I have a 91 formula with 350 tpi. The engine shakes when you rev it in nuetral, it feels like it is out of balance. But the rpms are smooth and don't jump around, and the car hasn't lost noticable power.
I have had people tell me it sounds like an ignition issue which is what I thought as well.
I checked timing, knock sensor, tps, map, iac, O2 everything good.
The engine was rebuilt less than 100 miles ago, and the heads remanned.
It has new plugs wire cap rotor. I double checked all those, and no problems.
It has new fuel injectors, and fuel filter.
I have done a solid complete tune up.

I am wondering if my problem could be with the ecm. I had the car at what I know now to be a very shadey GM service center for an unrelated problem, and they told me to diagnose the problem I was having at the time that they tapped on my ecm with a hammer to see if it changed the way the car ran.
Im wondering if they may have hurt the ecm by doing that.
Can the computer cause such a problem? and what does the computer control as far as ignition is concerned besides the timing? because I unplugged the EST wire and the car still had the same symptoms.

 

your sure there isnt a a couple of plug wires swapped around?

 

look at your firing order thats on the intake base, check all your wires to be sure you have it in the proper order, IIRC its clockwise start with one and go thru the entire order, check your cap and rotor to be sure theyre not corroded etc etc also check your plugs to be safe

 

hitting or taping an ecm not a good idea

 

I checked the firing order all is good there. The cap and rotor are brand new and this is the second set of plugs that the car has the same symptoms with. Plug wires all tested good resistance as well

 

I figured hitting the ecm was a bad idea, but do you think the ecm could cause this problem. I have no codes, and everything besides this little problem seems to be great

 

where did you get the fuel injectors

 

I got them from a place called five motor sports. They are stock 22lb replacements. With 4 hole spray. Do you think they might be a culprit?

 

let's hope not. new injectors sometimes have problems what brand?

 

Check your motor mounts. Sounds like one is loose to me.

 

Shakes when you rev it in neutral, what about drive?
Does it "run" rough or does idle just shake the car?
Probably an engine mount. I agree.

 

 

The fuel injectors are bosch. The RPMs are smooth at idle the needle doesn't even jump a beat. I will check the motor mounts but I don't think that is it, because I had that problem in a different car once and it isn't the same as that.

 

It might not help, but I would report the hitting the ecm to the boss of this mechanic, or someone in management at the dealership where this happened. It sounds like the injectors to me. When you idle it's smooth. When you ask more out of your engine it runs rough. Not getting enough fuel to all cylinders.

 

tapping on the ECM isn't a problem, its a very useful test when trying to find a possible problem with the ECM as is lightly flexing the computer housing.
the fact is, tapping on the ECM is listed in service manuals. ford first listed it as a valid test many years ago.
now, beating on the ECM is not useful and will cause damage.
my tool of preference for tapping on an ECM is a medium sized screw driver, but i have been known to use wrenches, sockets, ratchets, extensions and the occasional small hammer.


trying to figure out if you have a vibration problem or a misfire problem over the internet isn't always the easiest thing.
you say the motor was rebuilt, but not if the problem was there before the rebuild.
did it show up with the motor job? was the flex plate, balancer or torque converter changed? what RPM does the shaking show up at? is it worse at certain speeds or is it there all the time?
this works best with a known good running motor. to see what a misfire feels like, pull 1 plug wire and take an old spark plug and put it in the pulled plug wire. put the spark plug some place where it won't move or get into anything. under the metal part of one of the windshield wiper arms works great. now start the motor and notice how it feels, rev it up some. drop the trans in gear and feel the rhythmic pattern.
with it still in gear, give it a little throttle and see how the miss feels worse.
an intermittent misfire will feel different, but still similar.

if its there at certain engine speeds, with the trans in park, rev the motor up to 3500~4000 rpm and let off the throttle and turn the key off at the same time, do you feel a vibration that comes and goes as the engine slows to a stop?
the wrong or a bad flex plate, balancer or torque converter could be the cause of harmonic shaking. bent pulleys and damaged accessories can also cause vibration problems.

 

So help me I have never heard of taking a hammer to an ECM, but I learn new things all the time. So you are saying that vibrations manifest at different RPMs? I have had the same thing with tires. Makes sense. I still have a feeling it's induction related.. rough running engine.. I didn't re read the whole thread, but it didn't sound like he was describing a vibration.

 

most people do freak when i tell them to tap on the ECM.
of course i am talking about lightly tapping and not beating. dropping a standard 1/2 inch wrench from about 6~8 inches is about as hard as needed.
when i say a small hammer, i mean really small, i would guess my small hammer is around 4~5 ounces tops.
tapping is also very useful for checking the MAF and MAP sensors too.
talking about tapping on components to check them for problems in a room full of seasoned professional auto mechanics won't even get you noticed.

he said it feels like it is out of balance, so need to figure out if its just a misfire, or a vibration.
just like with tires, an out of balance motor will be worse at certain speeds.
i once had a defective new water pump that caused a vibration around 4000~4200 RPM. below or above that area and it felt fine. with no coolant in the system you didn't feel any vibration. the pump was missing 2 of the vanes on the impeller.
Horse Power on Spike TV bought a used Dodge motor for cheap. they tore into it and found it had some decent parts. they didn't really see anything wrong, other than it had the wrong balancer on it that would have caused a very bad vibration, which is probably why they got it cheap.

 

I never noticed this problem until after I put the new injectors on it, but my old stock injjectors had low resistance and were dumping too much fuel causing the engine to miss like crazy at an idle, but because it was running so poorly before I never noticed this problem. I will do the rev to about 4000 rpm and shut it off. If that isn't it I am going to put my old injectors in and check for vibration. If there isn't any I will assume it is the injectors.
I haven't seen any reviews on the bosch four hole injectors before, but Im thinking maybe those little tiny holes restrict the flow and the car is starving at higher rpms.

 

maybe southside would know but 4 hole injector? spray pattern might be difficult for your engine