Low Idle and Misfire for 6 months... Help Please!

[lexus082]

I have searched alot looking to resolve this problem. I have a 94 Lexus SC400 (CA emissions) 200,000 miles. I have a low idle to the point where the car shakes. It looks like it idles at around 200 rpms in drive and 400 in park.

So far in attempt to fix this problem I have changed air filter, pcv valve, spark plugs, distributor cap and rotors, fuel filter, mass air flow sensor, swapped out throttle bodies, tps sensor, checked for vacuum leaks, cleaned iacv I believe thats it. When I first start the engine (cold): the cold start idle is around 750 rpms then it will gradually decrease down to 400 rpms in park.

Sometimes after driving for thirty minutes and then putting it in park the idle won't even get back up to 400rpms. The car shakes due to the low idle. I also have a misfire. I have just recently tried to fix the misfire by replacing the plugs, caps and rotors. The oem plug wires are only two years old.

I am lost with the low idle. I don't know what is causing it. The only thing I have left to try myself that I know of is to replace the idle air control valve (iacv). And with the misfire the only thing I know of to check would be to check each plug wire to make sure it ohms within specs. Other than the low idle and the misfire, the car seems to drive fine.

So here are my questions:

A. What else could cause an engine to misfire
B. What else could cause this low idle to occur all the time?
C. Could the low idle and the misfire be related?
D. If my spark plug seals are starting to leak would that cause it to misfire?

Sorry for the long post, I was trying to describe it the best I could. Thanks in advance for any help. I have been a member on other lexus forums just not this one until recently.

 

[gloverman]

Try taking out #5 and #7 leads and check for spark at each of these leads. You can simply put some old spark plugs into them and earth the plugs against the intake. You may have a coil out - cheers

 

[Lextreme II]

Check your coils, wires and spark plugs along with ICV.

 

[lexus082]

Thanks for the quick reply. If one of the coils were bad then wouldn't it just be running on 4 cylinders? Not too familiar with the symptoms of a bad ignition coil. Car seems to drive fine and I haven't noticed a significant drop in horsepower. I will try to ohm the coils and see if they are within spec.

 

[gloverman]

I have a box of stuffed coils and on most the ohms are still within spec. Yes there should be lack of power when a coil is bad but sometimes the first noticable symptom is a misfire. It is easier to check the spark the way I discribed than get to the coils as well. Cheers

 

[cribbj]

As a quick check, just hook up a timing light and while the engine is running, clip it to each plug wire & look for misfires.

Also, at night, look in the engine bay and see if you can see any arcing going on around the wires, coils and distributor caps.

Last, I'm not that familiar with the distributor setup on the 1UZ as I've pulled mine off, however I understand it's "possible" to put a rotor back on 180 degrees out of phase, which would certainly cause it to run rough, if it would run at all?

 

[lexus082]

The rotors are installed correctly. I will check for arcing tonight. So a worn ignition coil could start causing it to misfire. Good to know. I'll let you all know what I find.

 

[Lextreme II]

If the coil is bad, you will only run 4 cylinders and the car will idle ok but not perfect. However, when you put your car in DRIVE. You will notice the car craw little an old lady...

 

[gloverman]

Incorrect there David. only a completely failed ignition coil will cause it to run on 4. If the coil is getting tired - say producing only 15kv then it will idle fine and on all eight but misfire underload. I have replaced 4 coils lately on 1uz's. V6 commodore coils also tend to idle fine but misfire underload. My test onto the manifold only check for completely stuffed coils. cheers

 

[lexus082]

Its misfiring at idle when its not under any load. Still have yet to check for arcing. Will post up what I find.

 

[cribbj]

Put the timing light on every plug lead and check for irregular flashing.

Can't beat it for a quick diagnostic to see which cylinders are misfiring, if you even have an ignition problem.

 

[lexus082]

I ohmed all the plug wires and found them within spec. I left off all the timing belt covers and I didn't see anything arcing. I don't have a timing light so I guess I'll be pulling each wire one by one to see if there's any problems. I guess if I can pin point it to one cylinder it might be related to an injector?

 

[Aphox]

Have you checked that the ECU coolant temp sensor is operating within correct range? I had very similar issues with my ls400 and replacing it fixed the problem. Under the Engine RH coil (In my celsior).

 

[lexus082]

No, I haven't checked the coolant temp sensor. I'll try and find that and see if I can ohm that to see if its within spec today. Thanks! I willing to check anything you guys can suggest to check. I think I have heard of this going bad and altering the idle before.

The misfire could also be unrelated to the low idle or it could be related. Is there any easy way to check to make sure a fuel injector is working properly? Or should I just ohm it out once if I find a dead cyclinder?

 

[MAKENTRACS]

Have you checked that the ECU coolant temp sensor is operating within correct range? I had very similar issues with my ls400 and replacing it fixed the problem. Under the Engine RH coil (In my celsior).

How would the coolant temp sensor affect the idle?

 

[Aphox]

The ECU uses the temperature of the coolant to know if the engine has warmed up. Obviously if the ECU coolant sensor is reporting the temp is higher than it is or is sending erratic signals the ECU will keep trying to lower the idle as it thinks it is hotter than what it really is.

 

[MAKENTRACS]

The ECU uses the temperature of the coolant to know if the engine has warmed up. Obviously if the ECU coolant sensor is reporting the temp is higher than it is or is sending erratic signals the ECU will keep trying to lower the idle as it thinks it is hotter than what it really is.

So with the signals sent by the sensor, the ecu then controls the idle via the idle speed control. Would that be right? Following that thread, if there's rough idling, it could be either the sensor or the isp causing the problem?

 

[och]

To solve the low idle problems, try the following:

1) Remove and clean the throttle body. With 200k you probably have 2-3 milimeters of carbon buildup on it.

2) Bypass power steering air valve.

 

[duza]

Did you ever solve this problem, as I have a very similar one on mine? I've replaced coils, leads, plugs, caps and buttons and also replaced the electrical plugs which fit onto the coils, all to no avail. The ECU temp sensor sounds like an interesting possibility though. Will have to figure out how to check it without replacing it.
Duza

 

[earthman]

Hi Guys,

Sorry to drag up an old post, my vehicle is now displaying similar symptoms. Did anyone ever get to the bottom of this issue? I have changed the AFM, cleaned the ISC, checked for vacuum leaks, no codes on the ECU, and am pretty sure it is not a fuel issue as it runs the same on Petrol and LPG.

Duza and Lexus082, how did you go with your idling dramas?

Thanks