Drivetrain Crazy vibration: need some help!

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
Hello everyone!

I just recently bought a 90 notch mustang. Most of the mods on this car was done 10-15 years ago.

It has a mild 306 h/c/I and a old msd 6A ignition box. It has a t5 transmission, and a 03-04 cobra IRS with 3:55 gears.

The previous owner said he never had this problem, but maybe he didn't drive like I did to create this whole car shaking!

The problem only happens at 1500 rpms mainly in 5th gear. I initially thought it was mph specific, because it would do this in 5th gear between 40-50 mph. But I created the same symptoms in 4th gear at 1500 rpms but it wasn't near as violent. It feels like it's right under my seat! The car also feels like it hesitates while cruising around town. If I put the car in neutral and rev up to 1500 rpms the car doesn't shake?

Some stuff that I replaced:
- Spark plugs, wires, distributor cap and rotor.
- Fuel filter.
- Driveshaft u-joints.
- Harmonic balancer, the old one had a wobble.
- Motor mounts look worn, I have new energy suspension ones, but haven't installed yet.

I verified the tps voltage, cleaned the maf sensor, and changed out the old mechanical fuel pressure gauge. The new one was only showing about 20 psi with the vacuum line on. I bumped it up to 39 psi. I also checked vacuum at idle which was 14.

I plan on having the tune checked out, but it's about an hour drive, so I want to make sure it's road worthy.

I haven't checked the timing yet. I had a mechanic friend drive the car. He's been a mechanic for a long time. He built my old 347 back in the early 2000's. He described the feeling as "lugging". That's what he thinks it is, but I'm not so sure.

I'm going to check codes again soon, I did it on a cold motor, scan tool said to do it on a warm motor.

Sorry for the long post, looking for some insight. I posted as drivetrain, but feel clueless as to what it is.
 
  • Sponsors(?)


nickyb

WAIT,you now have a pair?
10 Year Member
Apr 3, 2009
1,416
723
143
59
nevada
When you pulled the driveshaft did you mark it at the rear and put it back in the same location?
If not try marking spot it's at now and remove and turn 1\4 turn and replace,you might have to do this 2 or 3 times to get back to original spot.
 

Rdub6

So while I wait to figure out my rear end issues
Dec 29, 2017
3,021
2,295
143
Long Island, NY
My first fox years ago had some ball bearings fall out of the fan clutch. It caused a vibration so bad, it felt like the whole tranny was blowing up. Simple fix once I found it.
 

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
When you pulled the driveshaft did you mark it at the rear and put it back in the same location?
If not try marking spot it's at now and remove and turn 1\4 turn and replace,you might have to do this 2 or 3 times to get back to original spot.

Yes I did mark driveshaft, had the symptoms before I replaced u joints.
My first fox years ago had some ball bearings fall out of the fan clutch. It caused a vibration so bad, it felt like the whole tranny was blowing up. Simple fix once I found it.

I'm running a electric fan.
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,472
2,816
234
76
Dublin GA
lowendmac.com
I verified the tps voltage, cleaned the maf sensor, and changed out the old mechanical fuel pressure gauge. The new one was only showing about 20 psi with the vacuum line on. I bumped it up to 39 psi. I also checked vacuum at idle which was 14.

I plan on having the tune checked out, but it's about an hour drive, so I want to make sure it's road worthy.
Check fuel pressure:
The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
Turn the ignition switch on & start the engine. Observe the pressure: you should see 38-41 PSI at idle.
Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

Fuel pump pressure test
Disconnect the larger of the two fuel lines up by the Schrader valve. It is the return line and does not have the Schrader valve on it. Find a piece of rubber fuel hose and clamp it on the return line coming from the regulator. Stick a bolt in the other end of the hose and make sure that all your connections are tight and leak proof as possible. When this powers up, you don't want fuel squirting everywhere. Hook up the fuel pressure test gauge. Turn the ignition switch on and watch for leaks. You may want to use a helper inside the car to cut the switch off quickly if you have a leak. To trick the fuel pump into running, find the EEC test connector and jump the connector in the Upper RH corner to ground.

attachments\68357


Caution!!! You have blocked the return line for the fuel pump! Pressure will rise very quickly past safe levels with a good pump
If the pressure goes up past 55 PSI, the pump is good and the fuel pressure regulator is bad. If the fuel pressure does not hit 55 PSI or more in a few seconds, the pump is bad or you have electrical problems.
 

