95 GT Rough Idle and Stumble - SOLVED!


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Apr 17, 2015
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I thought I would pass along my findings regarding the ubiquitous 94-95 5.0 rough idle/stumble/slight miss issues, as they have been described. I've researched several of the Mustang forums, but no one seems to have solved the issue entirely. I'm happy to say I have achieved a smooth idle!

Some history: I purchased my 95 GT 5 speed from a friend last year with 97,800 miles, all stock and well maintained. I intended to use it in another state for local transport when there, and it has become a side project to fix the little nagging 20 year old issues, such as the thermostat housing leak, power steering hose and reservoir leaks, radiator leak from a bad rebuilt radiator, harmonic balancer, seat adjustment gears, new distributor, correct plugs, new diff and transmission fluids, and old vacuum lines ... Yes, those damned vacuum leaks ... And whence that annoying idle issue.

After cleaning the MAF, IAC, replacing the PCV and CANP vacuum lines under the manifold to the canister, and checking all the sensors for correct function, I still had the nagging rough idle. All vacuum lines were sealed, and the car would smoothly idle cold, but once heat soaked would pick up the stumble. Also present was the slight rough feeling from 900-1400 RPM. The base idle was set correctly and by the book: 0.010" feeler gauge to the set screw, then one full turn clockwise. Base idle without IAC was smooth and 400 RPM, with about 1 turn CCW from closed on the air adjustment screw. Perfect idle with IAC and great drivability, except for that rough stumble ... When hot ... Mostly. If your stock 94-95 won't idle with the book setting, you have more leaks to find.

I found a decent deal on O2 sensors, didn't expect it to fix the issue, but figured they were tired at 106,000 and it wouldn't hurt. The car passed emissions before the O2 sensors and intake removal and vacuum leak fixes with flying colors ... About as good as new. The idle improved only slightly after new O2 sensors to my surprise. I really didn't expect any. Once again on the hunt for any possible vacuum leak, and finding none, I also removed and cleaned up the ignition coil, on a hunch. I had noticed a rougher idle on humid, warm days, of which there have only been a few recently. Not much change, but I had also shot a little cleaner into the PCV valve for good measure. I had checked it back when I bought the car and the usual checks said it was good ... But there is more to that story. I discovered a strange noise while under the hood ... And now that all the vacuum leaks were sealed, I could hear a surging/flapping/squealing noise from the PCV valve at idle. It should have been the constant low volume suction. Wait a minute! This EEC-IV is too fast for it's own good, and there is a surging flow from what should be a constant unmetered flow through the PCV ... Eureka! The valve must be changing the metering as it becomes heat soaked, as the condition was worse when hot. A bit of sleuthing and searching showed some are having issues with aftermarket PCV valves as well ... So I popped in a new Motorcraft EV-98B, and I have a smooth idle and no roughness 900-1400 RPM anymore. The valve has the correct metering for the idle vacuum and the transition to open.

The moral of this story is that any vacuum leak or unmetered leak that is not what the EEC-IV expects will cause it to chase it's tail. Do not use aftermarket PCV valves, but Motorcraft. If your valve is old, replace it. The so called tests of 'does it rattle?' to 'can you blow through it or feel a vacuum?' are not valid. There is a spring loaded valve that is set for the correct high vacuum (idle) flow and it loses calibration. If this flow rate surges (like mine) or is the wrong amount, the EEC-IV will not be happy on your 94-95 model. Given the faster OBD II PCM's, I'm sure this is applicable to them, as well.

A hint for those of you with modded engines and idle issues, especially for cams that have lowered your vacuum below 18", you might consider trying a 1969 Camaro 350 PCV valve, as it meters for idle in the 14-16" range.

Hope this helps some of you.

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