Rough Idle only when engine is warm

saturn
sl

#1

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. I’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 ‘cylinder 4 misfire’ 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


#2

How many miles on that engine?


#3

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


#4

I would clean the IAC.


#5

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


#6

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


#7

no compression test yet. It does consume oil but that’s kinda ‘normal’ for Saturns. Would have to do the test some time eventually.


#8

I don’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 .


#9

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


#10

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?