will misfire until warmed up for about a minute and then the problem goes away. On occasion the "hiccup" is so bad that i have to dab the throttle pedal to keep it running. Once I've gently touched the throttle and immediately released it, the engine will idle at 1300 rpm (cold). Hot is 700 rpm.

Tests i have done to date

I have checked the valve for free movement and it was moving very freely, no resistance at all, which surprised me. I did re-clean it and both O rings were present.

I have scoped the idle air control valve and there was battery voltage, good ground and the valve had control from the PCM with a nice signal. Once warmed up i switched on the A/C and the duty cycle number changed showing control. The duty cycle also changed when blipping the throttle.

Questions

1) Is it possible the IAC Motor is weak causing the temporary "hiccup" upon cold start up?
2) Cleaning was done with the unit on the vehicle, should i remove it from the vehicle and clean it again? (Although the valve was moving freely)

Thanks

Steve

Might be worth checking for a vacuum leak when the engine is cold.

A single cylinder a cold start that clears after a few seconds is often an indicator of a cylinder head gasket or manifold gasket seeping water into the cylinder overnight.

Most Toyotas of this era are MAF engines you don't have diagnostic data but carefully cleaning it wouldn't do any harm.

Thanks for the reply. This engine does not have a MAF sensor, it uses a MAP sensor.

This problem has existed ever since i bought the car 2 years ago. The accelerator pedal was really stiff when i bought it. Cleaning the throttle body cured the stiff accelerator pedal but the cold start "hiccup" remained.

Earlier Tests
- Compression showed 180 psi across all 4 cylinders and the vehicle has never lost any coolant in the 2 years i have owned it. Really baffled with this one.
- Fuel pressure well within specification
- Sensors all reading normal voltages
- Smoke test for vacuum leaks

Forgot to mention that for 1 or 2 days, the "hiccup" went away on a cold start after cleaning the IAC valve but then the problem returned. However, removing the motor showed the valve was moving very freely. Baffled with this one.
The IAC vacuum hose is also new

My thinking went to a wonky ect input or some kind of temperature problem giving iac bad info well back then it was called a coolant temperature sensor

Thanks for the reply. I tested the ECT sensor against the IAT sensor when cold and both sensors read the same voltage. The ECT sensor came down to just under 1 volt when hot

Been a while since I saw a 98' corolla in the shop here in the NE. Most have turned into dust by now lol! Any chance it has a cold start injector?

Hi thanks just came off the top of my head

No it doesn't have a cold start injector.

Could we do waveform of injector current as engine warms up

chief eaglebear wrote: Could we do waveform of injector current as engine warms up

I can get an injector waveform on my scope, but putting it on here may prove difficult. Worth a try doing a current test definitely.

The o2 sensor does stay at 800 mv when full throttle is applied and the injectors all sound the same with a stethoscope

Has it ever set any misfire codes? Or do you have the ability to watch misfire index when it's doing it? My thoughts are possibly an injector leaking off pressure overnight.

Repair and despair since 1990 let's taken this tthe next level

Busted Knuckle wrote: Has it ever set any misfire codes? Or do you have the ability to watch misfire index when it's doing it? My thoughts are possibly an injector leaking off pressure overnight.

Unfortunately it has never set any codes. I really wish I could use a scanner on this vehicle but it is OBD1. The Toyota runs like clockwork after the engine has been running for 1 minute from a cold start. It has constant good mpg, no power issues and easily passes the emissions tests. I can smell no fuel in the engine oil and the level stays constant. I did a residual fuel pressure test when i did the fuel pressure and it held pressure easily after 15 minutes. Still worth putting a leaking injector on the list.