Rpm Drops To 0 When Downshifting...

James.Little · Oct 14, 2013

I unsure of the problem may be with my fox, but here is a description of what is going on and, I was hoping you guys could give me some ideas:
So I have noticed something is wrong with the car for a couple of weeks now, but the problem seems to be getting worse and worse.

* When I am driving, speeding up, the stick shift vibrates pretty bad, worse than ever.
* When I am coming to a stop, slowing down, SOMETIMES the steering wheel shakes like crazy. (Not sure if related)
* When slowing down, I can hear a lot of noise coming from under the car, like the drive shaft slowing down, but super loud or something in the transmission.
* I have also discovered a new "feature", if you will: when I downshift and let off the clutch the RPMs jump, then I push the clutch back in the RPMs drop close to 0 or sometimes it hits 0 and stalls the motor.
* Also when taking off, I have to give way more gas than I should, or else the engine will stall.
* The car is also very sluggish compared to when I first got it.

Any ideas on what I should take a look at or replace would be awesome, I have heard several things over the last couple of weeks that it may be, like:
* Bad U-Joints
* Out of balanced tires
* Something with the transmission
* Bad EGR valve
* Clogged Catalytic Converters

Thanks a ton,
James.

Mustang5L5 · Oct 14, 2013

What model car is this? Your sig is a little confusing.

Is this a speed density car converted to mass air? Is the VSS installed and wired?

Have you run the check engine codes (engine off and engine running)

James.Little · Oct 15, 2013

The body is a 1989, the transmission and motor is a 1988. The motor has been converted to MAF for about a year now. I did not install a VSS, but I am 100% that is not the problem and I have not tried pulling codes yet. The lack of a VSS only stalls the engine when I press the brake and down shift at the same time.

Gearbanger 101 · Oct 15, 2013

Vibration and driveline noise could be a bad U-joint...could be your balancer on the way out, could be some component of your exhaust touching the frame, could be a loose wheel...could be a lot of things. Driveline vibrations are the worst.

Stalling when you let off the gas could be a dirty IAC

First things first....run the car for codes and see if anything pops up...then get back to us.

James.Little · Oct 15, 2013

Awesome, sounds like I have a lot of stuff to look at. Also, I cleaned my IAC a few months back, is it time again?

James.Little · Oct 15, 2013

Awesome, sounds like I have a lot of stuff to look at. Also, I cleaned my IAC a few months back, is it time again?

Noobz347 · Oct 15, 2013

The VSS can in fact, cause the motor to die when the clutch is pushed in and you're rolling or coming to a stop. Don't ask me to explain that because when I had a problem similar to what you describe, I hooked the VSS up at the recommendation of a friend. When the symptoms went away, I stopped there and didn't look into it any further.

On the IAC well... I've got a couple lying around that will work on one car but not so well another (silly, I know). I now have them marked for which of my cars they best work.

The vibrations are the thing I would look at first and try to get resolved (at least until you have confirmed the culprit and know whether it will cause harm). Damper, U-joints, clutch, flywheel, TOB and/or retainer, driveshaft, yoke, rotors, tires, wheels, wheel bearings.

You gotta pull codes. Even if they're clear that tells you SOMETHING.

James.Little · Oct 15, 2013

I know the VSS can stall the motor, but I did my MAF conversion over a year ago and it has never given me any trouble, and I seriously doubt that is what is causing the issue now.

Also I had a clutch kit installed about four months ago, including throw out Bering, clutch fork, pressure plate, clutch and I think some other things. I also had them turn the flywheel.

I could try cleaning the IAC, again, but I don't see any changes by doing so, but its worth a shot.
How certain are we that the problems are two different things, and not one problem?

This saturday I am going to rod out the cats and replace the U-joints, and I will try pulling codes, unless somebody convinces my otherwise.

Thanks.

Mustang5L5 · Oct 15, 2013

I think you have a few different issues, not just one here.

1st things first...always pull the codes when you have an issue with how the car is running. Helps cut down on the guesswork.

Your first 3 issues sound mechanical in nature. Something is out of balance, loose, broken, etc. in the drivetrain. None of that is normal obviously and I don't beleive it to be an issue with regards to how the engine is running.

Clogged cats....are you running the air pump?

Possible bad U-joint, transmission mount, engine mounts, harmonic balancer. Definitely check these.

James.Little · Oct 16, 2013

Air pump? I still have a smog pump if that's what you mean. Since I got the car I have been meaning to rod them out or straight pipe the exhaust, so this only gives me a reason to rod them out. Someone told me the car wouldn't start if the harmonic balancer was out, is that true? Also, I won't get a chance to pull codes until this Saturday, but yeah, I have been meaning to do that.

Also, I notice that the RPM doesn't actually go all the way to 0, but really close, like 80-200 RPM, but on rare occasions, it will hit 0 and stall the motor.

Thanks for all the help!

Mustang5L5 · Oct 16, 2013

Car would still run, but it would shake badly.

Doubt it's the cats

James.Little · Oct 16, 2013

Yeah the car doesn't really shake all that bad.

Mustang5L5 · Oct 16, 2013

Still...inspect it

James.Little · Oct 16, 2013

I will! I'm still learning about mechanics, with that being said, where is the harmonic balancer and what should I be looking for?

jrichker · Oct 16, 2013

Simple explanation: The harmonic balancer is the gizmo that mounts all of the belt pulleys on the crankshaft.

Complex explanation:
Every mechanical device that moves has a primary resonant frequency. Did you ever watch the radio antenna on your car vibrate while you are driving at highway speeds? The ends are still and it wiggles in the middle. That' an example of a resonant frequency in action. A good balance job eliminates the most of the vibrations at the primary resonant frequency. The engine's crankshaft and rotating assembly (pistons, rods, piston pins, etc.) generates vibrations at frequencies that are a multiple of the primary resonant frequency. The harmonic balancer helps eliminate/dampen those harmonic vibrations which would damage the engine if left untreated.

Mustang5L5 · Oct 16, 2013

http://www.stangnet.com/mustang-forums/threads/harmonic-balancer.783210/

Some more pics and explanation. This may not be the cause....but it's something to inspect and check off the list.

flstang65 · Oct 19, 2013

Out of balance drive shaft causing the vibration? I have seen that before.

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