3.4 L rough idle
98Silhouette
03-11-2007, 03:59 AM
Hi folk, :)
I recently installed the head gaskets for the number one cylinder leak. Everything went back together fine but there is one small symptom that is becoming a pain to troubleshoot. During the first ten to 15 seconds while stopped at a light the idle is perfect. Little by little the idle gets rough. Sometimes the pcm will correct and cycle through a smooth idle for a second or two and then the idle gets rough again. Sometimes the idle will get so rough the engine quits or the van tries to lunge forward. Oddly too it seems that when the temperature gets up above 75 F the symptom gets worse. Below 75F the rough idle is less noticeable. :rolleyes:
I've checked all the vacuum lines and can not find any leaks...or don't know how.
I did find that if I disconnect the vac hose to a module between the fire wall and the engine it seems to run just a little better. To date I can't find reference to this module but I do see one line comes from the pcv valve and the other goes to what looks like line to the fuel tank and there is a wire harness connection on the module.
The gas mileage is better than it ever was(21.8mpg) and it runs beautifully at all other speeds.
The specs are : 1998 Silhouette, 3.4 L / 115K miles.
Parts replaced so far: Both O2 sensors, Air sensor unit next to air filter, Ambient air temp sensor, and pcv valve. :banghead:
So I ask, if there is anyone that might have better suggestions than taking it to the dealer, would you please shed some of your great wisdoms and findings this way? What should I look for next? Thanks.
I recently installed the head gaskets for the number one cylinder leak. Everything went back together fine but there is one small symptom that is becoming a pain to troubleshoot. During the first ten to 15 seconds while stopped at a light the idle is perfect. Little by little the idle gets rough. Sometimes the pcm will correct and cycle through a smooth idle for a second or two and then the idle gets rough again. Sometimes the idle will get so rough the engine quits or the van tries to lunge forward. Oddly too it seems that when the temperature gets up above 75 F the symptom gets worse. Below 75F the rough idle is less noticeable. :rolleyes:
I've checked all the vacuum lines and can not find any leaks...or don't know how.
I did find that if I disconnect the vac hose to a module between the fire wall and the engine it seems to run just a little better. To date I can't find reference to this module but I do see one line comes from the pcv valve and the other goes to what looks like line to the fuel tank and there is a wire harness connection on the module.
The gas mileage is better than it ever was(21.8mpg) and it runs beautifully at all other speeds.
The specs are : 1998 Silhouette, 3.4 L / 115K miles.
Parts replaced so far: Both O2 sensors, Air sensor unit next to air filter, Ambient air temp sensor, and pcv valve. :banghead:
So I ask, if there is anyone that might have better suggestions than taking it to the dealer, would you please shed some of your great wisdoms and findings this way? What should I look for next? Thanks.
jrdwyer
03-11-2007, 04:59 PM
I'm having a similar problem with a loping idle on our '01Olds minivan with 64K miles. It's not overheating or losing coolant and the heat seems to work fine.
The loping occurs every now and again when idleing (not continuous). It did cause the Service Engine Light to come on which showed a misfire in cylinder #2.
The spark plugs only have 15K on them and the wires are good. I also used a calibrated ignition tester and got good spark from all coils/wires. Changing the fuel filter did nothing to correct the problem. I hear clicking from the #2 fuel injector using a screwdriver to the ear.
The Haynes manual says to first check for vacuum leaks from the intake manifold. The hoses/PCV valve are fine. The specs for the upper bolts say 18 ft/lbs., but I am only getting around 10. That said, almost all of the upper IM bolts have metal connectors/washers below them and so maybe 18 is not correct anyway.
This van had new IM gaskets put on by a dealer just over a year ago under warranty (second time). I fear the head gasket is failing, but I am hopeful it is less drastic then that. Maybe there is someone out there who has solved this problem?
The loping occurs every now and again when idleing (not continuous). It did cause the Service Engine Light to come on which showed a misfire in cylinder #2.
The spark plugs only have 15K on them and the wires are good. I also used a calibrated ignition tester and got good spark from all coils/wires. Changing the fuel filter did nothing to correct the problem. I hear clicking from the #2 fuel injector using a screwdriver to the ear.
The Haynes manual says to first check for vacuum leaks from the intake manifold. The hoses/PCV valve are fine. The specs for the upper bolts say 18 ft/lbs., but I am only getting around 10. That said, almost all of the upper IM bolts have metal connectors/washers below them and so maybe 18 is not correct anyway.
This van had new IM gaskets put on by a dealer just over a year ago under warranty (second time). I fear the head gasket is failing, but I am hopeful it is less drastic then that. Maybe there is someone out there who has solved this problem?
jrdwyer
03-15-2007, 08:26 AM
I did an EGR valve test and it came up fine. My gut feeling is that it is the coil packs or the ignition control module underneath the packs. I will be rotating the engine forward in the near future to test these components.
jrdwyer
04-08-2007, 08:46 AM
I ended up cleaning the idle control valve, throttle plate, and passages (as best as possible without removing throttle body) of the carbon buildup. I then disconnected the negative battery terminal to reset the computer. It took about two weeks, but the rough idle has gone away and the van runs great again. So either my cleaning worked, or it was just bad gas/partially clogged fuel injector that finally cleared itself out.
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