Strange engine vibration or misfire

[ponkotsu]

I got my rough idle drivability issues about 99% fixed. But still had an occasional rough idle. What I've determined is that it is only happening at 900-1000 RPM. Condition doesn't matter. In P, N, driving, cold, hot, etc. Happens when idling steady at 1000rpm. It happens when revving up past 1000 or down. Seems like it always runs rough at that point. Other than that, it seems to run perfectly. It idles at 600rpm as smooth as can be. Get's great mileage. All the scan data looks normal, and I've never had a code.

I'm going to do some testing to try and figure this out. If anyone has any suggestions I'd appreciate it. I'm going to start with a cylinder drop test while trying to maintain 1000rpm.

I'm not 100% sure it's a misfire, or if it's some vibration in the engine. Maybe harmonic balancer or fan? Chain?

 

[apanello]

See if you can get it to throw a code by keeping it at 1000rpms. If you can't, have you checked your IAC valve outputs?

 

[ponkotsu]

See if you can get it to throw a code by keeping it at 1000rpms. If you can't, have you checked your IAC valve outputs?

Already tried that. Ford PCM is notorious for not throwing codes for minor misfires. Probably not going to happen. IAC is new and seems to be functioning correctly.

So I did a cylinder drop test by pulling the COP connector. It's very ambiguous. Sometimes I can pull a connector and it has a very clear affect on the engine. And other times I can pull the plug and it has no effect at all. It does seem like some cylinders are having more effect than others though, but it's not super clear. So I did a compression test on two adjacent cylinders. Getting about 130psi. That's on the low side for sure, but not horrible. Could be this engine is just plain worn out and it won't get any better.

The way the drop test seems, it's almost like there is some fuel saving strategy at idle where the PCM is cutting out cylinders, but rotating it around with different cylinders. Maybe I'll try the drop test again with my scanner so I can more clearly see the RPM change.

 

[Benjam]

I signed in to subscribe to this.. My dad's 05 aviator has a rough idle but as soon as you gas it a bit over idle it clears up. I also have no codes. His is worse when the truck is hot, and even then it isn't terrible.

There is a tsb out for the cylinder heads on these 4v 4.6's. Apparently the rear cylinders have poor cooling from a bad design. What happens is the heads get hot, and the valves burn up and no longer seal. This is commonly on the exhaust valve side. The mustang guys have a kit to remedy the cooling issue, otherwise you have to purchase revised cylinder heads from Ford. I believe those run about $800 a piece.

I haven't dug too far into my dad's truck, just doing spark plugs and a couple of coils that have gone bad. Using the scan tool at work I can monitor which cylinders aren't contributing as well as the others, and on his truck it seems to be cylinder 4(pass rear).

When I get some time I am going to swap injectors around and HOPE it follows and the slight miss doesn't stay on cylinder 4. I have already swapped coils.

Unfortunately, if swapping injectors doesn't make a difference, I will be doing a compression test across all cylinders. I have a sinking feeling thats where the problem is, however.

I'd rather not pull heads off an engine with only 115k on it!

Aside from my Dad's truck, we have an 03 aviator at work with a much rougher idle, again only when hot. Supposedly another shop has put injectors in it. That truck we put on a harmonic balancer because the rubber was starting to look bad. It made no change, still has a rough idle when hot. That truck also has about 115k on it.

I hope I didn't ruin your day with bad news! I have a feeling we will be communicating a lot over this issue!

Benjam

 

[ponkotsu]

I signed in to subscribe to this.. My dad's 05 aviator has a rough idle but as soon as you gas it a bit over idle it clears up. I also have no codes. His is worse when the truck is hot, and even then it isn't terrible.

There is a tsb out for the cylinder heads on these 4v 4.6's. Apparently the rear cylinders have poor cooling from a bad design. What happens is the heads get hot, and the valves burn up and no longer seal. This is commonly on the exhaust valve side. The mustang guys have a kit to remedy the cooling issue, otherwise you have to purchase revised cylinder heads from Ford. I believe those run about $800 a piece.

