2002 Crown Vic rough idle when hot

C

coldfusion11

Forum Member
I have a 2002 crown Victoria. I have replaced just about every emissions control related part on this car in the last year and am getting around 18mpg (which is 2 mpg above EPA est.) 70 highway 30 city driving. The problem that I have is that it seems to idle rough but only when it is hot, or the motor is hot. It is very slight and it feels kind of like a wave....smooth, then a second later is a little rough, then smooth again. The COP's are fairly new and they are accell cops. But I tried some new motorcraft cops and it was the same. MAF is brand new as well Ford OEM. I do have quite a few bolt-ons and modifications but I dont think that any of those are causing this. I am leaning more towards a slight manifold leak. I do have aftermarket OEM replacment (Walker brand) H-pipe and cherrybomb turbo (cheapo universal mufflers at meijer) mufflers dumped before the axle. When I had my flowmaster original 40's on with full tailpipe to the back I did not notice this issue but this was a few years ago and about 30-40k miles ago. I have been reading alot of info on exhaust backpressure and velocity and since it feels like it comes in waves, and exhaust deals with sound waves and exhaust gas waves, it leads me to believe that the exhaust setup is making the exhaust pressure to low. Thanks for any help.
 
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coldfusion11 said:
Could you please explain your reasoning behind this?
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A lot of CV's are used as Taxi's in the Metro Detroit Area and Tri-County Area... The intakes are plastic... They heat up expand and then cool off and shrink back down... A lot of CV's have this rough idle problem from the intake heating up and expanding in which causes the crack to open up and let un-metered air into the motor... When the motor cools off the intake shrinks back down and the crack seals and it's harder for unmetered air to enter the motor...
 
ReiKo said:
A lot of CV's are used as Taxi's in the Metro Detroit Area and Tri-County Area... The intakes are plastic... They heat up expand and then cool off and shrink back down... A lot of CV's have this rough idle problem from the intake heating up and expanding in which causes the crack to open up and let un-metered air into the motor... When the motor cools off the intake shrinks back down and the crack seals and it's harder for unmetered air to enter the motor...
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My crown victoria has 103k miles and I have owned it since 27k miles. It was a civilian vehicle and was owned by an elderly couple. How can one test for leaks?

Also this is the PI intake manifold with the aluminum crossover and not the plastic ones that had the recall.
 
coldfusion11 said:
My crown victoria has 103k miles and I have owned it since 27k miles. It was a civilian vehicle and was owned by an elderly couple. How can one test for leaks?
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Let the motor warm up and start rough idle... Then pop the hood and take starting fluid and start spraying all over the intake manifold... If there is a crack the RPM of the motor will start climbing... No crack no RPM increase...
 
Would this make the motor burn coolant? I have not noticed any coolant smells or burning.

I would also like to add that I can hear ticking noise when it is a cold startup. Which is why I originally thought it was a manifold leak. It does not idle rough when cold but there is a noise, it idles rough when hot but then there is not as much ticking/chugging noise.
 
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Sounds like an exhaust manifold leak from your description. Mine does it but its' not a problem like yours is.. mine is loud when cold then almost goes completely away when warm. Exhaust leaks upstream of the O2's can cause what you're describing too.
 
Mike, this has been my thinking for a while now.

I will try a seafoam to pinpoint the leak. It may be very very minor in the first place and from a visal and quick feel around the manifold and cat-to-manifold connection, I couldnt feel anything.
 
coldfusion11 said:
Would this make the motor burn coolant? I have not noticed any coolant smells or burning.

I would also like to add that I can hear ticking noise when it is a cold startup. Which is why I originally thought it was a manifold leak. It does not idle rough when cold but there is a noise, it idles rough when hot but then there is not as much ticking/chugging noise.
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No
 
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