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Q: How can I identify engine Misfire? on 1996 Ford Probe

Intermittent but not Random. No misfire under load or while driving. It idles and run rough while stopped after 15 minutes of driving in temperatures above 50 degrees. Runs smoother in temperatures under 50 degrees and sometimes while raining.

Maintenance - 2011-2013

No codes

Vacuum - 22 in. HG Steady = Good

Compression - Good

Fuel Pressure - Key on, not running, 40 psi = Good

Fuel Pressure - Key on, engine running, 32.5 = Good

Spark plugs replaced - 2012

Spark plug wires replaced - 2012

Distributor and cap replaced - 2012

Coolant Temperature sensor replaced - 2011

Throttle position sensor replaced - 2012

Camshaft position sensor replaced - 2013

I listened to each of the 6 fuel injectors. 4 sounded smooth but 2 are a bit noisy. But all are working.

Oil changed - Jan. 2013

Fuel filter changed - March 2013

Coolant Changed - Feb. 2013

Valve cover gaskets replaced - June 2012

Fuel system cleaned - Feb. 2013

Throttle body cleaned - March 2013

EGR valve - applied vacuum, the engine stalled. It passed.


Ignition timing. May have been adjusted when distributor was replaced. Not sure.

The fuel strainer on the fuel pump not checked but fuel presser is good.
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Hi, my name is Dan. It would be nice to know what engine you have. It would be great to know what the fuel trim is, just so you know whether the engine is running lean. I mention this because you say that the misfire gets worse as the engine warms up. As the engine warms up, the fuel mixture goes leaner, because a cold engine needs a richer air/fuel mixture.
If the long term fuel trim is say 11-14%, that can cause a rough idle and may not set a lean code. You say the rough idle never sets any misfire codes, have you sprayed carb cleaner or used propane to see if you have any vacuum leaks i.e intake manifold/plenum gasket etc ? Does the EGR valve stay fully seated? I test for this by raising the valve and then SLOWLY closing it to see if there is any leakage. Then I raise it and snap it closed by suddenly pulling the vacuum line off. If the idle is different when snapped shut, you have discovered something real. Also, check for any oil on the ends of the spark plug boots, as I have repaired many misfires due to the spark plug wells leaking oil on the spark plug boots. The oil acts as a conductor and the spark voltage bypasses going to the plug and goes directly to the head.
I assume you are using OEM plugs and wires i.e genuine Ford/Motorcraft. This really helps as I have chased so many issues with aftermarket parts, so keep this in mind.
Also, a fuel system cleaning can really help, I always do this whenever I repair a misfire in the shop where I work. I have a machine to do this. You can use Chevron Textron, just put a bottle ( as per the instructions )in your tank. I would use it for at lest 2 full tanks, Mercedes uses it on all of their vehicles when they come in for a service, that speaks volumes.
To check long term fuel trim ( lean or rich compensation. a high positive number i.e. 15% means the power train computer is adding 15% more fuel than normal etc ) you can get an Autel CAN/OBD II code reader at AutoZone. ($50 ? or so) It has a data stream feature that shows long term fuel trim, I use one on test drives to set the OBD-II monitors, because I don't want to lug around the Modis or another heavy, full featured scan tool.

Let me know if any of these suggestions shed some light, I will help you any way I can.



Hello Dan - thanks for putting up such a good, detailed answer - I have the exact same problem with my 1997 Ford Prode GTS (V6). I ran sea foam cleaner through the injecter, which cured the missing for about 1000 miles. I going to try the Chevron Textron cleaner tomorrow. Ill let you know if it works.

Well 30 miles into running with injector cleaner - still misfires after warmed up. I only have 75K on the car, so hoping not to have to replace injectors - was it expensive, the labor, to have the new injectors installed?
I installed 6 fuel injectors myself for $180 for 6. I saved on labor cost.

Just read your shop manual and take your time.
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