Diag help?

[Jardon]

Hey yall! My fiances 1994 mustang gt 5.0 manual has been having this very rough engine vibration the last couple of months. It initially started when it got cold outside. Ive taken a peek at the motor mounts (look fine), the harmonic balancer (doesnt have any wobble to it), replaced the spark plugs, etc. It gets the worst at about 1k rpm and smooths out at higher rpms and basically shakes the shifter at any load range/rpm range and im trying to think and figure out what the most probably cause is. Its got just over 100k miles. It accelerates great after I replaced the spark plugs (3 were loose before I did it which explained a hesitation issue before this happened). I would love any insights on how to diag this or what yall think the most probable causes are. Thank you!

 

[96blak54]

So you say the engine idles and runs great, but has a rough vibration at low rpm? And this vibration is very noticeable compared to how the car use to be?

Some one correct me, but does te factory harmonic balance fail on the 5.0l after time? A fail balancer would explain vibration. The flywheel/flexplate are counter balanced as well and possible weights detached, but not likely. If it were me, I would investigate the balancer 1st. Pulling it off to inspect would be the easiest and doing so would probably reveal the failure. If im thinking right, age simply does them in.

 

[Mustang5L5]

Some one correct me, but does te factory harmonic balance fail on the 5.0l after time? A fail balancer would explain vibration.

The rubber fails and the outer ring slips on the inner ring. However it usually vibrates at all RPMs, but not always. Unfortunately no easy way to determine if the ring has slipped other than looking at a pic of one and trying to compare.

But 5.0 vibrations at low RPM could be other things as well. In my experience a misfire/dead cylinder presents as a low RPM vibration that clears up. I had a dead injector on my Fox for about 9 months. Car ran awesome but had a vibration under 1500 and would leave two black strips from the tires...on 7 cylinders. Only reason It took me so long was because I just AOD-T5 swapped the car and thought the vibration was related to that.

I'd start with ruling out a misfire. The old OBD1 5.0's had a very crude misfire test called a cylinder balance test that you run during the code reading process. It turns off one injector at a time and can sense the RPM drop. It will tell you if one cylinder is dead or not (how i found my issue). A good code reader for the OBD1 cars is the Innova 3145 which you can get on amazon for about $40.

You can also proactively ohm out the injectors by checking resistance values. Should be about 14.4 ohms with a meter if you unplug them and test across the two connection prongs. You can also use an IR gun, cold start the engine and see if all 8 are warming up evenly.

There's also the question of condition of spark plug wires, cap/rotor, fuel filter and overall injector health as well.

 

[Jardon]

So you say the engine idles and runs great, but has a rough vibration at low rpm? And this vibration is very noticeable compared to how the car use to be?

Some one correct me, but does te factory harmonic balance fail on the 5.0l after time? A fail balancer would explain vibration. The flywheel/flexplate are counter balanced as well and possible weights detached, but not likely. If it were me, I would investigate the balancer 1st. Pulling it off to inspect would be the easiest and doing so would probably reveal the failure. If im thinking right, age simply does them in.

I have done some individual digging and so far it seems to be the balancer. The clutch is original and doesnt change the vibrations whether the pedal is depressed or not. And it accelerates rather well despite the engine vibration. I do also have engine mounts ready to go on his car if its not the balancer (ordered the mounts for my own mustang lol). Im also considering ordering out a new trans mount with the harmonic balancer if I do end up doing to engine mounts so everything is nice and new

 

[Jardon]

The rubber fails and the outer ring slips on the inner ring. However it usually vibrates at all RPMs, but not always. Unfortunately no easy way to determine if the ring has slipped other than looking at a pic of one and trying to compare.

But 5.0 vibrations at low RPM could be other things as well. In my experience a misfire/dead cylinder presents as a low RPM vibration that clears up. I had a dead injector on my Fox for about 9 months. Car ran awesome but had a vibration under 1500 and would leave two black strips from the tires...on 7 cylinders. Only reason It took me so long was because I just AOD-T5 swapped the car and thought the vibration was related to that.

I'd start with ruling out a misfire. The old OBD1 5.0's had a very crude misfire test called a cylinder balance test that you run during the code reading process. It turns off one injector at a time and can sense the RPM drop. It will tell you if one cylinder is dead or not (how i found my issue). A good code reader for the OBD1 cars is the Innova 3145 which you can get on amazon for about $40.

You can also proactively ohm out the injectors by checking resistance values. Should be about 14.4 ohms with a meter if you unplug them and test across the two connection prongs. You can also use an IR gun, cold start the engine and see if all 8 are warming up evenly.

There's also the question of condition of spark plug wires, cap/rotor, fuel filter and overall injector health as well.

From what ive seen, the wires look pretty new, cap looks decent from an external view (i dont trust myself with cap and rotors designed ignition systems). I do also have one of those scanners. We did put in fuel cleaner into the tank a bit ago and that hadn't really done anything to help it out at all. It initially started when it got cold out this winter which leads me to believe its the balancer since rubber gets stiff in cold climates

 

[RAU03MACH]

see if any oil is on the harmonic balancer
oil will eat that rubber
if any leaks at the crank shaft seal replace seal while removing harmonic balancer