One other point for question: if the garage removed the plenum to replace the new plugs and wires, is there a gasket for the plenum and if broken in replacing when removed, would that cause the misfires for some reason?
OP, I think you are almost there. At this point I think the next step is to find a good inde shop that works on your make/model, and tell them what you’ve discovered, no spark on 1 & 4. But spark is ok on the other cylinders.
GeorgeSanJose, thanks for the suggestion but two shops have told me the same thing, you have a misfire at 1 & 4 but we have no idea why - could be the computer or it could be something else. Perhaps they didn’t want to trace the problem down or whatever, but no help to me. I had a garage replace the plugs and wires -$226 - replace the coil and cam sensor $295 - and still no solution.
Did you tell the garage staff to replace the plugs and wires, or did you ask them to diagnose what was causing the problem? You’ll need to find a shop who is capable and properly equipped to do the latter. I don’t expect this will prove to be that difficult if you can find a shop properly equipped and staffed. You may have to go to a dealership shop if you can’t find a suitable inde shop.
George, I asked a shop to change the plugs and wires because the mpg dropped by about 2 mpg. When they changed the plugs they scanned and no code was thrown. About 2 weeks later the misfire problem happened.
Don’t need to go the dealership for this
All you need is a good mechanic and the wiring diagram for the pcm and ignition system
Since this van is old, it might be something as simple as a pigtail needing replacement, for that ignition coil which feeds #1 and #4
We have many trucks in the shop that have misfires caused by bad/old pigtails
I will just add this. You need to be careful about cash investment in the van at this point because those 3 cylinders at 125 PSI means the end is approaching no matter what the other 3 are doing.
That leads to my earlier point that sometimes an engine may not be running as well as perceived with the situation being similar to becoming acclimated to worn shocks.
125psi is . . . in my opinion . . . acceptable, given the high mileage
They’re not great numbers, no doubt, but what I would expect for the mileage
Clearly, 125 would not be acceptable for a recent model car with 50K, but that’s not what we have here
I agree that 125 is not bad for the mileage but the fact remains that it’s still way low and engine performance is going to be dependent upon that compression number.
The only engines that pressures in that range could be considered good is on flat head or air cooled motorcycle, car, and aircraft engines.
A 120 PSI on an old air cooled VW Beetle would be great; not so great on a water cooled where good is 190 and 150 is going in the tank.
Let’s keep things in perspective . . .
I’d venture to guess that MOST engines with well over 300K do not have 190psi, probably closer to 125
I never saw a 3.3/3.8L Chrysler with over 160 PSI of compression (@ 2200 ft elevation) when they were new. 125 is reasonable if the throttle is closed during the test. As long as all cylinders are even I wouldn’t be critical of an amateur’s test equipment, procedure or results.
Nevada_545, thanks for the picture that helps, and thanks for the no-go on the Map sensor. The pin is a great idea, but do I not attach another clamp somewhere else to check the wire? And, I would assume that if the light is solid, I need to go back down the wire until it is no longer solid and the light flashes?
One other point for question: if the garage removed the plenum to replace the new plugs and wires, is there a gasket for the plenum and if broken in replacing when removed, would that cause the misfires for some reason?
BTW, elevation where I live is 978 ft.
Connect the test light ground clip wire to the engine or battery ground then touch the probe pin with the tip of the test light. (Search “test light”)
If the wire is broke finding the break may be a challenge without an ohm meter. The first step, gently pull each wire at the coil pack, if one feels elastic/stretchy the copper is broken inside the insulation. Than is not an unusual find on Chrysler products, their copper wiring seems to fatigue easy.
Normally the rear spark plugs are replaced from below the engine. If the plenum was removed and is leaking the engine would be idling fast off the extra air entering. It may have a misfire at idle but at open throttle it would be smooth.
I assume the gentleman who was asking about an engine miss some years back on his Chrysler 3.3 which had 180 PSI on all cylinders at 120,000 miles must have owned a mutant…
I stand by my statement that 125 is on the way out. If a 125 PSI existed on a near new car with 20k miles it would be generally accepted that 125 is bad news. A 125 is a 125 no matter the miles.
Other than that, we’ll just have to agree to disagree. 
125 PSI is failing but recheck it with your gauge, with the throttle open, after at least 4 revolutions and at your elevation.
Thanks guys. I have no doubt that the 125 psi is not good. I was trying to address the misfire problem as being from compression or not from compression problem. Other than getting the van fixed, if I can find the problem, there may be people with this type issue show it’s head prior to getting down to 125 psi and with less miles and if, with your help, I can resolve this other people may benefit.
When I did the compression test, the engine was not totally up to operating temperature and the throttle was not open. I am going to redo the test twice. Once at warm and once at operating temperature with the throttle open in both cases. I don’t know if the results will be different, but I have read two different opinions on how hot the engine should be.
I do have an ohm meter, and I will be doing the light test as Nevada_545 suggested. Hopefully that will be revealing and I will let you know what I found, and I’m sure, ask a question or two more.
I appreciate the help very much.
180psi and 120K versus 125psi and well over 300K is not a valid comparison, in my opinion
I continue to disagree that 125psi is a real problem . . . given the mileage
I would worry more about getting this van into the hands of a good mechanic, along with those wiring diagrams I mentioned
Don’t bother with the ohmmeter if you’re checking wires. If you have one single strand of wire, it will measure 0.3 ohms for example, end to end
But it wont’ carry a load
You need to do a voltage drop test
The rule of thumb on compression pressures on a good engine is 20 X the compression ratio.
Someone needs to explain to me why 125 is good on a high miles engine and 125 is bad on a low miles engine since that seems to be the train of thought.
If it sucks it sucks no matter the mileage.
Was the coil pack and wires replaced? That’s all there is to it.