Rough Idle only when engine is warm

I have a 1999 Saturn SL1. Right after a cold start the engine runs OK with no apparent problems or roughness. Once warmed up, the car idles really rough in gear (Drive), RPM really starts to fluctuate between 500 and 700, the car kinda shakes, and the engine is likely to stall if I leave it in gear (and with car not moving) for much longer. If I immediately shift to Neutral at this point (bacially I have to shift to neutral every time the car is stopped) the car would not stall but RPM would surge briefly to 2000+ before going back to normal (900 ish). The problem started suddenly one day after a long (3+ h) drive. Prior to that the car ran smooth. Iā€™ve replaced spark plugs, plug wires, EGR, PCV, IACV, exhaust pipe, catalytic converter, and cleaned the throttle body, without resolving the issue. My code reader sometimes reads a ā€˜cylinder 4 misfireā€™ code. What could be the problem?

Right now Im guessing maybe a bad O2 sensor or sensor connection, a vacuum leak, or just really worn-out leaky engine cylinders (less likely). What else can it be? Suggestions appreciated

How many miles on that engine?

EGR valve may be clogged up and not allowing it to seal. that forms a leak that buggers up the idle.

I would clean the IAC.

about 200K on the engine. it had a blown cylinder head a while back and got a replacement head. Iā€™ve replaced spark plugs, plug wires, EGR, PCV, IACV, exhaust pipe, catalytic converter, and cleaned the throttle body, without resolving the issue.

Did you do a compression test? Is the engine burning oil?

no compression test yet. It does consume oil but thatā€™s kinda ā€˜normalā€™ for Saturns. Would have to do the test some time eventually.

I donā€™t know about your vehicles condition but there comes a time when it no longer makes sense to keep throwing parts and money at a 19 year old vehicle that the brand has been discontinued .

sure. If I buy a new vehicle now for 20K and keep it for 20 years the per year cost is $1000 + eventual repair costs. If I spend <1K on an old car per year for repairs I think it makes sense to keep for the time being

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I sort of wondered whether there could be a (automatic) transmission problem that causes hydraulic clutch to bind or otherwise creates drag on the engine. The engine shake and stall Im experiencing while idling in Drive feels very much like how a stick shift car would shake and stall if you slow down too much without depressing the clutch. But my problem only happens when car is warmed up. Could there be a transmission problem that causes hydraulic clutch binding or something only when the fluid is warmed up?

Hey, I am having this problem with my 2004 Mitsubishi Eclipse recently (2023). So far I have replaced a few things. Fuel filter, a belt, O2 sensor, spark plugs, no vacuum leaks, not the fuel pressure regulator. No codes are popping up and even the guys at the shop couldnā€™t figure it out. Did you ever get this problem fixed? If so, what was going on?

The OP had a 1999 Saturn SL1, not the same thing you are asking about and the OP has not been on the forum since September 2018, this is a 5 year old thread, donā€™t think the OP will see your questionā€¦ And again, two different vehiclesā€¦

It would be best to start your own topic with all the information you can giveā€¦ including engine sizeā€¦

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No harm to ask original poster if they ever discovered what was causing the problem. But in the case of the Mitsubishiā€™s owner, suggest to click ā€œmaintenance/repairsā€ upper left this page, then ā€œnew topicā€. Include w/query the make/model/year/engine & transmission configuration.

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Hey did you ever figure out what the problem was ? Im dealing with the sme issue

Welcome to the forumā€¦

The OP has not been on the forum since September 2018, this is a 5 year old thread, donā€™t think the OP will see your questionā€¦

It would be best to start your own topic with all the information you can giveā€¦ Details are everythingā€¦

including
*engine size, *
*year, *
*make, *
*model, *
*mileage *
and transmission type, * ā€¦