I haven't dug too far into my dad's truck, just doing spark plugs and a couple of coils that have gone bad. Using the scan tool at work I can monitor which cylinders aren't contributing as well as the others, and on his truck it seems to be cylinder 4(pass rear).

When I get some time I am going to swap injectors around and HOPE it follows and the slight miss doesn't stay on cylinder 4. I have already swapped coils.

Unfortunately, if swapping injectors doesn't make a difference, I will be doing a compression test across all cylinders. I have a sinking feeling thats where the problem is, however.

I'd rather not pull heads off an engine with only 115k on it!

Aside from my Dad's truck, we have an 03 aviator at work with a much rougher idle, again only when hot. Supposedly another shop has put injectors in it. That truck we put on a harmonic balancer because the rubber was starting to look bad. It made no change, still has a rough idle when hot. That truck also has about 115k on it.

I hope I didn't ruin your day with bad news! I have a feeling we will be communicating a lot over this issue!

Benjam

Swapping injectors is a pain on this engine. Really hard to do without pulling the lower manifold. Lot's of risk of contamination and dropping o-rings. And I think you need to pull the engine to pull the heads on this engine, but not sure. I'd check the compression first, since that's way easier than swapping injectors and it's gotta be done anyway. These engines were plagued with drivability issues and I think it gets worse when they are older.

My cylinder drop test is really frustrating. I pulled the coil on #1 and it acted like it was a totally dead cylinder. Then rev the engine a little and it comes back to life and problem goes to another cylinder. Truly a random misfire, but everything checks out ok.

 

[Benjam]

I've talked to someone who has worked with this engine in the mustangs as well as the lincolns and he says you're best to pull the engine out. Timing the beast with 4 different timing chins, as well as the head bolts are long to the point where they come out with the head, you can't pull them individually.

I share your frustration on the cylinder drop test.. I've done the same thing with the same results.

Benjam

 

[ponkotsu]

I do plan to rebuild this engine in a couple years, But I would hate to go to all that trouble only to find out I have the same issues I do now. It's really a tough engine to deal with. It's fairly high performance and not a lot of help available. They only made a total of 40k of them and more than half of those are already gone. They never made any PCM updates, even though it appears there are some issues that need correcting. All in all, it's still a very cool truck though and I don't plan on giving up.

 

[2000StreetRod]

drop test at idle?

I'm not familiar with your engine but I would think the PCM would attempt to control the IAC valve to maintain the commanded engine speed at idle. I would try setting the engine speed above the range of IAC valve control when performing your drop test. I suspect that 1500 rpm might be a good engine speed to use.

 

[ponkotsu]

I'm not familiar with your engine but I would think the PCM would attempt to control the IAC valve to maintain the commanded engine speed at idle. I would try setting the engine speed above the range of IAC valve control when performing your drop test. I suspect that 1500 rpm might be a good engine speed to use.

Any drop test I've seen is always at low RPM. I think at higher RPM you wouldn't notice at all on an 8 cyl. Like I said, when I pulled the plug it would sometimes be a very noticeable change in engine tone, and other times it had no noticeable effect. I think the IAC will adjust, but it takes a second or two.

 

[Benjam]

Which cylinders didn't make a difference in rpm change? Or are they appearing random?

Benjam

 

[ponkotsu]

Which cylinders didn't make a difference in rpm change? Or are they appearing random?

Benjam

It was at random. Seemed like I was chasing the misfire around the engine. First #3 seemed dead. Then I checked #1 and it was dead. Then back to #3 , and it was reborn, etc. It was very clear though when the drop test had an effect though. Very strange indeed.

 

[Benjam]

Basically the same things I've ran into.. I was/am hoping you will make some headway with this.


Soldier on

Benjam

 

[ponkotsu]

Basically the same things I've ran into.. I was/am hoping you will make some headway with this.


Soldier on

Benjam

I seriously wish I had a Picoscope. No doubt I could track this problem down with the right tools.