One more way out there suggestion, if you don’t want to do the troubleshooting I suggested above and just want to throw parts at it to see what sticks, then maybe replace your PVC valve and hose. You had to disturb these to remove the manifold so it may be leaking. Also look for any air leaks in the intake duct between the air filter and the throttle body. Any leaks here throw off the fuel trim.
Thanks Rod Knox and keith. Keith, I especially like your ideas because those are easy things to check. I will look into those ASAP.
More interesting developments tonight! First a little back story first though. The second time my neighbor and I changed my plugs, we forgot to plug the air temperature sensor back into the intake. I started the car up, and of course all hell broke loose. The check engine light was on solid, the VDC slip light was on, I believe the TCS Off light was on, and the engine kept revving itself up and down at idle. We turned the car off, plugged the sensor back in, and I took it for a quick drive. The check engine light kept flashing briefly every 5-10 seconds, but the car ran fine, minus the slight shuddering. When I got back from the drive I put the car in Diagnostic Test Mode II and followed the procedure to clear all engine codes.
Now, the reason I explained all of this: this evening I stopped by Auto Zone and borrowed their scan tool. The check engine light has not come on since the incident I described above, but the scan tool did show an ABS code - C1130 Engine Signal 1. I’ve Googled this code, and what I’m finding is “ECM judges communication between ABS/TCS control unit and ECM is abnormal”. I’m guessing this was thrown by the incident mentioned above. I cleared this code using the scan tool, and I’m planning to stop by in a day or two to see if it has been thrown again. I do wonder if it has anything to do with my engine shudder problem, though.
The other thing that happened was that apparently my battery was almost dead; good thing I was at Auto Zone when it happened! I got a new battery installed, and one of the guys there claimed that perhaps the low voltage had something to do with the shuddering. We’ll see if things get any better when I drive to work tomorrow. If nothing else, I’ll definitely try keith’s idea of unplugging the battery for a while to see if that makes any difference.
By the way, the guy at Auto Zone heard the hissing noise too, and like me, he said that it sounded like it was coming from the back of the intake manifold, but we couldn’t pinpoint a source. I’ll spray some carb cleaner back there, take it by the local mechanic and get his updated feedback, and hopefully we’ll find an easy fix.
You’ve clearly got an intake leak
A smoke machine will find it for you, if your ears and carb cleaner can’t
Another dirty trick for finding intake leaks . . . hook up the scanner and look at live data for fuel trims at idle. Spray carb cleaner at suspected leak points. Immediate changes in the fuel trim mean you’re zeroing in on that leak. But you should know your cylinder layout, and understand the theory behind fuel trim
Thanks db4690. I’ll take that under consideration.
One other thing that I forgot to mention is the fuel trim value readouts from the scan tool at idle. Someone asked about this earlier:
STFT, Bank 1: Varied between 0 and 4.
STFT, Bank 2: Varied between 1 and 3.1. Sometimes in sync with STFTB1, but not always.
LTFT, Bank 1: Steady at -3.1.
LTFT, Bank 2: Also steady at -3.1.
So I think everything checks out okay since the short term values stayed in the single digits. Not sure if the long term negative values are an issue since they are relatively close to 0.
Many people have suggested carb cleaner; a propane torch (unlit) could also disclose an intake leak, maybe more precisely. Maneuvering it might be difficult back there, so maybe stick a flexible hose over it.
So last night my wife and I sprayed all around the intake manifold area where the gasket is. We also sprayed lots of vacuum hoses toward the rear where it sounded like the hissing sound might be coming from. No change in idle.
I also disconnected the battery for ten minutes and then reconnected it. I then took it for a test drive and didn’t notice any difference.
One other note - not only is there a hissing sound coming from either inside the intake manifold or behind it, when I was sitting in my driver’s seat with the door open, I also noticed a hissing sound coming from near my left rear tire. I bent down and realized that it was probably coming from the back part of the exhaust system. This is probably completely unrelated to the other hissing noise, I just thought I’d mention it since I don’t recall hearing it before; it’s quiet loud, like the sound of a strong leak in the tire.
Unless anyone has any more ideas tonight, I’m going to break down and take my vehicle to the mechanic in the morning and ask him to do whatever it takes to diagnose it and possibly fix it.
Thanks again for all of the help.
Hi guys, another update for you. I took my car to the local mechanic yesterday, and he did a test drive while monitoring various data on his tool. He said that everything looked normal and that the car seemed to drive okay. The ECU did not throw any codes. However, he only got the car up to 70mph, and I have been noticing problems between 70 and 80. So I think I’ll ask a different mechanic to try it out in the 70 to 80mph range and see what they think because I definitely noticed misfires on the interstate at that speed today, and while under load at that speed (going uphill).
I inspected the front three (easy to get to) coils, and they all seemed OK. One of them did make a slight “boing” sound when I tapped it against my hand, while the other two did not. So i don’t know if that’s an issue. I guess I’ll break down and take the intake manifold off one more time so that I can inspect the rear coils as well. How do I know if the coil is defective when visually inspecting it, other than something obvious like the spring being completely MIA? Is there some sort of tool that I can use on it?
By the way, I don’t think I’m dealing with an intake leak here; from my research, a coil or spark plug problem seems more likely, and it’s definitely not the plugs. I say it’s not an air intake issue because it idles smoothly and misfires at high speed. From what I’ve read, if there was a leak, it would idle roughly and then run more smoothly at higher speed, as the extra air would have a negligible effect at that point.
Wish me luck. Hopefully the next time that I post I will be telling you what the problem was and how great the car runs again.
The part about the vacuum leak is only partially true. It would be true if any leak were BELOW the throttle plate; meaning from the throttle plate on down through the intake manifold and into the cylinder heads along with any associated vacuum hoses from the intake manifold.
If a leak exists in the intake tract ABOVE the throttle plate (meaning from the plate back up through the air filter housing and MAF area) then the rough idle and running smoothly at speed will not necessarily be true.
You might try unplugging the MAF sensor and taking it for a drive to see what happens. This will set a MAF code and turn on the CEL but this is for test purposes only.
A coil should not make a “boing” sound. It’s internals should be encapsulated as protection against shock and vibration. You may have an intermittent broken connection between the coil itself and the sparkplug, as in an internal break in the coil “stem” (for lack of being able to find the proper word).
I readily admit to never having clapped a coil assembly against my hand to listen for a boing, or open one up even, so this is somewhat of a wild guess, but I’ve been involved in many designs wherein electrical conductors were to be subjected to heat, vibration, and shock, and I cannot imagine not encapsulating the conductor as protection.