Well i bought a 89 gt and threw on longtubes, o/r h-pipe, 65mm throttle body new plugs, wires, distributor cap and rotor. It has a miss at low rpm and i cant figure out why.It had it before i put all that on. Could it be head related? Valvetrain? Please any help would be appreciated. thanks
yes its a constant miss. at idle up to about 1800 rpm is when you can really tell. Even on the freeway at about 65mph you can hear it. It sounds like what a burnt plug wire sound like basically. I put all new motorcraft plugs, wires, msd cap and msd rotor in.It revs up fine and runs strong when you get on it. if anything its just lacking a lil power when i release the clutch coming out of first gear slowly. Thanks for the help
If it were me, id pull wires untill i founf the cyl thats missing, then i would ohm out the wire just to be sure.. then id swap the plug with a known good one and see if that changes.. if not id do a dry and wet compression test on it and see if i had a base engine concern with that cyl. if the compression is good, id start looking at the injector, and after than id be looking at the injector harness..
Find out is one or two cylinders, or is it all of them missing? Here's how to find out...
Cylinder balance test:
Warm the car's engine up to normal operating temperature. Use a
jumper wire or paper clip to put the computer into test mode. Start
the engine and let it go through the normal diagnostic tests, then
quickly press the throttle to the floor. The engine RPM should exceed
2500 RPM's for a brief second. The engine RPM's will increase to about
1450-1600 RPM and hold steady. The engine will shut off power to each
injector, one at a time. When it has sequenced through all 8 injectors,
it will flash 9 for everything OK, or the number of the failing cylinder
such as 2 for cylinder #2. Quickly pressing the throttle again up to
2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is
weak and isn’t putting out power like it should. See the Chilton’s Shop
manual for the complete test procedure
Dump the codes and see what the computer says is wrong…Codes may
be present in the computer even if the Check Engine light isn’t on.
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check
engine light, or test light or voltmeter. I’ve used it for years, and it works great.
You watch the flashing test lamp or Check Engine Light and count the flashes.
Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.
Dumping the Engine Running codes: The procedure is the same, you start the engine with
the test jumper in place. Be sure the A/C is off and the transmission is in neutral.
You'll get an 11, then a 4 and the engine will speed up to do the EGR test.
After the engine speed decreases back to idle, it will dump the engine running codes.
Here's the link to dump the computer codes with only a jumper wire or paper clip and the
check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
You watch the flashing test lamp or Check Engine Light and count the flashes.
If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method.
There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
The STI has a gray connector shell and a white/red wire. It comes from the same
bundle of wires as the self test connector.
89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.
The STI has a gray connector shell and a white/red wire. It comes from the same
bundle of wires as the self test connector.
WARNING!!! There is a single dark brown connector with a black/orange wire.
It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector.
If you do, you will damage the computer.
What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's
internal workings are OK, and that the wiring to put the computer into diagnostic mode is good.
No code 11 and you have some wiring problems.
Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder
balance test. Low compression on one of these cylinders rules out the
injectors as being the most likely cause of the problem. Look at cylinders
that fail the cylinder balance test but have good compression. These
cylinders either have a bad injector, bad spark plug or spark plug wire.
Move the wire and then the spark plug to another cylinder and run the
cylinder balance test again. If it follows the moved wire or spark plug,
you have found the problem. If the same cylinder fails the test again,
the injector is bad. If different cylinders fail the cylinder balance test,
you have ignition problems or wiring problems in the 10 pin black &
white electrical connectors located by the EGR.
How to do a compression test:
Only use a compression tester with a screw in adapter for the spark
plug hole. The other type leaks too much to get an accurate reading.
Your local auto parts store may have a compression tester to rent.
If you do mechanic work on your own car on a regular basis, it would
be a good tool to add to your collection.
With the engine warmed up, remove all spark plugs and prop the
throttle wide open, crank the engine until it the gage reading stops
increasing. On a cold engine, it will be hard to tell what's good &
what's not. Some of the recent posts have numbers ranging from
140-170 psi. If the compression is low, squirt some oil in the cylinder
and do it again – if it comes up, the rings are worn. There should be
no more than 10% difference between cylinders. Use a blow down
leak test (puts compressed air inside cylinders) on cylinders that
have more than 10% difference.
See the link to my site for details on how to build your own blow
down type compression tester.
I hope I have a bad injector too! don't like messing around with all the sensors all that much. I haven't got a chance to check the codes but I got an adjustable clutch quadrant for $20 brand new from Pacifica Mustang that this guy is gonna trade me for his set of Motorsports #24 injectors. Kinda a good swap i'd say! now just got to get a tube a and bigger MAF.I found the dead cylinder by just unplugging the #4 plug,which was suspect due to a little lifter noise, and the idle remained the same. So I hope the injectors work! I think I got a bad deal on the car, should have been more patient.........I'm fixing all sorts of problems! next is a hacked up torque box