Viper RPM fall to stall on clutch in / throttle chopped

OK so I plan to chase down this year an annoying problem. I lived with it the last two viper seasons but now I want it solved

In order to explain the problem there is a story...

2 years ago I had cooling /manifold gasket leak, this motivated me to get new gaskets installed at the local "viper dealer" in town. So when I got the car back the leak had stopped and all seemed ok for the smell of burning coolant. However in a short time after picking up the car I noticed it ran a little bit different. It seem more touchy off the bottom end and the sound the car made when just the engine retarding on decel... seemed louder and different tone. AND it began stalling when coming to a stop sign or traffic light etc...

Essentially as the the car is decelerating, if you clutched in to drop the gearbox into neutral the result RPM drops as soon as you clutch in...the needle drops down to 0 RPM the engine stutters and I have to race to feather the gas to keep it idling. The dance I have to do to keep it running is ridiculous.

I took the car back to the dealer and they checked the throttle bodies and reset the PCM to "relearn" the throttle map for how I drive. (this sounds like BS to me, most fuel curves are set by design but with programmers can be shifted about). It kept happening so I took it back and asked them to set the low idle higher , they said it was set fine and idled good when sitting.

Now some people have suggested that the work on the exhaust manifold may be "pulling too much vacuum" but I am not sure what this terminology means.

Why would the RPMS drop so low to where a stall occurs? It is true that the engine idles fine at rest.

Any ideas?

N

How is the idle when its just sitting and not driving? Could be a vacuum leak, look for lose hoses etc. Also, you can do a carb cleaner / starter fluid spray test around the manifold to see if RPM's pick up. Might want to start there first.

You can try the following. Don't know if it will work for you, but it only takes a few seconds to do. There is a little adjuster screw on the bottom part of the driver side thottle body, which will open up both the tb's when adjusted. Just give it a slight turn (maybe a quarter-half turn), to open the throttle bodies slightly. I believe it's a small allen head screw. The idle should be the same as before, as the computer will compensate, but you should get slightly higher rpms when releasing the throttle.
If that doesn't work, just turn the screw back to where it was before (make a mark on it before turning). 30 second procedure...just a suggestion

Thanks Indy , I will try your method this weekend and report back... I have the battery out and cleaning up some new speaker wiring/ amp install...

daveg, the car idles perfectly, my goal is to understand the mechanics of this problem as well.

1. Red Light Approaches, must slow down engine is at 3500 rpm, start to brake, foot is off gas.
2. At this time the throttle bodies are in closed position.
2.5 Clutch in and intake pressure changes as engine is disengaged from transmission. This pressure change seems to initiate the condition...
3. Retarding RPM is likely creating a high pull on the air intake since RPM is high , however the Air/Fuel mixture is applied for position of the throttle body = 0 position.
4. Combustion is fundamentally stopping, either not enough air or not enough fuel.
5. (Vacuum Leak would imply too much air? thus O2 sensor may override from measure in intake manifold?)
6. (Or could the throttle body position sensor be send too low of a signal due a pull on the intake thus resulting in low fuel contribution?)


daveg method sounds like great fun - spraying a flammable vapor into an engine compartment. I have eased up on the darwin applications as I have aged. Might be time to join back up! Would the small quantity of the localized spray be enough to get a noticeable rev change?

N

6. (Or could the throttle body position sensor be send too low of a signal due a pull on the intake thus resulting in low fuel contribution?)

Click to expand...

The TPS isn't adjustable on these cars (at least the stock ones aren't). The way it's bolted on "should" give you the correct reading. I know with other cars, you can loosen the TPS and turn it to either side to change the voltage. It's easy to test with a multimeter though. Have someone inside the car while checking the voltage at various throttle positions.

Hey Fed,
I use this method all the time when suspecting a leak. I Learned it from my Dad (old time mechanic). As you spray light spurts around places that could have a leak, the rpm's jump right up because if there is a leak at that spot, it will draw in a flammable mist and not just air which causes the RPM's to increase for a moment. The only reason I suggested this first is because its “cause in affect” and since the problem happened right after manifold work………… And yes, its definitely a noticeable rev change...

Very interesting, I don't know what generation viper you have but I know how my Gen II works. As soon as you lift off the gas completely, Your engine computer only fires the injectors for three to four more seconds, After that you're coasting with the totally dead motor. When you push the clutch in there is a clutch actuated switch under your dash on the pedal, a signal is sent to your computer to bring the injectors back online and Idle, that doesn't explain a lot of things but it might clue you into how the system is supposed to work. check the pedal switch adjustment. good luck

Good stuff, I now have a few methods to try, thanks for the help. I'll add to my signature ... the car is a Gen I , 1995 RT/10,

thx again, I will report back my findings, if I manage to burn all my eyebrows/eyelashes off I will also post a picture for everyone to have a cheap laugh.

N

Darwin / eyebrows.

Indy, what do you use to dress the engine / compartment?

daveg said:

Indy, what do you use to dress the engine / compartment?

Click to expand...

Just interior detailer on the platics/rubber. And I try to clean the underhood stuff fairly regularily. It's not that clean The pic just doesn't show the details...

I've had the exact thing you describe happen on my Gen II. What I did that corrected it was to spray Mopar combustion chamber cleaner directly into the ICM (idle control motor) with the engine running.

Here some insights:

1. The idle is fixed at approximately 680.

2. The Deaccel rpm is set a little bit higher. These are both set in the PCM and cannot be changed unless you have access to a SCT programmer.

3. The Idle Air Control (IAT) is the primary control of the idle and Deaccel rpm. Timing is also used, however, to a lesser degree.

4. If something causes the Deaccel rpm to drop below idle rpm, the car will stall. The highest likelyhood is that IAT is dirty or faulty. A bad vacuum leak could also reduce the ability of the IAT to maintain both the idle and Deaccel rpm = therefore, causing a stall.