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Discussion Starter #1
Hi Guys

My Toyota Corona ST170 with a 4S-FI (Throttle Body Injection - TBI ) 1.8L has been having a rough idle. This rough idle with a popping sound from the exhaust only starts happening after the car has been started for 3-4 minutes then the rough idle with start.. Driving there are no problems but as soon as I am stopping at the lights it will be rough.

Engine misfire/popping from exhaust
Spark plugs fouled – too rich or lean misfire?
Cold start – okay
Warmed up – misfire problem only when at idle – driving is good
Engine oil - good, no smell of fuel and no colour difference.
Diagnostic terminal jumping pins – Error code: 21 – which is oxygen sensor but this error code has been present for a long time before the rough idle started

Things I have done:

1. Checked engine compression – okay 195/200 across all 4 cylinders
2. Replaced engine coolant temp sensor
3. Replaced spark plugs
4. Replaced spark leads
5. Replaced ignition coil and igniter
6. Replaced distributor unit
7. Distributor cap and rotor less than 5 months old
8. Replaced timing belt and got mechanic to check cam timing and crankshaft timing – okay
9. Replaced fuel injector and seal
10. Replaced fuel pressure regulator
11. Bled radiator coolant and found no bubbles left
12. Sprayed around throttle body and hoses and did not find any leaks

This idle is driving me crazy and really hope you guys could help...

Thank you Marc

2 Posts
Discussion Starter #2
I did a test on the fuel pressure regulator and when I unplug the vacuum hose the idle will idle higher and the miss with be less.. I do have a few spare fuel pressure regultors and the all test the same so mayne the fuel pump?

How would I do a test on fuel pump? I don't have gauges

441 Posts
If the spark plugs are sooty, most likely you are running rich, It makes sense that it runs better cold when it needs a rich mixture. If that is true, you need to find out what makes it run rich / too much fuel. The O2 sensor code might be telling you something. It could have coded earlier but now the parts has fully failed and is giving worst data to the computer. I'm old school and I read spark plugs to tell me a lot of motor issues.

You need good clean spark plugs for a motor to run smooth.

I also use fuel injector cleaners in the gas tank a few times a year, It helps keep the motor running great and keeps the MPGs up to a new car level..

Does your car have an " Idle air control valve?? Is the throttle body clean? Air filter clean?
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