I have a pristine 1999 dodge ram 1500 with a 5.2 L and 38,000 miles. However it has been driven for the last 5000 miles or so with a known failing Plenum. Just keeping up with the oil and if anything adding additive and gas tank.though we plan on doing the Plenum very soon. We had no heat. And the thermostat was recently replaced to resolve this problem. My actual issue is:
It began running very poorly to the point of no start just over a two day period. I have no codes and no apparent issues to point me in a particular direction.

I have good spark and I have fuel pressure (45psi). I have looked at fuel and the ASM relays I have replaced the crank sensor (twice due to questionable quality), cam sensor, throttle positioning sensor, cap /rotor with new plugs.

I was concerned about a clogged catalytic converter but can’t see this being an issue for the no start.

Anyway, we need your assistance from anyone that may have dealt with this or experiences this before.

thanks in advance for any responses

 

Need some noid lights, and see if you are getting injector pulse.

 

I don’t have the Noid lights yet. But I did tap in to one of the injectors with a multimeter and can see that I was getting some signal but not sure what to expect.

 

If the cat is plugged just right way it won’t breathe and can’t start. It’s rare it’s gets the perfect clog but it happens. Try unscrewing pre cat 02 sensor to see if the pressure relief will make it start

 

Agreed, I planned to remove the upstream O2 sensor to see what I got when it was running poorly but haven’t done that yet, I did have some concerns regarding the CAT as it felt hotter then normal coming out the tail pipe when it began running poorly. Again, this whole time no engine codes.

I’m just having a difficult time suspecting the cat at this time as when attempting to start i believe have enough flow at the tail pipe that I’m not quite convinced this is the problem for the no start.

do you think it’s possible an 02 sensor could be sending a fail code to prevent the ECM from allowing to start without showing an engine code?

thanks again the responses thus far, I appreciate the feedback and assistance troubleshooting this. I don’t want to have to have towed

 

No, the O2 sensor readings are ignored until the engine goes into closed loop operation. If you get some flow out of the exhaust during cranking then I would agree it's likely not a totally plugged cat. Hot catalytic converter could be excess fuel being burned in it, stuck injector or a cylinder is not firing because of the plenum issue. Are the plugs wet with fuel ?

Another thought as the cam sensor was replaced it could be the infamous "fuel sync" is off and the injectors fire at the wrong time.

 

Hard to imagine an engine with only 38k on it has a bad plenum and cat. If it sat for a long time, check the grounds. There are several and just one bad ground will cause lots of problems, including starting.

 

cam sensor fuel sync - this could be onto something. How does this usually happen? Process to resolve?

 

yes, this has been part of the problem with me accepting some direction of troubleshooting. However, with just taking a view down the intake, it does appear that there is oil residue and protruding gasket from what I can view on the edges.
but other than some oil consumption and porter gas mileage it has been a good now daily driver.

one thing I’d like to state related to the plenum is that during these troubleshooting weekends lately I did notice what appears to be a loss of vacuum (maybe to completely). But I didn’t think this would stop the vehicle from starting but perhaps just allowing it to run poorly.

 

Regardless, a leaking plenum gasket has no bearing on the engine starting. All you need is fuel, spark, and air. EFI and computers make it a bit more complicated but don't overthink as you diagnose. Not talking down just keep it simple as you work the problems.