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#7 miss on 4.6 SOHC from 2000 Mustang GT

I took a 4.6 SOHC from a crashed 2000 Mustang GT with 103k miles and unknown background, and put it in a FFR Roadster. The clean, painted heads appear to be new and there was no wear on the clutch, so I suspect a rebuild.

The problem is a fairly consistant miss on #7. It usually starts at idle and goes to about 2400 RPM. Cold compression test gave me 165 PSI (155 - 165 all around). Replaced plugs with motorcraft OE replacements. I swapped injectors and coils (one each per cylinder) and even replaced them with new as per suggestion from Ford mechanic. Replaced IAC in case of sticking. Some wiring and computer changes in swapping the engine. The randomness of the miss (consistantly random, or randomly consistant) seems to me like the computer is responding to something it’s not sure about. Also swapped front O2s and deleted rears with Sniper program due to exhaust system change. Anyone see this before, or have an idea what to do next?



If you swapped/replaced coils/injectors and the miss is still there, then something must be happening inside the engine for that cylinder. The compression appears to be good. 103,000 miles on the engine? This problem might be caused from carbon deposit buildup on the intake valves. At this point I would try a poor man’s decarbonization of the engine.

Purchase a can of SeaFoam Engine Treatment. Disconnect the brake booster vacuum hose from the brake booster. Adapt a hose that will fit inside the end of the brake booster hose and into the can of SeaFoam. Pinch this hose off with a pair of pliers. Have someone start the engine and bring the RPM’s to 2,000. Slowly open the pliers so the SeaFoam starts to drawn into the engine. It’s here where the RPM’s must be kept up and the pliers opened and closed to prevent the engine from stalling. Once all the Seafoam is drawn into the engine, shut the engine down and let it sit for a half hour. After this time, restart the engine and bring the RPM’s back up to 2,000 until the smoke clears from the exhaust. Take her for a drive and see if the miss is gone.


Look like you covered mostly everything leaving only a couple more things to check.

Intake manifold leak, Air or coolant getting into cylinder.

It looks like you have eliminated spark, compression and fuel as the problem. I think I’d look closely at the lifters on that #7 cylinder. Due to the unique lifter arraignment, they may not make the clacking sound that you normally get from a collapsed lifter.

The lifter is on the pivot end of the rocker and is mounted in a hole in the head. Oil is fed to the lifter through a drilled passage. The cam rides on the center of the rocker. Because of this, in the case of a collapsed lifter, gravity causes the rocker to rest on both the lifter and the valve so neither gets “slapped” when the cam lobe comes around.

The oil return passages are located in the center of the head, on the exhaust side. If this head were to crack, this is where the cracks would occur. If they are not draining properly due to sludge, look for the sludge here and clean them out so that oil doesn’t pool at the back of the head. The drain holes are rather small BTW, there are three in a row, close together.

Did you try replacing the plug wires? How old are they? Maybe a good idea to replace them anyway.

Curious . . what did you pay for the engine? Rocketman

There are no spark plug wires on that engine!

The compression is a bit low for a 4.6 it seems to me although it can run fine at 155+.
The 4.6 in my Lincoln Mark still has right at 190 all the way around at 235k miles.

If the car were mine I’d connect a vacuum gauge to it and see what’s going on.
The vacuum gauge is one of the handiest tools on the planet. The gauge could also let you know if there’s an intake leak on that particular manifold runner.

Since you mention clean painted heads and suspect a rebuild a miss could be caused by something in the valve train. A weak lash adjuster or weak valve spring could do it; either of which could easily be overlooked if the rebuilder was not using care.

This should show up on a vacuum gauge reading if this particular problem exists and it should be most noticeable at idle/low RPM.

Haven’t used many forums, but this is the best I’ve seen. I went to the garage for a while, and came back to some serious help.

Tester: I’m going to try to find some seafoam this afternoon during a trip to Cornelia.

Americar: I don’t see signs of water, but still looking for air leaks. I’ve sprayed water, carb cleaner (and even starting fluid) everywhere I can get to, but I’m unsure of the bottom side of the intake. Bought an intake gasket, but not quite ready to drain the radiator yet. Looks like that may be coming up soon.

Keith: Your input is excellent, since I have no knowledge or experience with this modular engine. I broke down and ordered a helm manual. They have always been great for me. I may have to pull the engine (at least partially) to get the valve cover off in this Cobra application, but it’s on my list after the sea-foam trick.

Joseph: This one is a coil on plug (8 coils) situation, but I did have a mechanic connect a small display device to show the ground pulse initiated by the computer. However it still doesn’t tell me how solid the ground is, so this is still on my mystery list.

Rocketman: I got the engine as part of a complete car from an insurance auction. The FFR kit uses the engine, xmission, drive shaft (shortened to 13 inches), fuel tank, pedal box, hydroboost, rack and pinion, wiring harness (modified), brakes, rear end, and lots of other etcetras. I’ve got my moneys worth out of the other parts, but if I can’t use this engine, I’m out whatever a replacement costs me.

To all, thanks for the input. I’ll be sure to post the final answer but my weekend mechanic work is usually slow. In the mean time, any more ideas are welcome.

Any chance you’ve posted pictures of your project somewhere on the web. I’m sure at least a few of use would like to see it. Not just the engine but the whole project.

I can understand why Joseph suggested spark plug wires. When this engine first came to market, it had what appeared to be a distributor at the back of each head with 4 plug wires coming from each to the plugs on that bank. The DOHC in the Mark VIII I believed used the coil on plug, but they hid everything under a big shroud.

Trying to attach a test picture. Unsure of sizing etc.

There are no spark plug wires on that engine!

I guess that shows how much I know.


Here are illustrations of what the vacuum gauge indications look like for each type of problem: Click on the little green scenarios for each indication. I keep this link in my Mozilla Firefox Bookmarks (a handy file).

Random misfire issues with this engine can be maddening. I once used a pressure=washer on a Crown Vic and it took me a YEAR to get it straightened out… The plug wells must be bone dry. The coil packs are problematic. The silicone boots that cover the plugs and connect them to the coils through a spring and carbon resistor all must be perfect and can never have been wet. The coil primary connectors must be clean, dry and tight.

Good Luck.

More info on the problem.

I tried the seafoam. Don’t understand the results though. I have side pipes with no connection between. I may have chosen a bad connection input. I used an easy connection from the evap system in a small tube a couple feet long that inputs near the throttle valve. Used the same connection for vacuum test.

During the foam injection, I noticed a large amount of white smoke/steam from the right bank, but almost none from the left side (where the #7 miss is). However, it did bring my attention to an exhaust leak on the front of the exhaust manifold. I lso need to point out that this installation required reversing the manifolds so that the exhaust exits from the front of the engine. I have not been able to get in yet to see what causes the leak. It’s below freezing outside and everything on the engine was too hot to deal with when I pulled back into the garage.

Snow day has given me an extra day to work, but I slow way down when I’m stumped.

Forgot to mention the vacuum test held steady around 17. Dropped a little during rev and settled back to about the same at 2000 RPM.


I gotta ask. Why didn’t you go with the 302/347/351/383?