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Low Idle And Misfire For The Last 6 Months! Help.


lexus082
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I have searched alot looking to resolve this problem. I have a 94 Lexus SC400 (CA emissions) 200,000 miles. Same engine as LS400's. 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. And no check engine light.

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. I just recently learned of the misfire. Not sure how long its been missing.

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Seen similar problem with two other 94's that my family and I owned. Both times it was the plug wires. Be real careful not to pinch or cut the new ones putting it back together. I replced both using wires from auto zone not OEM with no problems. I think around $100. I also made sure I used Denzo plugs.

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Seen similar problem with two other 94's that my family and I owned. Both times it was the plug wires. Be real careful not to pinch or cut the new ones putting it back together. I replced both using wires from auto zone not OEM with no problems. I think around $100. I also made sure I used Denzo plugs.

I would vote for a bad coil myself. Are the cats REAL hot after a missing run or do you smell a strong exhaust odor? A HOT cat is fuel getting burned in the cat due to faulty ignition. Plug wires that ohm out are good in my eyes-- if they are not cracked, thatll cause cross fires and poor performance. I dont think the coils are all that proprietary, you may be able to try another coil from another car if you can get them wired up for testing. I would pull the neg for about an hour while you are swapping coils just to get the ECU to reset the fuel curve to baseline. Trick on the plug wires, pull the covers and idle the car in complete dark garage, look for any jumping sparks but remember to vent the garage, dont kill yourself looking for a engine problem!

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I plan to check the resistance of all the plug wires and or check for spark also. Alot of people are suggesting it may be a faulty coil, but would a bad coil leave me running on just 4 cylinders? I haven't really noticed any change in horsepower. Someone else suggested it could be a faulty injector and of course the great idle air control valve. I plan to buy a used working iacv and swap that out as well. Thanks alot for the quick replies. If anyone can think of anything else please reply to this post. I have been trying to fix this for a really long time. I haven't noticed the cats getting hot. I'll check them again at night to see if they glow. Its been so long since the car was running properly that I am able to tell any change in exhaust odor. Thanks again for the help!

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  • 7 months later...
Tonight I ohmed all the plug wires and they were all within spec. So I'll be pulling each plug wire one by one to make sure they're all working properly.

See a reply I made under a LS 400 forum - basically my misfire was caused by oil leaking through a bad rocker cover gasket and into two of the spark plug recesses.

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