Sometimes stalls when coming to a stop.

maverickagm

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I have a 1993. As the title says, when coming to a stop my rpms will drop to near zero. It's even stalled a couple of times. At operating temperature the idle seems low. It shows closer to 575 that 704+-25. Yes I am reading the gauge correctly. Yes I realize the first mark just after 0 is actually 500 rpm, and the second is 750 rpm.

I've been changing things one at a time and retesting. The problem still persists.

1) Cleaned throttle bodies.
2) Had battery load tested. It was like new. This would inevitably include a PCM reset.
3) cleaned Idle Air Control (IAC) valve.
4) replaced the IAC

I had an instance where the throttle would suddenly surge back and forth from 1500 rpm to ~2200 rpm. I became suspect of the Throttle Position Sensor (TPS).

5) Measured voltage from TPS. As a reference, here's how I did that:
i. Open hood. Turn the key to ON (but don't actually start the car). The TPS is on the driver's side throttle body.
ii. There's a plug with 3 wires. Top is Ground. Middle is the sense wire. Bottom is 5 volt supply. I can fit my multimeter probes int the back of the plug while it is connected. You could also use a sewing needle to put in the back of the connector then touch the probe to that.
iii. I checked between 5v supply and ground and got an even 5 volts.
iv. I checked between ground and the sense wire. 0.72 volts at the idle stop. 3.63 volts when I fully open the throttle body. The service manual states I should get aprox 0.65v at idle and 3.8v at wide open throttle.
v. I slowly open and close the throttle and watch the voltmeter slowly increase and decrease.

Here's a picture of what it all looks like. Negative probe is in the back of the top plug wire. Sewing needle is in the middle. Positive prove is touching the sewing needle. The reading where the same with and without the sewing needle. It just made it easier.
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6) When measuring the volts of the TPS, I opened up the throttle and rapidly closed it. I did this about 30 times. Two of the times I saw the volts drop briefly to 0.45 volts and 0.65 volts. I then replaced the TPS.

It's still happening. My next step is to (mark first) then play with the throttle position stop screw. My current theory is the throttle blades close and choke the engine of air, then the IAC opens the pintle to compensate. However the IAC motor is too slow to react. The service manual mentions an Idle RPM Test:

"With the engine at normal operating temperature
and the idle air control motor pintle fully extended,
the engine should idle at 704 ±25 RPM. Use the
DRBII scan tool to fully extend the idle air control
motor."

With the IAC pintle fully extended, that should close off air coming through the IAC and all the air should be coming through the throttle bodies. That leads me to believe the IAC is mostly responsible for adding extra air when the air conditioning or headlights are turned on (which adds slight engine load that would normally lower the RPM). I don't have a DRB 2 scan tool, but perhaps I can open up each throttle body in increments and stop when the idle rpm rises beyond 700.

Has anyone else ever actually solved this problem?
 
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Steve-Indy

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Have you looked at MAP sensor?

Our 95 idles at 500 rpm when fully warm on hot day with A/C off.

By the way, a DRB III should work if you are using the correct "Y" cable.
 
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maverickagm

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Thanks. It's a good point that the gauge may not be accurate. I ran across another thread where someone said the RPM reading from the gauge was lower than whattheir scanner was reporting. I haven't looked at the MAP sensor yet. I understand there's 3 wires. 5v supply, ground and the middle wire is the sense wire. I haven't found what a normal voltage reading is at idle though. The Service manual doesn't list it.

A friend of mine suggested it could be dirty fuel injectors. I'm going to run a bottle of Lucal fuel injector cleaner. If that actually helps, I'll remove the injectors and have them cleaned. I'm not sure how to remove an injector without having fuel spill into the cylinder though.
 
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maverickagm

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Thanks all for your replies. I've had a busy week so I've been slow to work on this problem.

I haven't touched the throttle position stop screws. I looked at both throttle bodies and both have the plates stop just behind the first small hole in the bottom of the throttle body.

I added Lucas fuel injector cleaner and ran through a quarter of the tank. This appears to be helping my rough idle. My shifter no longer has the shakies when at a stop.

