My TF 3 litre idles rough, but not missing when driving. it has new plugs and dizzy cap. The heater core had been leaking but i disconnected it.. Can coolant leaking on to the ECU cause a bad idle? it has done 280,000 k's so maybe its just worn valves? anyone have any ideas what it maybe? cheers

My TF 3 litre idles rough, but not missing when driving. it has new plugs and dizzy cap. The heater core had been leaking but i disconnected it.. Can coolant leaking on to the ECU cause a bad idle? it has done 280,000 k's so maybe its just worn valves? anyone have any ideas what it maybe? cheers

It could be;


Oxygen Sensor (old, worn out, dirty)
Distributor - damaged / worn / damp / dirty.
Fuel Filter / Fuel Lines / Fuel Tank (dirty , leaking)
Fuel Pressure (leak, pump or regulator as cause)
Leads (damaged is a number of ways)
Spark Plugs (worn, mis-gaped, oil soaked)
Vaccum leak
Throttle Body (dirty, leak)
Idle Speed Control Valve (dirty, worn out, electrical issue)
MAF Sensor (dirty, electrical issue)
Engine Mounts (rubber gone hard, sagging, cracked)
Coolant Temperature Sensor
Injectors - Dirty or malfunctioning
Uneven compression across the cylinders.
...or in the case of my vehicle an idiot previous owner who obviously thought the EGR Valve and the other EGR Pipe (on the other end of the Plenum) didn't need a gasket.


...a combination of some of these things or even unrelated to any of these things.

Does your RPM drop to around 500 when you put your vehicle into drive or reverse, but otherwise rev at 750 to 800? If so it is probably the ISC or idle speed control valve. Could be just dirty or malfunctioning electronically.

Coolant leaking onto the ECU is a known issue I've read about here. So yes that could definitely cause problems.

It could be;


Oxygen Sensor (old, worn out, dirty)
Fuel Filter / Fuel Lines / Fuel Tank (dirty , leaking)
Fuel Pressure (leak, pump or regulator as cause)
Leads (damaged is a number of ways)
Spark Plugs (worn, mis-gaped, oil soaked)
Vaccum leak
Throttle Body (dirty, leak)
Idle Speed Control Valve (dirty, worn out, electrical issue)
MAF Sensor (dirty, electrical issue)
Engine Mounts (rubber gone hard, sagging, cracked)
Coolant Temperature Sensor
Injectors - Dirty or malfunctioning
Uneven compression across the cylinders.
...or in the case of my vehicle an idiot previous owner who obviously thought the EGR Valve and the other EGR Pipe (on the other end of the Plenum) didn't need a gasket.



Thanks so much for a very detailed description of what might be causing my bad idle Madasacutsnake, I will checkout each thing on the list you provided, thanks so much again, cheers

You are most welcome

http://www.youtube.com/watch?v=X_tbksFYhl4

This video could help you answer the question about worn valves.

You are most welcome

http://www.youtube.com/watch?v=X_tbksFYhl4

This video could help you answer the question about worn valves.

Thanks for that, very interesting video, compression test might be the best way to go first, cheers

Thanks for that, very interesting video, compression test might be the best way to go first, cheers

http://www.youtube.com/watch?v=WgrfT0LFMhc

If you discovered a problem when you did the compression test, the next step would be a "leak down test". A little more complicated, but will give you more specific answers as to where the problem is located. Eric The Car Guy does waffle somewhat in this video but it get's quite interesting towards the end.

Ideally, if you didn't discover any issue(s) during the compression test, you can disregard the above "leak down test" video.

Something also that can give the impression of a rough idle, but largely unnoticeable while driving, is a stuffed driver's side engine mount

I just thought of another one,... and it's a cheap fix (if it is the problem).

PCV Valve could be gummed up. You can just clean it, or replace it. They cost about $12 and are simple to replace. They should be replaced every 40,000km but are often overlooked.

A gummed up PCV valve can cause, engine sludge, rough idle or engine leaks.

http://www.youtube.com/watch?v=c1Jb_sfpZsI

The PCV is easy to test if it's working properly. Just take it out while the engine is running and put your finger over it to see if the valve opens/closes

The PCV is easy to test if it's working properly. Just take it out while the engine is running and put your finger over it to see if the valve opens/closes

Also a good one rattles freely, a gummed up one the balls inside will hardly move. Just quickly unplug it and give it a shake.

They are so cheap and easy to replace, so it's a case of why wouldn't you?

Thanks so much guys, I will start with the PCV valve today. The right hand engine mount is stuffed but don't think that is causing the bad idle as listening to the sound coming out of the exhaust its like missing rough sound where as my KH has a very even smooth sound, maybe I should do a video and post it on here., Cheers guys and thanks so much for all the time you've taken out help me with this.

Hi all, could this symptom be the same in the 2.4cyl TF model?? Any info would be appreciated....