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Chris, Shop Foreman
Category: Dodge
Satisfied Customers: 12582
Experience:  Dodge/Cummins and Chrysler Certified Master Tech
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5.9L loud vacuum rushing noise from carburetor while idle

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Hello, so there's no easy answer for this - but I'll do my best to explain. 1998 5.9L gas engine. It starts cold just fine, then according to the dash temp guage, around 130 F a very, very loud vacuum rushing sound starts and then engine begins to shake like misfiring. Yea, I smell something from the exhaust - not normal exhaust like one would expect. After about 1 minute, this loud vacuum rushing sound slowly goes away like a valve closing. So the engine is warming up - right? The noise starts after a few minutes of idling - like pretty suddenly. It goes away after about a minute (perhaps after a temp change?) defintely LIKE a valve is slowly closing (takes about 10 seconds to go away and then we hear normal idle air hissing noise (barely with air filter installed). Mechanics are all stumped - even Dodge's expert trained mechanic. We already did the obvious testing - like smoke to see where there's a vacuum leak - negative. Replace the mass air sensor - no difference. If you try to drive this - wow, the engine can't take gas - stalls - big banging noise like trans going into and out of gear, like the engine is starved for either gas or air. But no lean/rich/backfire computer codes ... no dash lights. To reproduce we have to wait for the entire engine to cool down to say about 100 degrees or less? one person said sounds like air restriction - but if the engine is huffing and puffing and I smell what seems like unburned gas from the exhaust (no smoking), then shouldn't the computer send out an alarm? What can cause a high intensity vacuum like sound ? (with air cleaner all closed and mounted - or without air cleaner )
Welcome and thank you for allowing me to assist you! You may have a lazy o2 sensor that is taking too long to warm up and start switching rich/lean. Have the o2 sensors been replaced? I have to drive to the shop and will look for your reply in about 30 minutes.

Customer: replied 4 years ago.
Hum... yea, I could see that. No, I don't believe we ever replace an O2 sensor. it always passes emissions testing :). So for you, that could explain the huge change in sound ?
Customer: replied 4 years ago.
so, an obvious other question. o2 sensors - they either work or they don't work - right? So, it would not be unusual to see not "warning - about to fail" from the computer? I guess I don't grasp how a computer monitors an o2 sensor for proper operation.

Then what is implied is that the engine is trying to adjust fuel and timing? (no backfire ever) - which can be drastic enough for me to hear a loud noise - yet the engine is not revving up? I don't see any movement for what I consider the carb's butterfly - so mechanically I look from the side and don't see any actuator moving the trottle (sorry if I'm talking from the 80's in technology). So, can I simply unattach the electrical connection from one or all of the o2 sensor(s) - can I still drive the truck? no safety hazard since I drive boy scouts with the truck and need to leave for summer camp this coming Sunday (with a 2,000 trailer in tow). I don't know if I can (myself) remove them since they are probably in the exhaust and very rusted.

Well here is how it works on your truck. When you first start the engine, the fuel system is in an "open loop" mode. In other words the PCM adjusts fuel and timing solely based on the MAP, TPS, RPM, and coolant temp sensors. It ignores the O2 sensor(s) at this time. A closed loop timer starts counting, and when the timer runs out, (around 2 to 3 minutes depending on the vehicle). When this timer stops, then the PCM looks at the O2 sensor voltage. If the sensor is worn and lazy, then it's voltage is too high. The PCM sees this as a over rich condition and starts to lean the fuel mixture. Well it leans it so much that the RPM, start to drop. So then the PCM tells the IAC motor to open up the air passage in the throttle body (the sucking noise you hear). If you drive the truck, it runs poorly because it is too lean. Eventually the O2 sensor starts switching and all is fine until the next cold/cool start. If you have not replaced them, then I recommend doing that as your next move. From the description of your problem, I am 98% sure that this will fix it.



Customer: replied 4 years ago.
I'm an electrical engineer - this is beginning to make sense to me... yea, about a 2 or 3 minute delay before noise starts. So, I'm looking for the sensor(s)... I found one easily behind the catalytic converter but from what I read, there's another one before the converter but I can't find it. What happens if I disconnect one or the other? - I see the sensor behind the converter - would that in theory bypass the o2 sensing since the computer might need "before" and "after" to gauge something?

It is the upstream O2 sensor that controls fuel. It is in front of the catalytic converter closer to or on the exhaust manifold. The sensor behind the cat is for catalytic converter monitoring. Look again for that upstream O2. If you need, I can maybe find a picture of it for you.


Customer: replied 4 years ago.
Found it - yea, top of the converter - the connector is on top of the trans.

So, YEP IT"S THE PROBLEM... But I pulled the 2nd sensor out just in case then. I idled for about 10 minutes - just a slight roughness perhaps around 3 minutes or when the RPMs fell from about 800-ish to about 650-ish.

Now can I ask another question? As I disconnected the front sensor, I notice a bunch of wiring harnesses - so this is the driver's side of the trans. Now I just happen to look near the skid plate I guess and see a fat red colored hose I guess - vacuum hose with a funny very thick, rubbery fitting that looks like it for connecting to a metal vacuum line. So this is disconnected due to corrosion but I definitely see a hole inside the connector. So there are actually 2 "hoses" bundled together - black and red. The black on is OK. I traced to a short bend of 2 tubes that goes to another set of black/red hose connection then to the front axle (so right side of the engine (below passenger's feet I guess) . Is this something to control locking in front 4WD?

Don't worry, You get a bonus/tip for the o2 sensor issue. from me for sure!

Yes those vacuum lines are forXXXXXcontrol of the front axle. Engine vacuum goes to the vacuum switch on the transfer case. Then when you switch toXXXXX vacuum is sent to the actuator on the front axle which locks the hubs. If you have a corrosion or connector problem, get that fixed so that yourXXXXXworks properly.


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