2001 LS 3.9 Rough Idle PO174

[Allemay]

I have seen a lot of these posts but nothing that exactly matches. Here are my symptoms:

1. Very rough idle
2. Car runs smooth at highway speeds
3. If you rev engine in park it sounds like it's hitting a rev limiter at 3,000 rpm
4. PO174 code
5. No other codes. No misfires.
6. 174,000 miles
7. Checked for vacuum leaks by spraying carb cleaner. None found.

So, what is it? PCV Valve, Fuel pressure, Timing chain? I find it hard to believe that it will run perfectly at highway speed.

I would sure appreciate any help you guys may have.

Thanks,
Brad

[joegr]

1. Misfires due to one or more of (a) marginal COP, (b) bad fuel injector, (c) secondary timing chain has jumped a tooth (probably this), (d) bad or dirty MAF, or (e) intake air leak.

3. In park and neutral, there is a 3K rev limiter. What you are experiencing there is normal and expected.

4. See (1b) to (1e) above.

5. Absence of codes does not indicate absence of misfires. You will often see them in the pre-code counts, if your scan tool can get those. Sometimes it takes a stress test to find them.


The V8 has no PCV valve.
Fuel pressure problems would trigger a check engine code.
Timing chain: No, but probably the timing chain guides have worn out and let one of the chains jump.

[Allemay]

Thanks for the response. That was very helpful. I had no idea about the rev limiter or lack of PCV on the V8. I'll be someone has documented the slack adjust issue huh? Any idea where I might find an article on it?

I have a diagnostics program. I'll attach that and check for misfires, etc before we dig into it.

Thanks again for the great information.

Brad

[joegr]

Originally Posted by Allemay

... Any idea where I might find an article on it?
...

Here's a couple of posts about it.

http://www.lincolnvscadillac.com/showthread.php?t=59886
http://www.lincolnvscadillac.com/showthread.php?t=78017

[Allemay]

Thanks again. I just read through those and it looks just like what we are experiencing and not too bad of a job. Thanks once again for all the help!

Brad