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
Okay guys,

The car is at my storage unit right now, but I was able to run codes again. I used the Ford code reader device. Yesterday it would say 8 cylinder, but then 6 cylinder. And today it said 6 cylinder. Makes me wonder how accurate this scanner is.

Also my fuel pressure gauge was reading 42 psi today. I had it set at 39 psi. But yesterday I believe I seen it around 37 psi.

Here are the codes:

KOEO:
10
15
33 continuous memory

KOER:
13
33
44
94

Now the car doesn't have a egr, smog pump, or cats.

Hopefully I can run this fuel test mentioned above soon. I may also do a compression test to see what each cylinder is at.
 

Blown88GT

Founding Member
Nov 13, 1999
2,177
521
164
Palm Beach Gardens, FL
Hello everyone!

I just recently bought a 90 notch mustang. Most of the mods on this car was done 10-15 years ago.

It has a mild 306 h/c/I and a old msd 6A ignition box. It has a t5 transmission, and a 03-04 cobra IRS with 3:55 gears.

The previous owner said he never had this problem, but maybe he didn't drive like I did to create this whole car shaking!

The problem only happens at 1500 rpms mainly in 5th gear. I initially thought it was mph specific, because it would do this in 5th gear between 40-50 mph. But I created the same symptoms in 4th gear at 1500 rpms but it wasn't near as violent. It feels like it's right under my seat! The car also feels like it hesitates while cruising around town. If I put the car in neutral and rev up to 1500 rpms the car doesn't shake?

Some stuff that I replaced:
- Spark plugs, wires, distributor cap and rotor.
- Fuel filter.
- Driveshaft u-joints.
- Harmonic balancer, the old one had a wobble.
- Motor mounts look worn, I have new energy suspension ones, but haven't installed yet.

I verified the tps voltage, cleaned the maf sensor, and changed out the old mechanical fuel pressure gauge. The new one was only showing about 20 psi with the vacuum line on. I bumped it up to 39 psi. I also checked vacuum at idle which was 14.

I plan on having the tune checked out, but it's about an hour drive, so I want to make sure it's road worthy.

I haven't checked the timing yet. I had a mechanic friend drive the car. He's been a mechanic for a long time. He built my old 347 back in the early 2000's. He described the feeling as "lugging". That's what he thinks it is, but I'm not so sure.

I'm going to check codes again soon, I did it on a cold motor, scan tool said to do it on a warm motor.

Sorry for the long post, looking for some insight. I posted as drivetrain, but feel clueless as to what it is.
Mine did the same thing about 6 months ago.
Replaced balancer & drive shaft...nothing.
It was the Megasquirt tune for the new & larger injectors.

Could be the motor mounts. Replaced mine a long time ago with the same you have ready to go in. Did the transmission mount, too.

Your fuel pressure is OK, it varies with vacuum.

UPDATE:
The root cause was none of the above. A spark plug clip had come off the plug but the boot was still tight. Therefore that one cylinder had to jump 2 gaps, wire to plug & plug tip. This delayed ignition to that one cylinder & was only noticeable at lower rpm, since the delay was insignificant at higher rpm.
 
Last edited:
  • Like
Reactions: 1 user

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
Mine did the same thing about 6 months ago.
Replaced balancer & drive shaft...nothing.
It was the Megasquirt tune for the new & larger injectors.

Could be the motor mounts. Replaced mine a long time ago with the same you have ready to go in. Did the transmission mount, too.

Your fuel pressure is OK, it varies with vacuum.

Maybe I really need to get the tune checked out. I also thought the vibration could be something clutch related or maybe the tailshaft bushing. I can move the yoke up and down a little.
 

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
Update:

I just replaced the motor mounts and transmission mount. Vibration is gone!
 
  • Like
Reactions: 1 user

91GTstroked

5 Year Member
Jun 14, 2007
390
175
63
Pictures of old ones.
 

Attachments

  • IMG_20200407_014300814.jpg
    IMG_20200407_014300814.jpg
    407.9 KB · Views: 62
  • IMG_20200407_014228815.jpg
    IMG_20200407_014228815.jpg
    351.5 KB · Views: 66
  • IMG_20200407_014224467.jpg
    IMG_20200407_014224467.jpg
    327.8 KB · Views: 69