Welcome aboard freeb
. Your problem can be any number of things, but Toyota's injectors are pretty tough and so are their ECUs. So IMO both of these would be low on the list of possibilities.......maybe the 2 very last things I would suspect. The improvement in running by tapping on the fuel rail seems to suggest an injector issue, but it could also be a coincidence. I'm not saying it isn't injector related, I'm just saying I'd check a bunch of other things before splitting the manifold and/or removing injectors.
My 1st question would be about trouble codes. Did you check for codes in your ECU? There may or may not be any codes present, but checking for them can often provide valuable clues, so anytime you experience a problem, codes should be checked before any corrective action is taken.
My 2nd question would be maintenance related. When was the last time you tuned up your van? Do you know how long it's been since the spark plugs, plug wires, distributor cap & rotor were replaced? If you don't know, then I would suggest replacing these parts 1st. Often times a miss can be caused by lack of maintenance. Even if the problem isn't here, if it's time anyhow then you've got nothing to lose.
Speaking of maintenance, when was the last time the alternator belt was replaced
? Unless too tight or too loose, belts stay reliable for around 60k miles / 5 years, so having one break indicates incorrect tension, lack of maintenance, or both. Regardless, having multiple cluster lights come on at the same time usually indicates a charge system problem, so unless you're sure they came on before the belt broke, I'd just chalk it up to the broken belt. If this continues to happen after the belt is replaced, then I'd take look at your charging system 1st.
If you've had a recent tune-up and still have the miss problem, then the next thing you should check for is vacuum leaks. Pay particular attention to your air intake hose as these tend to crack near the throttle body. To verify it's condition, give it a good squeeze while the engine is idling. If the engine stumbles or stalls, then chances are you've got a cracked intake tube. You should also inspect all of your vacuum lines to make sure they are secure and in good condition. If all these look good, then the next thing to check is the fuel pressure. Other possibilities could be a failing ignition coil or a failing igniter, but these items are further down the list of likely possibilities. IMO, due to the access issue and the proven reliability of these injectors, splitting your manifold just to check would be way, way down on that list. Tim