However I still experienced momentary low rpm when coming to a stop (though it hasn't been stalling). I also still have bucking and surging when lightly pressing the throttle. Of course, In a pinch the simple work around is the accelerate, let off the gas, accelerate, etc, rather than just lightly press the throttle. So as suggested I revisited syncing the throttle bodies. That is when I noticed something weird:

Passenger Side:
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Driver Side:
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There is a millimeter or two of slack in the cable on the driver's side. That would cause the driver's side to open after the passenger side. The service manual states that they should open simultaneously, or the left side should open up just before the right. So either I was doing the sync process wrong, or the cables are working themselves loose. I doubt they've been working themselves loose because, although it's not in the picture, I have screws and washers that screw into the cable push button to hold them in place. I believe there was a Technical Service Bulletin about this.

This time when I synced them I pressed the gas pedal to the floor, listened for both clicks, but then continued to press on the accelerator harder before finally letting go. When I checked my cables there was no slack on either side and both throttle bodies appeared to be opening simultaneously. I'll be out driving for a good amount of time tomorrow, so I'll see how things go.

For reference, Here's what the service manual has to say about adjusting both throttle cables at the same time:

(1) Push the button in on the throttle cable ratchet.
The shaft will pull in. Do both adjusters.
Ensure the shafts pulls in fully.

(2) To adjust the throttle cables to the proper position,
push the accelerator pedal to the floor. The ratchet
will click while adjusting to the proper position.

(3) Check the throttle cable adjustment.
The throttle bodies should open simultaneously or the left
throttle body should open just before the right throttle body.
If not, loosen the right side accelerator cable support bracket.
Push the bracket forward and tighten the mounting screws.
Adjust both throttle cables, starting at step (1).
 

vprtech

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Hi, I worked as a dealer Viper tech in the 1990’s and I also understand how they work at the code level as I tune these. You must adjust the cables so that the drivers side throttle body is opening just before the passenger side . Don’t adjust them to open the same, it would be better to have more lead on the drivers side than no lead. It is critical because on a 92-95 Viper the ecu will adjust the idle valve to achieve a specific map pressure when in “decel mode” i.e. is recognizes that the throttle is closed and the car is moving. If the driver side throttle is closed but the passenger side is open the idle valve will close to increase the vacuum, then when you de-clutch the car will stall or the idle will dip. Not saying that this is your issue but it certainly could be. Also, if you don’t have a Chrysler specific scan tool I highly recommend you get one otherwise you won’t really understand what’s going on. A used snap on MT2500 or DRB2 can be found on eBay for a reasonable price.
 

GTS Dean

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The TB nearest the MAP sensor (Driver's Side) should sense the pressure change first, so the cable response should be immediate. This allows stepping the IAC down on the opposite (Passenger Side) as the lagging TB opens on that side.

I believe the IAC moved from PS rear to PS front on the 3-rib manifolds in '94, maybe '95.
 
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vprtech

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@ GTS Dean, I'm not sure I understood 100% what you were trying to say in your post. Do you agree with what I said or are you saying that the IAC should compensate for the throttle opening on the passenger side therefore it make no difference ?

The MAP sensor location has more to do with getting a more accurate and fast MAP signal to the ecu for better transient fueling control. Since there is an equalization tube between both intake runners there is no issue with the map signal varying between both banks at low throttle angles.

Besides the stalling issue think of it like this from a drivability standpoint. When the throttle voltage increases past .04 volts from the closed position the ecu then goes into open throttle mode, below that it is in either decel mode or idle mode. If the right side throttle opens first from a stop the ecu will still be trying to control idle speed retarding ignition timing and closing the IAC. When the driver side throttle finally open enough to cross that .04 volt threshold the engine torque will suddenly increase causing a buck or ****.

if you ever observe the throttle opening on a G2 you will see that the factory setup the drivers side throttle to lead the passenger throttle .
 
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maverickagm

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I did manage to solve my problem.

For synchronizing the throttle bodies, I did the usual routine then used my iPhone slow motion camera at 240 fps to capture both throttle plates opening while a friend slowly pressed the accelerator pedal. From reviewing the footage frame-by-frame they seemed to be opening at the exact same time. So I cut a small washer and wrapped it around the tip of the throttle cable just to ensure the driver's side opens slightly sooner. It sure makes sense to ensure the driver's side throttle body is leading since that's the side with the throttle position sensor.

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I also think the fuel injector cleaner helped quite a bit.

It's also worth mentioning that I replaced my plugs and cables over the winter with Autolite XP985 and Quality Wires. I'm not sure if this contributed in any practical way.

Also worth mentioning that I went and snugged down all my intake bolts. Most didn't turn, but with light pressure, some turned and tightened down. I may have had minor vacuum leaks, but no codes were ever thrown.

Throttle tip-in and tip-out is fine now. Hopefully this helps someone in the future.
 
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