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Alright so my 2009 e 350 engine has been rough idling for a while and recently I’ve found a hissing noise that might be the culprit. I’ve changed my spark plugs and checked my air filters so I’m fairly certain those aren’t the issue. Although, I haven’t checked my ignition coils. But, the hissing noise is in the front drivers side of the engine close to where the coolant is and beside the farthest right belt. I think this might be the problem as my A/C has a hissing noise as well. Does anyone know what this might be and if so any suggestions on how to find it / fix it. If it helps, as soon as I start the car (cold start) it idles normally around a thousand rpm and then after a little while or until the engine is warm enough the engine gets quieter and vibrates more and then the idle drops to around 5-6 hundred rpm. I have told it could be a cylinder misfire but I am also unsure how to diagnose that. I would appreciate any feedback thank you! Also if anyone needs video I can provide it.
Second, “misfire”. Why do you think it is misfiring - a truly rough idle after changing spark plugs suggests to me you have a coil that didn’t fully “clip” and “click” when it is reattached to the spark plug and engine wiring harness. But, when that happens one usually gets a “check engine” light and the General misfire code.
So, your “rough idle”
may be a transmission or engine mounts.
Third, “hissing” sound around the A/C. I’ve never had this noise. If you have not changed your serpentine belt, I’d look to that.
I have the '08 E350 with the same engine. There is a PCV oil separator in the area you describe. It is plastic and vacuum driven, so it has the potential to crack and hiss. I've attached a picture of it for reference. It costs $25 and is easy to replace.
Noises can be devilishly difficult to pinpoint because sound reverberates throughout the engine. I've used an
to help narrow the source of air leaks. Another trick is to spray carburetor cleaner on a suspect area (hose, gasket, etc.) at idle and watch for a temporary improvement in idle, which would identify the leak.
If you're looking for cylinder misfire, get an OBDII code reader and connect it to the port under the steering wheel and above the brake pedal. I used the
and downloaded OBD Fusion software on my phone (I also tried TorquePro).
You mentioned that you recently replaced the spark plugs. Did you remember to replace the coil boots? It's recommended that they be replaced at the same time as plugs because of wear.
There are some other common sources of rough idle, once you've eliminated air leaks. A dirty throttle plate will interrupt air flow at idle and a dirty MAF sensor will create trouble; both are easy to clean. Dirty fuel injectors and intake valve stems, sticky/stuck piston rings and valve seats, and dirty valve trains in the crankcase are further causes, all of which I've experienced with this car. Carbon deposits seem to form quickly, especially in cold weather, short trips to the corner grocery store, or if I don't use the car often. Another time, I had a shaky engine due to carbon buildup on the pistons and combustion chambers from oil getting past the PCV valve. Thankfully, the PCV valve was inexpensive and easy to replace.
Last edited by TechSurfer; May 30, 2022 at 01:22 AM.
Alright so the mileage is around 159,000 miles which yes I know is probably the reason I should expect stuff like this to happen. Also the misfire comment is from another post I made a while ago and that’s what the person suggested I check but I didn’t really know how to check if it was or not. Also the coils are probably a good idea to check so thank you for the advice I’m definitely going to make sure they are all intact just in case even though I don’t have check engine light. Also for the engine mounts I believe those we replaced a little bit ago or my dad had a mechanic look at them while he was doing a check up one time I’m not too sure but I did suggest that and he said they should be fine, but I’ll check just in case. The belts were replaced recently as we thought that might’ve been the problem however unfortunately it was not. Thank you for all the feedback and I will definitely try all the advice you gave me.
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