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Discussion Starter #1 (Edited)
Flash back to way back when, before I did the splitport swap, my old singleport engine not only had a rod knock but also idled rough. I then decided to swap in a used splitport motor from a 2002 mustang with around 80k. After I swapped in the new engine, I had to deal with little problems here and there just like any other engine swap, however one problem that I just can't seem to figure out is the rough idle this car still has. The car runs great and has plenty of power but at idle it shakes the whole car and you can hear it misfire in the exhaust tone. It seems like whatever is causing this rough idle wasn't swapped out when I did the engine swap (sensor, injectors, etc.). I also did a cylinder balance test and the misfire is effecting every cylinder.

Things I have replaced or tested...
- Plugs, Wires, Cap/Rotor
- Cleaned IAC, then swapped another in
- PCM
- EGR
- Ignition control module/Coil/ Distributor
- checked for vacuum leaks, checked all vacuum lines in both fenders
- TPS
- cleaned fuel injectors
- Fuel pressure was checked when I had the singleport and was within spec
- Cleaned MAF
- No check engine light
- Motor mounts

It seems like I've tried everything and Im running out of ideas. If anyone has any input of what to check next would be very helpful. Thanks.
 

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I would check the ignition control module (ICM) that's on the side of the distributor; it's either grey or black (they ARE different so you have to put the same color module back on the vehicle). We change a lot of those on old Ford 6 cylinders at the shop - usually on Ford straight 6 cylinders on pickups. It looks like this:




EDIT: Oh, wait - I see you said you've already changed this.....


Well, what kind of pressures did you get when you did a cylinder compression test? And did you do a leak-down test?
 

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Discussion Starter #3
Doubt there's anything wrong with the engine internally, same rough idle, two different engines, so I haven't done either of those tests yet.
 

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Doubt there's anything wrong with the engine internally, same rough idle, two different engines, so I haven't done either of those tests yet.
Did you swap everything from your old motor to your new motor? When you said same rough idle, two different motors, is everything on the motor different?

If so, are the mounts good? Have you refilled your gas tank with good gas?

Same torque converter? I'm just trying to figure common things that would have kept the problem with the car...
 

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Discussion Starter #5
Did you swap everything from your old motor to your new motor? When you said same rough idle, two different motors, is everything on the motor different?

If so, are the mounts good? Have you refilled your gas tank with good gas?

Same torque converter? I'm just trying to figure common things that would have kept the problem with the car...
Swapped in a completely different engine. The only things that would remain the same are a few sensors (MAF, O2s, IAT, ECT), injectors, egr, wiring harness, ecu, distributor and ignition components.

I don't have a torque converter its a manual car.

Another problem I have it when I push the clutch pedal in the RPMs hang for a few seconds before coming down to idle.
 

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On the Cylinder balance test .. you can do it manually also instead of relying on the Ford tool.
Un-plug the IAC and hold the engine RPM @ 1200 or so .. disable the EGR, or you might want to unbolt it and put a coke can or block it off.
Then you can un-plug each fuel injector to see if you get an RPM drop to determine if its fuel / spark related.
 

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The 'hanging RPM' thing can be solved by the 'beer can mod'.

Cut a piece of beer can out to match the IAC valve and make the hole smaller for the air - it will idle down quicker.
 

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Discussion Starter #8
I did the cylinder balance test by cutting spark to each cylinder with a test light at idle. All cylinders had the same RPM drop.
 

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Sounds like low compression which is more apparent at idle and disappears as RPM's are increased...

Low compression in an engine usually results in hard-starting, as well.
 

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I did the cylinder balance test by cutting spark to each cylinder with a test light at idle. All cylinders had the same RPM drop.
That sounds "normal" .. they should drop RPM with Fuel / Spark cut .. so its not a problem with your EGR, Fuel or Ignition system.
 

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Discussion Starter #12
So I guess Im looking at a vacuum leak that I can't find or maybe a sensor problem? Going to try the propane method to try and find this vacuum leak.
 

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my bird was doing this as well, everything had been replaced according to the dealer whom had the car for 3 months fixing all the issues, recently i had a rough idle, it freaked me out i would be sitting at a red light and it felt off had no idea what was going on.
i got home parked and checked in the morning, heard a loud hissing sound.

i went ahead and found the issue, a hose had cracked, and went ahead and replaced all the near cracked hoses.
good luck! hope this helps.
 

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Discussion Starter #14
Well I ended up hooking up a vacuum gauge again when the engine was warmed up and idling. I noticed the needle move maybe a 1/2 in HG. ever so slightly up and go back down again. From what I read this indicates a small vacuum leak, so Im going to start by removing all the cruise control components (since its never worked, and is not even hooked up now) to eliminate those vacuum lines and go from there.
 

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Well I ended up hooking up a vacuum gauge again when the engine was warmed up and idling. I noticed the needle move maybe a 1/2 in HG. ever so slightly up and go back down again. From what I read this indicates a small vacuum leak, so Im going to start by removing all the cruise control components (since its never worked, and is not even hooked up now) to eliminate those vacuum lines and go from there.
What did the gauge read?
 

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Discussion Starter #17
I blocked off the vacuum tee going to the cruise control vacuum lines and the rough idle is still there so that eliminates that section of vacuum line. Im thinking of making a homemade smoke machine at this point.
 
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