Retorquing bolts after gasket replacement

@enargins
How many miles/ how much driving time have been done since you picked this van up with the vibration?

I ask because it is possible that since repairs were done and DTC’s were seemingly cleared, that the ECM has not yet completed a drive cycle and thus not yet relearned all of the data it needs to know where baselines are. Essentially, the engine is running on defaults until it relearns everything, then your idle may smooth out.

If it hasn’t been long (in terms of miles or drive time- NOT calendar time,) I suggest to drive for a week or so and see if that helps anything.

That’s a good idea – though the degree to which the vibrations occur would seem to indicate something more severe than that. Still, worth considering. Driven probably less than 100 miles since the vibrations started. I’ll try to drive it some more and see what happens. Thanks.

Clearing a diagnostic code shouldn’t cause the engine to run rough at all.

Could you imagine every time any code was cleared that the engine vibrated badly until the readiness monitors completed?

There’s something else wrong with this vehicle.

Tester

I don’t necessarily disagree with you- but I’ve seen it happen on older vehicles.

I thought he meant that with the repairs done, the computer would need to adapt to get the right settings, and, thus, it may take a little time until the engine runs smoothly.

I can’t tell you how many diagnostic codes I’ve cleared over the years.

And I’ve never had an engine operate as bad and for as long as you describe doing so.

Tester

OK, something we haven’t looked at yet. The valve cover gaskets were replaced. On most V-6 engines with a 60 degree V, the intake manifold has to be removed if one piece, or the top half (plenum) has to be removed if two piece. This can result in a vacuum leak that results in uneven vacuum between the intake ports, especially if the whole manifold has to be replaced.

I don’t see anything about the intake manifold gasket or the plenum gasket being replaced. Often the plenum gasket is a series of O-rings that supposedly can be reused, but if one is leaking, it will cause an uneven idle, especially in gear where the idle speed is at its lowest.

One indicator of this is that the idle speed in neutral will be 200 rpm or more above the normal idle speed. That usually turns out to be about 950 rpm or higher when at operating temperature. If this is the case, your shop did a lot of unnecessary work, except if you did have an exhaust leak and it appears that you did, that has to be fixed.

Re. vacuum leak, that was one of the things I asked about, and they said that the air/fuel mixture was spot on, and if there was a vacuum leak, it would be off.

They also said they checked with a smoke machine and didn’t detect any leaks.

Not really following this. But this all started by trying to track down a noise the new cat was making when accelerating. They found a tiny leak from the exhaust manifold, and so replaced that gasket.

The valve cover gaskets had been leaking oil for some time, so they replaced those as well (though not in connection with the noise).

Then replacing the exhaust manifold gasket didn’t stop the noise, they replaced the cat, and there was no noise with the new cat. So the problem was the cat, not the exhaust manifold.

Only after all of this was done (new valve cover gasket, new exhaust manifold gasket, new cat w/o the noise) did the vibrations start.

The air/fuel mix for the whole engine would be spot on, but the air/fuel mix can still vary from cylinder to cylinder. Using the OBDII data is not a test for this.

A smoke test for this is also somewhat unreliable because access to the full circumference of the port is limited. The usual test is to spray a light oil (WD-40) around the circumference of each port and watch for a change in the idle speed or smoothness. When you find the leaking port, the idle speed will drop and smooth out. But any change in the idle indicates that you found the leak. The problem with doing this on a V-6 is access to the full circumference of each port.

If you have a propane torch, with the engine running, use the torch, unlit, to spray raw propane around the intake manifold ports and if the idle speed jumps, you have a leak.

I actually mentioned that to the service manager, about using brake cleaner to check for vacuum leaks, as I had seen that in a video. He said that’s what’s done when you don’t have the testing equipment they do, and that their equipment would have found any leaks.

Anyway, when I bring it to the other mechanic this week, that’s probably something he’ll do when looking for a leak.

Keep us informed.

Funny you should say that. I was just coming here to post a message that I just heard from the mechanic who told me that he wouldn’t be able to look at the car this week, and it would probably be at least about a week before he’s able to do so.

So I wanted to update everyone who’s following this thread to let them know that there won’t be any updates right away. But as soon as there’s more information to post, I’ll definitely update this thread.

By the way, not that it’s that significant, but I was driving tonight and I determined that 5 miles an hour is the cutoff point where the vibrations stop. Once you hit that speed, the car feels normal. FYI.

When that happened on my 2003 Dodge Caravan, turned out to be a fouled spark plug.

That’s very interesting! The spark plugs and wires were just replaced 6,000 miles ago (Champion Double Platinum plugs), so they’re not old. I don’t know if something could have happened to them when they were working on it. But definitely something to look into. Thanks!

One thing I just noticed yesterday was a shaking that happens if I go above 60 MPH. The issue I’ve been discussing here is a strong vibration that happens when idle or less than 5 MPH. Over 5 MPH it’s fine.

But I noticed that now once I hit 60 MPH, there’s kind of slight shaking, or perhaps a lower-frequency vibration.

It might be totally unrelated to the idle vibration. But just thought I’d mention it.

When you had this problem, did you have any issues at high speeds, as well?

Thanks!

Tried to be brief. There was a chunk of carbon stuck across the spark gap. Mechanic dislodged it and the plug was fine. If its not a plug or wiring problem, could be valve related. The important thing is to eliminate the simple and inexpensive possibilities first.

So, to confirm, you had a problem with strong vibrations when idling or at low speeds, right?

And did you experience any issues at freeway speeds?

Thanks!

I didnt mess around! I started the engine. There was some vibration, like one cylinder was missing. I went straight to my trusted mechanic ( the late Benny Blackstone ). He opened the hood, Pulled and replaced one plug wire at a time to locate the trouble cylinder. Pulled the suspect plug and showed me the carbon particle in the plug gap. Problem fixed in 10 minutes. Your problem could be unrelated to the prior service. Or they might have dislodged or disconnected something and not have properly put it back.

I am not going on a freeway if the engine is not running properly!

Thanks for the info!

A diagnostic scan tool will show which cylinder is misfiring without high voltage being discharged with the ignition wires disconnected.

Must have been one of the front cylinders, the rear bank is not easily accessible.