Rough Idle

Topic: Rough Idle (Read 967 times) previous topic - next topic

Rough Idle

May 29, 2010, 10:06:05 AM

My Thunderbird (see sig for details on my setup) is experiencing a rough idle condition. When you 1st start the car after it has been sitting awhile, it will start idling very rough. If you give it throttle it responds & runs fine. Running down the road it seems to run smooth with no real bucking or surging. There is a slight gas smell, but I think it is coming from the filler neck gasket. I noticed the gasket is cracked and in bad shape when I changed the fuel pump last fall (I have a new one that that will be put on today).

Once you drive the car awhile and it is in drive, it idles fine and runs great when stopped (Idles perfectly at 850 rpms in drive). If you put it into Park...the car usually revs up to 1000 rpms or so and generally stays there. Very rarely the car will idle correctly in park.

I have replaced the IAC and the car has a new fuel pump. It has less than 1500 miles on the 5.0 HO swap but has had this problem ever since the swap was completed. What should I be looking for here? I am thinking maybe a vacuum leak? MAP sensor?

Trying to get her ready to leave on Power Tour next Sunday and time is ticking.

Rough Idle

Reply #1 – May 29, 2010, 11:40:07 AM

Is it throwing any codes? When I had the computer in mine it used to idle fine, for having a non stock cam. I had the idle raised a little but so there was enough vac to make the computer happy. You shouldn't have to do that as you have a stock HO. You could try doing a base idle rest and see if that helps: http://sbftech.com/index.php/topic,1031.0.html

Rough Idle

Reply #2 – May 29, 2010, 12:22:21 PM

Reset the base idle as stated above. If that doesn't clear you issue, hook up a vacuum gauge and see what's going on.

Rough Idle

Reply #3 – May 29, 2010, 01:33:41 PM

Guys I already did the idle reset back when I installed the new IAC. I need to check the vacuum and go from there I guess...

Rough Idle

Reply #4 – May 29, 2010, 02:57:00 PM

Vacuum leaks would be a place to start or better yet, a vacuum gauge would give you a quick hint. You can't go off the rpm's on the stock cluster - mine shows some 800 and such when the eec actually shows 625rpm's. Since it smooths out after closed loop, it sounds like the o2 sensors are doing theirjob but before that point, unmetered air is getting in.

I have the opposite problem where my idle is usually rough in closed loop and the eec is adjusting the spark timing up and down rapidly for whatever reason. Everything is new on this engine and there's no surging, delayed throttle response, or anything. Since mine's "rough" also (imo but not alarming to anyone else that drives my car), "rough idle" isn't really a good description of the problem you're having so I would try to learn more about how it behaves and then describe exactly what is happening if you think we can be of further help.

As for finding the issue, I would recommend unplugging the iac, increasing the throttle plate's gap open a little more, then see what it does. If it continues, start to look at the hoses on the drivers side firewall, fuel pressure regulator, and charcoal canister. You can disconnect the smog vacuum lines fairly easily to check to see if they are a source of a vacuum leak. I would hope you already have this line removed since you removed the smog pump. I'd just run a new hose from the upper intake straight to the MAP sensor, new hose to the fpr, and cap off everything else. Beyond that, you only have gaskets (most likely throttle body to upper intake or iac to throttle body but there's still the more rare issues with the upper intake to lower intake, and lower intake to heads).

I'm curious, what does a "smooth" idle mean to you? Can you still feel the motor at idle or does it feel like the engine isn't turning (only valid with soft motor mounts)?

Edit: There's also the EGR system and its gasket but I doubt that is causing your problem at cold start.

It's also possible to be having a tps issue - that is the first sensor in the line for a speed density system. This should be about 1v at closed throttle but if the throttle plate stop screw is turned in, it may be as high as 1.5v. I assume your timing is set to a base 10 degrees for troubleshooting purposes?

Rough Idle

Reply #5 – May 29, 2010, 11:20:47 PM

Quote from: Seek;323584

1.) You can't go off the rpm's on the stock cluster

2.) "rough idle" isn't really a good description of the problem you're having

3.) If it continues, start to look at the hoses on the drivers side firewall, fuel pressure regulator, and charcoal canister.

4.) I'm curious, what does a "smooth" idle mean to you?

5.) It's also possible to be having a tps issue

6.) I assume your timing is set to a base 10 degrees for troubleshooting purposes?

1.) I have a Stewart-Warner buttstuffog tach so my rpm readings should be accurate...but I agree I should use a vac guage to check to get an accurate reading.

2.)& 4.) I call this a rough idle because I know how this car idled before the 5.0 HO swap and it is not quite right now. It always has idled smooth and now it idles almost like it has a more aggressive/choppy cam than what it actually has. If its not a rough idle, then I really don't know what else to call it because it sure as heck isn't smooth...:hick: I drove the car today and when I first fired it up, it idled really choppy. After I drove the car and stopped with the car in drive before I shut her down, it idled fine. Throw it into Park and it idled up to around 1000 rpms. I let it sit for awhile (maybe a minute or two) and it gradually worked down to 850 rpms or so. I'm not really sure how this makes sense...but that's what it does.

3.) Tomorrow I am going to go over the vacuum lines...especially the ones you mentioned.

5.) Good idea, but I have already checked out the tps and everything is within' spec. I feel like since the tps and IAC check out OK, I really need to be looking into vacuum as the prime suspect. Correct me if I'm wrong. We put on all new gaskets on the motor when I did the HO swap from pan to intake...so I really doubt that its leaking anywhere on the intake.

6.) I really don't remember what we set the timing to when we did the HO swap...I probably need to look into that tomorrow too. I am sure we bumped the timing up somewhat from stock.

Thanks for the help.

Rough Idle

Reply #6 – May 30, 2010, 05:50:43 PM

I checked the vacuum and it fluctuates between 14 and 18 when idleing. Checked all the vacuum lines that I could get to and they all look OK. How much vacuum should there be?

Rough Idle

Reply #7 – May 31, 2010, 09:45:58 AM

Also; check this!
http://sbftech.com/index.php/topic,3020.0.html

Go through every scenario to find which one is closest to yours

FWIW -- I was getting 15" of vacuum from my Trick Flow cam. I'm getting about 16 with the new cam. I would expect 20 with a stocker....

Rough Idle

Reply #8 – May 31, 2010, 01:55:57 PM

Awesome info...thanks v8demon!!

Based on how my reading compares with the animations you linked to, I really think I need to check the timing. It shouldn't be valves because I had the heads hot tanked and a valve job done. Everything looked good (valvesprings, etc) and the heads came off a motor with only 80,000 miles.

Thanks again that animation is awesome and VERY helpful!