At our Jeep TJ Wrangler forum, we've been tackling the #1 Cylinder Misfire P0301 DTC issue. Member Belvedere brought up the weak valve spring issue and offered some great suggestions on changing out the valve springs on a Jeep inline six!
Regarding weak valve springs on the 2.5L four or 4.0L and 4.2L inline Jeep six, weak springs can show up in a simple manifold vacuum check. At any steady throttle setting, there will be an erratic, shaky vacuum needle movement with the vacuum gauge hooked up to an intake manifold vacuum source. (Not to be confused with the wider swinging needle movement associated with a valve that is steadily leaking.) As Belvedere shared, weak valve springs can build up carbon, as the valves do not seat firmly when closed.
Valve springs can actually be tested for valve seat pressure on the engine. Above is a photo of a simple tester available for that purpose. (Click here for a more upscale Moroso 62388 design available at JEGS.) With the rocker arm(s) removed, head and valves still in place, this over-the-top spring pressure tester can indicate the actual seating pressure, which is a true test of each valve spring's function. This is a sensible testing method with the least amount of teardown work: simply removing the valve cover and rotating the crankshaft to close the valve(s) to be tested. (Caution: Disconnect the negative battery cable to prevent starter engagement when turning the crankshaft by hand.)
There are two off-the-engine tests for weak valve springs. If Belvedere still has the original springs, measuring and comparing the free standing height of the springs can be one test. Another method, commonly used by automotive machine shops and race engine builders, is actual spring compression testing (read in actual pounds force or as PSI) with a special gauge. This measures pressure as the spring compresses.
As for removing the valve springs, Belvedere's classic rope-in-cylinder method works. (See the exchange at the Jeep Wrangler TJ forum under the #1 Cylinder Misfire thread.) So does an air hold, and this is especially easy for #1 cylinder, since the timing mark for TDC on the crankshaft damper is a quick way to find TDC for #1 piston.
Here's how I do an AMC-design Jeep 4.2L or 4.0L valve spring removal:
1) Disconnect the battery negative cable to disable the starter. Remove the valve cover and spark plugs, at least #1 plug in this case, all of them to make rotating the crankshaft easier by hand. Rotate the crankshaft by the damper bolt, turning the crankshaft in its normal direction of rotation. Watch the valves open and close to be sure #1 piston is coming up on its compression stroke as you bring the damper pulley around to TDC on the compression stroke.
2) Set the damper mark at TDC to be sure the #1 piston is at the top. This will prevent fears of "losing" a valve into the cylinder.
3) Use an air hold fitting in the #1 spark plug hole to keep the valves up in position. These adapter/tools are commercially available and inexpensive, or you can make an air hold tool with an air coupler and an old spark plug. (See my comments below. Summit Racing lists the KD 901 adapters for $4.97, the best price I've seen anywhere! For that price, no need to make your own.)
4) Remove the #1 cylinder intake and exhaust rocker arms. The pedestal bolts simply get torqued back into place, there is no valve "adjustment" to be concerned about when you reassemble the rockers. (Just align the arms carefully with the pushrod tips and valve stems when you reinstall the rocker arms.)
5) With a stream of air applying pressure from any reasonable size home shop or garage air compressor, you can remove the valve springs using the "over the top" method. (80-90 PSI should be plenty, there will be some leak down, so your tank compressor should be full when you begin the spring change out. You can recharge the compressor if necessary between each spring removal.) Belvedere's approach with a pry tool attached to the rocker stud can be effective, and this tool is readily available. KD has made an affordable rocker pedestal pry bar for many years. Even if "universal" fit, however, make sure the tool is designed for the Jeep 4.0L engine application, or you will be fighting this task.
6) There is also an over-the-top valve spring compressor available, which can be easier to control for the less practiced mechanic. (OTC's version is shown at the Summit Racing page link. KD makes a tool like this, too.) This is a two jaw compressor that can compress the spring between the valve spring retainer and the spring coils. This tool is great—as long as there is enough installed spring height and adequate coil gaps for the jaws to fit. You must be able to compress the spring enough to safely remove the valve keepers. With either tool, stay centered on the valve spring retainer to prevent valve stem or keeper damage, and carefully remove the keepers like Belvedere cautions. Belvedere's magnet suggestion works for lifting out and installing the keepers.
Note: If you do use Belvedere's rope method for holding the valves, make sure the piston is coming up on the compression stroke before inserting the rope. Otherwise, rope could get caught between a valve head and seat, which would reduce exposed valve stem height (or chew up the rope under valve spring pressure).
As for making an air hold tool, above is a photo of the hold I made in ten minutes for a tight-access Honda four-valve motorcycle engine. The KD type adapter is so inexpensive that unless you need the tool this minute or have a unique situation (like the narrow access Honda four-valve thumper motorcycle engine, which will not accommodate the air hold adapter!), buy the KD or similar air adapters.
Steps involved in making and using an air hold tool:
1) Remove the ground strap from the old spark plug shell.
2) Knock the porcelain and center electrode out of the plug shell. Use eye protection, you're breaking porcelain/glass here.
3) If the shell size allows for tapping with a pipe tap, you can drill, cut and thread for an air coupler fitting's male threads.
4) Use Teflon tape on threads of the air coupler fitting if you do use the threaded method. I find brazing works very well and can be a much quicker way to attach the air coupler fitting—if you have an oxy-acetylene welding/brazing torch.
5) Surface grind away any rough areas, like the remainder of the ground strap weld. Wire brush your "new tool" as required. You don't want loose material to blow into the engine's cylinder.
6) Thread your KD type or homemade tool into the spark plug hole. With the piston at TDC and the valves closed, hook your hose coupler to the air fitting and apply compressed air at the fitting and into the cylinder.
The air hold tool is a way to hold the valves in position and also run a crude cylinder leak down test. (For details on a leak down test, see my HD video how-to at the magazine site.) Though you cannot measure the percentage of leak with an air hold adapter, you can certainly find a badly leaking valve or leaky piston rings by the volume of air leaking out of the cylinder through the exhaust pipe, intake manifold/throttle body/carburetor or into the crankcase. The leak down test is only reliable when performed with each piston at TDC on its compression stroke and both valves closed.
That the PCM would send a #1 Cylinder Misfire DTC, code P0301 in this case, due to weak valve springs is "interesting". The diagnostic tie-in here would be incomplete combustion, since fuel flow volume through the injectors is uniform (whether the valves seat properly or not), and the ignition spark reliability can be easily determined with an oscilloscope analysis. Poor injector flow or weak spark can also create incomplete combustion and a misfire.
So, that means that the DTC reflects poor combustion at #1 cylinder, which could also be the result of inadequate valve sealing from the weak springs. AMC-design engines do not have a history of weak valve springs, so weak valve springs should not be an epidemic or wide-ranging concern. However, it would be a factor in some cases, and "weak valve springs" can result from valve seat recession/wear, overheated valve springs or over-revving the engine to the extreme and "floating the valves". The 4.0L and 2.5L engines are known to run 250K miles without valve spring issues.
Belvedere, thanks for sharing. This kind of information is very helpful to the forum members! I'm very pleased that you take time to contribute at this level!