I have not used a back pressure gauge yet. Kinda hard to justify 100 bucks for a one time use.

Trying to figure this out when the truck runs great some times, and not so great at other times is frustrating.

If the MAP sensor was clogged would it affect how the catalytic converters operated? I was checking it out today, volts test fine today, but then the truck was working just fine. I took it out and noticed a piece of of the rubber was hanging off, flapping around. Wondering it it would cover the hole every once in a while and make it get inaccurate readings, thus affecting how the truck ran.

 

That's a very good observation ... the cats are basically only "on" and "off" devices. When your exhaust heats them up, the honey comb, or monolith, begins to chemically change your exhaust as it flow through. So it's not really a filter so to say ... it's what its name implies. It converts your exhaust's N0x emissions into better ones. I can't remember the exact chemicals that it converts, but it cleans your exhaust purely by a chemical reaction.

So can a MAP sensor change how they work ..... well yes and no. The MAP can change how the engine runs which would in turn change how fast the catalytic converter heats up ... which changes how much they convert your exhaust. So ... yes and no.

What I'm more interested in, is your what your fuel trim and oxygen sensors say when your truck is running poorly. You might just be onto something with your MAP sensor, but in reality, it's only a diaphragm that measures engine vacuum and reports a signal to the PCM. So the rubber ... would only bother that sensor if it was sealed up around the little orifice that's on the end of the sensor. An intermittent vacuum leak however, will most certainly cause your engine to run poorly.

Consider that when you open your throttle, your engine vacuum goes down, because your restricted orifice (your throttle body) just got bigger, and equalized atmospheric pressure to the inside of your intake. So, the lower the vacuum in the intake, the richer the engine will run (black exhaust, high fuel trim numbers, and (I think this is right, could be wrong .. it's been a long day) higher oxygen sensor numbers.

So the question is. Why is the truck running poorly, given you just discovered that you might have a vacuum leak at your MAP sensor. Squirt some carburetor cleaner on and around the MAP sensor and see if your idle jumps up. If it does, replace the MAP with one that has a good grommet on it and see if that fixes your vacuum leak, assuming that it's even leaking.

Information is powerful. Digest, learn, discover. We'll figure it out, we just need to make sure you have the right tools for the job

 

So after having it run good for the past few days, it started acting up again on the way home this morning. This tells me it was not the little rubber piece making it act up. I did take and spray some starter fluid all around the MAP sensor and the RPMs did not change at all. I have used up a whole can spraying it everywhere there is a hose running in the engine. This would lead me to believe it is not a vacuum issue.

It is so strange that it is hit or miss like it is. Now back to the catalytic converters again I guess. I will have to find some way to test the back pressure to make sure they are clogged before dropping the coin on a new set.

 

I knew a guy who used to take a self tapping set hex head screw and drill a super small hole up in a nice place on the exhaust pipe pre-catalytic converter and use the pointy tip on his exhaust pressure gauge in that hole instead of using it in an 02 sensor hole.

It has a pretty distinct sound when the catalytic converters are plugged. It hisses pretty loudly. I guess you can try that and see if the sound it makes leads you to believe it's a catalytic converter. He would weld up that hole when he was done though. You can just leave that self tapping screw in the exhaust to seal up the hole though.

 

well I checked the back pressure, and it tested just fine. Less that 1 PSI on idle and at 2500-3000 rpm. Guess that ruled the catalytics out as a problem.

So far I have replaced both forward O2 sensors, MAF sensor. Checked the catalytics, the MAP sensor, the coolant temp sensor. Checked vacuum pressure and for leaks ( best I know how). New fuel filter a month ago, tested fuel pressure 56 on start about 52 when reved up. New plugs and wires a month ago as well.

Still cannot get the dang thing to run well. Step on the gas and it stars to bog down, backfire and eventually stalls.

This sucks.

 

What year truck and what engine are we talking here? I guess I don't think I've asked that yet.

I'm guessing you have a 88-98 GMT400 with a TBI 350 or Vortec CPI 350. More likely a Vortec with that fuel pressure. Now, what happens if you break torque the engine with the fuel pressure gauge on it? The pressure can easily be dropping due to not enough flow. Plugged fuel filter can cause these symptoms. Good thing you didn't replace the catalytic converter, huh ...

 

it is a 97 with the 5.7 l Vortec 350, It also has a new fuel filter about a month ago, forgot to mention that. I retested the fuel pressure today, at idle it is about 60 psi, when reved it is about 56 psi. Put in new cap and rotor today, nothing. NO kidding about not getting the new cats. Though I find it odd that the temps are not correct but there is no back pressure.

Guess it may be time to take it to a mechanic and try to diagnose this problem for me. i hate to do it, but I am at a loss.