3500 hard start

[bxd]

OP here.

I think we have the problem fixed.

I did take it to the dealer, and of course they couldn't reproduce the problem. But they did flash the computer with an update under warranty, and said that might help. Well it didn't help.

So then I decided to go ahead and try some "maintenance" and go with the easy stuff first -- cleaning the throttle body. I was surprised at how dirty it was. Carboned up. I sprayed B12 into the TB while my wife kept the engine running, did about 1/2 can and so far after 2 days, it hasn't had one rough start. So I am keeping my fingers crossed.

When I removed the air intake hose (the accordion tube) I did find some oil residue in the hard plastic line that inserts into the top of it. It runs somewhere towards the back of the engine. The 3500 is pretty tight in the RDV, so I can't feel where it goes. What is this hose for? Is it PCV, although I thought the PCV was at the front of the engine.

Thanks.

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[TGP37]

PCV is in the front, where air is drawn out of the crankcase. There is a hose that goes to the back to feed air in. The PCV line connects behind the throttle, the fresh air feed connects before the throttle body. This way, at idle conditions, air that is already metered from the MAF sensor is used to draw vapors from the crankcase from blow by. If either hose is disconnected then fresh unmetered air will enter, causing a lean condition.

You MUST have an air tight intake after the MAF sensor.

[bxd]

OK - let's see if I understand what you are saying.

Fresh air is supposed to flow through the air filter, through the accordion, past the MAF, and some of it is supposed to enter that hard plastic hose in the accordion tube, head to the back of the engine and create air pressure that takes some crankcase gases and helps them get sucked back into the engine, but past the throttle body.

Since I found oil in the hard plastic line that should have air flowing AWAY from the intake, the PCV valve might not be working right? And yes fortunately the PCV valve is right in front where its easy to replace. I probably should use some pipe cleaners to try to get as much oil residue out of the lines as possible before dropping in the new PCV valve.

[TGP37]

The PCV is a spring load valve. Under idle conditions there is a vacuum in the plenum behind the throttle blade. This pulls the PCV almost closed for minimal ventilation. When the throttle is opened all the way, the vacuum is gone and the PCV opens up for greater ventilation as blowby can generate pressure if not ventilated.

Under low/no load, the one plastic tube feeds fresh, filtered, measured (MAF) air to the crankcase in the back side. But it is a passive air feed, it doesn't provide any positive pressures. It is the vacuum present at the PCV valve that draws upon the fresh air feed, through the crankcase.


Shake the PCV valve, if it doesn't rattle replace it. Also check the hoses and grommets to make sure you have an air tight seal. You do not want any air leaks past the MAF sensor. Or the filter but that is another issue.